A genus of nonmotile, non-spore-forming, facultatively anaerobic bacteria (family Actinomycetaceae) containing gram-positive, non–acid-fast, branched, diphtheroid rods (0.2–0.3 by 3.0–5.0 mcm and longer). These organisms produce filamentous microcolonies. Their metabolism is fermentative. Primarily propionic and acetic acids are produced from glucose. Catalase is not produced. The cell wall contains diaminopimelic acid but not arabinose. These organisms are pathogenic for humans, causing lacrimal canaliculitis and typical actinomycosis. The type species is Arachnia propionica.
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