A comparative study of serotype 1/2a and serotype 4b strains of Listeria monocytogenes in biofilms using a simulated food processing system

The majority of Listeria monocytogenes isolates from foods and the environment are serotype 1/2a strains. However, serotype 4b strains cause the majority of human listeriosis outbreaks. The purpose of the research has been to compare the growth of serotype 1/2a strains and serotype 4b strains in biofilms. A method to enumerate viable L. monocytogenes cells of each serotype in mixed culture biofilms was developed using real-time PCR with propidium monoazide. To determine the competitive fitness of strains of serotype 1/2a and 4b, cocktails of each serotype were mixed to form biofilms. The biofilms were treated with a simulated food processing (SFP) system composed of repeated cycles of growth, sanitation treatment, and starvation. Data show that the serotype 1/2a strains were generally more efficient than the 4b strains at forming biofilms and predominated in the mixed culture biofilms. The growth of 4b strains was not inhibited in mixed culture biofilms compared to the single serotype (4b) biofilms in the SFP system. To compare the density of biofilms formed by strains of the two serotypes, 18 strains of each serotype were examined for biofilm formation under a variety of conditions, including varying concentrations of glucose, sodium chloride and ethanol at different temperatures using a microplate assay. Results indicate that the serotype 1/2a strains formed higher density biofilms than the 4b strains under most conditions. The data from this project support the hypothesis that L. monocytogenes serotype 1/2a strains are more efficient in biofilm production than 4b strains and may help to explain the higher percentage of 1/2a isolates from foods and the environment.