A Plaque Assay for Rickettsia Rickettsii

A plaque technique for the assay of Rickettsia rickettsii is described. The method employs primary chick or African green monkey kidney (VERO) monolayer cell cultures with either an agarose or a special Noble agar overlay. Plaques were counted after 6 days; resultant titers correlated well with LD50 end points obtained by a standard assay in embryonated eggs. The plaque-forming organisms were identified by direct observation of rickettsia- like bodies in the monolayer lesions, inhibition of plaques by antibiotics, sensitivity of plaques to specific immune serum, and failure to cultivate other microorganisms from the infected cells. Versatility of the test was demonstrated by assaying samples of rickettsiae from several different sources commonly used in our laboratory.