Effects of temperature on the detection of methicillin resistance in Staphylococcus aureus using cefoxitin disc diffusion testing with Iso-Sensitest agar.
OBJECTIVE
Cefoxitin is today the substance of choice for the phenotypic detection of methicillin-resistant Staphylococcus aureus (MRSA). We investigated the influence of incubation temperature in the standard range, i.e. 35-37 degrees C, and time, i.e. 18-20 h, versus a full 24 h.


METHODS
Cefoxitin disc testing was examined at incubation temperatures of 35 and 36 degrees C and times of 18-20 and 24 h, respectively, for 94 mecA-negative and 49 mecA-positive S. aureus on Iso-Sensitest agar using a semi-confluent inoculum.


RESULTS
Cefoxitin inhibition zones on Iso-Sensitest agar were larger at temperatures above 35 degrees C; two isolates (4%, 95% confidence interval=0.5-14%) incubated at 36 degrees C were falsely categorized as susceptible to methicillin. Incubation time across 18-24 h did not impact results.


CONCLUSIONS
Detection of methicillin resistance in S. aureus using the cefoxitin disc method with a semi-confluent inoculum on Iso-Sensitest agar is influenced by incubation temperature, and the temperature should not exceed 35 degrees C for the reliable detection of MRSA.
Skov
R
R
National Center for Antimicrobials and Infection Control, Statens Serum Institut, Copenhagen, Denmark. rsk@ssi.dk
Smyth
R
R
Yusof
A
A
Karlsson
A
A
Mills
K
K
Frimodt-Moller
N
N
Kahlmeter
G
G
D000362:Agar; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002440:Cefoxitin / Q000494:pharmacology*; D003470:Culture Media / Q000737:chemistry*; D006801:Humans; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests / Q000379:methods / Q000592:standards; D013211:Staphylococcus aureus / Q000187:drug effects*; D013696:Temperature; D013997:Time Factors
D000900:Anti-Bacterial Agents; D003470:Culture Media; D002440:Cefoxitin; D000362:Agar