Microbiology Questions and Answers

Microbiology Questions and Answers Proper media for culture of a urethral discharge from a man include: a sheep blood and phenylethyl alcohol agars b eosin-methylene blue and sheep blood agars c thioglycollate broth and chocolate agar d chocolate and modified Thayer-Martin agars d chocolate and modified Thayer-Martin agars d Chocolate agar and chocolate agar- based selective media should be used for recovery of Neisseria gonorrhoeae from urethral discharge. Chocolate agar provides the nutrients required by N gonorrhoeae and selective media contains antimicrobial agents that inhibits other organisms and permits recovery of pathogenic Neisseria. A sheep blood agar plate inoculated with 0.001mL of urine grows 70 colonies of Staphylococcus aureus. How many colony forming units per mL of urine be reported? a 70 b 700 c 7,000 d 70,000 d 70,000 d When 0.001 mL of urine is plated, the growth of one colony is equivalent to 1000 CFU/mL. Thus 70 colonies is 70,000 CFU/mL. The lowest concentration of antibiotic that inhibits growth of a test that inhibits growth of a test organism is the: a minimum inhibitory concentration b serum inhibitory concentration c minimum bactericidal titer d maximum inhibitory titer a minimum inhibitory concentration a The MIC is a basic laboratory measurement of the activity of an antibiotic against an organism. It is the lowest concentration of antibiotic that inhibits visible growth of the organism. It does not represent the concentration of antibiotic that is lethal to the organism. Which of the following clean catch urine culture colony counts indicates the patient likely has a urinary tract infection? a 10^1 CFU/mL b 10^3 CFU/mL c 10^5 CFU/mL d no growth b 10^3 CFU/mL b Patients with infection often have at least 100,000 bacteria/mL of urine in the bladder. However one third of young women with symptomatic cystitis have less than 100,000 bacteria/mL of urine. The Infectious Disease Society of America consensus definition of cystitis is greater than or equal to 1,000 CFU/mL of a uropathogen. The steam autoclave method of sterilization: a uses 15 lbs of pressure for 15 minutes b utilizes dry heat for 20 minutes c produces a maximum temperature of 100 C d requires a source of ethylene oxide a uses 15 lbs of pressure for 15 minutes a The traditional gravity displacement steam sterilization cycle is 121°C for 15 minutes at 15 pounds per square inch. Ethylene oxide is an alternative sterilization method. The expected colony count in a suprapubic urine from a healthy individual is: a 0 CFU/mL b 100 CFU/mL c 1,000 CFU/mL d 100,000 CFU/mL a 0 CFU/mL a Urine in the bladder is normally sterile. Suprapubic aspiration removes urine directly from the bladder and yields a specimen free of urethral contamination. An aspirate of a deep wound was plated on blood agar plates and incubated aerobically and anaerobically. At 24 hours there was growth on both plates. This indicates that the organism is a(n): a nonfermenter b obligate anaerobe c aerobe d facultative anaerobe d facultative anaerobe d Facultative anaerobes are organisms that can grow under both aerobic and anaerobic conditions. The proper blood-to-broth ratio for blood cultures to reduce the antibacterial effect of serum in adults is: a 1:2 b 1:3 c 1:10 d 1:30 c 1:10 c Human blood contains substances that may inhibit microbial growth. Diluting blood in culture broth reduces the concentration of these substances as well as any antibiotics that may be present. The recommended blood broth ratio is 1:5-1:10. Dilutions less than this may cause the blood to clot trapping organisms in the clot. Greater dilutions may increase the time to detection A penicillin-resistant Neisseria gonorrhoeae produces: a alpha-hemolysin b beta-lactamase c enterotoxin d coagulase b beta-lactamase b Antimicrobial resistance in Neisseria gonorrhoeae is widespread. The production of beta-lactamase (penicillinase) breaks open the beta lactam ring of penicillin, destroying its activity. Thus, Ngonorrhoeae that produce beta lactamase are resistant to penicillin. Which selective medium is used for the isolation of gram-positive microorganisms? a Columbia CNA with 5% sheep blood b trypticase soy agar with 5% sheep blood c eosin methylene blue d modified Thayer-Martin a Columbia CNA with 5% sheep blood a Columbia CNA agar contains colistin and nalidixic acid, which inhibit most gram-negative organisms. Eosin methylene blue is selective and inhibits gram-positive organisms and modified Thayer Martin is selective and inhibits gram-positive organisms, gram-negative bacilli and yeast. The most sensitive substrate for the detection of beta-lactamases is: a penicillin b ampicillin c cefoxitin d nitrocefin d nitrocefin d The chromogenic cephalosporin test using nitrocefin is the most sensitive and specific test for detection of beta lactamase. Acidimetric tests employing penicillin are less expensive, but not as sensitive, as the nitrocefin assay. A Staphilococcus aureus isolate has an MIC of 4ug/m: to axacillin. There is uncertainty as to whether this represents an oxacillin (heteroresistant) resistant strain or a hyperproducer of beta-lactamase. Strain Oxacillin Amoxicillin-clavulanic acid strand A Sus Sus stand B Sus Res strand C Res Sus strand D Res Res Based on the above results for oxacillin and amoxicillin, which strand is heteroresistant? a strand A b strand B c strand C d strand D d strand D d A Staphylococcus aureus isolate with an MIC of 4 pg/mL is resistant to oxacillin.. As per the Clinical and Laboratory Standards Institute (CLSI) recommendations a heteroresistant strain would be defined as a methicillin (oxacillin) resistant S aureus (MRSA/ORSA). An MRSA/ORSA isolate would be reported as resistant to all beta lactam agents, beta lactam/beta lactamase inhibitor combinations, such as amoxicillin- clavulanic acid, and carbapenems. 