Changes in Listeria innocua isolates preserved in a biphasic medium for eight months

Abstract

Listeria innocua is a bacterium phylogenetically related to Listeria monocytogenes, the causative agent of foodborne listeriosis. Published studies regarding the long-term survival of L. innocua have been performed on short periods and single cell observation from liquid media, but studies in biphasic media with an emphasis on colonies have not been published. The objective of this work was to describe the survival, during 8 months, of 30 Listeria innocua isolates in a biphasic médium composed in its liquid part by Brain Heart Infusion Broth added with 0.6 % Yeast Extract and in its solid part by Tripticasein Soy Agar added with 0.6 % Yeast Extract. Observations with the naked eye and photographic record were made monthly, focusing the observations on the presence of colonies in the slant front side área. Through descriptive statistics, the changes in the frequency and type of colonies, as well as in the generation time were identified.

All the isolates showed at least one type of change, consisting of the emergence, increase, decrease and disappearance of colonies with different morphologies, ora n increase in the biomass density of some colonies. Chages can be attributed to factors such as pH, temperatura, nutrient limitation, quorum sensing and genetics. The procedures and results described in this work can serve as an educational model for showing bacterial long term persistence, research on genetic diversity, survival in starvation and persistence studies of strains related to resistance to antimicrobials or disinfectants.