Date Published: December 10, 2015
Publisher: Public Library of Science
Author(s): Ann-Katrein Bär, Niha Phukan, Jully Pinheiro, Augusto Simoes-Barbosa, Philip J. Cooper. http://doi.org/10.1371/journal.pntd.0004176
Abstract: Infections by parasitic protozoans are largely neglected, despite threatening millions of people, particularly in developing countries. With descriptions of the microbiota in humans, a new frontier of investigation is developing to decipher the complexity of host–parasite–microbiota relationships, instead of the classic reductionist approach, which considers host–parasite in isolation. Here, we review with specific examples the potential roles that the resident microbiota can play at mucosal interfaces in the transmission of parasitic protozoans and in the progress of infection and disease. Although the mechanisms underlying these relationships remain poorly understood, some examples provide compelling evidence that specific components of the microbiota can potentially alter the outcomes of parasitic infections and diseases in humans. Most findings suggest a protective role of the microbiota, which might lead to exploratory research comprising microbiota-based interventions to prevent and treat protozoal infections in the future. However, these infections are often accompanied by an unbalanced microbiota and, in some specific cases, apparently, these bacteria may contribute synergistically to disease progression. Taken together, these findings provide a different perspective on the ecological nature of protozoal infections. This review focuses attention on the importance of considering polymicrobial associations, i.e., parasitic protozoans and the host microbiota, for understanding these human infections in their natural microbial context.
Partial Text: Parasitic protozoans contribute significantly to the burden of infectious diseases worldwide and represent a major public health problem. Most of the people affected by these infections live in developing countries, and these diseases remain neglected, receiving little funding and intervention. Parasitic protozoans that infect and colonise or infect and transit human mucosas are extremely prevalent; examples of these are illustrated in Fig 1. While Toxoplasma gondii is one of the most prevalent human infections acquired via ingestion, Trichomonas vaginalis is the most prevalent sexually transmitted infection of non-viral cause worldwide [1,2].
Infections by parasitic protozoans are generally associated with changes in the structure and composition of the commensal bacteria. By separately examining intracellular and extracellular forms of parasitic protozoans during their development in humans, this section of the review reveals intimate relationships between the host, the native bacterial microbiota, and protozoans, which impact the outcomes of many of these medically important infections.
The number and diversity of microbial cells living on the mucosal surface of humans should impact infections by parasitic protozoans that transit or reside on mucosal surfaces. Commensal microorganisms contribute a large repertoire of unique genes to their hosts, whose products are likely to impact the functioning of the host and invading parasites. This review presents evidence that the microbiota of humans can significantly alter the outcomes of various protozoal infections. However, the cellular and molecular mechanisms underlying these mutual responses are yet to be deciphered in most cases.