Lj. Halverson et al., VARIABLE STABILITY OF ANTIBIOTIC-RESISTANCE MARKERS IN BACILLUS-CEREUS UW85 IN THE SOYBEAN RHIZOSPHERE IN THE FIELD, Molecular ecology, 2(2), 1993, pp. 65-78
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.