Abstract: In the present study， sensitivity of Escherichia coli strains to tetracycline （TE） was detected by broth micro-dilution method， and tetA， tetB genes of E.coli strains were detected by PCR methods to explore the possible mechanisms. It was shown that addition of different concentrations of nitrogen and phosphorous induced resistance of E.coli to TE. But， there was no significant correction between TE resistance of E. coli and nitrogen or phosphorus dose. For 37 strains with high TE resistance， the detection rate of tetA gene was 100%; for 66 strains with lower TE resistance， the detection rate of tetA gene was 1667%; and no tetA gene was detected in the strains sensitive to TE. Interestingly， no tetB gene was detected in either TE resistant or sensitive E.coli strains， which indicated that tetA gene， other than tetB gene， might be involved in the nitrogen and phosphorus induced TE resistance of E.coli.

Key words: microcosm, Escherichia coli, tetracycline