MLPAO Microbiology and Parasitology Clinical Applications 2 — Questions and Answers
Question 1: What is the appropriate specimen collection for anaerobic culture of a wound infection?
- Swab of wound surface only
- Tissue biopsy or aspirate of deep wound, transported in anaerobic transport system (Correct answer)
- Swab in aerobic transport medium
- Swab air-dried and transported at room temperature
Correct answer: Tissue biopsy or aspirate of deep wound, transported in anaerobic transport system
Proper anaerobic culture requires tissue or aspirate specimens from the deep wound (avoiding surface flora), collected and transported in anaerobic transport systems that maintain an oxygen-free environment. Surface swabs are unacceptable because they contain aerobic colonizers and expose anaerobes to lethal oxygen.
Question 2: A sputum Gram stain shows many squamous epithelial cells (>25 per low power field) and few white blood cells. What should the technologist do?
- Process the specimen for culture
- Reject the specimen as it represents oral contamination and request a new specimen (Correct answer)
- Report only the Gram stain findings
- Process for AFB culture only
Correct answer: Reject the specimen as it represents oral contamination and request a new specimen
A sputum specimen with >25 squamous epithelial cells per low power field indicates oral/salivary contamination rather than a true lower respiratory specimen. The specimen should be rejected according to Murray-Washington or Bartlett quality criteria, and a new properly collected specimen requested. This prevents misleading culture results.
Question 3: Which organism is the most common cause of bacterial meningitis in Canadian adults?
- Haemophilus influenzae type b
- Streptococcus pneumoniae (Correct answer)
- Listeria monocytogenes
- Escherichia coli
Correct answer: Streptococcus pneumoniae
Streptococcus pneumoniae is the most common cause of bacterial meningitis in adults in Canada. It appears as gram-positive lancet-shaped diplococci on CSF Gram stain. Since the introduction of Hib vaccine, H. influenzae meningitis has decreased dramatically. In neonates, Group B Streptococcus and E. coli are more common.
Question 4: A culture from a diabetic foot wound grows a polymicrobial mixture including Bacteroides fragilis. What is the significance?
- Bacteroides fragilis is a contaminant and can be ignored
- B. fragilis is a significant anaerobic pathogen that is resistant to many antibiotics and requires specific anaerobic coverage (Correct answer)
- B. fragilis is susceptible to all antibiotics
- B. fragilis only requires topical treatment
Correct answer: B. fragilis is a significant anaerobic pathogen that is resistant to many antibiotics and requires specific anaerobic coverage
Bacteroides fragilis is the most clinically significant anaerobic pathogen, commonly found in polymicrobial diabetic foot infections, intra-abdominal abscesses, and wound infections. It produces beta-lactamase and is resistant to many antibiotics. Metronidazole, carbapenems, or beta-lactam/beta-lactamase inhibitor combinations are typically effective.
Question 5: What is the clinical significance of identifying Streptococcus agalactiae (Group B Strep) in a vaginal/rectal screening culture at 35-37 weeks of pregnancy?
- No significance; it is normal flora
- Positive screening requires intrapartum antibiotic prophylaxis to prevent neonatal GBS disease (Correct answer)
- The patient should be treated with oral antibiotics immediately
- Cesarean section is recommended
Correct answer: Positive screening requires intrapartum antibiotic prophylaxis to prevent neonatal GBS disease
GBS colonization detected on prenatal screening (35-37 weeks gestation per Canadian guidelines) requires intrapartum antibiotic prophylaxis (IAP), typically IV penicillin or ampicillin, to prevent neonatal early-onset GBS disease (sepsis, pneumonia, meningitis). Without IAP, approximately 1-2% of colonized mothers' newborns develop invasive GBS disease.
Question 6: A patient returning from travel to sub-Saharan Africa presents with cyclic fevers. What should the laboratory perform?
- Blood culture only
- Thick and thin blood smear examination for malaria parasites (STAT) (Correct answer)
- Stool culture for Salmonella
- Urine culture
Correct answer: Thick and thin blood smear examination for malaria parasites (STAT)
Malaria must be considered in any febrile patient with travel to endemic areas. Thick and thin blood smears should be prepared and examined urgently (STAT) as P. falciparum malaria can be rapidly fatal. Multiple smears may be needed (at least 3 over 12-24 hours if initial smears are negative). Rapid diagnostic tests (malaria antigen) provide complementary results.
MLPAO Microbiology and Parasitology Clinical Applications 2
What is the appropriate specimen collection for anaerobic culture of a wound infection?