Expelled Food Vacuoles as Protozoan-Derived Vehicles for Bacterial Persistence and Transmission

Abstract

Abstract Protozoa are major bacterial predators in aquatic, soil and host-associated environments. During feeding, bacteria are internalised into food vacuoles, which normally mature into digestive vacuoles containing the molecular machinery required for bacterial digestion. However, several bacterial species can resist this process and are instead expelled from protozoa packaged within these vacuoles now commonly referred to as expelled food vacuoles (EFVs), and previously described as “expelled vesicles”, “faecal pellets” or “multilamellar bodies”. Rather than being viewed simply as waste products of protozoan digestion, EFVs should be considered as biologically generated reservoirs that package, protect and transmit viable pathogenic bacteria. In this review, we synthesise evidence from studies in Vibrio cholerae , Salmonella enterica , Legionella pneumophila , Campylobacter jejuni , Listeria monocytogenes , Escherichia coli , Burkholderia cenocepacia and Mycobacterium smegmatis indicating that EFV-associated bacteria display increased resistance to acid, starvation, antibiotics, biocides, oxidative stress and desiccation, together with enhanced infectivity and horizontal gene transfer. Based on this research, we propose that EFVs create a distinct environmental transmission state in which bacteria that survive protozoan digestion are packaged into protective vesicles and exit the predator already primed for persistence, dissemination and host colonisation. This process connects environmental persistence with infection biology and suggests that EFV-associated pathogens could serve as useful targets for water surveillance and outbreak prediction.