No presente trabalho foi feito o isolamento de mutantes de Xanthomonas campestris resistentes a 64,0 ug/ml de estreptomicina e a 8,0 ug/ml de aureomicina. Foi estudado o comportamento dêsses mutantes “in vitro” e “in vivo” pelo tratamento de sementes com os referidos antibióticos. Dentre os resultados obtidos podemos tirar as seguintes conclusões: a) a linhagem mutante resistente à estreptomicina foi resistente a tôdas as doses de antibiótico usadas “in vitro” e “in vivo”. O tratamento de sementes com estreptomicina na concentração de 1000,0 ug/ml não previne o aparecimento de mutantes resistentes a êsse antibiótico; b) a linhagem resistente à aureomicina começou a ser inibida “in vitro” com doses superiores a 8,0 ug/ml e “in vivo” com doses superiores a 16,0 ug/ml. O tratamento de samentes com aureomicina na concentração de 1000,0 ug/ml previne o aparecimento de mutantes resistentes a êsse antibiótico; c) os mutantes resistentes à estreptomicina e à aureomicina foram mais patogênicos do que a linhagem da bactéria sensível; d) o mutante resistente à aureomicina foi mais patogênico do que o mutante resistente à estreptomicina; e) os mutantes resistentes inoculados em plântulas não tiveram sua resistência alterada em relação aos dois antibióticos.

Black rot of crucifers, caused by the bacterium Xanthomonas campestris (Pam.) Dowson is world wide spread. The black rot bacteria which naturally contaminate the seed are reported to remain viable for 3 years. Seed disinfection by antibiotics treatment have enhanced efforts to reduce incidence of the disease. This investigation has the objective of providing some information about the development of antibiotic-resistant pathogens as a result of seed treatment with antibiotic. Streptomycin and aureomycin-resistant mutants were obtained from an isolate of Xanthomonas campestris. Antibiotic treated cabbage seeds and antibiotic treated seed plants were inoculated with these two mutants strains in laboratory and green-house experiments. From the results obtained the following are the main conclusions achieved: the streptomycin mutant strain showed resistance to the streptomycin concentrations tested “in vitro” and “in vivo”; treatment of the seeds with 1000,0 ug of streptomycin per ml do not prevent the development of streptomycin resistant mutants; the aureomycin resistant strain was inhibited “in vitro” by concentrations of aureomycin higher than 8,0 ug/ml and “in vivo” by concentrations higher than 16,0 ug/ml; treatment of the seeds with 1000,0 ug of aureomycin per ml do prevent the development of aureomycin resistant mutants; the streptomycin and aureomycin-resistant mutants were more virulent than the sensitive strain while the aureomycin-resistant mutant was more virulent than the streptomycin-resistant mutant; the antibiotic-resistant strains inoculated on cabbage plants and reisolated were still resistant to the same doses of antibiotics.