The influence of heteroresistance on minimum inhibitory concentration (MIC), investigated using weak-acid stress in food spoilage yeasts

Abstract

AbstractPopulations of microbial cells may resist environmental stress by maintaining a high population-median resistance (IC50) or, potentially, a high variability in resistance between individual cells (heteroresistance); where heteroresistance would allow certain cells to resist high stress, provided the population was sufficiently large to include resistant cells. This study sets out to test the hypothesis that both IC50and heteroresistance may contribute to conventional minimal-inhibitory-concentration (MIC) determinations, using the example of spoilage-yeast resistance to the preservative sorbic acid. Across a panel of 26 diverse yeast species, both heteroresistance and particularly IC50were positively correlated with predicted MIC. A focused panel of 29 different isolates of a particular spoilage yeast was also examined (isolates previously recorded asZygosaccharomyces bailii, but genome resequencing revealing that several were in fact hybrid species,Z. parabailii and Z. pseudobailii). Applying a novel high-throughput assay for heteroresistance, it was found that IC50but not heteroresistance was positively correlated with predicted MIC when considered across all isolates of this panel, but the heteroresistance-MIC interaction differed for the individualZygosaccharomycessubspecies.Z. pseudobailiiexhibited higher heteroresistance thanZ. parabailiiwhereas the reverse was true for IC50, suggesting possible alternative strategies for achieving high MIC between subspecies. This work highlights the limitations of conventional MIC measurements due to the effect of heteroresistance in certain organisms, as the measured resistance can vary markedly with population (inoculum) size.ImportanceFood spoilage by fungi is a leading cause of food waste, with specialised food spoilage yeasts capable of growth at preservative concentrations above the legal limit, in part due to heteroresistance allowing small subpopulations of cells to exhibit extreme preservative resistance. Whereas heteroresistance has been characterised in numerous ecological contexts, measuring this phenotype systematically and assessing its importance are not encompassed by conventional assay methods. The development here of a high-throughput method for measuring heteroresistance, amenable to automation, addresses this issue and has enabled characterisation of the contribution that heteroresistance may make to conventional MIC measurements. We used the example of sorbic acid heteroresistance in spoilage yeasts likeZygosaccharomycesspp, but the approach is relevant to other fungi and other inhibitors, including antifungals. The work shows how median resistance, heteroresistance and inoculum size should all be considered when selecting appropriate inhibitor doses in real-world antimicrobial applications such as food preservation.