Comparison of cefoxitin and moxalactam 30 microg disc diffusion methods for detection of methicillin resistance in coagulase-negative staphylococci.
OBJECTIVE
To compare cefoxitin and/or moxalactam 30 microg disc diffusion (DD) methods to detect methicillin resistance in coagulase-negative staphylococci (CoNS) using both high- and low-density (HD/LD) inoculum techniques.


METHODS
A challenge set of 192 CoNS was tested. DD test results were compared with PBP2a detection.


RESULTS
With the LD inoculum, the sensitivity/specificity of cefoxitin and moxalactam were 94.4%/100% and 100%/92.4%, respectively, using the DD breakpoints of the Comité de l'Antibiogramme de la Société Française de Microbiologie. With the HD inoculum, the sensitivity/specificity of cefoxitin and moxalactam were 93.7%/100% and 100%/96.9%, using the cefoxitin DD breakpoints of the CLSI and a resistant/susceptible breakpoint of < 20 mm/>or=20 mm for moxalactam. Comparison of receiver operating characteristic AUCs did not show significant difference between studied assays, but the overlapping zone where both PBP2a-positive and PBP2a-negative isolates were observed concerned a lower number of strains with moxalactam than with cefoxitin (P < 0.001). Combination of cefoxitin and moxalactam DD methods demonstrated that all isolates with a concordant cefoxitin/moxalactam phenotype were correctly classified. Interestingly, all isolates misclassified by each DD method used alone were cefoxitin-susceptible and moxalactam-resistant.


CONCLUSIONS
Although all DD methods studied here performed well for detecting methicillin resistance in CoNS, moxalactam had a higher accuracy than cefoxitin to differentiate heteroresistant isolates from PBP2a-negative strains. Identification of isolates that should be submitted to a confirmatory test to conclude on methicillin resistance can be easily obtained by combining cefoxitin and moxalactam DD methods.
Join-Lambert
Olivier F
OF
Microbiology Laboratory, Necker-Enfants Malades Hospital, Faculté de Médecine Paris-Descartes, 149 rue de Sèvres, 75015, Paris, France. join@necker.fr
Clauser
Sylvain
S
Guillet
Christelle
C
Jais
Jean-Philippe
JP
Abachin
Eric
E
Quesnes
Gilles
G
Carbonnelle
Etienne
E
Le Monnier
Alban
A
Zahar
Jean-Ralph
JR
Kayal
Samer
S
Berche
Patrick
P
Ferroni
Agnès
A
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000378:metabolism; D002440:Cefoxitin / Q000494:pharmacology*; D003030:Coagulase / Q000378:metabolism; D016106:Methicillin Resistance / Q000187:drug effects*; D008826:Microbial Sensitivity Tests; D009070:Moxalactam / Q000494:pharmacology*; D046915:Penicillin-Binding Proteins / Q000378:metabolism; D012372:ROC Curve; D013210:Staphylococcus / Q000187:drug effects* / Q000201:enzymology; D013212:Staphylococcus epidermidis / Q000187:drug effects
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D003030:Coagulase; D046915:Penicillin-Binding Proteins; C487789:mecA protein, Staphylococcus aureus; D002440:Cefoxitin; D009070:Moxalactam