Bacteriology MCQs: Laboratory Diagnosis of Bacterial Infections

Laboratory diagnosis is where microbiology meets the patient: choosing the right specimen, the right culture conditions, and the right confirmatory test often decides whether an organism is found at all.

These ten questions work through those decisions, from which specimen and medium suit a given pathogen, to the special growth conditions organisms like Campylobacter demand, to the serologic tests that identify infections culture cannot. Write down your answers first, then check them against the key and explanations at the end.

1. Which of the following bacteria is not the most common bacterial pathogen causing diarrhea?
   a. Shigella spp
   b. Salmonella spp
   c. Enterococcus faecalis
   d. Campylobacter spp
2. Which of the following bacteria is rarely associated with urinary tract infections?
   a. E.coli
   b. Enterobacter spp
   c. Proteus spp
   d. Shigella spp
3. Which of the following statement regarding Campylobacter jejuni is not correct?
   a. It is commonly cultured in antibiotic-containing media
   b. Incubation temperature is 42°C
   c. It is cultured in an atmosphere containing 5% O₂ and 10%CO₂
   d. It is a normal flora of intestine
4. Which of the following bacteria is not the most important cause of acute bacterial meningitis?
   a. Neisseria menigitidis
   b. Streptococcus pneumoniae
   c. Haemophilus influenzae
   d. Streptococcus pyogenes
5. Which of the following Gram-negative rod is not a blood-borne bacterial pathogen?
   a. Shigella spp
   b. Escherichia coli
   c. Klebsiella pneumoniae
   d. Pseudomonas aeruginosa
6. Throat culture is not useful to diagnose
   a. Streptococcal sore throat
   b. Diphtheria
   c. Thrush
   d. Pneumonia
7. Stool culture is primarily recommended when the complaint is
   a. bloody diarrhea (dysentery, enterocolitis)
   b. watery diarrhea
   c. for both bloody and watery diarrhea
   d. an indication of anaerobic infection
8. Which of the following bacteria causing sexually transmitted diseases can not be grown on artificial media?
   a. Neisseria gonorrhoeae
   b. Chlamydia trachomatis
   c. Treponema pallidum
   d. Treponema pallidum and Chlamydia trachomatis
9. Cold agglutinin test is useful for the diagnosis of
   a. Mycoplasma pneumoniae
   b. H. influenzae
   c. N. menigitidis
   d. Cryptococcus neoformans
10. Which of the following test is not recommended for the diagnosis of Syphilis?
    a. VDRL test
    b. Culture
    c. FTA-ABS test
    d. MHA-TP test

Answer key:

1. c. Enterococcus faecalis
2. d. Shigella spp
3. d. It is a normal flora of the intestine
4. d. Streptococcus pyogenes
5. a. Shigella spp
6. d. Pneumonia
7. c. For both bloody and watery diarrhea
8. d. Treponema pallidum and Chlamydia trachomatis
9. a. Mycoplasma pneumoniae
10. b. Culture

Why these are the answers

1. Enterococcus faecalis. Shigella, Salmonella, and Campylobacter are three of the most common bacterial causes of diarrhea worldwide. Enterococcus faecalis is a normal gut commensal and a cause of urinary tract infection, endocarditis, and wound infection, but it is not a recognized diarrheal pathogen. The pattern across this set: Enterococcus keeps appearing as the gut organism that lives there without causing diarrhea.
2. Shigella spp. E. coli, Enterobacter, and Proteus are all recognized urinary tract pathogens (E. coli by far the commonest; Proteus notable for urease and struvite stones). Shigella is a cause of dysentery, an invasive colonic pathogen, and essentially never a urinary pathogen. The clue is habitat: Shigella's whole biology is confined to the gut mucosa. Read more about Shigella in this article.
3. The false statement is that Campylobacter jejuni is normal intestinal flora. It is a pathogen acquired from animals and contaminated food or water, not part of the human normal flora. The other three statements are true and worth knowing together as the Campylobacter culture recipe: selective antibiotic-containing media (such as Skirrow or Campy media), incubation at 42°C, and a microaerophilic atmosphere of about 5% O2 and 10% CO2. That combination, warm and low-oxygen on selective media, is exactly what distinguishes Campylobacter isolation from routine stool culture.
4. Streptococcus pyogenes. The big three causes of acute bacterial meningitis across age groups are Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae (the last much reduced where Hib vaccine is used). Streptococcus pyogenes (group A strep) causes pharyngitis and skin infection and is an uncommon cause of meningitis. Group A versus the meningitis trio is the distinction being tested.
5. Shigella spp. The question asks which gram-negative rod is NOT a bloodstream pathogen. E. coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa are all common causes of gram-negative bacteremia and sepsis. Shigella is characteristically non-invasive beyond the gut wall: it stays in the colonic mucosa and rarely enters the blood, which is why blood cultures are not how you diagnose shigellosis (stool culture is).
6. Pneumonia. A throat swab usefully diagnoses conditions of the throat itself: streptococcal pharyngitis (culture or rapid antigen), diphtheria (on tellurite and Loeffler media), and oral thrush (Candida). It cannot diagnose pneumonia, which is a lower respiratory tract infection: the pathogen sits in the lungs, so the right specimen is sputum, not a throat swab. The trap is that all four options are infections you might loosely associate with the mouth or airway, but only pneumonia lives below the reach of a throat swab.
7. For both bloody and watery diarrhea. Stool culture is worthwhile whether the diarrhea is bloody (dysentery from Shigella, Campylobacter, invasive E. coli) or watery (Salmonella, Vibrio), because bacterial pathogens cause both patterns and culture identifies the organism and guides therapy and public-health reporting. Option d is wrong because stool culture targets aerobic and microaerophilic enteric pathogens, not anaerobic infection.
8. Treponema pallidum and Chlamydia trachomatis. Both cause sexually transmitted infections and neither grows on cell-free artificial media: T. pallidum has never been cultured on ordinary lab media, and Chlamydia trachomatis is an obligate intracellular organism that requires living host cells, not agar. Neisseria gonorrhoeae, by contrast, grows well on enriched selective media such as Thayer-Martin. This is the fact that makes diagnosis of syphilis and chlamydia rely on serology or molecular tests rather than culture.
9. Mycoplasma pneumoniae. Infection with M. pneumoniae classically triggers cold agglutinins, IgM antibodies that agglutinate red cells at cold temperatures, and a positive cold agglutinin test supports the diagnosis of Mycoplasma atypical (walking) pneumonia. It is a nonspecific bedside clue rather than a confirmatory test, but the association with Mycoplasma is the classic teaching point. The other organisms listed do not produce this pattern.
10. Culture. Treponema pallidum cannot be grown on artificial media, so culture is not a diagnostic option for syphilis. Diagnosis instead uses serology, split into two groups: nontreponemal tests (VDRL, RPR) for screening and monitoring treatment response, and treponemal tests (FTA-ABS, MHA-TP) for confirmation. All three of the non-culture options here are legitimate syphilis tests; culture is the impossible one.