Bacterial Persister-Cells and Spores in the Food Chain: Their Potential Inactivation by Antimicrobial Peptides (AMPs)

Abstract

The occurrence of bacterial pathogens in the food chain has caused a severe impact on public health and welfare in both developing and developed countries. Moreover, the existence of antimicrobial-tolerant persisting morphotypes of these pathogens including both persister-cells as well as bacterial spores contributes to difficulty in elimination and in recurrent infection. Therefore, comprehensive understanding of the behavior of these persisting bacterial forms in their environmental niche and upon infection of humans is necessary. Since traditional antimicrobials fail to kill persisters and spores due to their (extremely) low metabolic activities, antimicrobial peptides (AMPs) have been intensively investigated as one of the most promising strategies against these persisting bacterial forms, showing high efficacy of inactivation. In addition, AMP-based foodborne pathogen detection and prevention of infection has made significant progress. This review focuses on recent research on common bacterial pathogens in the food chain, their persisting morphotypes, and on AMP-based solutions. Challenges in research and application of AMPs are described.

Keywords: antimicrobial peptides; bacterial spores; foodborne pathogen; persisters.

Figure 1

The possible sources of foodborne pathogens. Every step from farm to table can ultimately lead to foodborne diseases, including polluted farming environments, diseased food-producing animals, contaminated raw materials, non-hygienic food processing, and non-hygienic food consumption. In this process, food handlers can cause (post-processing) contamination, which they can also acquire themselves from animals or in the food processing environment even though the latter is less likely. The review focuses on persister-cells of various Escherichia coli (EPEC, EHEC, STEC, EIEC, EAEC, ETEC), Salmonella, and Listeria species both in food and upon human infection. For the major bacterial cause of gastroenteritis, Campylobacter jejuni, data on persister-cells is emerging but still scarce. Staphylococcus aureus and Bacillus cereus are considered human and animal contaminants of the food chain with the former being present on the skin of food handlers and the latter originating from spores present in the environment.

Figure 2

Different types of persister-cells and bacterial spores. Green images: susceptible bacteria cells. Red images: persisters. Images are not drawn to scale.

Figure 3

Functions of ppGpp associated with persister formation after being generated by a RelA/SpoT homolog family. RPDP model: ribosome dimerization persister model.

Figure 4

Models of Toxin-antitoxin (TA) systems associated with persister formation. (A) Type I TA system: it contains a small RNA (sRNA) antitoxin and a protein toxin. In normal conditions, antitoxin inhibits the translation of toxin mRNA. In a stress condition, the antitoxin is degraded, freeing the toxin mRNA to be translated. The type I toxin causes bacterial membrane depolarization. (B) Type II TA system: it contains a DNA-binding protein antitoxin and a protein toxin. In normal conditions, the antitoxin binds to the toxin and inhibits its activity. The antitoxin, as well as most of type II TA, the complex can target on the promoter and repress the transcription of the toxin gene. In stress conditions, the antitoxin is degraded by Lon or Clp proteases, releasing the toxin to inhibit several essential metabolic processes like DNA or protein synthesis. P: promoter.

Figure 5

Model of the vegetative cycle and sporulation, germination, and outgrowth of B. cereus. Under severe stress conditions, B. cereus can initiate a sporulation process and form spores. Under favorable conditions, the spore can germinate and grow out to vegetative cells [56].

Figure 6

Various schematic models of antimicrobial peptides (AMPs) induced membrane disturbance. Models (A–C) are derived from in vitro membrane lipid model studies [123,124,125], and models (D–F) are based on single cell live-imaging studies [126,127,128]. See the manuscript text for a detailed explanation of each model.