Recent findings on the viable but nonculturable state in pathogenic bacteria

This paper is a review of what is currently known about the viable but nonculturable state (VNBC) in pathogenic bacteria, the paper outlines what is the VNBC state? The factors regulating it, gene expression, and probably the most relevant in medical terms the virulence of cells in the VNBC state.
The viable but nonculturable state, or VNBC state, refers to when bacteria are unable to be cultured on laboratory mediums in which they would usually grow, however these cells still remain viable and alive. RT-PCR (reverse transcriptase) is one of the most important methods in not only showing that the cells are alive but also in showing that these cells retain their “pathogenic potential” while in the VNBC state. The half-life of bacterial mRNA is 3-5 minutes, so if RT-PCR shows continuing gene expression in cells which are unculturable, these cells must remain viable. Furthermore it has been shown that genes for the 16S rRNA synthesis continued to be expressed in non culturable E.coli, the author concluded that this was strong evidence that these cells remained viable.
The review states that cells in the VNBC state are unlikely to cause infection, however that they retain their virulence and once resurrected can cause reinfection in the host organism. The review uses Mycobacterium tuberculosis as an example of this, the dormant stage in these bacteria is in fact the VNBC state, and if reinfection occurs many years after the primary infection it is because the bacteria have been resurrected from the VNBC state. In the review there are several examples of studies which have found cells which are non culturable, but continue to produce known toxins and also express virulence gene. For example cells of V.cholerae O1, V.parahaemolyticus O3:K6 and V.vulnificus expressed known toxins (ctxAB, rtxA, hlyA, tl, tdh and vvhA), while also expressing the virulence genes (tcpA and TTSS) therefore retaining the ability to infect the host organism when not in the VNBC state.
The exact role the VNBC state plays in bacteria is still not fully understood; however it is thought to be survival strategy and has been shown to be induced by various environmental factors, such as nutrient starvation, incubation outside the normal temperature range and changes in the osmotic concentrations. The VNBC state can only be a means of survival if the cells can be resurrected, resurrection has been most extensively studied in V.vulnificus, like many vibrio species the VNBC state can be induced due to a temperature downshift, V.vulnificus enters the state when temperatures drop below 10 0c. It has been shown both in situ, in vivo and in vitro in V.vulnificus that reversing the stressor, e.g. a temperature upshift will allow the cells to be resurrected.
Perhaps one of the most important consequences to humans of cells into the VNBC state is the effect that this has on antibiotic resistance. As cells in the VNBC state have such low metabolic activity, they effectively become resistant to antibiotics, and then are able to resurrect and reinitiate the infection. Furthermore it was found that, only when the concentration of Vancomycin was 500 times the MIC was it effective against cells of E.faecalis in the VNBC state. The author suggests that the antibiotic resistance of cells in the VNBC state is a reason why many bacterial infection seem to clear with antibiotics, however the course of antibiotics is finished, cells revert to their metabolically active phase and cause reinfection in the host organism. The author outlines two examples of this, H.pylori which are responsible for gastric and duodenal ulcers, and E.coli which can be responsible for urinary tract infections, the antibiotic resistance in the cells which are in the VNBC state likely accounts for high amount of recurrent infections.
The author concludes the review by stating that a large number of bacteria, including those which are pathogenic to humans enter the VNBC state, maintaining cellular structure and biology and continuing gene expression even when they are not able to be cultured by standard laboratory methods. He says that the VNBC state plays a critical role in the survival of pathogens and their ability to produce disease.
A review of: Oliver, J.D. 2010. Recent findings on the viable but nonculturable state in pathogenic bacteria. FEMS Microbiol Rev. 34, 415-425.