Bordet–Gengou agar is a type of agar plate optimized to isolate Bordetella, containing blood, potato extract, and glycerol, with an antibiotic such as cephalexin or penicillin and sometimes nicotinamide. The potato extract provided nitrogen and vitamins, and potato starch absorbed fatty acids present in nasal secretions or collection-swab cotton that inhibited growth; glycerol was a carbon source.