Today I heard a talk by Carmen Buchrieser from the Institut Pasteur about Legionella, the bacterium which causes Legionnaire’s disease. This disease was first observed in 1976. At a meeting of US military veterans in a hotel in Philadelphia many fell ill with an unknown respiratory disease and an alarming number of those affected died as a consequence. The bacterium was discovered early in the next year and given the name Legionella pneumophila. Infection with this disease can be fatal but is usually only so in older people or those who are immunocompromized. The disease is not passed directly between humans. In the words of the speaker, the human lung is a dead end for Legionella. A bacterium of this kind which enters one human host will never leave. The sources of infection in humans are water droplets from cooling towers,air conditioning systems, showers etc.
The natural hosts of Legionella are amoebae. In humans they infect pulmonary macrophages which could be thought of as surrogate amoebae in this context. Because of the lifestyle of the species there is no evolutionary pressure on it resulting from its time spent in humans. The pressure comes from its natural life in the amoeba. I have mentioned molecular mimicry in a previous post as a phenomenon relevant to immunology. In this post I am using it to mean something more primitive, namely a similarity in the genes of hosts and their parasites. When the host is a vertebrate this has an implication for immunity but in the case of Legionella the principal host is the amoeba, so that this is not relevant. This bacterium does have a wide variety of proteins which resemble proteins of eukaryotes. The speaker mentioned two ways in which molecular mimicry could have arisen in Legionella. One is horizontal gene transfer, i. e, the transfer of genes between different species. The other is convergent evolution. I did not have the impression that it is clear which if these two is more important.