peptococcus peptococcus peptococcus is a gram-positive bacterium genus in the family peptococcaceae. peptococcus scientific classification domain: bacteria phylum: bacillota class: clostridia order: clostridiales family: peptococcaceae genus: peptococcuskluyver and van niel 1936 type species peptococcus niger(hall 1930) kluyver & van niel 1936 species p. niger p. simiae species in the genus are part of the human microbiome, especially in the bacteria that form the gut flora. they are part of the flora of the mouth, upper respiratory tract and large intestine. mezlocillin is an antibiotic that is effective against peptococcus species. peptococcus niger is the only species left in the genus. all others have been moved to peptostreptococcus.