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Antibiotic resistance carried on R factors was transferred by conjugation fromEscherichia coli B/r andShigella flexneri 1a toErwinia spp. Tetracycline resistance (TetR) carried on R factor R100drd-56 was transferred fromE. coli B/r to strains ofErwinia amylovora, E. aroideae, E. atroseptica, E. chrysanthemi, E. cytolytica, E. dissolvens, E. herbicola, E. nigrifluens , andE. nimipressuralis , but not to strains ofErwinia carotovora, E. carnegieana, E. dieffenbachiae, E. oleraceae , andE. quercina . Multiple antibiotic resistance (chloramphenicol, streptomycin, tetracycline; ChlR-StrR-TetR) carried on R factor SR1 was transferred from a clinical isolate ofS. flexneri 1a to strains ofE. aroideae, E. chrysanthemi, E. herbicola , andE. nigrifluens , but not to strains of otherErwinia spp. The frequency of this transfer was low with receptive cultures ofErwinia spp. andE. coli (F− strain). Antibiotic resistance in the exconjugants showed varying degrees of stability in the presence or absence of acridine orange, depending on the strain tested. The frequencies of segregation to drug susceptibility in the presence of acridine orange, though low, suggest that the elements exist as plasmids in the majority of theErwinia exconjugants. Multiple antibiotic resistance (ChlR-StrR-TetR) was found to segregate into various resistance classes (ChlR-StrR, StrR-TetR, TetR, StrR, and none) in these exconjugants. The exconjugants ofE. amylovora, E. herbicola , andE. nigrifluens , to which R100drd-56 was transferred fromE. coli B/r, were sensitive to the male (F)-specific phage M13. There was a positive correlation between the susceptibility of exconjugants to the F-specific phage M13 and their ability to transfer R100drd-56 to the recipient cultures ofEscherichia coli, Erwinia herbicola, Salmonella typhimurium , andShigella dysenteriae . Exceptions were, however, noted withErwinia dissolvens andE. nimipressuralis exconjugants harboring R100drd-56 ; these exconjugants, although not susceptible to M13, transferred R100drd-56 to the recipient cultures. The frequency of transfer of R100drd-56 and the levels of resistance to tetracycline inErwinia exconjugants were found to differ markedly depending upon the strain employed. Transfer of multiple antibiotic resistance (ChlR-StrR-TetR) fromErwinia exconjugants was not obtained in preliminary trials with anE. coli F− strain as the recipient culture.

Transfer Among Erwinia spp. and Other Enterobacteria of Antibiotic Resistance Carried on R Factors