Methylobacterium is a taxonomic genus of bacteria whose type species is Methylobacterium organophilum.[ref] The 1976 genus description concerned pink, rod-shaped isolates and emphasized facultative methylotrophy: using one-carbon compounds while also being able to grow with more complex organic carbon sources. That is a description of the original isolates, not a promise that every bacterial strain subsequently assigned to Methylobacterium has identical physiology.[ref]
Methylobacterium, a New Genus of Facultatively Methylotrophic Bacteria by T. E. Patt and others. Abstract, original isolates and carbon-source comparison
Plant experiments show why identifying the genus is only a starting point. In 2011, researchers established candidate bacterial colonists on Arabidopsis thaliana, then tested their effects during challenge by Pseudomonas syringae pv. tomato DC3000. The tested Methylobacterium strains did not reduce disease development, whereas some Sphingomonas isolates did.[ref]
Protection of Arabidopsis thaliana against leaf-pathogenic Pseudomonas syringae by Sphingomonas strains in a controlled model system by Gerd Innerebner and others. Abstract and Figure1, Methylobacterium comparison
The negative result distinguishes presence from protective function. Detecting a Methylobacterium sequence on a healthy leaf does not establish that the bacterium caused the leaf to remain healthy. Testing that idea needs a controlled comparison with a specified strain, host and outcome. Equally, failure to protect in this experiment does not rule out other interactions in different biological settings.
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