Several conditions of growth of Bordetella pertussis cause a reversible phenotypic alteration in properties termed modulation. Growth in medium containing nicotinic acid induces normal (X-mode) cells to change to modulated (C-mode) cells. We examined several pyridines and compounds resembling pyridines for their ability to affect modulation, using envelope protein patterns and serological reactivity as indicators of modulation. We found that 6-chloronicotinic acid and quinaldic acid were more effective modulating stimuli than was nicotinic acid on a molar basis. Both 2-chloronicotinamide and isoniazid interfered with nicotinic acid-induced modulation, and can be called antimodulators. Picolinic acid inhibited growth.
