Residency · Residency · Pathology
Bacteriology for the Pathologist: Culture Interpretation and Susceptibility Testing
Introduction
The pathologist must understand the principles of bacterial culture interpretation, including specimen quality assessment, identification methods, and antimicrobial susceptibility testing (AST). Effective communication of culture results impacts antibiotic stewardship, patient outcomes, and infection control practices.
Specimen Quality and Rejection Criteria
Pre-analytical quality is the most important determinant of a meaningful culture result. For sputum, quality is assessed using the Murray-Washington criteria: more than 25 PMNs and fewer than 10 squamous epithelial cells per low-power field indicates a valid lower respiratory specimen. Specimens with heavy squamous cells represent oropharyngeal contamination and should be rejected or reported with a caveat. Anaerobic cultures require proper collection by aspirate rather than swab and transport in an anaerobic transport system. Duplicate cultures within 24 hours generally should not be processed.
Culture Media and Incubation
Common Media
Blood agar (BAP) is a general-purpose medium that demonstrates hemolysis patterns (alpha, beta, and gamma). MacConkey agar is selective for gram-negatives and differentiates lactose fermenters (which appear pink) from non-fermenters. Chocolate agar is enriched for fastidious organisms such as Haemophilus and Neisseria. Selective and differential media include mannitol salt agar for Staphylococcus, CNA agar for gram-positives, and Hektoen enteric agar for Salmonella and Shigella.
Incubation Conditions
Standard incubation is at 35-37 degrees C in ambient air or 5% CO2 for 18-24 hours, with extended incubation for slow growers. Anaerobic organisms require a dedicated anaerobic chamber or jars. Special conditions are needed for certain organisms: Campylobacter grows at 42 degrees C under microaerophilic conditions, and Legionella requires BCYE agar.
Bacterial Identification Methods
Traditional Methods
Traditional identification relies on colony morphology (size, shape, color, hemolysis, and odor), Gram stain (which reveals gram-positive cocci in clusters for Staphylococcus, chains for Streptococcus, and gram-negative rods for Enterobacterales), and biochemical tests including catalase, coagulase, oxidase, indole, and urease.
Modern Methods
MALDI-TOF mass spectrometry has become the standard of care in most laboratories, providing rapid identification from colonies in minutes. It works by matching a protein fingerprint to a reference database. Some closely related species (such as Streptococcus pneumoniae versus S. mitis group), certain anaerobes, and some fungi may require supplementary testing.
Antimicrobial Susceptibility Testing (AST)
Methods
Disk diffusion (Kirby-Bauer) measures zones of inhibition compared to CLSI or EUCAST breakpoints. Broth microdilution is the gold standard and determines the minimum inhibitory concentration (MIC) in mcg/mL. Automated systems such as VITEK 2, MicroScan, and Phoenix perform identification and AST simultaneously. Gradient diffusion (Etest) uses strips with an antibiotic gradient to read the MIC directly.
Interpretation Categories
Results are reported as Susceptible (S) when infection is likely to respond to standard dosing, Susceptible-dose dependent (SDD) when higher doses or more frequent administration is needed, Intermediate (I) which EUCAST has redefined as susceptible at increased exposure, or Resistant (R) when response is not expected.
Breakpoint Standards
CLSI (Clinical and Laboratory Standards Institute) is used predominantly in the US, while EUCAST (European Committee on Antimicrobial Susceptibility Testing) is used internationally. Breakpoints may differ between the two organizations for some drug-organism combinations.
Resistance Detection
Key Resistance Mechanisms
| Resistance | Organism(s) | Mechanism | Detection Method |
|---|---|---|---|
| MRSA | S. aureus | mecA → PBP2a | Cefoxitin disk/PCR |
| ESBL | Enterobacterales | Extended-spectrum β-lactamase | Cephalosporin synergy test |
| CRE | Enterobacterales | KPC, NDM, OXA-48 | mCIM, molecular testing |
| VRE | Enterococci | vanA (high-level), vanB | Vancomycin MIC/PCR |
MRSA (methicillin-resistant Staphylococcus aureus) carries the mecA gene encoding PBP2a and is screened with a cefoxitin disk. ESBL (extended-spectrum beta-lactamases) in Enterobacterales are detected by cephalosporin synergy testing. CRE (carbapenem-resistant Enterobacterales) involve mechanisms including KPC, NDM, and OXA-48. VRE (vancomycin-resistant enterococci) carry vanA (high-level, inducible) or vanB genes.
Reporting Considerations
Cascade reporting is a stewardship tool that reports narrow-spectrum agents first, with broader agents shown only if the organism is resistant to the narrower options. Selective reporting supports antimicrobial stewardship goals. Critical resistance patterns such as CRE and VRSA must be reported to infection control and public health authorities.
Blood Culture Interpretation
Detailed blood culture interpretation is covered in Lecture 52. The key principle is distinguishing clinically significant isolates from contaminants such as coagulase-negative staphylococci in a single set. Time to positivity and the number of positive sets guide assessments of clinical significance.
Common Interpretive Challenges
Mixed flora in wound cultures should be reported with predominant organisms and clinical context considered. Normal flora sites such as throat, stool, and vaginal cultures require selective approaches. Breakthrough bacteremia may indicate inadequate therapy or a new source. Polymicrobial bacteremia should prompt consideration of a GI source, line infection, or contamination.
Clinical Pearls
Specimen quality assessment prevents reporting misleading results from contaminated specimens. MALDI-TOF has revolutionized bacterial identification with same-day results from positive cultures. Cascade reporting is an essential stewardship tool that reduces unnecessary broad-spectrum antibiotic use. Critical resistance patterns including CRE and VRSA are reportable to public health authorities and require immediate infection control notification.
References
- CLSI. Performance Standards for Antimicrobial Susceptibility Testing. M100, 33rd ed. Wayne, PA: Clinical and Laboratory Standards Institute; 2023.
- Patel R. MALDI-TOF MS for the diagnosis of infectious diseases. Clin Chem. 2015;61(1):100-111.
- Leber AL. Clinical Microbiology Procedures Handbook. 4th ed. ASM Press; 2016.
- Bauer KA, et al. An antimicrobial stewardship program's impact on clinical and economic outcomes. Clin Infect Dis. 2014;59(9):1236-1245.