Growth media and assay plate material can impact on the effectiveness of cationic biocides and antibiotics against different bacterial species.

The effectiveness of several cationic disinfectants as well as colistin and polymyxin B were assessed under different growth conditions against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa strains. These conditions included different media (MH1, MH2, TSB and LB) and plate material (polypropylene and polystyrene). Results showed that Minimum inhibitory and bactericidal concentrations (MIC/MBC) values of colistin and polymyxin B were significantly lower on polypropylene plates when compared to polystyrene plates regardless of media used. There were also differences in MIC/MBC values to certain biocides e.g. chlorhexidine and octenidine particularly for S. aureus and E. coli strains, with polypropylene again showing lower values. Other biocides appear to be mostly unaffected by plate type. Whether biocide efficacy was altered by media composition was organism dependent with S. aureus and E. coli more affected than P. aeruginosa. Lower MIC values were more commonly associated with MH2 media and higher MIC values with TSB media for both polypropylene and polystyrene plates, although there were exceptions. Results obtained for standard strains were, in general, indicative for other S. aureus, E. coli and P. aeruginosa strains tested. This study demonstrates the importance of media composition and plate material on biocide effectiveness and highlights the need for optimized disinfectant testing methods. SIGNIFICANCE AND IMPACT OF THE STUDY: There are an increasing number of reports of bacterial strains that are multi-drug resistant. The use of biocides as part of infection control is crucial in helping to combat the spread of these particular strains. Unlike for antibiotics, there are few standardized measuring techniques to understand if an isolate has become more resistant to biocides. This study demonstrates the importance of media composition and plate material on variation and reporting of susceptibility of several bacterial species to specific cationic biocides. It is a useful comparison study to highlight the need to standardize biocide susceptibility testing.