Listeria monocytogenes is a foodborne pathogen associated with various foods including ready-to-eat products such as deli meats and cheeses. L. monocytogenes is the etiological agent of listeriosis, which mainly affects immunocompromised individuals, pregnant women, children, and the elderly. One proposed avenue of L. monocytogenes entering into food products is contamination due to persistent strains of bacteria found in food processing environments. These persistent strains are more likely to form biofilms on various surfaces in the processing environment, which enable the organisms to survive adverse environmental and processing conditions, and become a contamination source of the pathogen. While several molecular attributes contributing to biofilm formation have been elucidated, further studies are greatly needed to reveal the key mechanisms in biofilm formation (Bfm) for targeted mitigation. In this study, I investigated molecular attributes essential for L. monocytogenes biofilm formation by revealing the molecular difference in Bfm between a strong biofilm (Bfm ^S) strain and a weak biofilm forming (Bfm^W) strain, evaluated the difference in Bfm between wild type and mutant strains, and examined the effectiveness of selected sanitation agents on inactivating LM biofilms.

The results from these experiments showed that the presence or absence of a plasmid and a cell wall associated surface protein did not affect biofilm formation. After comparing the transcriptomes of two L. monocytogenes strains that differ in Bfm by microarray assessment, a gntR-family response regulator (LMOf2365_0414) designated LbrA, which was greatly expressed in the Bfm^S strain, was selected to further assess its impact on L. monocytogenes Bfm. A L. monocytogenes ScottA lbrA deletion mutant AW3 exhibited defective Bfm as compared to the wild type strain, and recombinant plasmid with a functional copy of the lbrA was able to partially complement the mutation in AW5, a derivative of AW3. However, the functional lbrA was not able to restore strong Bfm in the derivative of Bfm^w F2365. A comparison of the transcriptomes between Scott A and its lbrA deletion mutant AW3 revealed a set of genes that differ in expression, but the number of genes were much less than that between ScottA and F2365. The data indicated that additional factors likely also contributed to weak biofilm formation by F2365.

Three commercially available sanitizers ProSan LC, ProSan and Oasis 145 were examined for their efficacy on inactivating both planktonic and biofilm cells of L. monocytogenes strain Scott A. All sanitizers were effective in inactivating L. monocytogenes planktonic cells, but much less effective against biofilm cells with 2 log or less reduction even after 30 min treatment. Overall ProSan LC, ProSan are more effective than Oasis 145 in reducing both L. monocytogenes planktonic and biofilms cells.

Results from this study contributed to the scientific understanding of L. monoctogenes biofilm formation, and enabled further investigation to reveal potential primary biofilm attributes regulated by lbrA. The sanitizer assessment provided helpful information on the efficacy of the sanitizers against both planktonic and biofilm cells, which is important for industrial applications.