[Experimental study on phenotypic conversion of clinical chloromycetin-resistant strains of E. coli to drug-sensitive strains by using EGS technique in vitro].

Objective: To explore the possibility of phenotypic conversion of clinical chloromycetin (Cm)-resistant isolates of E.coli to drug-sensitive ones with external guide sequences (EGS) in vitro.

Methods: Recombinant EGS plasmids directed against Cm acetyl transferase (cat) and containing kanamycin (Km) drug-resistance gene and control plasmids only containing kanamycin-resistance gene without EGS were constructed. By using CaCl(2) method, the recombinant plasmids were introduced into the clinically isolated Cm-resistant E.coli strains. Extraction of plasmids and PCR were applied to identify the EGS positive clones; The growth rate in liquid broth culture of Cm-resistant bacteria after EGS containing plasmid transformation was determined by spectrophotometer A(600). Drug sensitivity was tested in solid culture by using KB method.

Results: Transformation studies were carried out on 16 clinically isolated Cm-resistant E.coli strains with pEGFP-C1-EGS + cat1 + cat2 recombinant plasmid. Transformants were screened on LB-agar plates containing Km after transformation using EGS. In 4 tested strains of them, transformants with specific EGS plasmid showed growth inhibition when grown in liquid broth culture containing 100 approximately 200 micro g/ml of Cm. They were sensitive to Cm on LB-agar plates containing 100 approximately 200 micro g/ml of Cm in drug-sensitivity test. Extraction of plasmids showed the existence of EGS bands. PCR amplified products of EGS. The above facts indicated that the 4 strains out of the 16 clinical isolates had been converted to drug-sensitive phenotype, and Cm-resistant clinically isolated E. coli resumed sensitivity to Cm.

Conclusion: EGS has the capability of converting the phenotype of clinical drug-resistant isolates to drug sensitivity.