Extrapolating key antibiotic susceptibilities for Staphylococcus aureus

Indicator antibiotic tested Result Extrapolates to Also susceptible (but not first line)
Betalactamase production and/or penicillin susceptibility

 

Susceptible

i.e betalactamase negative

 

 

benzylpenicillin

ampicillin/ amox

dicloxacillin

flucloxacillin

cephalexin

cefazolin

Later generation cephalosporins

Carbapenems

Betalactam+betalactamase inhibitors including amoxycillin+clavulanate

Cefoxitin Susceptible = the strain is NOT MRSA di/flucloxacillin

cephalexin

cefazolin

Cefoxitin Resistant =

MRSA detected

Confirmation – presence of mecA (or mecC) gene

No betalactams other than ceftaroline can be used for treatment  Ceftaroline
Erythromycin Susceptible Azithromycin

Clarithromycin

Clindamycin

Erythromycin

 

Resistant

Positive ‘D test’ to look for inducible clindamycin resistance

Clindamycin is susceptible if the D test is negative (ie shows no evidence of inducible resistance (see below) Note: as only a minority (10-20% ) of erythromycin resistant strains are clindamycin susceptible (D test negative), most labs do not do the D test and report Clinda as resistant based on the Erythromycin test result. This is a reliable strategy.
Tetracycline Susceptible Doxycycline Tigecycline
Vancomycin

(broth micro-dilution method required)

Susceptible

 

Teicoplanin

 

Note: most labs will also conduct an MIC test by the E-test method for isolates of MRSA from blood. Raised MICs 1.5mg/L or higher within the susceptible range are associated with worse patient outcomes.  Testing for vancomycin-intermediate strains may also be indicated in some circumstances.

Inducible r

Image from microbeonline.com.

3 comments

  1. […] MRSA is done by a cefoxitin 30 microgram disc test by the correct EUCAST or CLSI methodology.  This posting discusses susceptibility of S. […]

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  2. […] 80% of local methicillin-susceptible S. aureus is penicillin resistant by this mechanism.  This post explains how to extrapolate penicillin and other antibiotic susceptibilities to related […]

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