13 An Enterococcus isolated from multiple blood cultures in a patient endocarditis should be: a screened for high level aminoglycoside resistance b checked for tolerance c assayed for serum antimicrobial activity d tested for beta-lactamase production a screened for high level aminoglycoside resistance a Systemic enterococcal infections, such as endocarditis, are commonly treated with a cell-wall-active agent and an aminoglycoside. These agents act synergistically to kill the organism. If the organism is resistant to one or both, there is no synergy, and the combination will fail. It is important to detect aminoglycoside and beta-lactam resistance in these cases. Enterococci have intrinsic moderate level resistance to aminoglycosides. Acquired resistance corresponds to very high MICs (greater than 500 pg/mL) for gentamicin and is termed high level resistance. In the Kirby-Bauer disc diffusion susceptibility test, which variable is critical when testing Pseudomonas species for antibiotic susceptibility to aminoglycosides? a incubation temperature b duration of incubation c cation content of media d depth of agar c cation content of media c Variations in the concentrations of divalent cations primarily calcium and magnesium affect the results of aminoglycoside, tetracycline and colistin tests with P aeruginosa isolates. A cation concentration that is too high results in smaller zone sizes, and a concentration that is too low increases zone sizes. The procedure that assures the most accurate detection of mecA-mediated oxacillin resistance in routine broth microdilution susceptibility testing against S aureus is: a addition of 4% NaCl b incubation at 30°C c incubation for 48 hours d use of cefoxitin for testing d use of cefoxitin for testing d Cefoxitin is used as a surrogate for mecA-mediated oxacillin resistance in S aureus. S aureus with cefoxitin MICs 4 pg/mL are considered oxacillin resistant. The Clinical Laboratory Standards Institute (CLSI) recommends addition of 2% NaCl, incubation at 35±2°C, and incubation for 24 hours when performing susceptibility testing of S aureus against oxacillin. When performing a stool culture, a colony type typical of an enteric pathogen is subcultured on a blood agar plate. The resulting pure culture is screened with several tests to obtain the following results: TSI: acid butt, alkaline slant, no gas, no H2S phenylalanine deaminase: negative motility: positive serological typing: Shigella flexneri (Shigella subgroup B) The serological typing is verified with new kit and controls. The best course of action would be to: a report the organism as Shigella flexneri without further testing b verify reactivity of motility medium with positive and negative controls c verify reactivity of the TSI slants with positive and negative controls for H2S production d verify reactivity of phenylalanine d aminase with positive and negative controls b verify reactivity of motility medium with positive and negative controls b All of the biochemical and serological reactions listed are consistent with an identification of Shigella flexneri, with the exception of motility. All Shigella are nonmotile. Susceptibility testing performed on quality control organisms using a new media lot number yielded zone sizes that were too large for all antibiotics tested. The testing was repeated using media from a previously used lot number, and all zone sizes were acceptable. Which of the following best explains the unacceptable zone sizes? a the antibiotic disks were not stored with the proper desiccant b the depth of the media was too thick c the depth of the media was too thin d the antibiotic disks were not properly applied to the media c the depth of the media was too thin c All Mueller-Hinton agar used for disk diffusion susceptibility testing should be poured to a depth of 4mm. If the depth of the media is 4mm, this may be associated with excessively large zones and false- positive susceptibility results. Agar that is 4mm in depth may cause excessively small zone sizes. Three sets of blood cultures were obtained from an adult patient with fever and suspected endocarditis. The aerobic bottle of one set had growth of Staphylococcus epidermidis at 5 days of incubation. This indicates that: a there was low-grade bacteremia b the organism is most likely a contaminant c the patient has a line infection d the blood culture bottles are defective b the organism is most likely a contaminant b Coagulase-negative staphylococci are commonly associated with contaminated blood cultures; however, they are also increasing as a cause of true bacteremia. Significant bacteremia in a patient with endocarditis is usually continuous and low grade. In most cases, all blood cultures drawn will yield positive results. The facts that only 1 bottle of 1 set was positive, and that the bottle did not become positive until day 5 of incubation, indicate that this isolate is most likely a contaminant. In order to isolate Campylobacter coli/jejuni, the fecal specimen should be: a inoculated onto selective plating media and incubated in reduced oxygen with added CO2 at 42°C b stored in tryptic soy broth before plating to ensure growth of the organism c inoculated onto selective plating media and incubated at both 35°C and at room temperature d incubated at 35°C for 2 hours in Cary-Blair media before inoculating onto selective plating media a inoculated onto selective plating