Multidrug-resistance plasmid of Enterobacter cloacae: transfer to Erwinia herbicola on the phylloplane of mulberry and weeds

A mulberry epiphytic Enterobacter cloacae MUL1 harbors plasmid pMUL1 encoding five drug-resistance genes. This plasmid was examined upon its conjugal transfer into epiphytic Erwinia herbicola on the phylloplane of mulberry and 12 species of weeds. The plasmid was transferred into Er. herbicola at a frequency of 10^–5–10^–3/recipient in mulberry and Lolium multiflorum LAM. 1–8 days after wound inoculation with 10⁶–10⁸/ml suspensions. In Chenopodium album L. and C. album L. var. centrorubrum, however, it was transferred only after wound inoculation with a 10⁸/ml suspension, but not with 10⁷/ml or 10⁶/ml suspensions, owing to the weak epiphytic fitness of Ent. cloacae on these weeds. Transconjugants were also obtained for seven other species of weeds in the case of inoculation with a 10⁸/ml suspension. In contrast, when bacterial suspensions were sprayed on mulberry leaves with or without fresh wounds, transconjugants were obtained only in wounded leaves, which were considered suitable for bacterial conjugation. These findings suggest that epiphytic bacteria, including Ent. cloacae and Er. herbicola, may be carriers of drug-resistance genes distributed among plant pathogenic bacteria in nature.