Skip to content
SearchProgress
Display

Display

Theme
Density
Text size

Sign in

Add your earlier progress to your account?

Study progress is waiting to be saved

Methicillin resistance in staphylococci

15 min

  • Detect methicillin resistance in staphylococci with a cefoxitin screen
  • Recall the altered penicillin-binding protein encoded by mecA
  • Name what a negative mecA, nitrocefin, or other targeted test cannot detect

Read the full reference

Try first

Try first

A Staphylococcus aureus from a blood culture gives a cefoxitin disk zone of 19 mm at 18 hours. The laboratory's cefoxitin criteria for S. aureus are susceptible 22 mm or more and resistant 21 mm or less. What is reported?

Right. The next section explains why.

The next section explains it.

The next section explains it.

The next section explains it.

Get the idea

One altered protein

Methicillin resistance in staphylococci comes from PBP2a, a penicillin-binding protein with low affinity for beta-lactams. The mecA gene encodes it, and the rarer mecC encodes a variant. PBP2a makes the isolate resistant to nearly every beta-lactam, including combinations with a beta-lactamase inhibitor. The cephalosporins made for MRSA, such as ceftaroline, are the exception.1,2

Expression can be heterogeneous. A small resistant subpopulation can make an isolate look susceptible, which is why testing above 35 °C or reading too early can miss it.2

The test depends on the species

The Clinical and Laboratory Standards Institute (CLSI) sets the valid tests and their reading times species by species.2,3

TestValid forRead at
Cefoxitin diskS. aureus, S. lugdunensis16 to 18 hours
Cefoxitin diskS. epidermidis and other listed coagulase-negative staphylococci24 hours
Cefoxitin MICS. aureus, S. lugdunensis only16 to 20 hours
Oxacillin MICAll staphylococci, with species-specific breakpoints24 hours

Cefoxitin induces mecA strongly and gives a clearer endpoint than oxacillin.1 A PBP2a latex or lateral-flow test on a colony detects the protein directly and gives a rapid confirmation.2 S. pseudintermedius, S. schleiferi and S. coagulans fall outside the cefoxitin disk's validation, and they are tested with oxacillin.1,2

What a negative targeted test cannot see

A targeted test detects only its own target:

  • A mecA PCR misses mecC.
  • Older PBP2a reagents can miss the mecC protein.
  • A negative nitrocefin test in Haemophilus influenzae misses beta-lactamase-negative, ampicillin-resistant (BLNAR) strains, whose resistance comes from altered PBPs.2

The phenotypic result supplies the category when a targeted test is negative. A resistant genotype also stands when the phenotype looks susceptible. Some isolates carry mecA and still test oxacillin susceptible, so a susceptible result from a serious infection is confirmed with a molecular test before it is reported. An S. aureus whose genotype and phenotype cannot be reconciled is reported resistant.2

References
  1. Centers for Disease Control and Prevention. Laboratory testing for methicillin (oxacillin)-resistant Staphylococcus aureus (MRSA). Reviewed June 27, 2025. Accessed September 27, 2026. https://www.cdc.gov/mrsa/php/laboratories/index.html
  2. Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. 36th ed. CLSI supplement M100. CLSI; 2026. Accessed September 27, 2026. https://clsi.org/shop/standards/m100/
  3. Clinical and Laboratory Standards Institute. M100, 30th ed., combined corrections. September 14, 2020. Accessed September 27, 2026. https://clsi.org/media/3824/m100ed30_combined_correction_notice_20200914_web.pdf

Watch one

A Staphylococcus epidermidis grows from two sets of blood cultures in a patient with a prosthetic hip. The cefoxitin disk was read at 18 hours with a zone of 27 mm. The laboratory's cefoxitin criteria for S. epidermidis are susceptible 25 mm or more and resistant 24 mm or less.

What is reported for oxacillin?

  1. Confirm the species. S. epidermidis is within the cefoxitin disk's validation.

    The species decides which test and which criteria are valid.

  2. Check the reading time. S. epidermidis is read at 24 hours, so the 18-hour zone is not yet a result.

    Coagulase-negative staphylococci need the full reading time for slow resistant growth to show.

  3. Read at 24 hours. Fine growth has moved in, and the zone now measures 23 mm.

    Heterogeneous resistant growth can fill in the zone late.

  4. Compare. 23 mm is 24 mm or less, so the isolate is resistant.

    The cefoxitin disk has two categories only.

  5. Report oxacillin resistant. The other beta-lactams follow the laboratory's rule for methicillin-resistant staphylococci.

    Cefoxitin stands for oxacillin, and PBP2a covers nearly every beta-lactam.

Oxacillin (methicillin) resistant, from the 24-hour cefoxitin zone of 23 mm.

Your turn

Problem 1 of 3

Why is cefoxitin useful in phenotypic testing for methicillin resistance in S. aureus?

Cefoxitin supports detection of methicillin resistance. Vancomycin susceptibility needs its own appropriate method.

This is a growth-based test. A nucleic acid assay directly detects its specified genetic target.