media and incubated in reduced oxygen with added CO2 at 42°C a Campylobacter coli/jejuni require a microaerophilic atmosphere for optimal recovery. The use of selective media is recommended for recovery from fecal specimens. Selective media for Campylobacter contains antibiotics to inhibit the growth of enteric gram-negative flora. Unlike other enteric pathogens, C coli/jejuni grow well at 42°C. An expectorated sputum is sent to the laboratory for culture from a patient with respiratory distress. The Gram stain of the specimen shows many squamous epithelial cells (25/lpf) and rare neutrophils. The microscopic appearance of the organisms present include: moderate gram-positive cocci in chains and diplococci moderate gram-negative diplococci moderate palisading gram-positive bacilli all in moderate amounts This Gram stain is most indicative of: a a pneumococcal pneumonia b an anaerobic infection c an Haemophilus pneumonia d oropharyngeal flora d oropharyngeal flora d Sputum specimen quality is assessed to determine if the specimen is representative of the site of infection. The presence of white blood cells is an indicator of infection, and presence of squamous epithelial cells is an indicator of oropharyngeal contamination. In this specimen, 25 epithelial cells per low power field is an indicator of poor specimen quality, and the bacteria present are representative of oropharyngeal flora. Which type of microscope would be most useful in examining viruses and the structure of microbial cells? a electron b phase-contrast c dark-field d bright-fleld a electron a Due to the small size of viruses, they are not visible using light microscopy. Electron microscopy is used to visualize viruses and the internal structure of microorganisms. After satisfactory performance of daily disk diffusion susceptibility quality control is documented, the frequency of quality control can be reduced to: a twice a week b every week c every other week d every month b every week b Daily disk diffusion quality control can be converted to weekly testing when 30 days of consecutive testing demonstrates no more than 3 antibiotic/organism combinations outside of the acceptable limits. In disk diffusion susceptibility testing, as an antimicrobial agent diffuses away from the concentration of antibiotic is: a increased b decreased c unchanged d inoculum dependent b decreased b The amount of antibiotic used in disk diffusion susceptibility testing is standardized and constant. Once the disk is placed on the inoculated plate and makes contact with the agar, the antibiotic in the disk begins to diffuse out. As it diffuses into the media, the concentration of antibiotic gets lower the further it diffuses from the disk. The most common cause for failure of a GasPak™ anaerobic jar to establish an adequate environment for anaerobic incubation is: a the failure of the oxidation-reduction potential indicator system due to deterioration of methylene blue b the failure of the packet to generate adequate H2 and/or C02 c condensation of water on the inner surface of the jar d catalysts that have become inactivated after repeated use d catalysts that have become inactivated after repeated use d The 2 most common causes of failure of the GasPak™ system is a defective gasket in the jar lid that allows escape of gas from inside the jar and inactivated catalyst pellets. Which of the following is the most appropriate method for collecting a urine specimen from a patient with an indwelling catheter? a remove the catheter, cut the tip, and submit it for culture b disconnect the catheter from the bag, and collect urine from the terminal end of the catheter c collect urine directly from the bag d aspirate urine aseptically from the catheter tubing d aspirate urine aseptically from the catheter tubing d Indwelling catheters are closed systems, and should not be disconnected for specimen collection. Urine samples should not be collected from catheter bags, and Foley catheter tips are unsuitable for culture because they are contaminated with colonizing organisms. Urine from indwelling catheters should be collected by aseptically puncturing the tubing (collection port). Which one of the following specimen requests is acceptable? a feces submitted for anaerobic culture b Foley catheter tip submitted for aerobic culture c rectal swab submitted for direct smear for gonococci d urine for culture of acid-fast bacilli d urine for culture of acid-fast bacilli d Urine is an appropriate specimen for the detection of renal tuberculosis. Since feces contain anaerobic organisms as part of the indigenous flora, it is an unacceptable specimen for anaerobic culture. Foley catheter tips are also not acceptable for culture, because they are contaminated with colonizing organisms. Gram stain smears of rectal swabs for Ngonorrhoeae should also not be performed, since the presence of organisms with similar morphologies may lead to overinterpretation of smears. Which of the following groups of specimens would be acceptable for anaerobic culture? a vaginal, eye b ear, leg tissue c pleural fluid, brain abscess d urine, sputum c pleural fluid, brain abscess c Materials collected from sites not harboring indigenous flora (sterile body fluids, abscess exudate and tissue) should be cultured for anaerobic bacteria. However, since anaerobes normally inhabit the skin and mucus membranes as part of the indigenous flora, specimens such as urine, sputum, and vaginal, eye and ear swabs are not acceptable for culture. A liquid fecal specimen from a three-month-old infant is submitted for microbiological examination. In addition to culture on routine media for Salmonella