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&#13; South Texas. <b>Problem statement: </b>Pesticides may become environmental contaminants due to&#13; overuse, runoff and other mechanisms and may impact non-target organisms and ecosystems.&#13; Pesticides may be degraded by indigenous microorganisms or by abiotic means. In this study, waterborne&#13; bacteria from agricultural canals were examined to assess potential atrazine and oxamyl&#13; degradation in the Lower Rio Grande Valley.<b> Approach: </b>Water samples were collected March 2007&#13; and June 2009 and inoculated onto agar media containing either atrazine or oxamyl to estimate&#13; densities of atrazine-tolerant and oxamyl-tolerant bacteria. Bacterial isolates were characterized&#13; morphologically by visually observing colony shape and size and by gram-staining. Commercial test&#13; strips and microplates were used to differentiate biochemical and nutritional capabilities of bacteria&#13; and an inhibition disk assay was employed to determine pesticide sensitivity. <b>Results:</b> Average density&#13; of atrazine-tolerant bacteria was 2,233 cfu mL^-1 in March 2007 and 12, 845 cfu mL^-1 in June 2009.&#13; Average density of oxamyl-tolerant bacteria was 330 cfu mL^-1 in 2007 and 1,158 cfu mL^-1 in&#13; 2009.66.7% of bacteria were Gram-negative. Most isolates were resistant to atrazine or oxamyl&#13; regardless of which pesticide medium they were originally grown. Only 2 of 30 tested isolates&#13; displayed intermediate and sensitive inhibition phenotypes, respectively, to oxamyl. Biochemical&#13; profiles were generally 70% or greater in similarity but still displayed diverse phenotypes. About half&#13; of isolates exhibited a unique biochemical phenotypic profile. Microbial communities in the canals&#13; could metabolize a variety of organic compounds and demonstrated high carbon substrate utilization&#13; and activity. <b>Conclusion:</b> Overall, indigenous pesticide-tolerant microorganisms were present in lowto-&#13; moderate densities, displayed diverse phenotypes and were able to use many organic substrates.&#13; This may increase the likelihood that atrazine and oxamyl compounds will be degraded in the Lower&#13; Rio Grande Valley