Effect of Pseudomonas graminis strain CPA-7 on the ability of Listeria monocytogenes and Salmonella enterica subsp. enterica to colonize Caco-2 cells after pre-incubation on fresh-cut pear

Abstract

To further gain insight into the mechanism by which the biopreservative bacterium Pseudomonas graminis CPA-7 develops its antimicrobial activity, we have examined the effect that the prior interaction stablished by this bacterium and two foodborne pathogens on fresh-cut pear, has on their capacity to colonize human epithelial cells (Caco-2 cell line) which is crucial for establishing infection. CPA-7 inhibited the growth of L. monocytogenes and S. enterica subsp. enterica ser. Enteritidis by 5.5 and 3.1 log10, respectively, after 7 d of interaction at 10 °C. Furthermore, CPA-7 attenuated the adherence of S. enterica to Caco-2 cells by 0.8 log10 regardless of the pre-adaptation on the fruit. Conversely, the adhesiveness of L. monocytogenes was not influenced by the interaction with the antagonist but it was reduced by 0.5 log10 after incubation on the food matrix. Pathogen-antagonist-food matrix interaction was associated to a significant reduction of the relative invasiveness of both pathogens, by 1.3 log10 in the case of L. monocytogenes and to an undetectable level (below 5 CFU/g fruit) for S. enterica. CPA-7 can adhere to and internalize into intestinal epithelium which enables it for competition. Its adherence positively correlates to the multiplicity of infection (MOI) with respect to Caco-2 cells, increasing by 0.6 log10 in an MOI range of 0.1:1 to 100:1. For the same levels of inoculum, internalized cells could only be detected after 7 d of pre-adaptation in the fruit (pH 4.5–5.0). However, the combination of gastrointestinal digestion and habituation on the fruit resulted in a significant reduction of CPA-7 populations (by 2 log10 more after 7d of incubation than on inoculation day) as well as in the decrease of its adhesiveness (by 0.8 log10) and invasiveness (to undetectable levels).

This work has been supported by the CERCA Programme/Generalitat de Catalunya, by the Fons Social Europeu and by the Secretaria d'Universitats i Recerca del Departament d'Economia i Coneixement de la Generalitat de Catalunya (grants 2016-FI-B1-00064 and 2015-FI-B100156). The authors are grateful to Dr. Sonia Marín and Dr. Jordi Voltas for their advice regarding the analysis of the results.