and Shigeli. this specimen should be routinely: a examined for the presence of Entamoeba hartmanni b examined for the presence of Campylobacter sp c screened for the detection of enterotoxigenic Escherichia coli d placed in thioglycollate broth to detect Clostridium botulinum b examined for the presence of Campylobacter sp b Campylobacter continues to be the most common enteric pathogen isolated from patients with diarrhea. Routinely fecal specimens should be cultured for Salmonella, Shigella and Campylobacter. Fecal specimens are not routinely cultured for enterotoxigenic E coli or C botulinum. E hartmanni is a onpathogenic parasite and does not cause diarrhea. Cerebrospinal fluid from a febrile 25-year old man with possible meningitis is rushed to the laboratory for a stat Gram stain and culture. While performing the Gram stain, the technologist accidentally spills most of the specimen. The smear shows many neutrophils and no microorganisms. Since there is only enough CSF to inoculate one plate, the technologist should use a: a blood agar plate b chopped meat glucose c chocolate agar plate d Thayer-Martin plate c chocolate agar plate C Enriched media such as chocolate agar has no inhibitory effects on bacterial growth and contains additional nutrients that supports the growth of fastidious organisms such as H influenzae and Neisseria. A diabetic foot swab from a 82-year-old woman with recurrent infections is submitted for culture. The Gram stain reveals: many neutrophils, no squamous epithelial cells many gram-negative bacilli many gram-positive cocci in chains The physician requests that all pathogens be worked up. In addition to the sheep blood and MacConkey agar plates routinely used for wound cultures, the technologist might also process a(n): a CNA agar plate b chocolate agar plate c XLD agar plate d chopped meat glucose a CNA agar plate a CNA agar is a selective medium rutinelly used in the isolation of gram positive aerobic and anaerobic organisms. Since the Gram stain indicates a mixture of gram-positive and gram-negative organisms, use of CNA will aid in the recovery of the gram-positive cocci in culture. Which of the following is the most appropriate specimen source and primary media battery? a endocervical-chocolate, Martin Lewis b sputum-sheep blood, Thayer-Martin, KV-laked blood c CSF-Columbia CNA, MacConkey d urine-sheep blood, chocolate, Columbia CNA a endocervical-chocolate, Martin Lewis a Chocolate agar and chocolate agar-based selective media (Martin Lewis) are routinely used for the recovery of Neisseria gonorrhoeae from genital specimens. Sputum and urine specimens are routinely processed using a general purpose media (blood agar) and a selective agar (EMB or MacConkey). In addition chocolate agar is routinely included to enhance recovery of fastidious organisms such as H influenzae. CSF is routinely processed using blood and chocolate agars. Which of the following is the most appropriate organism and media combination? a Legionella species—Regan Lowe b Clostridium difficile—phenylethyl alcohol (PEA) c Campylobacter species—charcoal yeast extract d Yersinia enterocolitica—cefsulodin-irgasan-novobiocin (CIN) d Yersinia enterocolitica—cefsulodin-irgasan-novobiocin (CIN) d CIN agar is a selective and differential medium for the isolation and differentiation of Y enterocolitica. This medium contains sodium desoxycholate, crystal violet, cefsulodin, irgason (triclosan), and novobiocin as selective agents, and mannitol as the carbohydrate. A Gram stain from a swab of a hand wound reveals: moderate neutrophils no squamous epithelial cells moderate gram-positive cocci in chains moderate large gram-negative bacilli Select the appropriate media that will selectively isolate each organism. a KV-laked agar, Thayer-Martin b sheep blood, MacConkey c Columbia CNA, chocolate d Columbia CNA, MacConkey d Columbia CNA, MacConkey d Columbia CNA agar is a selective medium used for the isolation of gram- positive organisms. The medium contains colistin and nalidixic acid, which inhibits gram-negative organisms. MacConkey agar is a selective and differential medium used for the isolation of gram-negative organisms. The medium contains bile and crystal violet, which inhibits gram-positive organisms. Upon review of a sputum Gram stain, the technician notes that the nuclei of all of the neutrophils present in the smear are staining dark blue. The best explanation for this finding is: a the slide was inadequately decolorized with acetone/alcohol b the sputum smear was prepared too thin c the cellular components have stained as expected d the iodine was omitted from the staining procedure a the slide was inadequately decolorized with acetone/alcohol a Problems with analysis of Gram staining generally result from errors including interpretation of the slide (smear prepared too thick), excessive heat fixing, and improper decolorization. Inadequate decolorization with acetone/alcohol results in a smear in which host cells (neutrophils and squamous cells), as well as bacteria, all appear blue. When performing a Kovac indole test, the substrate must contain: a indole b tryptophan c ornithine d paradimethylaminobenzaldehyde b tryptophan b The indole test is used for determining an organism's ability to produce indole from deamination of tryptophan by tryptophanase. The ONPG test allows organisms to be classified as a lactose fermenter by testing for following? a permease b beta-galactosidase c beta-lactamase d phosphatase b beta-galactosidase b Through the action of the enzyme beta- galactosidase, ONPG cleaves into galactose and o-nitrophenol (a yellow compound). Sodium bicarbonate