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Title: Competition of Listeria monocytogenes Serotype 1/2a and 4b strains in mixed culture biofilms

Author
item PAN, YOUWEN - North Carolina State University
item Breidt, Frederick
item KATHARIOU, SOPHIA

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2009
Publication Date: 9/1/2009
Citation: Pan, Y., Breidt, F., Kathariou, S. 2009. Competition of Listeria monocytogenes Serotype 1/2a and 4b strains in mixed culture biofilms. Applied and Environmental Microbiology. 75(18):5846-5852.

Interpretive Summary: It has been observed that a foodborne disease causing bacterium Listeria monocytogenes has several different subtypes. This organism can be found in processing equipment used for ready-to-eat foods. It forms films on stainless steel surfaces of that are hard to remove. One subtype of the organism is commonly found in food equipment, but another subtype causes disease. This discrepancy is the reason for this study. We investigated the growth of the two subtypes and found that the one commonly found in processing equipment is better at forming films on processing equipment. Furthermore, the subtype that causes disease can be harbored and protected in the films of the other subtype. This basic research may lead to a better understanding of the bacterial films and the ecology of Listeria. The study may lead to novel approaches for eliminating the pathogen from foods.

Technical Abstract: The majority of Listeria monocytogenes isolates recovered from foods and the environment are strains of serogroup 1/2. However, serotype 4b strains cause the majority of human listeriosis outbreaks. Our investigation of L. monocytogenes biofilms used a simulated food processing system that consisted of repeated cycles of growth, sanitation treatment, and starvation to determine the competitive fitness of strains of serotype 1/2a and 4b in pure and mixed culture biofilms. Selective enumeration of strains of a certain serotype in mixed culture biofilms on stainless steel coupons was accomplished using serotype-specific quantitative PCR and propidium monoazide treatment to prevent amplification of extracellular DNA or DNA from dead cells. The results showed that the serotype 1/2a strains tested were generally more efficient at forming biofilms, and predominated in the mixed culture biofilms. Growth and survival of strains of one serotype was not inhibited by strains of the other serotype in mixed culture biofilms. However, we found that a cocktail of serotype 4b strains survived and grew significantly better in mixed culture biofilms containing a specific strain of serotype 1/2a, with final cell densities averaging 0.5 log10 CFU/cm2 higher, than without the serotype 1/2a strain. The methodology used in this study allowed unique insights in determining how environmental stresses and microbial competition influence survival and growth of L. monocytogenes in pure and mixed culture biofilms.