Augmentation of antibiotic resistance in Salmonella typhimurium DT104 following exposure to penicillin derivatives

Antibiotic resistance in pathogenic bacteria has been a problem in both dev eloped and developing countries. This problem is especially evident in Salm onella typhimurium, one of the most prevalent foodborne pathogens. While pe rforming in vitro gentamicin protection-based invasion assays, we found tha t certain isolates of multiresistant S. typhimurium earn be 'induced' to ex hibit new resistance profiles. That is, bacteria become resistant to a wide r range of antibiotics and they also exhibit quantitative increases in MIC values for antibiotics that were part of their pre-induction antibiograms. This 'induction' process involves growing thr:bacteria to stationary phase in the presence of antibiotics such as ampicillin, amoxicillin or ticarcill in. Since the isolates studied exhibited resistance to ampicillin, amoxicil lin and ticarcillin prior to exposing the bacteria to these antibiotics, th e observed phenomenon suggests that resistant Salmonella not only have a se lective advantage over non-resistant Salmonella but their resistance phenot ypes can be accentuated when an inappropriate antibiotic is used therapeuti cally. Published by Elsevier Science B.V.