The propionic-acid producers, typically members of Propionibacterium, that run the propionic fermentation of glucose or lactate. In Swiss-type cheesemaking they are indispensable secondary flora: after lactic starters acidify the curd, these bacteria generate propionate for flavor, acetate, and the carbon dioxide that inflates the cheese’s eyes during warm ripening. They grow slowly and dislike oxygen, so cheesemakers manage salt, temperature, and ripening-room conditions to favor them at exactly the right stage. Their metabolism also underpins silage preservation and some probiotic concepts.