Evaluation of 10, 30 and 60 mcg cefoxitin neo-sensitabs for prediction of methicillin resistance in S. aureus

Abstract number: 1134_03_150

Skov R., Larsen A.R., Frimodt-Møller N.

Objective:  

Cefoxitin paperdisks have been shown to accurately predict methicillin resistance in S. aureus. In this paper we evaluate 10, 30 and 60 mcg Cefoxitin Neo-Sensitabs tablets for predicting methicillin resistance using a challenge set of S. aureus.

Methods:  

A total of 196 recently isolated S. aureus all tested for presence of the mecA gene (EVIGENE, SSI, Denmark) were investigated. The mecA positive isolates (N = 62) were a challenge set consisting of the most heterogeneous resistant MRSA isolates found in a recent investigation (Skov et al, J Antimicrob Chemother). The isolates represented 39 different PFGE types (at least one band difference) and included isolates with oxacillin MIC <2 mg/L. The mecA negative isolates were consecutive blood culture isolates (N = 99) as well as the mecA negative isolates with the smallest cefoxitin zones in a recent evaluation (N= 35). S. aureus ATCC 29213 and 25923 were included as control strains in each run. All isolates were tested with confluent growth and 18–22 h incubation at 35–37°C on Mueller Hinton Agar (BBLII, Becton Dickinson, US) using 10, 30 and 60 mcg Neo-Sensitabs tablets (Rosco, Taastrup, Denmark). Inhibition zone diameters were measured to the nearest millimetre at the inner zone edge–disregarding a faint haze, if present.

Results:  

Using a breakpoint of R < 19 mm (cefox 10); R < 24 mm (cefox 30) and R < 29 mm (cefox 60) two, three and two mecA positive isolates tested false susceptible, respectively. Five, four and six mecA negative isolates tested false resistant. Zone edges for the 10 mcg tablet were more distinct and easier to read. For S. aureus ATCC 29213 the following zone diameters were found 21–23 mm, 27–29 mm and 29–33 mm, respectively and for S. aureus ATCC 25923 22–23 mm; 27–29 mm, and 32–33 mm, respectively.

Conclusions:  

All three cefoxitin Neo-Sensitabs performed with high accuracy using this challenge set and as good as the results obtained in a recent evaluation of a 10 mcg paperdisk. The cefoxitin 10 mcg Neo-Sensitabs performed slightly better than the 30 or 60 mcg and zone edges were easier to read why we will suggest the use of 10 mcg tablet.