MLPAO Microbiology and Parasitology Clinical Applications 1 — Questions and Answers
Question 1: A Gram stain of a positive blood culture shows gram-positive cocci in clusters. What organism is most likely and what empiric therapy should be communicated?
- Streptococcus pneumoniae; penicillin
- Staphylococcus species; vancomycin pending identification and susceptibilities (Correct answer)
- Enterococcus; ampicillin
- Neisseria meningitidis; ceftriaxone
Correct answer: Staphylococcus species; vancomycin pending identification and susceptibilities
Gram-positive cocci in clusters from blood culture is most consistent with Staphylococcus species. Because MRSA must be considered, empiric vancomycin is recommended until species identification and susceptibility results are available. If methicillin-susceptible S. aureus (MSSA) is confirmed, therapy is narrowed to cloxacillin/cefazolin.
Question 2: A urine culture grows more than 100,000 CFU/mL of a single organism. What does this indicate?
- Contamination
- Significant bacteriuria consistent with urinary tract infection (Correct answer)
- Normal flora
- Specimen collection error
Correct answer: Significant bacteriuria consistent with urinary tract infection
Growth of ≥10⁵ CFU/mL (100,000 CFU/mL) of a single organism from a clean-catch midstream urine is considered significant bacteriuria. This threshold, combined with clinical symptoms, supports the diagnosis of UTI. Lower counts may be significant in catheterized specimens or symptomatic patients.
Question 3: A laboratory identifies a Salmonella species from a stool culture. What is the reporting obligation in Canada?
- No reporting is required
- Salmonella is a reportable/notifiable disease and must be reported to public health authorities (Correct answer)
- Only typhoid Salmonella needs reporting
- Reporting is optional and at the physician's discretion
Correct answer: Salmonella is a reportable/notifiable disease and must be reported to public health authorities
Salmonellosis is a reportable/notifiable disease in all Canadian provinces and territories. The laboratory must report the isolation to local public health authorities. Isolates may be sent to a reference laboratory for serotyping and molecular typing for outbreak investigation and surveillance purposes.
Question 4: Which infection control precaution is implemented when Clostridioides difficile infection is suspected?
- Standard precautions only
- Contact precautions with hand washing using soap and water (not alcohol-based hand rub) (Correct answer)
- Airborne precautions with N95 respirator
- Droplet precautions only
Correct answer: Contact precautions with hand washing using soap and water (not alcohol-based hand rub)
C. difficile requires contact precautions including gown and gloves, with hand hygiene using soap and water. Alcohol-based hand sanitizers do NOT kill C. difficile spores. Enhanced environmental cleaning with sporicidal agents (dilute bleach) is also essential as spores can persist on surfaces for months.
Question 5: A wound culture grows a beta-hemolytic, catalase-negative, gram-positive coccus that is PYR-positive. What is the most likely identification?
- Staphylococcus aureus
- Group A Streptococcus (Streptococcus pyogenes) (Correct answer)
- Enterococcus faecalis
- Group B Streptococcus
Correct answer: Group A Streptococcus (Streptococcus pyogenes)
The key features are: beta-hemolytic (GAS, GBS, or Group C/G), catalase-negative (streptococcus/enterococcus, not staphylococcus), gram-positive cocci, and PYR-positive. Among beta-hemolytic streptococci, Group A Streptococcus (S. pyogenes) is PYR-positive while Group B and other groups are PYR-negative.
Question 6: A patient with HIV has CD4 count below 200 cells/µL and presents with chronic watery diarrhea. Modified acid-fast stain of stool shows small round pink oocysts (4-6 µm). What is the diagnosis?
- Giardiasis
- Cryptosporidiosis (Correct answer)
- Cyclosporiasis
- Isosporiasis
Correct answer: Cryptosporidiosis
Cryptosporidium oocysts are 4-6 µm, round, and stain variably pink with modified acid-fast stain. Cryptosporidiosis causes chronic, potentially life-threatening watery diarrhea in severely immunocompromised patients (CD4 <200). Cyclospora oocysts are larger (8-10 µm) and stain variably. Antiretroviral therapy to restore CD4 count is the most effective treatment.
MLPAO Microbiology and Parasitology Clinical Applications 1
A Gram stain of a positive blood culture shows gram-positive cocci in clusters.
What organism is most likely and what empiric therapy should be communicated?