Desiccation Survival of Listeria monocytogenes in Mixed Biofilms with Pseudomonas fluorescens, Serratia liquefaciens and Shewanella putrefaciens

Introduction:  Listeria monocytogeneshas been found to withstand harsh environmental conditions including desiccation. The non-linear bacterial desiccation inactivation kinetics results in a significant tail and the resistant sub-population has been shown to resume growth once rehydrated. The pathogen is known to form biofilm when in co-culture with other bacteria found in food products.

Purpose: The objective of this study was to investigate the desiccation survival of L. monocytogenes in mixed biofilms with P. fluorescens, S. liquefaciens and Sh. putrefaciens.

Methods: Cultures of L. monocytogenes 568 (serotype 1/2 a), Sh. putrefaciens A2, P. fluorescens ATCC 11105  or S. liquefaciens 2RT were prepared in Tryptic Soy Broth (TSB) +1% glucose, adjusted to 10⁴ CFU/ml and deposited on stainless steel (SS) coupons to form mono- or binary-species biofilms with L. monocytogenes and the Gram-negative bacteria (48 h, 15°C & 100% RH). The coupons were subsequently desiccated (43% RH, 21 days at 15°C). To determine the kinetics of desiccation survival, selective agars (BHI, Oxford, Iron agar with PenicillinG, PseudomonasCFC agar and VRBG) were used to enumerate survivors. The resulting survivor curves were fitted to the double Weibull model. Biofilms were additionally characterized by Scanning Electron Microscopy.

Results:  The presence of the Gram-negative spoilage bacteria with the exception of Sh. putrefaciens reduced (P < 0.05) the number of L. monocytogenes cells on the SS coupons, indicating that competition decreased its growth and/or biofilm formation.  However, the survival kinetics of L. monocytogenes in dual-species biofilms was not (P > 0.05) different from that in the mono-species biofilm, meaning that the competitor species did not alter desiccation survival of L. monocytogenescells. Microscopy revealed different biofilm forming capabilities in the mono- and binary bacterial combinations.

Significance:  The presence of competitor bacteria may significantly decrease the initial level of L. monocytogenes populations but they do not affect the desiccation resistance of the pathogen.