Primed, sealed, and delivered: How protozoa package and prepare pathogens for host infection

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Controlling microbial pathogens and infections in an era of growing antimicrobial resistance and climate change-induced disease outbreaks requires improved understandings of microbial and biofilm biology in the broader context of environmental interactions. 

This approach led our team to step back and consider the interactions between microbes and higher organisms in the environment, in particular protozoa that prey on bacteria.

By studying pathogens and their interactions with protozoa, we observed that some protozoa expel bacteria in food vacuoles rather than digesting them. Intrigued by this process, we began to investigate what factors impact this release.

Amazingly, we also found that the ability of bacteria to resist stress and cause infections were significantly enhanced following ingestion by the protozoa and subsequent release to the environment in expelled food vacuoles (EFVs).

In the case of Vibrio cholerae (responsible for the diarrheal disease cholera), the EFV-associated bacterial cells were more resistant to antibiotics, acid stress, and starvation than free-living cells. Intriguingly, the EFVs remained intact in pH equivalent to human stomach acid for at least two hours (longer than the time it takes for food to pass through the stomach) with no decrease in viability to the cells they harboured. However, the EFVs immediately released their microbial contents when exposed to bile salts which they would encounter in the small intestine, the site in the human digestive system where they colonise and cause infection.

Hence, the protection imparted by the EFV, coupled with enhanced fitness traits of the bacterial cells, equips the pathogen with the ability to persist longer in hostile environments such as nutrient limited marine waters and the high acidity of a host stomach, while being primed for infection upon release into more favourable environments.

The likelihood of bacterial-sized cells associating with larger organisms and structures was highlighted by Prof. Rita Colwell, who demonstrated that filtering drinking water through folded sari cloth can decrease cholera cases by 48% in a study conducted in villages in Bangladesh. The original presumption was that this was due to microbial association with higher organisms such as copepods, but the possibility remains that EFVs could also be trapped in the cloth sieve.

Legionella pneumophila is also found in EFVs, where it has been demonstrated to be resistant to cooling tower biocides for up to 24 hours1. Other pathogens known to be packaged into EFVs include Listeria monocytogenes, Pseudomonas aeruginosa, Helicobacter pylori, Salmonella enterica, Escherichia coli O157and Enterococcus faecalis. This wide-ranging EFV phenomenon thus presents a potentially powerful new mechanism to target for microbial control in the environment, as EFVs are not recognised as pathogen transmission vectors and are overlooked in current disease and outbreak mitigation protocols. The challenge remains to devise tools to identify and remove EFVs from recreational waters, drinking water, cooling towers and other environmental settings, to reduce the infectious disease burden associated with antimicrobial resistance.

1. Berk, S. G., Ting, R. S., Turner, G. W. & Ashburn, R. J. Production of respirable vesicles containing live Legionella pneumophila cells by two Acanthamoeba spp. Appl Environ Microbiol 64, 279-286 (1998)

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Diane McDougald

Associate Professor, University of Technology Sydney