VARIABLE STABILITY OF ANTIBIOTIC-RESISTANCE MARKERS IN BACILLUS-CEREUS UW85 IN THE SOYBEAN RHIZOSPHERE IN THE FIELD

We compared the stability of antibiotic-resistance markers in strains derived from Bacillus cereus UW85 in culture media and in the soybean rhizosphere in a growth chamber and in the field. We studied two indep endent, spontaneous mutants resistant to neomycin, three independent, spontaneous mutants resistant to streptomycin, and strains carrying pl asmid pBC16, which encodes tetracycline resistance. Antibiotic-resista nce markers were maintained in populations of all UW85 derivatives in culture and in the rhizosphere of soybeans grown in soil in a growth c hamber. In two field experiments, antibiotic resistance was substantia lly lost in rhizosphere populations of B. cereus as early as 14 or as late as 116 days after planting. To distinguish between death of the i noculated strain and loss of its marker, we tested populations of B. c ereus for other phenotypes (orange pigmentation, plasmid-borne resista nce to tetracycline, and biocontrol activity) that are typical of UW85 -derivatives used as inoculum, but atypical of the indigenous populati ons of B. cereus, and these phenotypes were maintained in populations from which the marker was lost. In general, neomycin-resistance marker s were maintained at a higher frequency than streptomycin-resistance m arkers, and maintenance of antibiotic-resistance markers varied with p osition on the root and with the year of the experiment. In a semi-def ined medium, the UW85 derivatives grew at the same rate as the wild ty pe at 28-degrees-C, but most grew more slowly than the wild type at 16 -degrees-C, demonstrating that antibiotic resistance can affect fitnes s under some conditions. The results suggest that the stability of ant ibiotic-resistance markers should be assessed in the ecosystems in whi ch they will be studied.