cysts can withstand unfavourable conditions to do this this they have an outer
1 plasma membrane
2 nucleus
3. protective cell
4. cell wall
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Answer:
The production of cysts, an integral part of the life cycle of many free-living protozoa, allows these organisms to survive adverse environmental conditions. Given the prevalence of free-living protozoa in food-related environments, it is hypothesized that these organisms play an important yet currently underinvestigated role in the epidemiology of foodborne pathogenic bacteria. Intracystic bacterial survival is highly relevant, as this would allow bacteria to survive the stringent cleaning and disinfection measures applied in food-related environments. The present study shows that strains of widespread and important foodborne bacteria (Salmonella enterica, Escherichia coli, Yersinia enterocolitica, and Listeria monocytogenes) survive inside cysts of the ubiquitous amoeba Acanthamoeba castellanii, even when exposed to either antibiotic treatment (100 μg/ml gentamicin) or highly acidic conditions (pH 0.2) and resume active growth in broth media following excystment. Strain- and species-specific differences in survival periods were observed, with Salmonella enterica surviving up to 3 weeks inside amoebal cysts. Up to 53% of the cysts were infected with pathogenic bacteria, which were located in the cyst cytosol. Our study suggests that the role of free-living protozoa and especially their cysts in the persistence and epidemiology of foodborne bacterial pathogens in food-related environments may be much more important than hitherto assumed.
INTRODUCTION
Foodborne pathogenic bacteria are a major cause of foodborne illness and have important implications for human public health along with economic consequences (1). Despite thorough disinfection protocols and hygiene monitoring during food production and processing, pathogenic bacteria often persist in food-related environments and on food, suggesting that our knowledge about the transmission routes and epidemiology of foodborne pathogenic bacteria is still incomplete.
Recent studies have shown that bacteria can benefit from intracellular associations with free-living protozoa (FLP) (2, 3), heterotrophic eukaryotic microorganisms that are common in natural aquatic and terrestrial ecosystems (4). Although FLP feed on bacteria, some bacteria resist digestion. These so-called “digestion-resistant bacteria” can survive and even grow inside their FLP hosts (5). These hosts thus effectively act as a reservoir, shelter, and vector for the bacteria and can as such play an important role in their ecology (3, 6). Intracellular association with FLP has also been demonstrated for human-pathogenic bacteria (7, 8), including food-related pathogens (e.g., see references 5, 9, and 10). As FLP have been isolated from diverse food-related habitats, such as broiler houses (11,–13), meat cutting plants (14), domestic refrigerators (15), and vegetables (16, 17), this suggests that FLP may be implicated in the epidemiology of foodborne pathogens.
Many FLP have two life cycle stages: the trophozoite and the dormant cyst. The former is the actively feeding stage, preying on bacteria, algae, viruses, yeast, and organic particles by phago- and pinocytosis (18). Encystment (i.e., conversion from trophozoite to cyst) is triggered by adverse environmental conditions, such as food shortage, hyper- or hypo-osmolarity, temperature, and pH extremes (18). Cysts usually possess a thick, often double or multilayered protective wall, consisting of lipids, (glyco)proteins, and carbohydrates like chitin and cellulose. This protects the protozoon against unfavorable environmental conditions, such as freezing (19), gamma and UV radiation (20), and chemicals used for disinfection in health care settings (21) and drinking water production (22, 23). Some cysts can withstand desiccation for more than 20 years (24). Under favorable conditions, excystment (i.e., reconversion into trophozoites) takes place.
To date, most studies on interactions between FLP and bacteria (including pathogenic bacteria) have focused on the trophozoite stage. In contrast, little is known about bacterial association with the cyst forms, which, given their high tolerance for adverse environmental conditions and hence also high dispersal capacity (18), are especially relevant from an ecological and epidemiological point of view. It is expected that FLP cysts are even more effective as a shelter and vector for internalized bacteria than the trophozoites. It has indeed been shown that some internalized digestion-resistant bacteria, including human pathogens, can survive the
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