and sodium citrate are components of which of the following? a JEMBEC system b MTM agar c NYC medium d ML agar a JEMBEC system a The JHMBEC system is a transport and inoculation medium used for direct plating of specimens for Ngonorrhoeae. Chocolate- based selective medium is inoculated with the specimen. This is placed in an impermeable plastic bag with a bicarbonate- sodium citrate pellet that absorbs moisture to generate a carbon dioxide-rich environment, Chocolate agar base containing vancomycin, colistin, anisomycin, and trimethoprim is also known a EMB agar b modified Thayer-Martin agar c Columbia CNA agar d Martin Lewis agar d Martin Lewis agar d Martin Lewis agar is a modification of the modified Thayer Martin formulation, and contains a higher concentration of vancomycin and anisomycin instead of nystatin. These modifications provide better inhibition of gram-positive organisms and Candida. A medium that aids in the presumptive identification of organisms based on their appearance on the medium is called: a enriched b selective c differential d specialized c differential c Differential media contain compounds, often carbohydrates, that provide a presumptive identification based on colony color or a precipitate around the colony. Examples include MacConkey, Hektoen and xylose lysine desoxycholate agar. SPS is used as an anticoagulant for blood cultures because it: a inactivates penicillin and cephalosporins b prevents clumping of red cells c inactivates neutrophils and components of serum complement d facilitates growth of anaerobes c inactivates neutrophils and components of serum complement c Most commercially available blood culture media contain sodium polyanetholsulfonate (SPS) in concentrations between 0.025 and 0.05%. SPS has anticoagulant activity, and inactivates neutrophils as well as some antibiotics including gentamicin and polymyxin. It also precipitates components of serum complement. When evaluating a new susceptibility testing system, if the new system characterizes a susceptible isolate as resistant, this is termed a: a very major error b major error c minor error d acceptable error b major error b When evaluating susceptibility testing systems the following conventions are used: a very major error occurs when the system characterizes a resistant isolate as susceptible; a major error occurs when the system characterizes a susceptible isolate as resistant; and a minor error occurs when the system characterizes a susceptible or resistant isolate as intermediate, or an intermediate isolates as susceptible or resistant. Which of the following methods is inadequate for the detection of vancomycin-intermediate S aureus? a broth macrodilution b agar dilution c gradient diffusion d disk diffusion d disk diffusion The disk diffusion procedure will not differentiate S aureus strains with reduced susceptibility to vancomycin (MICs 4-8 pg/mL) from susceptible stains even when incubated for 24 hours. In a disk diffusion susceptibility test, which of the following can result if disks are placed on the inoculated media and left at room temperature for an hour before incubation? a the antibiotic would not diffuse into the medium, resulting in no zone b zones of smaller diameter would result c zones of larger diameter would result d there would be no effect on the final zone diameter c zones of larger diameter would result c A delay of more than 15 minutes between placing the disks on an inoculated plate and incubation permits excess prediffusion of the antimicrobial agent from the disk. This would result in a larger than expected zone diameter. Which of the following factors would make an organism appear to be more resistant on a disk diffusion susceptibility test? a too little agar in the plate b too many organisms in the inoculum c the presence of 0.5% NaCl in the medium d a medium with a pH of 7.4 b too many organisms in the inoculum b To ensure the reproducibility of disk diffusion testing, the inoculum must be standardized. If the inoculum is too dense (too many organisms), zone sizes would be smaller than expected and appear falsely resistant. First generation cephalosporins can be adequately represented by: a cefotetan b ceftriaxone c cephalothin d cefoxitin c cephalothin c Cephalothin is a first-generation cephalosporin, cefotetan and cefoxitin are second-generation cephalosporins, and ceftriaxone is a third generation cephalosporin. An antibiotic that inhibits cell wall synthesis is: a chloramphenicol b penicillin c sulfamethoxazole d colistin b penicillin b Penicillin inhibits penicillin binding proteins that are essential to peptidoglycan (cell wall) synthesis. Chloramphenicol inhibits protein synthesis, colistin increases cell membrane permeability, and sulfamethoxazole inhibits folate metabolism. Which one of the following organisms does not require susceptibility testing to the antimicrobial indicated when isolated from a clinically significant source? a Staphylococcus aureus-clindamycin b Proteus mirabilis-gentamicin c Streptococcus pyogenes-penicillin d Escherichia coli-levofloxacin c Streptococcus pyogenes-penicillin c Susceptibility testing should be performed when the susceptibility of the organism cannot reliably be predicted and resistance is known or suspected. Susceptibility testing of penicillins for treatment of S pyogenes does not need to be performed routinely since resistance has not been documented. Which of the following antibiotics would routinely be tested and reported for isolates of Pseudomonas aeruginosa? a penicillin b erythromycin c clindamycin d gentamicin d gentamicin d Aminoglycoside antibiotics such as gentamicin are