The viable but non-culturable hypothesis and medical bacteriology

Abstract

Summary The ‘viable but non-culturable’ (VNC) hypothesis proposes that some bacteria that do not form morphologically-identifiable spores or cysts are, nevertheless, able to differentiate into dormant cells. Cells in this VNC state cannot be recovered by routine isolation methods. Evidence for retained activity in non-culturable cells and for recovery of certain pathogens from previously non-culturable populations by animal passage or gradual nutrient supplementation has been presented. This evidence is not conclusive. The physiological basis for the proposed VNC state remains obscure and, although its induction appears to be stimulated by stresses such as starvation, its relationship to the relatively well-characterized global regulatory responses to stress in bacteria has not been established. In spite of these difficulties, the possibility that VNC forms of pathogenic bacteria such as Campylobacter jejuni, Vibrio cholerae and Salmonella enteritidis constitute a reservoir or even a direct source for human infection makes it important to obtain a clear view of the evidence relating to the hypothesis and to establish whether occurrence of these forms is widespread. Whatever the basis for the phenomena which led to its proposal, investigation of the VNC hypothesis has stimulated a re-examination of the concept of viability and the extent to which its physical expression can be equated with culturability and other measures of bacterial activity.