Letters to the editor should be sent to:

patlacey@asmusa.org

Links to Other ASM Pages:

• Table of Contents
• ASM News Issues

Revisiting the Concept of Microbial Resuscitation

The March 2000 issue of ASM News (p. 123) carried an extraordinary letter from Shi V. Liu "revisiting the concept of microbial resuscitation." While we remain unclear regarding what overall message the author wished to convey, several assertions were made that we feel cannot go unchallenged.

At the outset, we state our view that the only validated operational test of bacterial viability is propagation in culture. This does not mean that everything that cannot be cultured is not viable. For those organisms that have never been cultured—but which clearly have

recently grown—it simply means we have no satisfactory test of viability. The possibility that viable cells of readily culturable organisms may be temporarily nonculturable (TNC) or not immediately culturable is also recognized, and was recently reviewed at length (Kell et al., Antonie van Leeuwenhoek 73: 169--187, 1998). Whether such phenomena reflect specific adaptive responses based on previously unrecognized physiological processes or experimental artifacts that can be explained on the basis of established knowledge remains, in our view, to be established in most instances. Moreover, with one exception (Kaprelyants et al., Appl. Env. Microbiol. 59: 3187--3196, 1993), in our experience "resuscitation" phenomena that appear to involve transitions from TNC to immediately culturable states are difficult to replicate and may be open to alternative interpretations, especially those involving the regrowth of a small number of genuinely viable cells (Weichart et al., FEMS Microbiol Lett 100:205--210, 1992; Bogosian et al., Appl. Env. Microbiol. 64:1736--1742, 1998; Ekweozor et al., FEMS Lett. 158:267--272, 1998).

Apparently, Liu finds application of the term resuscitation unacceptable unless the recovered organisms were "dead or dying." We feel strongly that this view is not justified either etymologically or by precedent. Moreover, the whole process of communication in this area is undermined when issues are discussed without reference to unambiguous (and indeed well-established) definitions of the terms used.

We have previously discussed the self-contradictory nature of the phrase "viable but nonculturable" in the context of our frame of reference (Barer, J. Med. Microbiol. 46: 629--631, 1997; Barer and Harwood, Adv. Micr. Physiol. 41: 93--137, 1999). This sort of contradiction reaches a new height in Liu’s view of resuscitation. Apparently only dead or dying cells can really resuscitate. Unless yet another term has lost its meaning, dead cells are not and cannot be viable. Surely the process to which Liu refers is resurrection not resuscitation?

In every etymological source we have consulted, resuscitation is a term primarily applied to humans in the context of consciousness. As with other nontechnical terms applied in a microbiological context (e.g., viability, dormancy, latency, live, dead, moribund, survival, etc.), our familiarity with the terms provides ample opportunity for confusion. Nonetheless, there is substantial precedent for the use of "resuscitation" to describe a return of inactive (but viable) cells to an active state (e.g., Kaprelyants et al., Appl. Env. Microbiol. 59:3187--3196, 1993; Mukamolova et al., Proc. Natl. Acad. Sci. USA 95:8916--8921, 1998), and the recognition of dormancy necessarily implies that dormant cells have the potential to resuscitate (Kaprelyants et al., FEMS Microbiol. Rev. 104: 271--286, 1993). This terminology does at least have the advantage of harmony with the consciousness/ unconsciousness and sleep/wakefulness pairings that use of the terms implies in common parlance.

In his discussion of the viability and culturability argument Liu clearly does not grasp the points we have made previously and repeat above concerning the need for clear separation between operational and conceptual domains of thinking. One may conceive of a bacterial cell that has the potential to propagate but for which we have no available culture method. If the organism concerned has never been cultured then the cell’s viability is operationally indeterminate; if it has been cultured then, until the hypothetical culture method is established, the cell must be considered nonviable. To accept any other conclusions moves us into the realms of metaphysics.

Finally, using microscopy, image analysis, flow cytometry, and dilution cultures, we and many others have made substantial attempts to determine the natural history of single microbial cells. These approaches allow characterization of biochemical and physiological events in single cells, and we are surprised that Liu feels he can state that "we still have not achieved a single successful observation on the whole lifespan of any microorganism." There are ample examples, dating back at least 50 years (e.g., Brieger and Fell, J. Hyg. 44:158--169, 1945) of sequential observations on single microbial cells from their birth by binary fission either to their subsequent fission or to their death by exposure to inimical agents. We suggest that Liu’s careful analysis could be directed to this and the above-mentioned literature to useful effect.

Mike Barer
University of Newcastle upon Tyne Medical School
Newcastle
United Kingdom
m.r.barer@ncl.ac.uk

Gregg Bogosian
Monsanto Company
Chesterfield, Mo.
gregg.bogosian@stl.monsanto.com

Douglas Kell
University of Wales
Aberystwyth
United Kingdom
dbk@aber.ac.uk

Huw Williams
Imperial College
London
United Kingdom
h.d.williams@ic.ac.uk