Low temperature induced non-culturability and killing of Vibrio vulnificus

Comparative proteomic analysis to characterize temperature-induced viable but non-culturable and resuscitation states in Vibrio cholerae

Vibrio cholerae can survive environmental adversities by entering into a viable but non-culturable (VBNC) state and is able to resuscitate under favourable conditions. In this study, an environmental strain of V. cholerae (AN59) showed a decrease in culturability from 4×10⁷ to ≤ 3 c.f.u. ml ^-1 in artificial seawater media at 4 °C within 35 days. During the course of VBNC progression, viability was confirmed by real-time RT-PCR which showed reduced but stable expression of molecular chaperones groEL and dnaK. Resuscitation was induced in VBNC microcosm by a temperature increase from 4 to 37 °C for 24 h. The results obtained from resuscitation and growth experiments suggest that 10³-10⁴  c.f.u. ml ^-1 of VBNC cells should recover upon temperature increase and grow to attain 10⁷  c.f.u. ml ^-1. We used comparative proteomics to differentiate recovery from the VBNC state and selected 19 proteins whose expression was significantly variable between these two states. These proteins were mainly related to carbohydrate metabolism, phosphate utilization, stress response, transport and translation. The main difference in the proteome profile was higher protein expression in the recovery state compared to VBNC state. However, during recovery Pi-starvation led to expression of PhoX, PstB and Xds, which might help in utilization of extracellular DNA to promote growth after resuscitation. In addition, the expression of EctC suggests that osmotic adaptation is necessary to grow at high salinity. Detection of AhpC in the VBNC and recovery state indicates the significance of the oxidative stress response. A temperature-induced VBNC and recovery state is a combination of adaptive and survival responses under nutrient limitation.

Recovery in culture of viable but nonculturable Vibrio parahaemolyticus: regrowth or resuscitation?

The objective of this study was to explore the recovery of culturability of viable but nonculturable (VBNC) Vibrio parahaemolyticus after temperature upshift and to determine whether regrowth or resuscitation occurred. A clinical strain of V. parahaemolyticus Vp5 was rendered VBNC by exposure to artificial seawater (ASW) at 4 degrees C. Aliquots of the ASW suspension of cells (0.1, 1 and 10 ml) were subjected to increased temperatures of 20 degrees C and 37 degrees C. Culturability of the cells in the aliquots was monitored for colony formation on a rich medium and changes in morphology were measured by scanning (SEM) and transmission (TEM) electron microscopy. Samples of VBNC cells were fixed and examined by SEM, revealing a heterogeneous population comprising small cells and larger, flattened cells. Forty-eight hours after temperature upshift to 20 degrees C or 37 degrees C, both elongation and division by binary fission of the cells were observed, employing SEM and TEM, but only in the 10-ml aliquots. The results suggest that a portion of VBNC cells is able to undergo cell division. It is concluded that a portion of VBNC cells of V. parahaemolyticus subjected to cold temperatures remain viable. After temperature upshift, regrowth of those cells, rather than resuscitation of all bacteria of the initial inoculum, appears to be responsible for recovery of culturability of VBNC cells of V. parahaemolyticus. Nutrient in filtrates of VBNC cells is hypothesized to allow growth of the temperature-responsive cells, with cell division occurring via binary fission, but also including an atypical, asymmetric cell division.

The survival response of Escherichia coli K12 in a natural environment

To verify the hypothesis of cryptic growth and viable but nonculturable (VBNC) state, survival responses of Escherichia coli cells were examined under oligotrophic microcosm conditions for an extended period. In the case of filtered distilled water at 4 degrees C, E. coli cells definitely entered the VBNC state within 56 days. However, culturability and viability increased while the total number of cells declined after 110 days. This phenomenon can be explained by considering three possible states. The first is the existence of the VBNC state, the second is cryptic growth, and the third is the death of E. coli cells. In the case of artificial seawater at 4 degrees C, VBNC E. coli cells confirmed the existence of two log units of elongated VBNC cells. Moreover, elongated VBNC cells showed the most significant change among all the other transformed cells. Also, E. coli cells in microcosms at 28 degrees C indicated the entrance to the classical starvation survival state. In resuscitation tests, 1% diluted Luria-Bertani agar medium showed the highest level of resuscitation among amended agar media. To evaluate the survival ability of E. coli cells in the activated sludge samples, we used an E. colistrain XL-1 blue containing plasmids pQ2 including GFPcDNA (XL/GFP). In supernatant of activated sludge (SUP) at 28 degrees C, XL/GFP cells entered the VBNC state after 10 days, whereas existence of VBNC cells was not detectable in resuspended activated sludge (ACT) at 28 degrees C.

Bacterial viability and culturability

Renewed interest in the relationships between viability and culturability in bacteria stems from three sources: (1) the recognition that there are many bacteria in the biosphere that have never been propagated or characterized in laboratory culture; (2) the proposal that some readily culturable bacteria may respond to certain stimuli by entering a temporarily non-culturable state termed 'viable but non-culturable' (VBNC) by some authors; and (3) the development of new techniques that facilitate demonstration of activity, integrity and composition of non-culturable bacterial cells. We review the background to these areas of interest emphasizing the view that, in an operational context, the term VBNC is self-contradictory (Kell et al., 1998) and the likely distinctions between temporarily non-culturable bacteria and those that have never been cultured. We consider developments in our knowledge of physiological processes in bacteria that may influence the outcome of a culturability test (injury and recovery, ageing, adaptation and differentiation, substrate-accelerated death and other forms of metabolic self-destruction, prophages, toxin-antitoxin systems and cell-to-cell communication). Finally, we discuss whether it is appropriate to consider the viability of individual bacteria or whether, in some circumstances, it may be more appropriate to consider viability as a property of a community of bacteria.