Abstract

 E. coli and enterococci have traditionally been used as indicators of sewage contamination, partly because of the assumption that fecal bacteria cannot grow in the nature. We examine tropical and temperate soils to determine whether these bacteria can grow in situ and to explore factors that limit re-population. In Hawaii, soil temperature and moisture are conducive for E. coli growth throughout the year, while in Indiana soil conditions are suitable only seasonally. In Hawaii, soil extract agar supported E. coli growth, demonstrating that there are enough nutrients in the soil to support multiplication of E. coli. Demonstrating multiplication of E. coli in natural soil is difficult because indigenous soil microorganisms are present in higher concentrations and are more efficient in obtaining nutrients. However, when bile salt was added to natural soil, indigenous soil microorganisms were inhibited and growth of E. coli could be observed. Moreover, growth of E. coli was evident the bacteria were added to cobalt-irradiated soil but not in native, non-sterilized soil. E. coli and enterococci were ubiquitous and numerous in Indiana forest sandy soils. In Indiana soils, E. coli repopulation of in situ heat-treated temperate soil was rapid and density was sustained for several months. In these experiments, moisture and temperature limited E. coli occurrence. Re-wetting of soils increased growth of E. coli. In summary, similar results showing growth potential of E. coli in tropical and temperate soils indicate that more studies should focus on the ability of E. coli to multiply in natural soils especially when their presence can act as non-point sources of bacterial contamination of water. These results may further influence the current interpretation that presence of E. coli in environmental samples represents direct evidence of fecal contamination.