Pork juice promotes biofilm formation in Listeria monocytogenes

Elucidating the biofilm formation process, microstructure and functional gene expression of Listeria monocytogenes in beef juice

Listeria monocytogenes biofilm is recognized as a frequent cross-contamination source in the food industry, with raw beef and beef products as common food reservoirs. L. monocytogenes sequence types 9 (ST9) and ST8 are frequently isolated in meats and meat processing environment. In this study, beef juice was selected and compared to a laboratory medium (tryptone soy broth with 0.6 % yeast extract, TSB-YE). The purpose of this work was to investigate the effect of beef juice on the biofilm formation of ST9 and ST8 strains, including biofilm microstructure and modelling the biofilm formation process. Then the expression of biofilm functional genes in two culture media was also investigated. L. monocytogenes ST9 and ST8 can form a dense three-dimensional structure biofilm with multilayers of cells in beef juice after 48 h of incubation, but both strains formed a monolayer biofilm structure in TSB-YE. The ST9 strain developed more sessile cells on the stainless-steel surfaces than the ST8 strain under the same culture conditions. The Logistic model showed a good fit for with the biofilm formation process, and the estimated model parameters in beef juice and TSB-YE were considerably different. Under the same conditions, the maximum specific biofilm formation rate (μmax) in beef juice was higher than that in TSB-YE. This indicated that beef juice can facilitate the biofilm formation of L. monocytogenes, suggesting that the particles in beef juice act as a surface conditioner to support attachment. However, the maximum counts of L. monocytogenes biofilm formed on stainless steel coupon (Ymax) in beef juice was smaller than that in TSB-YE. The ST9 strain exhibited a stronger biofilm formation ability than the ST8 strain, and this was consistent with the scanning electron microscopy images. In the corresponding culture suspensions, the number of adherent cells increases with the number of planktonic cells. Moreover, the expression of biofilm functional genes was significantly different in the two culture media. Compared to biofilm cultured in TSB-YE, the expression of the agrA gene of biofilm in beef juice was significantly down-regulated for both the ST9 and the ST8 strains, and the expression of the inlB and the actA genes were dramatically up-regulated for the ST8 strain. Our results suggested that beef juice promotes biofilm formation of L. monocytogenes in meat processing and provide new insights into controlling biofilm.

The mixed biofilm formed by Listeria monocytogenes and other bacteria: Formation, interaction and control strategies

Listeria monocytogenes is an important foodborne pathogen. It can adhere to food or food contact surface for a long time and form biofilm, which will lead to equipment damage, food deterioration, and even human diseases. As the main form of bacteria to survive, the mixed biofilms often exhibit higher resistance to disinfectants and antibiotics, including the mixed biofilms formed by L. monocytogenes and other bacteria. However, the structure and interspecific interaction of the mixed biofilms are very complex. It remains to be explored what role the mixed biofilm could play in the food industry. In this review, we summarized the formation and influence factors of the mixed biofilm developed by L. monocytogenes and other bacteria, as well as the interspecific interactions and the novel control measures in recent years. Moreover, the future control strategies are prospected, in order to provide theoretical basis and reference for the research of the mixed biofilms and the targeted control measures.

Biofilm Formation of Listeria monocytogenes and Pseudomonas aeruginosa in a Simulated Chicken Processing Environment

This study aims to investigate the mono- and dual-species biofilm formation of Listeria monocytogenes and Pseudomonas aeruginosa incubated in different culture mediums, inoculum ratios, and incubation time. The planktonic cell population and motility were examined to understand the correlation with biofilm formation. The results showed that chicken juice significantly inhibited the biofilm formation of L. monocytogenes (p < 0.05). Pseudomonas aeruginosa was the dominant bacteria in the dual-species biofilm formation in the trypticase soy broth medium. The dynamic changes in biofilm formation were not consistent with the different culture conditions. The growth of planktonic L. monocytogenes and P. aeruginosa in the suspension was inconsistent with their growth in the biofilms. There was no significant correlation between motility and biofilm formation of L. monocytogenes and P. aeruginosa. Moreover, scanning electron microscopy (SEM) results revealed that the biofilm structure of L. monocytogenes was loose. At the same time, P. aeruginosa formed a relatively dense network in mono-species biofilms in an initial adhesion stage (24 h). SEM results also showed that P. aeruginosa was dominant in the dual-species biofilms. Overall, these results could provide a theoretical reference for preventing and controlling the biofilm formation of L. monocytogenes and P. aeruginosa in the food processing environment in the future.