Microbiology sample processing

Antibiotic effectiveness is related to antibiotic concentrations in patients relative to the susceptibility of bacteria. To determine the susceptibility of colonic Enterobacteriaceae, which we considered to be the most likely cause of post-operative SSIs, rectal or stoma swabs were collected from patients pre-operatively [18]. Swabs were screened for Enterobacteriaceae resistance by two methods: the first isolated the Enterobacteriaceae strain that was numerically predominant, and the second isolated the most antibiotic resistant strain. The predominant strain was identified by inoculating a CLED agar plate with the swab, streaking for isolated colonies, and determining the minimum inhibitory concentration (MIC) of these colonies from the terminal streak. To identify the most resistant organism, a CLED agar plate was swabbed for confluent growth and a 30-μg cefuroxime antibiotic disc (Oxoid) was placed in the centre. The growth closest to the disc was cultured to purity, and MICs were determined. MALDI-TOF and a 0.16 to 256 mg/L cefuroxime gradient strip (biomerieux) were used to determine species and MICs respectively. Resistance was defined as cefuroxime MIC > 8 mg/L [22].