Chapter 27 : Protozoa and Lurking Pathogens

Abstract:

Michael Brown believes that while protozoal grazing of bacteria is well recognized, people have neglected a very different scenario in other environments. Here, bacteria invade protozoa, replicate, and emerge better fitted to enter other protozoa, as well as more resistant to antibiotics, biocides, and other stresses. One can envisage the two forms of microbial life coexisting and coevolving before we and other animals appeared. The principal selective factors affecting bacteria were predation by protozoa and environmental stresses, such as drying, freezing, and oxidation. A major plank in Brown's hypothesis is the significance of protozoa today as reservoirs of animal (including human) pathogens. Brown cites two examples of the possible medical relevance of the discoveries. Grazing cattle almost certainly ingest protozoa in silage and grass, so if the protozoa contain pathogens, this could be a significant route of infection between and within cattle herds. The link proposed by Brown between bacterial evolution in the environment and pathogenicity is the general stress response (GSR). Building on evidence of its dual role in survival and virulence, he argues that the coevolution of bacteria and protozoa equipped some species of bacteria both for persistence in the environment and for invasion of, and survival in, animal cells. Commenting on the apparent decline in Helicobacter pylori infection in developed countries, Martin Blaser has argued that “to maintain its niche in the human biosphere, H. pylori must be transmitted from person to person."