active against Pseudomonas and routinely tested and reported on these isolates. Penicillin, erythromycin, and clindamycin are not active against Pseudomonas. Which of the following must be incubated in a microaerophilic environment for optimal recovery of the organism? a Campylobacter jejuni b Escherichia coli c Pseudomonas aeruginosa d Proteus mirabilis a Campylobacter jejuni a Most Campylobacter species grow best under lower oxygen tension in an atmosphere of 5% oxygen, 10% carbon dioxide and 85% nitrogen. E coli and Proteus mirabilis are facultative anaerobes and Pseudomonas aeruginosa is an aerobe. Diagnosis of typhoid fever can be confirmed best by culture of : a stool b urine c blood d bone marrow d bone marrow d The sensitivity of blood culture is only 50%-70%. Stool cultures are positive in 50% of patients and urine cultures are positive even less frequently. Bone marrow has a sensitivity of up to 90%. Higher colony counts are present in bone marrow and counts are not decreased by up to 5 days of antimicrobial therapy prior to specimen collection. Vibrio parahaemolyticus can be isolated best from feces on: a eosin methylene blue (EMB) agar b Hektoen enteric (HE) agar c Salmonella Shigella (SS) agar d thiosulfate citrate bile salts (TCBS) agar d thiosulfate citrate bile salts (TCBS) agar d TCBS is a highly selective and differential medium for the recovery of most Vibrio species including V parahaemolyticus. Hektoen and Salmonella-Shigella agars are selective and differential for the isolation and differentiation of enteric pathogens such as Salmonella and Shigella. EMB is a selective and differential medium for gram-negative enteric bacilli. Which of the following media can be used to culture Campylobacter jejuni? a Skirrow medium b CIN agar c anaerobic CNA agar d bismuth sulfite a Skirrow medium a Skirrow medium is an enriched selective blood agar medium used for the isolation of Campylobacter from specimens with mixed flora. CIN and bismuth sulfite agars are selective and differential for Yersinia enterocolitica and Salmonella, respectively. CNA agar is selective for gram- positive organisms. Tests for beta-lactamase production in Haemophilus influenzae: a are not commercially available b include tests that measure a change to an alkaline pH c should be performed on all blood and CSF isolates d are not valid for any other bacterial species c should be performed on all blood and CSF isolates c As many as 20%-40% of H influenzae produce beta-lactamases. Detection of these enzymes should be performed on any isolate considered to be a pathogen using the chromogenic cephalosporin (nitrocefm) test. Media used to support to support growth of Legionella pneumophilia should contain which of the following additives? a X and V factors b hemin and Vitamin K c charcoal and yeast extract d dextrose and laked blood c charcoal and yeast extract c Buffered charcoal yeast extract medium is a specialized enrichment medium for the isolation of Legionella. The nutritive base includes yeast extract. Charcoal is added to the medium as a detoxifying agent. The best medium for culture of Bordetella pertussis is: a Regan-Lowe agar b cystine blood agar c Martin Lewis agar d Ashdown agar a Regan-Lowe agar a Regan-Lowe agar is an enriched and selective medium for the isolation of B pertussis. Cephalexin is added to inhibit nasopharyngeal flora. It provides better isolation of B pertussis than Bordet-Gengou medium. The best medium for culture of Francisella tularensis is: a Bordet-Gengou medium b cystine glucose blood agar c Loeffler medium d charcoal selective medium b cystine glucose blood agar b Francisella tularensis is fastidious and not readily recovered in culture. Cysteine blood glucose agar is an enriched medium with beef heart infusion, peptones, glucose and rabbit blood. It also includes cystine, which is required by F tularensis for growth. The most rapid method for detection of Francisella tularensis is: a serological slide agglutination utilizing specific antiserum b dye stained clinical specimens c fluorescent antibody staining techniques on clinical specimens d polymerase chain reaction d polymerase chain reaction d Methods for rapid diagnosis of Francisella tularensis include fluorescent antibody staining of smears and tissues, antigen detection in urine detection of lipopolysaccharide using specific monoclonal antibodies and PCR. Only PCR has gained widespread use. PCR is appealing because smears and cultures are usually negative, and organism isolation may be hazardous. Serological diagnosis may take weeks to confirm. When processing throat swabs for a group A Streptococcus culture, the medium of choice is: a sheep blood agar b rabbit blood agar c human blood agar d horse blood agar a sheep blood agar a Sheep blood agar is preferred because clear-cut patterns of hemolysis are obtained. The ability to detect oxacillin-resistant Staphylococcus aureus may be enhanced by: a shortening incubation of standard susceptibility plates b incubating susceptibility plates at 39°-41°C c using Mueller-Hinton broth with 2% NaCl d adjusting inoculum to 0.1 McFarland before inoculating susceptibility plates c using Mueller-Hinton broth with 2% NaCl c For optimum detection of oxacillin- resistant S aureus, a suspension with a turbidity equivalent to a 0.5 McFarland standard should be inoculated into a cation- adjusted Mueller-Hinton broth with 2% NaCl. Plates should be incubated at 35 ± 2°C for 24 hours. Temperatures above 35°C may not detect oxacillin resistance. The optimal wound specimen for culture of anaerobic organisms should be: a a swab of lesion obtained before administration of