8.5 Antimicrobial Susceptibility Testing, Resistance, and Virology

Antimicrobial Susceptibility Testing, Resistance, and Virology

Accurate antimicrobial susceptibility testing (AST) and the rapid detection of resistance mechanisms are central to the role of the clinical microbiology laboratory in guiding empirical and targeted therapy. Understanding both phenotypic and genotypic markers of resistance ensures appropriate clinical intervention.

Antimicrobial Susceptibility Testing (AST)

AST determines the in vitro activity of antimicrobial agents against specific bacterial isolates. The goal is to categorize the organism as Susceptible (S), Intermediate (I), or Resistant (R) based on established Clinical and Laboratory Standards Institute (CLSI) breakpoints.

Kirby-Bauer Disk Diffusion

The Kirby-Bauer method is a widely used, standardized qualitative assay.

• Medium: Mueller-Hinton Agar (MHA). It is chosen because it shows excellent batch-to-batch reproducibility, is low in sulfonamide, trimethoprim, and tetracycline inhibitors, and supports the growth of most non-fastidious pathogens.
• Parameters:
  • Depth: Must be exactly 4 mm. If the agar is too thin (<4 mm), the antibiotic diffuses further, creating falsely large zones (false susceptible). If the agar is too thick (>4 mm), the diffusion is restricted, creating falsely small zones (false resistant).
  • pH: 7.2 - 7.4. Changes in pH can alter the activity of certain drugs, such as aminoglycosides and macrolides.
  • Cations: Calcium (Ca2+) and Magnesium (Mg2+) levels must be tightly controlled. Increased cation levels decrease the activity of aminoglycosides against Pseudomonas aeruginosa, leading to false resistance.
• Inoculum: Standardized to a 0.5 McFarland turbidity standard (approximately 1.5 x 10^8 CFU/mL).
  • Exam Trap: If the inoculum is too heavy (>0.5), zone sizes will be smaller (false resistance). If too light (<0.5), zone sizes will be larger (false susceptible).

Minimum Inhibitory Concentration (MIC) Testing

The MIC is the lowest concentration of an antimicrobial agent that visibly inhibits the growth of the organism.

• Broth Microdilution: The reference standard method. Serial twofold dilutions of antibiotics are prepared in microtiter wells, inoculated with a standardized suspension, and examined for visible growth (turbidity).
• Agar Dilution: Antimicrobial agents are incorporated into solid agar plates at varying concentrations, and multiple isolates can be spot-inoculated onto the surface.
• E-test (Episiliometer Test): A quantitative gradient diffusion method. A plastic strip impregnated with a decreasing concentration gradient of an antibiotic is placed on an inoculated MHA plate. The MIC is read where the elliptical zone of inhibition intersects the scale on the strip.
• Minimum Bactericidal Concentration (MBC): The lowest concentration of drug that kills 99.9% of the bacterial population, determined by subculturing clear MIC wells onto solid agar.

Key Mechanisms of Antimicrobial Resistance

The rapid emergence of multidrug-resistant organisms (MDROs) requires specific detection strategies in the laboratory.

Methicillin-Resistant Staphylococcus aureus (MRSA)

MRSA strains are resistant to all beta-lactam antibiotics (penicillins, cephalosporins, carbapenems), with a few exceptions like the 5th generation cephalosporin, ceftaroline.

• Mechanism: Mediated by the mecA gene, which encodes an altered penicillin-binding protein, PBP2a. Normal PBPs are targeted by beta-lactams to halt cell wall synthesis. PBP2a, however, has a low affinity for beta-lactam antibiotics, allowing cell wall synthesis to continue even in their presence.
• Detection: The cefoxitin disk screen is the preferred phenotypic method, as cefoxitin is a strong inducer of the mecA gene. Molecular detection of mecA (via PCR) or latex agglutination tests for the PBP2a protein provide definitive, rapid identification.

Vancomycin-Resistant Enterococci (VRE)

Typically observed in Enterococcus faecium (most common) and Enterococcus faecalis.

