n 1  cap  :  a genus (the type of the family Propionibacteriaceae) of gram-positive nonmotile usu. anaerobic bacteria that form propionic acid by fermenting lactic acid, carbohydrates, and polyalcohols and that include forms found esp. on human skin and in dairy products  2  pl  -ria   :  any bacterium of the genus Propionibacterium .
 
 
 
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