ASCP Practice Questions Microbiology with Complete Solutions

If a blood agar plate and an EMB plate are both inoculated with 0.001 ml of urine, and 90 colonies of Staph aureus grew on the blood agar plate, what should be reported? Choose the single best answer A. 90 colony forming units/mL of Staphylococci aureus B. 90,000 colony forming units/mL of Staphylococci aureus C. 9,000 colony forming units/mL of Staphylococci aureus D. 9 colony forming units/mL of Staphylococci aureus B; 90,000 colony forming units/mL of Staphylococci aureus should be reported. Calculate the number of bacteria in the urine sample by multiplying the colony count X the dilution of the calibrated loop (1/1000). If there are 90 colonies on the EMB, 90 X 1000 dilution = 90,000 colony forming units/ml of urine. A; Salmonella spp. is the correct answer because the isolate is a non-lactose and sucrose fermenter on TSIA, positive for H2S, lysine decarboxylase positive, urea negative, citrate positive, phenylalanine deamination negative, indole negative, and ornithine positive. Shigella would be H2S negative, lysine negative, urea negative, citrate negative, phenylalanine negative, and nonmotile. Proteus vulgaris would not decarboxylase lysine and ornithine, and is citrate negative, phenylalanine positive, and produce indole. Citrobacter freundii would usually ferment lactose or sucrose, not decarboxylase lysine, and would be phenylalanine and indole negative. This member of the Enterobacteriaceae is shown on triple sugar iron agar, lysine iron agar, urea agar, citrate agar , phenylanine deaminase agar, and motility indole ornithine agar. It should be identified as: The correct answer is highlighted below A. Salmonella spp. B. Shigella spp. C. Proteus vulgaris D. Citrobacter freundii B; Of the Mycobacterium species listed in this exercise, M. kansasii and M. marinum are the two photochromogens, fitting the description of the colonies included in the question. Of these, M. marinum would not grow at 35°C; therefore, the intended answer in this exercise is M. kansasii. M. tuberculosis is neither a photochromogen nor scotochromogen, producing only buff-pigmented colonies no matter what the light exposure. M. gordonae is a scotochromogen, and produces yellow-pigmented colonies, both in the presence and in the absence of light. Illustrated in this photograph are two Middlebrook 7H-11 plates incubated at 35°C, on which are growing colonies of an unknown Mycobacterium species. The plate on the left was incubated in the dark; the plate on the right had been exposed to light for 24 hours. The most likely identification is: