6 Gaps in Your Environmental Testing Program: #1 - Viable but Non-Culturable (VBNC)

In this series, product manager Stefan Widmann investigates six things you might miss when testing your food production environment.

#1. Viable but non-culturable (VBNC) microorganisms

What is the viable but non-culturable state?

For a long time, microbiologists assumed that any bacteria that failed to grow on normal culture media were dead. Subsequent research revealed that there is a third state beyond culturable and dead: viable but non-culturable (VBNC). In general, bacteria in the VBNC state do not multiply but are still alive, as shown by their metabolic activity. Most relevant to us is the fact that they can become culturable after resuscitation. 

There are many reasons why bacteria can go into the VBNC state; starvation, incubation outside the temperature range optimal for growth, elevated osmotic concentrations, the degree of oxygen concentration, or exposure to white light are just some causes. The specific traits of the bacteria strain in question determine what exactly causes bacteria to enter this state.

Why should food producers care about VBNC bacteria?

Some bacteria able to enter the VBNC state are of concern for food producers. While we do not yet know all bacteria species that can become VBNC, we know some that do; they count indicator organisms (such as Enterobacter aerogenes and Klebsiella pneumoniae), adulterants (such as Lactobacillus plantarum and Lactococcus lactis) and pathogens (such as Salmonella typhimurium and Campylobacter coli) among their numbers.

Having identified them, we must now ask whether these bacteria could return to a fully culturable and potentially pathogenic state. Microbiologists were, for a long time, in the dark on this question, as it is difficult fully to separate VBNC bacteria from culturable ones. Researchers have solved this problem, in part, by using a statistical approach: they dilute high numbers of VBNC bacteria to the point that it is nearly impossible for any culturable bacteria to remain. The bacteria are then counted after a defined period of time. If high degrees of growth are observed, the only possible conclusion is that bacteria have left the VBNC state and have become culturable.  A further corollary is that if they can return to a culturable state, they can also become pathogenic again. There are examples of exactly this phenomena leading to outbreaks. For example, VBNC E. coli O157 were suspected in an outbreak in Japan in 1997, as the numbers of pathogenic E. coli were too low to cause infection.

How can food producers detect VBNC bacteria?

Food producers generally do not have many options at their disposal. There are ways to detect VBNC bacteria, but they are generally not applicable in specific food production circumstances or are too expensive for practical use. Flow cytometry shows some promise, as it directly counts cells and can distinguish between living or VBNC cells and dead cells. Another method would be to apply vital staining in combination with microscopes, but this works only with very clean samples and is extremely time-consuming. 

This article originally appeared in International Food & Meat Topics 31 (1).