Thirteen-year survival study of an environmentalEscherichia coli in field mini-plots

Abstract

A multiple-antibiotic resistantE. coli was applied to rye-grass covered field mini-plots to simulate point-source contamination. Using three mini-plots for testing and a fourth as a control, the ability of the tracer bacterium to survive under field conditions was studied. Three test plots each received separately 10⁷, 10⁸, or 10¹⁰ cfu mL⁻¹ E. coli grown for 24 h. in 5 L one-third strength Tryptic soy broth. In Phase I of the study, it was determined that the tracer disappeared from leaf surfaces of rye-grass covering the plots after 41 days. In Phase II, determination of the presence of the tracer in the top 2″ (5 cm) of soil after two months elapsed time indicated that tracer cfu/g dry wt. of soil had declined five, three, and three-logs for test plots 1,2, and 3. In Phase III, subsurface soil sampling using a soil auger on the three test minei-plots indicated the tracer had penetrated through the top-soil and into the underlying B horizon (20 to 50 cm down). In Phase IV, detailed sampling by excavation of the subsurface soil Horizons of the third test mini-plot showed that the tracer had also penetrated through the hardpan (C Horizon) located 0.6 m below the surface to enter the groundwater (1.06 m deep) (Phase V).E. coli counts fell precipitously to 10³ cfu g⁻¹ in soil and then, in the groundwater at the groundwater-soil interface, persisted at a concentration of 10³ cfu 100 mL⁻¹ for 2 yr. As time past, tracer counts fell to 145 cfu/100 mL in 6 yr. rose to 820 cfu 100 mL⁻¹ in 1986 (8 yr elapsed time), and then fell to 25 cfu 100 mL⁻¹ in 1991 after 13 yr. Serotyping of 1986E. coli isolates indicated that 62% were of the original tracer serotype (0.128:B12) while only 43% of the 1991 isolates were of the same serotype. The penetration rate of the tracer down through the mini-plot soil into the groundwater was 0.02 m day⁻¹ while downslope dispersion occurred at an estimated rate of 1.0 m day⁻¹. The implications of the above findings are discussed.