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Methylorubrum is a taxonomic genus of Alphaproteobacteria, established in 2018 by separating a group of species formerly assigned to Methylobacterium. Its type species is Methylorubrum extorquens. The type fixes the application of the genus name; it does not imply that every member has the same metabolism.^[ref]

The group belongs to a much-studied assemblage of pink-pigmented, facultatively methylotrophic bacteria. Facultative methylotrophy means that a bacterium can use substrates such as methanol while retaining the ability to use other carbon sources. Neither pink colour nor one metabolic test should be treated as a complete definition of a taxonomic genus.

The boundary with Methylobacterium illustrates how classification is tested. A 2021 study compared 62 type strain genome assemblies, supplementing existing data with newly sequenced material. Whole-genome comparisons placed members assigned to the two genera close together and interspersed, motivating reconsideration of their separation.^[ref] For example, a tree built from one ribosomal RNA gene and a tree built from many genomic regions may support different groupings. The resulting debate concerns which relatives a genus includes, rather than whether the experimental organisms suddenly changed their biological properties.

Methylorubrum extorquens is a species of bacteria and the type species of Methylorubrum. The older combination Methylobacterium extorquens names the same species: LPSN records it as a homotypic synonym, meaning that the two names share a nomenclatural type.^[ref] Experimental results still need a specified bacterial strain; a species name alone does not identify every tested culture.

Methylorubrum extorquens AM1 is a model for facultative methylotrophy. Its studied pathway converts methanol to formaldehyde, with carbon subsequently divided between oxidation toward carbon dioxide and carbon assimilation into cellular material.^[ref]

A 2005 experiment compared AM1-derived mutants with a reference strain during colonization of Medicago truncatula. Mutants defective in methylotrophy lost ground in competition, yet could reach comparable populations when tested separately. This distinction supports a competitive advantage from methanol use without making it the only usable carbon source.^[ref] In an illustrative count, a population share changing from $1/2$ to $1/4$ halves the strain's share; it does not show that its absolute number of cells halved. Both the fraction and the total count are needed to distinguish relative success from population decline.