15 The Family Methylobacteriaceae

The Methylobacteriaceae comprise a large family of Alphaproteobacteria within the Order Rhizobiales and currently contains three genera, Methylobacterium, Microvirga, and Meganema. The largest genus currently contains 44 validated species of Methylobacterium, most of which are facultative methylotrophs able to grow on methanol and other one-carbon compounds as sources of energy and carbon. Most are pink-pigmented, exhibit common fatty acid profiles, and contain ubiquinone Q-10. The eight species of Microvirga and the single species of Meganema are not methylotrophic. The phylogenetic and phenotypic properties of Meganema indicate that it is wrongly placed in this family. Methylobacterium species are ubiquitous in the natural environment,bothasfree-living organismsinsoilandwater,but also on the phylloplane of plants, and in the leaf, stem, and root tissues of plants: Some induce plant leaf and root nodule formation, and can promote plant growth by production of auxins. Some species are opportunistic human pathogens; others have been found in insect tissues. Some areimportant for their rolein the degradation of pollutants, and they may also cause commercial problems such as the fouling of aircraft fuels. Microvirga species occur in diverse habitats as free-living species, and others are colonists in plant root nodules. The filamentous Meganema has only been recovered as an organism involved in the fouling and blocking of water filtration systems. The genomes of several Methylobacterium species have been sequenced, and they show considerableinterstrain homology.Significantgenomeplasticity is indicated by the large number of insertion elements in some genomes, andsome methylotrophic functions seem to havebeen acquired by horizontal gene transfer.