In the quest for persistence markers in listeria monocytogenes

Abstract

Listeria monocytogenes (Lm) is an opportunisticand highly invasive foodborne bacterial pathogen. Although not life-threatening for immunocompetent individuals, Lm is deleterious among more susceptible populations. This pathogen's primary transmission route relies on contaminated foods and is often associated with ready-to-eat (RTE) products, such as soft cheeses. Even though considerable efforts are spent in removing Lm from processing facilities, due to its prevalence in the environment, this pathogen effortlessly colonizes food processing environments (FPEs), with certain strains more frequently isolated from FPEs than others, with some instances of strains continuously isolated throughout time in the same processing facility. However, no consensus has yet been reached on the causes of persistence. Some attribute it to failure in facility disinfection, while others hypothesize higher intrinsic tolerances from the persistent strains. The objective of the present work was to identify genotypic and phenotypic differences between persistent and sporadic isolates. Seventeen isolates were selected to carry out the proposed tasks, and all were subjected to growth under stressful environments with varying degrees of temperature, pH, and salinity. Moreover, infrared spectra were collected from all isolates grown at 11 and 30 ºC with two objectives: I) To develop classification models that could correctly distinguish isolates grown at both temperatures and persistent and sporadic isolates; II) To evaluate how membrane fluidity in both groups (persistent and sporadic) was affected by the exposure to lower temperatures, providing useful information on phenotypic expression of both groups. Additionally, the accessory genomes of all isolates were characterized for the presence of resistance genes associated with enhanced tolerance to adverse conditions present in FPEs. Growth under the selected stress conditions did not yield significant differences between the growth parameters of persistent and sporadic isolates. Additionally, the accessory genomes of both groups were highly heterogenous and did not contain genetic elements that could account for enhanced tolerance to unfavourable conditions commonly encountered in FPEs. Analysis of the infrared spectra of Lm isolates exposed to 30 and 11 ºC yielded promising results. PLSDA prediction models for 11 ºC correctly classified a significant portion of the isolates as persistent or sporadic, possibly implying that the FTIR technique possesses enough discriminatory power to distinguish phenotypical differences between both groups. Thus, in the author's opinion, further research needs to be conducted on the FTIR analysis of isolates grown in various stress conditions to provide reliable and accurate evidence corroborating the claim that persistence may also be attributed to phenotypic characteristics, not only to sanitation deficiencies.

Date of Award	14 Nov 2022
Original language	English
Awarding Institution	Universidade Católica Portuguesa
Supervisor	Paula Teixeira (Supervisor), Clara Sousa (Co-Supervisor) & Vânia Alexandra Borges Ferreira (Co-Supervisor)