Paul Nolan Water Hygiene Limited

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  Viable But Not Culturable (VBNC)

Viable But Not Culturable (VBNC)

Refers to bacteria that are in a state of very low metabolic activity and do not divide (sleeping), but are alive and have the ability to become culturable once resuscitated.

Bacteria in a VBNC state struggle to grow on standard agar growth plates.  

Bacteria can enter the VBNC state as a response to stress i.e. due to adverse\nutrient\temperature\oxygen and light conditions.  

The cells that are in the VBNC state are morphologically smaller and demonstrate reduced nutrient transport and rate of respiration.  Sometimes VBNC bacteria can remain in that state for long time periods.

It has been shown that numerous pathogens and non-pathogens can enter the VBNC state, and this potentially has significant implications when attempting to culture bacteria on an agar plate.

Quantitative Polymerase Chain Reaction (qPCR)

An international trial has compared the use of qPCR for Legionella with culture and aimed to establish guidelines for action and alert levels as determined by this technology.  

The study confirmed the relatively large discrepancy between the results for Legionella species by qPCR and culture and the better correlation for L. pneumophila.

In hot and cold water below 50 ºC the differences reported by Lee et al. (2011) between qPCR and culture could be explained simply by the culture method only recovering 40–60% of the organisms (i.e. the poor overall recovery by culture alone).

qPCR offers a rapid, reproducible means of monitoring water for the presence of L. pneumophila. It can be used for routine monitoring for L. pneumophila and is particularly useful for investigating outbreaks and failures in control, providing the data are correctly interpreted.

The application of qPCR can prevent unnecessary expenditure and importantly, can quickly rule out negative sites, enabling better focusing of control measures.

Detection of numbers of L. pneumophila dead or alive in a water system indicates that there has been amplification of L. pneumophila within the system.

If these are dead at the point of sampling this, at best, indicates that control measures are limiting the release of viable legionellae, but should these be relaxed for any reason the system could immediately present a significant risk to health.

The added benefit of qPCR is that sampling results are known within 24 hours, rather than 10 to 14 days for agar plate sampling.