Isolation of inducible Amp C β-lactamase producing <i>Enterobacter aerogenes</i> from a diabetic foot ulcer

Amp C enzymes may be constitutively expressed or inducible (expressed only in the presence of the antibiotic). When these enzymes are inducible, the organism will appear sensitive to the newer penicillins and thirdgeneration cephalosporins on antibiotic sensitivity testing (ABST) and be misreported. A 60 year old female was admitted to the National Hospital of Sri Lanka with an infected diabetic foot ulcer. Wound curettings yielded a growth of two "coliforms". One of these showed a blunting of the cefotaxime zone near the coamoxyclav disc in the ABST and it was suspected that this could be an Amp C -lactamase producer. A screening test using cefoxitin confirmed this suspicion. Therefore, we reported it resistant although the isolate appeared sensitive to cefotaxime on ABST. The isolate was speciated as Enterobacter aerogenes. Inducible Amp C enzymes are found in most Enterobacter, Serratia and Providencia spp and in Morganella morganii and Citrobacter freundii [1]. First generation cephalosporins and amoxicillin strongly induce Amp C enzymes and are destroyed by them, and this is readily detectable on ABST. Third generation cephalosporins and cefuroxime are labile but induce weakly. Therefore, on routine testing, the strains appear susceptible but there will be clinical failure when these drugs are used for therapy. There are no standard tests for identification of inducible Amp C producers [3]. They may be detected by using an inducing antibiotic to induce the enzyme [4]. This induction of resistance is the basis for the cefoxitin /