Increased resistance to chloramphenicol in Rickettsia prowazekii with a note on failure to demonstrate genetic interaction among.

Weiss, Emilio (Naval Medical Research Institute, Bethesda, Md.) and Harry R. Dressler. Increased resistance to chloramphenicol in Rickettsia prowazekii with a note on failure to demonstrate genetic interaction among strains. J. Bacteriol. 83:409-414. 1962.-A further effort to produce genetically labeled Rickettsia prowazekii was made by serially passing the Madrid E strain in eggs in the presence of increasing concentrations of chloramphenicol. After 40 serial and 1 limit-dilution passages, a substrain, called C(r), was isolated, which appeared to be the result of a two-step change, each involving a twofold increase in resistance to the antibiotic. Chloramphenicol resistance was retained after 10 drugless egg passages. Cross-resistance to thiocymetin, but not to unrelated drugs, was shown. Several attempts were made to demonstrate genetic interaction, in eggs or entodermal cell cultures, among strains resistant to chloramphenicol, erythromycin, and p-aminobenzoic acid. They have not been successful.