Suppressive Effect of Lactobacillus casei on the Drug Resistance Transfer in Escherichia coli

In the mixed culture of Escherichia coli S-58 and S-42 strains, CP^r gene is thought to be transferred from the former to the latter by conjugation. The cultured supernatant of Lactobacillus casei YIT 0003 exhibited a considerable suppressive effect on the drug resistance transfer between the two E. coli strains. However,it failed to inhibit the R-factor transfer between E. coli W 3630 and ML 1410. This factor in the supernatant was heat stable, ethanol-precipitable, and the molecular weight was estimated to be approximately 6,000. This factor probably constitutes a non-dialyzable complex with some other substances in the culture fluid.