Cefoxitin is a strong mecA inducer and provides endpoints that are easier to interpret than oxacillin in this application.

Hint
  1. Cefoxitin is used as a stand-in. Ask what it stands in for.
  2. The disk test reads growth on a plate. Ask what a growth test can show and what it cannot.

Review Staphylococcal methicillin resistance

Problem 2 of 3

A blood isolate of Haemophilus influenzae is β-lactamase-negative by nitrocefin. What does this mean for ampicillin?

Incorrect. BLNAR strains test nitrocefin negative and ampicillin resistant, and CLSI recommends ampicillin testing for blood and CSF isolates, so the test is still performed.

Incorrect. TEM-1 and ROB-1 are what a positive nitrocefin screen detects. This screen is negative, so any ampicillin resistance would come from altered penicillin-binding proteins.

Correct. β-Lactamase-negative, ampicillin-resistant (BLNAR) strains resist through altered penicillin-binding proteins that nitrocefin cannot detect. CLSI recommends ampicillin testing for blood and CSF isolates of H. influenzae, so this blood isolate still receives full susceptibility testing.

Hint
  1. Nitrocefin detects an enzyme. Ask whether an enzyme is the only way to resist ampicillin.
  2. Name another way a bacterium can resist a penicillin.

Review HACEK organisms and Haemophilus identification

Problem 3 of 3

An S. aureus from a dairy farmer's wound gives a cefoxitin disk zone of 16 mm, resistant by the laboratory's criteria. A mecA PCR on the colony is negative, and so is a PBP2a latex test. What is reported for oxacillin?

Both tests look only for mecA and its protein. They can miss mecC, and the cefoxitin phenotype shows resistance.

Called an isolate susceptible from a negative targeted test

A targeted test detects only its targets. A mecA assay misses mecC, and a negative nitrocefin screen misses beta-lactamase-negative ampicillin-resistant (BLNAR) Haemophilus with altered PBPs. Reporting susceptibility from the negative test can release a resistant isolate, so report the category from the phenotypic result.

The phenotypic result supplies the category. A mecC strain, seen in people with livestock contact, fits a resistant cefoxitin zone with a negative mecA test.

The cefoxitin result is valid and reportable now. Sequencing can explain the mechanism later.

Review Staphylococcal methicillin resistance

Use it

  • A Staphylococcus pseudintermedius grows from a dog-bite wound on a veterinary technician's hand.
  • The cefoxitin disk on the routine panel shows a large zone.
  • The laboratory's oxacillin MIC breakpoints for S. pseudintermedius are susceptible 0.25 µg/mL or less and resistant 0.5 µg/mL or more.
  • The oxacillin MIC, set up next, reads 2 µg/mL at 24 hours.
  • The ward calls to ask whether amoxicillin-clavulanate can be reported susceptible, since clavulanate inhibits beta-lactamase.
Decision 1 of 2

What do you do with the cefoxitin zone?

The cefoxitin disk is not validated for S. pseudintermedius. A large zone can hide mecA-mediated resistance.

Read a cefoxitin disk on S. pseudintermedius

The cefoxitin disk is validated for S. aureus, S. lugdunensis, S. epidermidis, and other listed coagulase-negative species. S. pseudintermedius, S. schleiferi, and S. coagulans fall outside that validation, so a cefoxitin zone on them can report a methicillin-resistant isolate as susceptible.

S. pseudintermedius falls outside the cefoxitin disk's validation. The oxacillin MIC with its species breakpoints gives the category.

Criteria for one species do not transfer to another. The cefoxitin disk has no validated criteria for S. pseudintermedius.

Read a cefoxitin disk on S. pseudintermedius

The cefoxitin disk is validated for S. aureus, S. lugdunensis, S. epidermidis, and other listed coagulase-negative species. S. pseudintermedius, S. schleiferi, and S. coagulans fall outside that validation, so a cefoxitin zone on them can report a methicillin-resistant isolate as susceptible.

Review Staphylococcal methicillin resistance

Decision 2 of 2

The oxacillin MIC of 2 µg/mL is resistant. How is amoxicillin-clavulanate reported?

Clavulanate inhibits beta-lactamases. PBP2a is an altered target protein, and clavulanate has no effect on it.

Confused the mecA product with a beta-lactamase

mecA encodes PBP2a, an altered penicillin-binding protein with low beta-lactam affinity. Calling it a beta-lactamase suggests an inhibitor combination would restore activity. PBP2a confers resistance to nearly all beta-lactams, inhibitor combinations included, except anti-MRSA cephalosporins such as ceftaroline.

mecA encodes PBP2a. It resists nearly every beta-lactam, inhibitor combinations included.

An oxacillin-resistant staphylococcus is resistant to beta-lactam-inhibitor combinations by rule, whatever a separate test shows.

Review Staphylococcal methicillin resistance

The clue that settles this case is the species name. S. pseudintermedius sits outside the cefoxitin disk's validation, so the large zone never counted. The oxacillin MIC showed PBP2a at work, and that protein ignores clavulanate.

Keep

Sources checked