The viable but nonculturable phenotype: a crossroads in the life-cycle of non-differentiating bacteria?

In nature, prokaryotes must face alternating periods of prosperity and adversity. Differentiating bacteria confront situations of adversity by developing resistant structures. When there is a plentiful period, they adopt a vegetative state and when the period is adverse, a resistant structure, thereby completing a cycle. Non-differentiating bacteria do not develop such morphological distinct resistant structures. It has been proposed that many of these bacteria withstand periods of adversity by adopting the viable but nonculturable phenotype (VBNC). Bacteria of this phenotype conserve detectable metabolic function but become unculturable. Is it possible that the VBNC phenotype can revert to a culturable state, and vice versa, thus establishing a life-cycle? This review presents and evaluates different hypotheses regarding this question. Moreover, it attempts to analyse and proffer answers to other questions related to this phenotype. Is this a successful phenotype which prolongs survival? Is this a strategy for the survival of individual cells, or is it a strategy for the survival of a population? Finally, is it possible that this phenotype is, in fact, an example of altruistic death?