Unravelling the mystery of dormancy in food pathogens

Listeria monocytogenes, a bacterium that causes listeriosis, can go into dormancy when confronted with environmental stress; for example, in the presence of detergents or in water devoid of nutrients. It then becomes undetectable by the tests commonly used in the food and other industries. Scientists at INRAE, in collaboration with the Institut Pasteur, have discovered the mechanisms behind this dormancy and have developed a specific detection method. These results, published in Nature Communications, shed new light on bacterial dormancy and open up prospects for better management of health risks.

The detection tests commonly used to check for the absence of microbes in hospitals or the agri-food industry are based on microbial growth, ie, the laboratory cultivation of microorganisms from a sample to be checked. If no microorganisms 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.

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.

Listeria bacterium losing its cell wall, in transition to the VBNC state of dormancy.  Image credit: INRAE – Alessandro Pagliuso

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, and the conditions that allow this bacterium to become active and pathogenic again, as well as developing tools to specifically detect it for better protection.

Top image credit: iStock.com/microgen