Description:
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135 years ago Ferdinand Cohn, the founder of bacteriology, microscopically observed a conspicuous filamentous bacterium with a complex life cycle and described it as
<jats:italic>Crenothrix polyspora</jats:italic>
. This uncultured bacterium is infamous for mass developments in drinking water systems, but its phylogeny and physiology remained unknown. We show that
<jats:italic>C. polyspora</jats:italic>
is a gammaproteobacterium closely related to methanotrophs and capable of oxidizing methane. We discovered that
<jats:italic>C. polyspora</jats:italic>
encodes a phylogenetically very unusual particulate methane monooxygenase whose expression is strongly increased in the presence of methane. Our findings demonstrate a previously unrecognized complexity of the evolutionary history and cell biology of methane-oxidizing bacteria.
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