13.1 MRSA/MSSA, VRE, ESBL/CRE Screening

13.1 MRSA/MSSA, VRE, ESBL/CRE Screening

Quick Answer: Infection-control screening detects colonization, not infection. Collect anterior nares (plus axilla, groin, and wounds) for MRSA/MSSA and rectal or perianal swabs for VRE and ESBL/CRE. Use chromogenic media, cefoxitin or vancomycin selective culture, and targeted PCR (mecA/mecC, vanA/vanB, carbapenemase genes). Never substitute a screening isolate for a diagnostic culture from a sterile site.

The ASCP M content guideline (revised 2025-09-25) lists II.L MRSA/MSSA, VRE, ESBL/CRE Screening as its own bacteriology topic: specimen sources, culture methods, and molecular methods. These assays exist to drive contact precautions, decolonization, and outbreak control. They are not a shortcut around blood, CSF, tissue, or other diagnostic cultures.

Screening versus diagnostic testing

Infection-control screening answers a single question: Is this patient colonized with a targeted multidrug-resistant organism? A positive screen places the patient into isolation and, in many MRSA programs, into decolonization. A negative screen is only as good as the anatomic sites sampled and the method's limit of detection.

Diagnostic testing answers a different question: What organism is causing this infection, and which drug will treat it? That work-up requires a specimen from the infected site, identification of the isolate, and antimicrobial susceptibility testing (AST) of that isolate.

Two rules follow and are frequently tested:

1. A nasal MRSA PCR does not diagnose pneumonia, wound infection, or bacteremia.
2. A rectal CRE or VRE screen must not be reported as the pathogen of a subsequent bloodstream infection. Draw blood cultures. Work up the blood isolate. Screening flora can differ from the invasive strain, can be mixed, and do not carry a validated MIC for the sterile-site organism.

Specimen sources

Match the swab to the anatomic reservoir.

Wounds, sputum, catheter urine, and throat swabs are adjuncts when those sites are clinically relevant; they do not replace the reservoir swab. Pooling swabs is a local operations decision. If swabs are pooled, a positive result cannot assign the body site—infection prevention still isolates the patient.

Collect screening swabs before bathing or chlorhexidine when possible, and label them as surveillance, not as diagnostic cultures, so the laboratory applies the correct media and reporting comments.

MRSA and MSSA: culture methods and the mecA definition

Methicillin-resistant Staphylococcus aureus (MRSA) carries mecA or, less often, mecC. Those genes encode PBP2a (or PBP2c), penicillin-binding proteins with low affinity for nearly all beta-lactams. Methicillin-susceptible S. aureus (MSSA) lacks mecA/mecC and tests susceptible to cefoxitin, the laboratory surrogate for oxacillin. Calling an isolate MRSA or MSSA is a genotype-defined distinction, not a marketing color on a plate.

Chromogenic MRSA agar is the usual screening plate. The medium contains cefoxitin (or oxacillin) plus chromogenic substrates so that MRSA colonies appear in a manufacturer-specified color after 18–24 hours. Read the plate exactly as the insert directs. Extending incubation can recover slow strains but lets breakthrough flora mimic a positive.

Cefoxitin screen—disk diffusion (30 µg) or MIC—is the preferred phenotypic surrogate for mecA in S. aureus. Cefoxitin induces mecA more reliably than oxacillin, which is why CLSI standardized on cefoxitin rather than oxacillin disks. Do not override a cefoxitin-resistant result with an oxacillin-susceptible MIC. Cefoxitin is a screen, not a drug that is reported for therapy of S. aureus.

Broth enrichment (tryptic soy or similar broth with salt, sometimes plus cefoxitin) raises sensitivity when the colonizing inoculum is low. After overnight incubation the broth is subcultured to chromogenic agar. Enrichment adds about 24 hours but is the method of choice when a negative direct plate will be used to discontinue isolation.

Confirm colored colonies as S. aureus (catalase-positive Gram-positive cocci in clusters; coagulase or MALDI-TOF) and as methicillin resistant (cefoxitin or PBP2a immunoassay). Borderline oxacillin-resistant S. aureus (BORSA) lacks mecA; do not report BORSA as MRSA. Some mecC strains are weakly reactive in older PBP2a latex kits—use a mecA/mecC PCR when the phenotype and antigen disagree.

VRE: culture methods

Infection-control vancomycin-resistant enterococci (VRE) are almost always E. faecium or E. faecalis that carry vanA (high-level vancomycin and teicoplanin resistance) or vanB (vancomycin resistant, teicoplanin susceptible). Intrinsic, low-level vanC in motile E. gallinarum and E. casseliflavus is not managed as VRE.

Chromogenic VRE agar recovers colored enterococcal colonies in the presence of vancomycin (commonly 6–10 µg/mL). The classic alternative is bile esculin agar containing vancomycin: enterococci hydrolyze esculin (black colonies) and grow despite the drug. Confirm Gram-positive cocci in pairs or short chains, catalase negative, PYR positive, then identify to species. Measure a vancomycin MIC. Do not report VRE from plate color alone, and do not treat vanC motile enterococci as an isolation trigger.

ESBL and CRE: culture methods, Hodge versus mCIM

Extended-spectrum beta-lactamases (ESBLs) hydrolyze oxyimino-cephalosporins (cefotaxime, ceftriaxone, ceftazidime, cefepime) and are inhibited by clavulanate. Carbapenem-resistant Enterobacterales (CRE) test resistant to meropenem, imipenem, or ertapenem. Carbapenemase producers—KPC, NDM, OXA-48-like, VIM, IMP—are the infection-control priority because the genes are plasmid-borne.

Chromogenic ESBL or CRE media recover colored colonies from rectal swabs. Laboratories that prepare their own screens use MacConkey agar with cefotaxime (often 1 µg/mL) for ESBL and MacConkey with a carbapenem—meropenem or ertapenem in the agar, or a carbapenem disk on the first streak—for CRE. Any colony on the selective plate or inside a disk zone is identified and tested.

The modified Hodge test (clover-leaf growth of a susceptible indicator strain around a test isolate plus a carbapenem disk) is the historical phenotypic carbapenemase screen. CLSI retired it. AmpC hyperproduction plus porin loss caused false positives; NDM metallo-beta-lactamases caused false negatives. The current CLSI phenotypic method is the modified carbapenem inactivation method (mCIM). Adding EDTA (eCIM) separates serine carbapenemases (KPC, OXA-48; eCIM negative) from metallo-enzymes (NDM, VIM, IMP; eCIM positive). Carba NP is an alternative colorimetric assay. Use mCIM/eCIM on recovered isolates; they do not replace molecular panels or public-health referral of unusual profiles.

Molecular methods

Direct PCR from a swab can return in 1–3 hours and is the fastest path to isolation. Culture remains necessary for outbreak typing, AST of a true pathogen, and any sterile-site diagnosis.

What to refuse on exam day

Do not treat a chromogenic colony color as a species-level identification. Do not use an oxacillin disk in place of cefoxitin to screen S. aureus. Do not call every vancomycin-resistant catalase-negative coccus VRE. Do not revive the modified Hodge test as the primary carbapenemase screen. Do not issue AST from a surveillance swab as if it were the patient's infected-site isolate.