Influence of polysorbate-80 when determining the tigecycline MIC by the reference method

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.

Effect of polysorbate 80 on oritavancin binding to plastic surfaces: implications for susceptibility testing

Oritavancin, a semisynthetic lipoglycopeptide with activity against gram-positive bacteria, has multiple mechanisms of action, including the inhibition of cell wall synthesis and the perturbation of the membrane potential. Approved guidelines for broth microdilution MIC assays with dalbavancin, another lipoglycopeptide, require inclusion of 0.002% polysorbate 80. To investigate the potential impact of polysorbate 80 on oritavancin susceptibility assays, we quantified the recovery of [(14)C]oritavancin from susceptibility assay plates with and without polysorbate 80 and examined the effect of the presence of polysorbate 80 on the oritavancin MICs for 301 clinical isolates from the genera Staphylococcus, Enterococcus, and Streptococcus. In the absence of polysorbate 80, [(14)C]oritavancin was rapidly lost from solution in susceptibility assay test plates: 9% of the input drug was recovered in broth at 1 h when [(14)C]oritavancin was tested at 1 mug/ml. Furthermore, proportionately greater losses were observed at lower oritavancin concentrations, suggesting saturable binding of oritavancin to surfaces. The inclusion of 0.002% polysorbate 80 or 2% lysed horse blood permitted the recovery of 80 to 100% [(14)C]oritavancin at 24 h for all drug concentrations tested. Concordantly, oritavancin MIC(90)s for streptococcal isolates, as determined in medium containing 2% lysed horse blood, were identical with and without polysorbate 80. In stark contrast, polysorbate 80 reduced the oritavancin MIC(90)s by 16- to 32-fold for clinical isolates of enterococci and staphylococci, which are typically cultured without blood. The results presented here provide evidence that the MIC data for oritavancin in the current literature significantly underestimate the potency of oritavancin in vitro. Moreover, the combination of data from MIC and [(14)C]oritavancin recovery studies supports the revision of the oritavancin broth microdilution method to include polysorbate 80 throughout the assay.