Describe antimicrobial susceptibility testing with its methods and quality control.

Purpose and Principles

• The purpose is to guide treatment for the individual patient, to generate cumulative antibiogram data for empirical policy, and to detect emerging resistance for surveillance
• It is not performed on every isolate — not on organisms with predictable susceptibility (group A streptococcus remains universally penicillin sensitive), not on contaminants, and not on normal flora
• Standards — the CLSI (used in India) and EUCAST publish breakpoints and methods, updated annually; results are reported as S (susceptible), I (susceptible, increased exposure) or R (resistant)

• MBC (minimum bactericidal concentration) is the lowest concentration killing 99.9% of the inoculum; it matters in endocarditis, meningitis and neutropenia, where bactericidal activity is required
• Serum bactericidal titre and synergy testing in selected severe infections

Special Tests and Quality Control

• MRSA detection — a cefoxitin disc (a better inducer of mecA than oxacillin), or detection of the mecA gene or PBP2a protein
• ESBL detection — screening with a third-generation cephalosporin, then a combined disc or double-disc synergy test showing restoration of activity by clavulanic acid, which inhibits ESBLs
• Carbapenemase detection — the modified carbapenem inactivation method (mCIM), the modified Hodge test (now largely superseded), and molecular detection of blaNDM, blaOXA-48 and blaKPC
• D-test — detects inducible clindamycin resistance in staphylococci: an erythromycin disc placed beside a clindamycin disc flattens the clindamycin zone into a "D" shape. A positive D-test means clindamycin will fail in vivo despite appearing susceptible — a clinically important trap
• Vancomycin must be tested by an MIC method, not by disc diffusion, which cannot detect reduced susceptibility in staphylococci
• Colistin requires broth microdilution; disc diffusion and gradient strips are unreliable because the molecule diffuses poorly in agar
• Quality control — standard reference strains (ATCC 25922 E. Coli, ATCC 25923 S. Aureus, ATCC 27853 Pseudomonas) are tested regularly, with results plotted against expected ranges; the inoculum density, medium depth, pH, cation content and disc storage are all controlled, since each affects zone size

Applied Aspects

• Read the whole report, not just the sensitivities; the specimen type, the Gram stain and the quantity determine whether the organism matters at all
• Choose the narrowest effective agent from the report rather than the broadest one listed; laboratories increasingly suppress reserve agents from the routine report for exactly this reason
• A susceptible report does not guarantee success where there is an undrained collection, a retained device, a biofilm, or poor penetration to the site — source control comes first
• Ask about the D-test before using clindamycin for a staphylococcal infection, since inducible resistance causes treatment failure that the routine report does not predict
• Use the local cumulative antibiogram for empirical choices; resistance differs between hospitals and between wards, and a national guideline cannot substitute for local data
• Report and act on unusual resistance immediately — a carbapenem-resistant isolate or a vancomycin-resistant staphylococcus requires confirmation, infection control action and notification, not simply a line on a report