Practice question

Veillonella represents anaerobic Gram-negative cocci within class Negativicutes of phylum Firmicutes exhibiting diderm envelope architecture that stains Gram-negative despite firmicute phylogenetic affiliation. Genomic analysis shows presence of outer membrane with lipopolysaccharide atypical for Gram-positives, interpreted as secondary acquisition within Firmicutes. These commensals metabolize lactate to propionate in oral cavity and gut. Exception demonstrates that Gram stain reaction does not always reflect phylogeny, challenging classical determinative criteria, as Veillonella clusters with Bacillus and Clostridium by 16S rRNA, unlike typical monoderm Firmicutes.