Filamentation of Listeria monocytogenes in the Presence of Sublethal Dose of Bacteriocins

Introduction: Pathogenic foodborne bacteria can form filaments under environmental stresses such as cold temperature, acid and salt. These filaments are comprised of long-strings of undivided cells and can rapidly divide into daughter cells and start to multiply when placed in favorable growth conditions. 

Purpose: The purpose of this study was to examine whether a sublethal dose of bacteriocins can cause filamentation in Listeria monocytogenes.

Methods: Listeria monocytogenes 08-5923 was grown in tryptic soy broth at 37°C and exposed to sublethal concentrations of the cell-free supernatant of Clostridium maltaromaticum UAL307. After 24 h, cells were fixed with 1% (v/v) formaldehyde for flow cytometry analysis to estimate the percentage of filamented cells in the population. For transmission electron microscopy (TEM) cells were fixed with 10% (v/v) formaldehyde and pelleted by centrifugation at 11,000 x g for 15 s prior to preparation for TEM.

Results: Treatment of L. monocytogenes 08-5923 resulted in a significant increase in the mean forward light scatter. Following 24 h exposure to the cell-free culture supernatant of C. maltaromaticum UAL307, nearly 90% of the Listeria population was filamented. TEM analysis of these cells showed that L. monocytogenes does not contain segments between the undivided daughter cells within the filaments and these filaments do not have flagella. In conclusion, the sublethal dose of the cell-free supernatant of C. maltaromaticum UAL307 induced filamentation in L. monocytogenes 08-5923.

Significance: This is the first evidence that a sublethal dose of bacteriocins caused filamentation in L. monocytogenes. The presence of filamented cells has the potential to underestimate the number of viable cells when determining counts of L. monocytogenes for regulatory compliance.