PUBLIC HEALTH MICROBIOLOGIST (PHM)
CERTIFICATION EXAMINATION
100 PRACTICE QUESTIONS WITH ANSWERS AND
RATIONALES
2026–2027 TESTING CYCLE
SECTION 1: BACTERIOLOGY (Questions 1–35)
Question 1: Which Gram-positive cocci are catalase-positive, coagulase-negative, and
frequently isolated from skin flora?
Answer: Staphylococcus epidermidis
Rationale: S. epidermidis is catalase-positive, coagulase-negative, and a common skin
commensal. It can cause device-related infections, particularly in patients with indwelling
catheters or prosthetic devices.
Question 2: The primary metabolic pathway used by most Enterobacteriaceae to ferment
glucose under anaerobic conditions is:
Answer: Mixed-acid fermentation
,Rationale: Enterobacteriaceae typically perform mixed-acid fermentation, producing acids
(lactate, acetate, formate, succinate) and gases. This characteristic is the basis for the Methyl
Red (MR) test used in enteric identification.
Question 3: Which genetic mechanism is most responsible for the rapid spread of β-lactamase
genes among Gram-negative bacteria?
Answer: Conjugation via plasmids
Rationale: Plasmid-mediated conjugation enables efficient horizontal transfer of β-lactamase
genes between Gram-negative organisms. This mechanism is responsible for the rapid
dissemination of extended-spectrum β-lactamases (ESBLs) and carbapenemases in healthcare
settings.
Question 4: A patient's stool culture grows non-lactose-fermenting, oxidase-positive rods that
produce a characteristic grape-like odor. Which organism is most likely?
Answer: Pseudomonas aeruginosa
Rationale: P. aeruginosa is oxidase-positive, non-lactose fermenting on MacConkey agar, and
produces a characteristic grape-like or corn tortilla odor due to production of 2-
aminoacetophenone. It is an opportunistic pathogen in immunocompromised patients.
,Question 5: Which selective medium is used for the isolation of Campylobacter jejuni from
stool specimens?
Answer: Skirrow agar
Rationale: Skirrow agar contains antibiotics (vancomycin, polymyxin B, and trimethoprim) that
suppress competing flora and supports growth of Campylobacter spp. It is incubated at 42°C
under microaerophilic conditions.
Question 6: Which of the following is a characteristic feature of the organism that causes
Legionnaires' disease?
Answer: Gram-negative rod requiring cysteine-enriched media (BCYE)
Rationale: Legionella pneumophila is a fastidious Gram-negative rod that requires Buffered
Charcoal Yeast Extract (BCYE) agar supplemented with L-cysteine and iron for growth. It does
not grow on routine blood agar or MacConkey agar.
Question 7: The gold standard method for drug susceptibility testing of Mycobacterium
tuberculosis is:
Answer: MGIT 960 liquid culture system
, Rationale: The MGIT (Mycobacteria Growth Indicator Tube) 960 system provides rapid,
automated, and standardized susceptibility results for M. tuberculosis. It detects growth by
fluorescence quenching and is the current gold standard for first-line drug susceptibility testing.
Question 8: Which Mycobacterium species is classified as a non-tuberculous mycobacterium
(NTM) and is often associated with pulmonary disease in patients with underlying lung
pathology?
Answer: Mycobacterium avium complex (MAC)
Rationale: MAC is an NTM that commonly causes pulmonary disease in patients with COPD,
bronchiectasis, or cystic fibrosis. It is also associated with disseminated disease in
immunocompromised patients.
Question 9: A stool culture grows non-lactose-fermenting, oxidase-negative, Gram-negative
rods that are motile and produce H₂S on triple sugar iron (TSI) agar. The most likely organism is:
Answer: Salmonella enterica
Rationale: Salmonella is a non-lactose-fermenting, oxidase-negative, motile Gram-negative rod
that produces H₂S on TSI agar (black precipitate). It is a common cause of foodborne
gastroenteritis and is identified by serotyping and biochemical tests.
