The present invention is to provide an agent for preventing and treating drug-resistant bacterial infection for livestock/fowls or fish and shellfish, using particular microbial agents as active components, without using synthetic antibacterial substances or antibiotics, and a method for preventing and treating its infection. By using lactic acid bacteria, their dead bacteria or treated substances thereof, or Mygasphaera elsdenii as active components, for an agent for preventing and treating drug-resistant bacterial infection for livestock/fowls or fish and shellfish carrying or being infected by drug-resistant bacteria such as Vancomycin, particularly by using Enterococcus faecalis, Enterococcus faecium as lactic acid bacteria, the above mentioned object was resolved.