Protozoa and pathogenic bacteria: lessons learned from Legionella pneumophila

The microhabitats favoured by protozoa are dynamic ecosystems comprising a heterogeneous microflora of bacteria and fungi as well as protozoa in a complex biofilm. Many protozoa feed by grazing on microbial biofilms, but they do not do so indiscriminately. A dynamic equilibrium will exist and the nature of the relationship changes depending on environmental conditions. The view of protozoa as the hunter and bacteria as the hunted is now recognised as overly simplistic. Some bacteria not only resist predation by protozoa, but also have become endosymbionts of free‐living protozoa, demonstrating adaptation to the otherwise hostile intracellular environment. The concept that protozoa can be reservoirs for some pathogens is more widely appreciated and the catalyst for much of this work came from the observation that Legionella pneumophila infects and multiplies within some species of free‐living amoebae. In addition to amplifying cell numbers, the phenotype of intra‐protozoal‐grown cells is quite distinct from their free‐living counterparts cultured on complex laboratory media: altered surface properties, decreased susceptibility to chemical inactivation and antibiotics, and enhanced invasiveness for mammalian cells. There is significant conservation in the genes used by L. pneumophila to parasitize protozoa and macrophages and it has been proposed that co‐evolution of bacteria and lower order eukaryotic cells has equipped bacteria both for environmental survival and virulence towards higher order eukaryotes. Other authors have even described protozoa as an “evolutionary gym” enabling L. pneumophila to “train” for its role in invading higher eukaryotic organisms. A growing list of bacteria and fungi that interact with protozoa confirm that the co‐evolution between L. pneumophila and protozoa is not unique in nature. An emerging theme amongst many of the micro‐organisms associating with protozoa is that they typically are recognised as opportunistic pathogens and many lack a recognised animal host. These features are suggestive of a relationship much closer to symbiosis which becomes parasitic only in special circumstances. The question remains why this is so? The answer could lie in co‐evolution with lower order eukaryotic organisms present in biofilms.