Listeria monocytogenes is well known as a hardy organism that can survive many environmental stresses. One response to stress is the formation of filaments (elongated cells without septation) that retain the ability to divide into multiple normal-sized cells. This study was based on our hypothesis that certain stresses encountered by L. monocytogenes in the ready-to-eat (RTE) meat processing environment induce filamentation, which contributes to the pathogen’s ability to survive in the food processing environment and enhances its virulence potential. Under favorable conditions, the filaments divide into multiple normal-sized cells, leading to a sudden increase in the number of L. monocytogenes, which can impact safety and risk assessment of food products.
Our objective was to determine the conditions that affect the formation and persistence of L. monocytogenes filaments, and growth and survival properties of these filaments.
L. monocytogenes developed filaments when exposed to osmotic stress induced by low water activity. When the stress was removed, the filaments septated and divided into normal-sized cells at 30°C and 4°C, leading to an increase in bacterial numbers more quickly than control (non-filamented) cells. The relative ability of filaments and control cells to survive stresses commonly encountered in the RTE meat environment varied depending on the stress and exposure time. Filaments were less sensitive than control cells to acid (pH 2) and drying on a stainless steel surface, but survival was similar at high temperature (55°C). Three sanitizers tested were all effective against both filaments and control cells. L. monocytogenes did not grow or develop filaments on three types of luncheon meat tested; however, there was a significant decrease in viability on pepperoni, which had the lowest water activity. Control cells and filaments decreased in viability to a similar extent when exposed to either nitrite or a nisin/rosemary extract blend. Both control cells and filaments increased in viable numbers in diacetate/lactate, with greater increases in the filament culture, while in sodium hexametaphosphate, the viable control cell number generally decreased but that of filaments increased. Overall, exposure to luncheon meats or an antimicrobial appeared to prolong the persistence of the filamentous phenotype.
The presence of L. monocytogenes filaments in meat and processing ingredients could impact the safety and risk assessment of meat products. If filaments form septa when favorable conditions are encountered, there will be a sudden increase in the number of L. monocytogenes that will significantly affect estimations of the total number of viable cells of L. monocytogenes in the food. This could affect retrospective assessments of the infectious dose. Likewise, occurrence of filamentous L. monocytogenes in the processing environment can influence detection, enumeration, effective processing parameters, and risk assessments.