antibiotics b a swab of lesion obtained after administration of antibiotics c a syringe filled with pus, obtained before administration of antibiotics d a syringe filled with pus, obtained after administration of antibiotics c a syringe filled with pus, obtained before administration of antibiotics c The use of swabs for collection of specimens for anaerobic culture is discouraged. Aspiration with a needle and syringe is recommended. Whenever possible cultures should be obtained before the administration of antibiotics to optimize organism recovery. A 21-year-old patient presents with pharyngitis. A throat swab is collected and submitted for anaerobic culture. This specimen should be: a set up immediately b rejected as unacceptable c inoculated into thioglycollate broth d sent to a reference laboratory b rejected as unacceptable b Many anaerobic bacteria are commensal flora in the oropharynx. Anaerobic bacteria do not cause pharyngitis. The most common cause of pharyngitis is Streptococcus pyogenes. Other causes include Arcanobacterium haemolyticum, Corynebacterium diphtheriae, Neisseria gonorrhoeae and viruses. Anaerobic susceptibility testing is helpful in the management of patients with: a synovial infections b rectal abscesses c streptococcal pharyngitis d pilonidal sinuses a synovial infections There is evidence that antimicrobial resistance among anaerobic organisms is significant, and that inappropriate therapy correlates with poor patient outcomes. Susceptibility testing is useful when anaerobes are isolated from normally sterile sites such as synovial fluid. Rectal abscesses and pilonidal sinuses are polymicrobial in nature, and often resolved by surgical management. An antibiotic used to suppress or kill contaminating fungi in media is: a penicillin b cycloheximide c streptomycin d amphotericin B b cycloheximide b Cyclohexamide, which inhibits protein synthesis, is the common agent used in Mycosel ' or mycobiotic agar to inhibit faster-growing saprophytic fungi. Penicillin and streptomycin do not inhibit fungi. Amphotericin B is not routinely used as an additive in fungal media. Production of beta-lactamase is inducible in which of the following: a Haemophilus influenzae b Staphylococcus aureus c Cory neb acterium diphtheriae d Streptococcus pyogenes b Staphylococcus aureus b Some bacteria such as Enterococcus sp, H influenzae and Neisseria gonorrhoeae continually produce beta-lactamase. S aureus produces beta-lactamase only after exposure 70 to an inducing agent (such as penicillin). C diphtheriae and S pyogenes do not produce beta-lactamase. A sputum specimen is received for culture and Gram stain. The Gram stained smear from this specimen is seen in the image (total magnification 100x): (image) The technologist's best course of action would be to: a inoculate appropriate media and incubate anaerobically b inoculate appropriate media and incubate aerobically c call the physician and notify him of this "life-threatening" situation d call the patient care area and request a new specimen d call the patient care area and request a new specimen d There are several sputum-screening systems for assessing the quality of respiratory specimens. In general, neutrophils are a positive indicator of quality, and squamous epithelial cells are a negative indicator of quality, suggesting oropharyngeal contamination. This specimen contains an abundance of squamous cells ( 10/low power field), and would be unacceptable for culture. A technologist is reading a Gram stain from a CSF and observes many neutrophils and lancet-shaped gram-positive diplococci. Which set of chemistry and hematology CSF results would most likely be seen in someone with this type of infection? CSF results WBC Glucose Protein A increased increased increased B decreased decreased decreased C increased decreased increased D decreased increased decreased a result A b result B c result C d result D c result C c The classic CSF alterations associated with bacterial meningitis are a high WBC count with a neutrophil predominance as well as a low CSF glucose and a high CSF 72 protein. Refer to the following illustration... Examine the broth microdilution suceptibility test shown above and determine the MIC for gentamicin. a 64ug/mL b 32 ug/mL c 16 pg/mL d 2 pg/mL d When reading a broth microdilution susceptibility test, growth in each well is determined by comparison with the growth control well and indicated by turbidity. The well with the lowest concentration of antibiotic displaying no growth is read as the minimum inhibitory concentration (MIC). Which of the following tests is used to monitor bactericidal activity during antimicrobic therapy in cases of endocarditis? a Elek b tolerance c Sherris synergism d Schlichter d Schlichter d The serum bactericidal, or Schlichter, test can be used to assess the activity of patient's serum when they are receiving long-term therapy for endocarditis or osteomyelitis. High titers of antibacterial activity in the serum suggest adequate dosing, a nontolerant isolate or normal elimination of the antibiotic. Quality control results for disk diffusion susceptibility tests yield the following results: aminoglycoside zones too small and penicillin zones too large. This is most likely due to the: a inoculum being too heavy b inoculum being too light c pH of Mueller-Hinton agar being too low d calcium and magnesium concentration in the agar being too high c pH of Mueller-Hinton agar being too low c Mueller-Hinton agar used for disk diffusion susceptibility testing is standardized at pH 7.2-7.4. Penicillins function better in an acidic environment, so zone sizes would become larger if the