Adaptation of Campylobacter field isolates to propionic acid and sorbic acid is associated with fitness costs

Aims: To reduce the burden of Campylobacter at different stages of the food chain, recent studies have shown the effectiveness of organic acids as a risk mitigation strategy. However, very little is known about possible adaptation responses of Campylobacter that lead to reduced susceptibility to organic acids. Here we investigated the adaptive responses of Campylobacter field isolates to organic acids and estimated the fitness costs.

Methods and results: Exposure of two Campylobacter jejuni and one Campylobacter coli isolate to subinhibitory concentrations of propionic acid or sorbic acid resulted in twofold to fourfold increased minimal inhibitory concentration values for the adapted variants. With one exception, the decreased susceptibility was stable in at least 10 successive subcultures without selection pressure. Growth competition experiments revealed a reduced fitness of adapted variants compared to the wild-type isolates. A linear regression model allowed an estimation of the fitness cost. Growth kinetics experiments showed significantly prolonged lag phases in five of six adapted isolates while there was not a direct correlation in the maximum growth rates compared to the wild-type isolates.

Conclusions: The results of the study showed that a stepwise adaptation of Campylobacter to organic acids is possible, but at the detriment of changes in growth behaviour and reduced fitness.

Significance and impact of the study: The study contributes to the understanding of adaptive responses of Campylobacter to organic acids treatments, for example, as part of risk mitigation strategies.