84 Chapter 5 Specificity of the ceftazidime 1mg/L + cefoxitin 8mg/L agar was calculated using following formula: 39 + (25/40 x number of ESBL isolates)/(39 + (25/40 x number of ESBL isolates) + (15/40 x number of ESBL isolates)). Prevalence of ESBL was calculated using the formula: number of ESBL isolates / (119 + number of ESBL isolates). Results The 159 E. coli isolates were evaluated for growth on the four different selective agar plates as shown in Table 1. Isolates hyperproducing cAmpC or producing pAmpC or ESBL E. coli grew on both cefotaxime and ceftazidime agar plates without cefoxitin. Of the AmpC/ESBL negative isolates, only one (2.6%) isolate grew on the cefotaxime agar. No growth of AmpC/ESBL negative isolates was seen on the ceftazidime agar. The cefotaxime + cefoxitin agar showed growth of 39 (97.5%) pAmpC isolates, 39 (97.5%) cAmpC isolates, 17 (42.5%) ESBL isolates and one (2.6%) AmpC/ESBL negative isolate. The ceftazidime + cefoxitin agar showed growth of 39 (97.5%) pAmpC isolates, 40 (100%) cAmpC isolates, 15 (37.5%) ESBL isolates and none of the AmpC/ESBL negative isolates. In both agars the pAmpC isolate that did not grow was the same and appeared to contain a blaACC gene. The ATCC control E. coli strain did not grow on any of the evaluated agars. Table 1. Ability of 159 E. coli isolates to grow on four different AmpC screening agars Cefotaxime 1 mg/L Ceftazidime 1 mg/L Characteristics No. of isolates No Cefoxitin 8 mg/L Cefoxitin No Cefoxitin 8 mg/L Cefoxitin pAmpC positive and ESBL negative 40 40 (100%) 39 (97.5%) 40 (100%) 39 (97.5%) blaCMY-2 31 31 (100%) 31 (100%) 31 (100%) 31 (100%) blaCMY-42 3 3 (100%) 3 (100%) 3 (100%) 3 (100%) blaCMY-79 1 1 (100%) 1 (100%) 1 (100%) 1 (100%) blaDHA 2 2 (100%) 2 (100%) 2 (100%) 2 (100%) blaACC 2 2 (100%) 1 (50%) 2 (100%) 1 (50%) blaMOX 1 1 (100%) 1 (100%) 1 (100%) 1 (100%)
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