Research Product

The population of a strain of Escherichia coli that was resistant to nalidixic acid and streptomycin declined rapidly in samples of sterile and nonsterile Cayuga Lake water and reached an undetectable level in nonsterile water at 24 and 72 h when counted on cosin-methylene blue (EMB) agar and half-strength Trypticase soy agar, respectively. In sterile lake water amended with 10 ug of amino acids per ml or 0.1 M phosphate, E. coli multiplied exponentially for more than 24 h. The addition of Rhizobium leguminosarum biovar phaseoli to unamended sterile lake water prevented the decline of E. coli and its addition to amended sterile lake water prevented E. coli multiplication. The data suggest that E. coli cells grown on rich media suffer a shock when introduced into lake water because of low hypotonicity, the indigenous competing flora, or both. This shock is prevented by either phosphate buffer or by amino acids at low concentration. The shocked bacteria formed colonies on half-strength Trypticase soy agar. Depending on environmental conditions, the presence of a second organism either has no effect or results in an increase or decrease in E. coli numbers.