MASKING OF ANTIBIOTIC-RESISTANCE UPON RECOVERY OF ENDOPHYTIC BACTERIA

During studies on internal plant colonization by rhizosphere bacteria and endophytic bacteria over several years, we frequently observed lac k of growth of rifampicin-resistant mutants (rif+) on tryptic soy agar amended with rifampicin (RTSA). Following seed treatment of cucumber with 6 species of rif+ rhizosphere bacteria in one experiment, all str ains were recoverable on RTSA when external root colonization was moni tored. Following trituration of surface-disinfested roots, only one st rain grew directly on RTSA; however colonies isolated on tryptic soy a gar (TSA) grew within 18 h after transfer to RTSA. We term this tempor ary loss of the antibiotic-resistant phenotype 'antibiotic masking'. A ntibiotic masking was also observed with isolation of 7 rif+ endophyti c bacterial strains from inside stems of cotton and with isolation of mutants of bacterial endophytes resistant to polymyxin B sulfate from cotton plants. Rifampicin-masking was not accounted for in vitro by in hibitory compounds from cotton plant extracts, by bacterial growth on low nutrient agar, or by competition with other bacteria. Collectively , these results suggest that expression of antibiotic-resistance may b e altered in planta, although causes for this antibiotic-masking remai n to be elucidated, methods for quantifying internal plant colonizatio n by rif+ bacteria should account for this possibility.