Recent developments in culture media have given rise to the use of chromogenic substrates as a means of differentiating bacteria particularly among the Enterobacteriaceae. Colorex™ ESBL (Extended Spectrum Beta-Lactamase) medium has been developed for the isolation of ESBL – producing organisms with the aim of simplifying the differentiation and presumptive identification of the causative organism. It should be noted that other non-ESBL and AmpC isolates will be inhibited on this medium reducing the incidence of false positives. The distinctive colonial colouration of the various species can reduce the need for additional follow up testing allowing many positive results to be issued within 24 hours of receipt of the sample. If necessary an Indole test for confirmation of Escherichia coli and TDA test for Proteus spp. can be performed directly from the medium.
Typical colour reactions are as follows:
Escherichia coli – Red colonies;
Proteus spp., Providencia spp. & Morganella spp. – Clear colonies with a brown halo;
Klebsiella spp., Enterobacter spp, Serratia spp. & Citrobacter spp. – Metallic blue colonies;
Salmonella spp. & Acinetobacter spp. – Clear colonies; Gram +ve bacterial species and yeasts – Inhibited.