STREPTOMYCIN RESISTANCE AND COPPER TOLERANCE AMONG STRAINS OF PSEUDOMONAS-CICHORII IN CELERY SEEDBEDS

Strains of Pseudomonas cichorii were isolated from celery plants exhib iting symptoms of bacterial blight at four seedbed sites in Florida. I n 1991, 70% of 87 isolated strains were resistant to 200 mug/ml of str eptomycin in vitro. In 1992, 32% of 106 strains were streptomycin-resi stant. Most of the reduction in streptomycin resistance in 1992 was ac counted for by a reversal from 100% to 0% resistance among strains col lected from one seedbed site. Strains were also assayed for tolerance to 0.64 mM CuSO4. In 1991, 32% of the strains were classified as sensi tive (growth 0-30% of that on controls), 44% as moderately tolerant (g rowth >30 to <60% of that on controls), and 25% as highly tolerant (gr owth greater-than-or-equal-to 60% of that on controls). In 1992, 36% w ere sensitive, 27% were moderately tolerant, and 37% were highly toler ant. When exposed to 1.2 g/L of a commercial copper hydroxide bacteric ide for 4 hr in the laboratory, populations of tolerant strains were r educed from 10(8) to only 10(6) cfu/ml. In contrast, populations of se nsitive strains were typically less than 100 cfu/ml. Strains identifie d as copper-tolerant became more sensitive as cryogenic storage time i ncreased. After 10 mo of storage, 10(3)-10(4) cells survived exposure to copper hydroxide. Phenotypic expression of copper tolerance (10(7) cfu/ml) could be retrieved by growing cultures from cryogenic storage on media containing a sublethal dose of copper (0.32 mM CuSO4).