CERTIFICATION EXAMINATION
100 PRACTICE QUESTIONS WITH ANSWERS AND
RATIONALES
2026–2027 TESTING CYCLE
SECTION 1: BACTERIOLOGY (Questions 1–35)
Question 1: Which Gram-positive cocci are catalase-positive, coagulase-negative, and
frequently isolated from skin flora?
Answer: Staphylococcus epidermidis
Rationale: S. epidermidis is catalase-positive, coagulase-negative, and a common skin
commensal. It can cause device-related infections, particularly in patients with indwelling
catheters or prosthetic devices.
Question 2: The primary metabolic pathway used by most Enterobacteriaceae to ferment
glucose under anaerobic conditions is:
Answer: Mixed-acid fermentation
,Rationale: Enterobacteriaceae typically perform mixed-acid fermentation, producing acids
(lactate, acetate, formate, succinate) and gases. This characteristic is the basis for the Methyl
Red (MR) test used in enteric identification.
Question 3: Which genetic mechanism is most responsible for the rapid spread of β-lactamase
genes among Gram-negative bacteria?
Answer: Conjugation via plasmids
Rationale: Plasmid-mediated conjugation enables efficient horizontal transfer of β-lactamase
genes between Gram-negative organisms. This mechanism is responsible for the rapid
dissemination of extended-spectrum β-lactamases (ESBLs) and carbapenemases in healthcare
settings.
Question 4: A patient's stool culture grows non-lactose-fermenting, oxidase-positive rods that
produce a characteristic grape-like odor. Which organism is most likely?
Answer: Pseudomonas aeruginosa
Rationale: P. aeruginosa is oxidase-positive, non-lactose fermenting on MacConkey agar, and
produces a characteristic grape-like or corn tortilla odor due to production of 2-
aminoacetophenone. It is an opportunistic pathogen in immunocompromised patients.
,Question 5: Which selective medium is used for the isolation of Campylobacter jejuni from
stool specimens?
Answer: Skirrow agar
Rationale: Skirrow agar contains antibiotics (vancomycin, polymyxin B, and trimethoprim) that
suppress competing flora and supports growth of Campylobacter spp. It is incubated at 42°C
under microaerophilic conditions.
Question 6: Which of the following is a characteristic feature of the organism that causes
Legionnaires' disease?
Answer: Gram-negative rod requiring cysteine-enriched media (BCYE)
Rationale: Legionella pneumophila is a fastidious Gram-negative rod that requires Buffered
Charcoal Yeast Extract (BCYE) agar supplemented with L-cysteine and iron for growth. It does
not grow on routine blood agar or MacConkey agar.
Question 7: The gold standard method for drug susceptibility testing of Mycobacterium
tuberculosis is:
Answer: MGIT 960 liquid culture system
, Rationale: The MGIT (Mycobacteria Growth Indicator Tube) 960 system provides rapid,
automated, and standardized susceptibility results for M. tuberculosis. It detects growth by
fluorescence quenching and is the current gold standard for first-line drug susceptibility testing.
Question 8: Which Mycobacterium species is classified as a non-tuberculous mycobacterium
(NTM) and is often associated with pulmonary disease in patients with underlying lung
pathology?
Answer: Mycobacterium avium complex (MAC)
Rationale: MAC is an NTM that commonly causes pulmonary disease in patients with COPD,
bronchiectasis, or cystic fibrosis. It is also associated with disseminated disease in
immunocompromised patients.
Question 9: A stool culture grows non-lactose-fermenting, oxidase-negative, Gram-negative
rods that are motile and produce H₂S on triple sugar iron (TSI) agar. The most likely organism is:
Answer: Salmonella enterica
Rationale: Salmonella is a non-lactose-fermenting, oxidase-negative, motile Gram-negative rod
that produces H₂S on TSI agar (black precipitate). It is a common cause of foodborne
gastroenteritis and is identified by serotyping and biochemical tests.
