Pesticide-Tolerant Bacteria Isolated from Agricultural Canals in the Lower Rio Grande Valley of South Texas

Abstract

Atrazine and oxamyl are commonly-used pesticides in the Lower Rio Grande Valley of South Texas. Problem statement: Pesticides may become environmental contaminants due to overuse, runoff and other mechanisms and may impact non-target organisms and ecosystems. Pesticides may be degraded by indigenous microorganisms or by abiotic means. In this study, waterborne bacteria from agricultural canals were examined to assess potential atrazine and oxamyl degradation in the Lower Rio Grande Valley. Approach: Water samples were collected March 2007 and June 2009 and inoculated onto agar media containing either atrazine or oxamyl to estimate densities of atrazine-tolerant and oxamyl-tolerant bacteria. Bacterial isolates were characterized morphologically by visually observing colony shape and size and by gram-staining. Commercial test strips and microplates were used to differentiate biochemical and nutritional capabilities of bacteria and an inhibition disk assay was employed to determine pesticide sensitivity. Results: Average density of atrazine-tolerant bacteria was 2,233 cfu mL^-1 in March 2007 and 12, 845 cfu mL^-1 in June 2009. Average density of oxamyl-tolerant bacteria was 330 cfu mL^-1 in 2007 and 1,158 cfu mL^-1 in 2009.66.7% of bacteria were Gram-negative. Most isolates were resistant to atrazine or oxamyl regardless of which pesticide medium they were originally grown. Only 2 of 30 tested isolates displayed intermediate and sensitive inhibition phenotypes, respectively, to oxamyl. Biochemical profiles were generally 70% or greater in similarity but still displayed diverse phenotypes. About half of isolates exhibited a unique biochemical phenotypic profile. Microbial communities in the canals could metabolize a variety of organic compounds and demonstrated high carbon substrate utilization and activity. Conclusion: Overall, indigenous pesticide-tolerant microorganisms were present in lowto- moderate densities, displayed diverse phenotypes and were able to use many organic substrates. This may increase the likelihood that atrazine and oxamyl compounds will be degraded in the Lower Rio Grande Valley.