Summary auto-generated
This taxonomic study reports the reclassification of Actinomyces humiferus, a soil bacterium originally described in 1969, into the genus Cellulomonas as Cellulomonas humilata. Although A. humiferus was classified as Actinomyces based on morphological features including branched filamentous growth and catalase-negative properties, its placement was always controversial. The bacterium differs from typical Actinomyces species in multiple ways: it inhabits organically rich soils rather than warm-blooded animals, has higher DNA G-C content (73 mol%), grows optimally at 30°C rather than 37°C, and is sensitive to lysozyme. Comparative 16S rRNA gene sequencing analysis provided definitive phylogenetic evidence showing that A. humiferus is only distantly related to Actinomyces species but clusters closely with Cellulomonas species, particularly C. turbata and C. flavigena. The phylogenetic tree constructed using neighbor-joining methods confirmed A. humiferus placement within a robust Cellulomonas clade with 86% bootstrap support. Although A. humiferus exhibits some phenotypic differences from typical cellulomonads, the authors argue these differences are insufficient to prevent reclassification, noting that C. turbata also produces mycelium and C. fermentans shares catalase-negative and fermentative characteristics. The new species can be distinguished from other Cellulomonas members by its ability to hydrolyze casein and inability to reduce nitrate.
Key findings
- 16S rRNA gene sequencing demonstrates A. humiferus is phylogenetically distant from Actinomyces species but clusters within the Cellulomonas genus with 95-97% sequence similarity to C. turbata and Promicromonospora enterophila
- A. humiferus is reclassified as Cellulomonas humilata based on overwhelming phylogenetic evidence from neighbor-joining, Fitch, and maximum-parsimony tree methods
- C. humilata is distinguished from other Cellulomonas species by casein hydrolysis and inability to reduce nitrate, and differs in being catalase-negative like C. fermentans
- The reclassification reconciles long-standing taxonomic controversy regarding A. humiferus placement despite phenotypic differences including soil habitat, higher growth temperature optimum (30°C), and lysozyme sensitivity
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Abstract
The placement of Actinomyces humiferus within the genus Actinomyces has always been controversial. A humiferus differs from typical members of the genus both phenotypically and in possessing a relatively high DNA G + C content. Comparative 16S rRNA gene sequencing has shown that A. humiferus is related only distantly to other species of the genus Actinomyces and is, in fact, a member of the genus Cellulomonas. On the basis of phylogenetic evidence, it is proposed that A. humiferus be reclassified in the genus Cellulomonas as Cellulomonas humilata nom. corrig., comb. nov.