Study uncovers how listeria monocytogenes survives in nutrient-poor environments

The detection tests commonly used to check for the absence of microbes in hospitals or the agri-food industry are based on microbial growth, i.e. the laboratory cultivation of microorganisms from a sample to be checked. If no micro-organisms appear during culturing, the sample is considered safe. But faced with the multiple stresses encountered in these environments (presence of disinfectants or detergents), some bacteria enter a dormant state, known as “viable but non-culturable” (VBNC), making them undetectable by growth-based tests. Pathogenic bacteria in a VBNC state can “wake up”, become virulent again and thus represent a serious health hazard.

Listeria monocytogenes is a bacterium that is ubiquitous in the environment (soil, rivers, lakes, plants). It is responsible for listeriosis, a food-borne disease with a mortality rate of up to 30% in humans. Previous studies have shown that this bacterium is capable of entering a VBNC state when exposed to artificial aquatic environments, but the underlying mechanisms remain poorly understood. This is why the research team set out to better understand this phenomenon, in order to develop effective detection and elimination strategies.

A change of form in dormancy and a test to detect them

To understand the mechanisms behind the dormancy of L. monocytogenes, the scientists exposed these bacteria to mineral spring water, a nutrient-poor environment that causes them to enter the VBNC state of dormancy. They discovered that during the transition to the VBNC state, the bacterium loses its initial rod shape and becomes round. This radical transformation is due to the loss of its cell wall, a structure that confers its shape and protects it. Despite the absence of a cell wall, the dormant forms of the bacterium are highly resistant and adapt to physico-chemical imbalances by modifying their membrane and producing specific proteins. The researchers have developed antibodies capable of specifically detecting L. monocytogenes bacteria in a dormant state, which will enable specific tests to be developed for detecting them.

These results reveal the crucial role of the bacterial wall in the formation of the VBNC state in L. monocytogenes. This adaptation could be frequent and therefore favour the formation of reservoirs of undetectable pathogens, posing a serious public health risk. The research team is continuing its work to identify other factors that govern the dormant state, the conditions that allow this bacterium to become active and pathogenic again, and to develop tools to specifically detect it for better protection.