media pH is too low. Aminoglycosides, on the other hand, are less effective in an acidic environment, so zone sizes would become smaller if the pH of the media is too low. Plates should not be incubated in a carbon dioxide atmosphere, which lowers the pH of the media. When using a control strain of Staphylococcus aureus, the technologist notices that the zone around the oxacillin disk is too small. Which of the following is the most likely explanation? a inoculation of the plates 10 minutes after preparing the inoculum b incubation of the Mueller-Hinton plates at 35°C c use of a 0.25 McFarland standard to prepare inoculum d use of outdated oxacillin disks d use of outdated oxacillin disks d Deterioration of the antimicrobial agent in the disk will cause the zone sizes to be too small (falsely resistant). Standardization of the inoculum turbidity to less than a 0.5 McFarland standard would result in an inoculum that is too light and resulting zone sizes that are too large. Incubation of the plates at 35°C and inoculating plates within 10 minutes of preparation would not have an adverse effect on zone sizes. In the disk diffusion method of determining antibiotic susceptibility, the size of the inhibition zone used to indicate susceptibility has been determined by: a testing 30 strains of 1 genus of bacteria b correlating the zone size with minimum inhibitory concentrations c correlating the zone size with minimum bactericidal concentrations d correlating the zone size with the antibiotic content of the disk b correlating the zone size with minimum inhibitory concentrations b The zone size observed has no meaning in and of itself. Interpretive standards are derived from a correlation between zone sizes and minimum inhibitory concentrations. Usually a large number of organisms from a given species or group (eg, Enterobacteriaceae) are tested. Which of the following organisms may be mistaken for Neisseria gonorrhoeae in Gram stained smears of uterine cervix exudates? a Lactobacillus species b Streptococcus agalactiae c Pseudomonas aeruginosa d Moraxella osloensis d Moraxella osloensis d Moraxella osloensis is a gram-negative coccobacillus that is often plump and occurs in pairs and demonstrates a morphology similar to Neisseria. The presence of this organism in endocervical specimens contaminated with vaginal secretions can lead to over interpretation of smears for N gonorrhoeae. A 73-year-old man diagnosed as having pneumococcal meningitis is not responding to his penicillin therapy. Which of the following tests should be performed on the isolate to best determine this organism's susceptibility to penicillin? a beta-lactamase b oxacillin disk diffusion c penicillin disk diffusion d Schlichter test b oxacillin disk diffusion b With the exception of the oxacillin disk screening test, disk diffusion is not recommended for testing S pneumoniae against beta-lactam agents. S pneumoniae does not produce beta-lactamase, so beta- lactamase testing would not be useful. The Schlichter test is not a method for determining an organisms susceptibility to a given agent. Salmonella enteritidis is isolated from multiple blood cultures in a patient with fever. Susceptibility results are as follows: ampicillin-susceptible ceftriaxone-susceptible ciprofloxacin-susceptible trimethoprim/sulfamethoxazole-resistant. What is the next best step? a report all of the susceptibility testing results with no changes b perform a beta-lactamase test on the isolate before reporting the ampicillin as susceptible c test the isolate against nalidixic acid and if resistant report the ciprofloxacin as resistant d report gentamicin since the trimethoprim/sulfamethoxazole is resistant c test the isolate against nalidixic acid and if resistant report the ciprofloxacin as resistant c Extraintestinal isolates of Salmonella should be tested for resistance to nalidixic acid in addition to fluoroquinolones. Fluoroquinolones susceptible stains of Salmonella that are resistant to nalidixic acid may be associated with clinical failure or delayed response to therapy. In the case where ciprofloxacin is susceptible and nalidixic acid is resistant the nalidixic acid result should be used for reporting. Which of the following antimicrobials would be inappropriate to report on an E coli isolated from a wound culture? a gentamicin b ampicillin c cefazolin d nitrofurantoin d nitrofurantoin d Certain antimicrobials, such as nitrofurantoin and norfloxacin, are used only or primarily to treat urinary tract infections. These agents should not be reported for pathogens recovered from other sites of infection. The susceptibility results below are reported on an Enterococcus faecalis isolated from peritoneal fluid. ampicillin: susceptible vancomycin: resistant clindamycin: susceptible levofloxacin: resistant linezolid: susceptible The physician calls questioning the results. Which of the following should have been done before the report was released? a the clindamycin result should have been removed from the report since it is inactive against Enterococcus b the ampicillin result should have been changed to resistant since the isolate is vancomycin resistant c the linezolid result should have been removed from the report since it is inactive against Enterococcus d ciprofloxacin should have been added to the report since levofloxacin was resistant a the clindamycin result should have been removed from the report since it is inactive against Enterococcus a Enterococcus species may appear active in vitro to clindamycin, cephalosporins and trimethoprim/sulfamethoxazole but are not effective clinically and should not be reported as susceptible.