In response the emergence of plasmid-mediated resistance due to mcr-1 and increasing evidence against agar based testing of polymyxins, the CDS reference laboratory undertook a review of its disc testing method. A combination of intrinsically resistant and naturally susceptible Enterbacteriaceae and pseudomonads were tested in parallel by agar dilution, disc diffusion and broth microdilution.
With the increasing worldwide prevalence of multi-drug resistant (MDR) Gram negative bacteria, polymyxin has re-emerged as a last resort treatment. Polymyxins have in vitro activity against Pseudomonas aeruginosa, Klebsiella spp., Enterobacter spp., Escherichia coli, Salmonella spp., Shigella spp., Citrobacter spp, Yersinia pseudotuberculosis and Haemophilus influenzae. It is not active against Burkholderia spp., Proteus spp., Providencia spp., Morganella morganii, Serratia spp., Brucella spp., Neisseria spp., Chromobacterium spp., and Gram-positive bacteria.
The minimal inhibitory concentration (MIC) determined by agar and broth dilution were compared to the annular radius obtained by disc diffusion for categorical agreement. The results are presented along with a commentary on the future of polymyxin B 300 u disc testing for CDS users.