Antonie van Leeuwenho ek73:169–187, 1998. 169 c 1998Kluwer Academic Publishers. Printed in the Netherlands. Viability and activity in readily culturable bacteria: a review and discussion of the practical issues Douglas B. Kell 1 , Arseny S. Kaprelyan ts 1 2 , Dieter H. Weichart 1 , Colin R. Harwood 3 & Michael R. Barer 3 1 Edward Llwyd Building, Institute of Biological Sciences, University of W ales, Aberystwyth SY23 3DA, U.K.; 2 Bach Institute of Biochemistry, Russian Academy of Sciences, Leninskii prospekt 33, 117071 MOSCOW , R ussia; 3 Department of Microbiology , the Medical School, Framlington Place, Newcastle upon Tyne NE2 4HH ( authorfor correspondence) Received 22 July 1997; accepted 23 October 1997 Key words: viability, culturability, anabiosis, cryptobiosis, dormancy, metabolic activity, thanatology Abstract In microbiology the terms ‘viabilit y’ and ‘culturability’ are often equated. However, in recent years the apparently self-contradictory expression ‘viable-but-nonculturable’ (‘VBNC’) has been applied to cells with various and often poorly defined physiological attributes but which, nonetheless, could not be cultured by methods normally app ropr iate to the org ani sm con cer ned . The seattribute s include app are nt cel l int egr ity , the pos sessio n of some form of measurable cellular activity and the apparent capacity to regain culturability. We review the evidence relating to putative VBNC cells and stress our view that most of the reports claiming a return to culturability have failed to exclude the regrowth of a limited number of cells which had never lost culturability. We argue that failure to differentiate clearly between use of the terms ‘viability’ and ‘culturability’ in anoperationalversus a conceptual sense is fuelling the current debate, and conclude with a number of proposals that are designed to help clarify the major issues involved. In particular, we suggest an alternative operationalterminology that replaces ‘VBNC’ with expressions that are internally consistent. Introdu ction and background : ‘viable’ and ‘non viable’ cells At the simpl est level, bacteria may be classifi ed into two physi ologica l groups : those that can and those that cannot readily be grown to detectable levelsin vitro. Leaving aside organisms such as Mycobacterium lep- rae that can only be seen to reproduce in a foreign host, and demonstrably syntrophic organisms (McIn- erney et al., 1981), the well-established view is that culturabilityreflectsviability. In this review we will be concerned only with bac- teria that are normall y readily culturable by standard methods. In partic ular , we will discuss the signi ficanc e of apparently intact cells which, at the time of sam- pling, are not able to grow on media appropriate for the organi sms conce rned. W e w ill also discus s how the lack of a widely accepted and consistently applied ter- minology has led to a debate that has centred as much on semantics as the underlying scientific issues. In an attempt to disentangle these issues, we have focussed on theoperational(practical) domain, aiming to avoid the philosophical problems which arise when such terms are used conceptually (i.e. plausibly , but without the possibility of direct experimental analysis in many cases). Moreover, by excluding from the discussion organisms which have yet to be grown axenically (in vitro), we have been able to concentrate on the area of curre nt controver sy relating to the culturabili ty ofspe cie s which are cla imed to mai nta in via bil ity in s pit e of failing to grow on media which normally support their growth. The notion that, for readi ly culturable organis ms, culturability and viability are synonymous, is support- ed by many rev ie ws and tex ts e.g . (H att ori , 198 8; P ost - gate, 1969)). From this point of view, one r ˆ ole of the