• Mechanism: Mediated primarily by the vanA and vanB genes, which alter the terminal amino acids of the peptidoglycan precursor from D-alanyl-D-alanine to D-alanyl-D-lactate. This structural alteration dramatically reduces vancomycin's ability to bind and inhibit cell wall cross-linking.
• Detection: Requires precise reading. Some vanB phenotypes can be missed on disk diffusion; prolonged incubation (24 hours) and viewing with transmitted light are recommended. E. gallinarum and E. casseliflavus possess intrinsic, low-level resistance via the vanC gene, which is chromosomally mediated and not transmissible.

Extended-Spectrum Beta-Lactamases (ESBL)

ESBLs are plasmid-mediated enzymes produced by Gram-negative bacilli (most commonly Klebsiella pneumoniae, Klebsiella oxytoca, and Escherichia coli) that hydrolyze penicillins, cephalosporins (including 1st, 2nd, and 3rd generations), and monobactams (aztreonam), but NOT carbapenems or cephamycins.

• Mechanism: Point mutations in classical beta-lactamase genes (e.g., TEM, SHV, CTX-M).
• Detection: Screening relies on demonstrating resistance or decreased susceptibility to indicator drugs like cefotaxime or ceftazidime. Confirmation involves demonstrating a ≥ 5 mm increase in zone diameter for either agent when tested in combination with clavulanic acid (a beta-lactamase inhibitor) versus the agent alone.

AmpC Beta-Lactamases

AmpC enzymes are cephalosporinases typically chromosomally mediated and inducible in the "SPACE" organisms (Serratia, Pseudomonas, Acinetobacter, Citrobacter, Enterobacter). Unlike ESBLs, AmpC beta-lactamases are not significantly inhibited by clavulanic acid and can hydrolyze cephamycins (e.g., cefoxitin).

Carbapenem-Resistant Enterobacteriaceae (CRE)

CRE represent a profound public health threat due to resistance against nearly all available beta-lactams, including last-resort carbapenems.

• Mechanism: Production of carbapenemases (enzymes that hydrolyze carbapenems). Common types include KPC (Klebsiella pneumoniae carbapenemase, Ambler Class A) and NDM (New Delhi metallo-beta-lactamase, Ambler Class B).
• Detection: The Modified Hodge Test (MHT) was classically used to detect carbapenemase production in vivo (using a cloverleaf-like indentation of indicator E. coli growth). However, modern laboratories heavily rely on molecular assays (PCR for blaKPC, blaNDM, blaOXA-48) or biochemical colorimetric assays (e.g., Carba NP test) for superior specificity and rapid turnaround.

Clinical Virology Overview

Viruses are obligate intracellular parasites containing either DNA or RNA, enclosed in a protein capsid, and sometimes a lipid envelope.

Diagnostic Methods

1. Cell Culture: Historically the "gold standard." Viral growth is detected by observing characteristic Cytopathic Effects (CPE) on monolayer cell lines (e.g., rounding, syncytia formation, cell lysis).
2. Molecular Diagnostics (NAAT/PCR): Now the standard of care for most viral infections. Highly sensitive, specific, and rapid.
   • Qualitative PCR: Used for primary diagnosis (e.g., Influenza A/B, RSV, Herpes Simplex Virus [HSV] in CSF for encephalitis, SARS-CoV-2).
   • Quantitative PCR (Viral Loads): Crucial for monitoring disease progression and antiviral therapy efficacy (e.g., HIV RNA, Hepatitis C Virus RNA, CMV DNA in transplant patients).
3. Serology: Detects patient antibodies (IgM for acute infection, IgG for past infection/immunity) or viral antigens in serum.
   • Hepatitis B Virus (HBV): HBsAg (surface antigen) indicates current infection. Anti-HBs indicates immunity (either from vaccination or resolved infection). Anti-HBc (core antibody) indicates past or current infection, but is NOT present if immunity is strictly from vaccination.
   • Epstein-Barr Virus (EBV): The causative agent of Infectious Mononucleosis. The classic rapid test is the heterophile antibody test (Monospot), which detects non-specific antibodies produced during acute EBV infection that agglutinate horse or sheep red blood cells.

Exam Trap: Remember that CMV and EBV can both cause mononucleosis-like syndromes, but CMV will be negative for heterophile antibodies.