<p>This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.</p> <p>Obligate intracellular bacteria commonly have much reduced genome sizes compared to
their nearest free-living relatives. One reason for this is reductive evolution: the loss of
genes rendered non-essential due to the intracellular habitat. This can occur because of the
presence of orthologous genes in the host, combined with the ability of the bacteria to
import the protein or metabolite products of the host genes. In this article we take a look
at three such bacteria whose genomes have been fully sequenced. <italic>Buchnera</italic> is an
endosymbiont of the pea aphid, <italic>Acyrthosiphon pisum</italic>, the relationship between these two
organisms being so essential that neither can reproduce in the absence of the other.
<italic>Rickettsia prowazekii</italic> is the causative agent of louse-borne typhus in humans and
<italic>Mycobacterium leprae</italic> infection of humans leads to leprosy. Both of these human
pathogens have fastidious growth requirements, which has made them very difficult to
study.</p>
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Comp Funct Genomics. 2001 Feb; 2(1):44-48