Technical Abstract: Antibiotic resistance in pathogenic bacteria has been a problem in both developed and developing countries. This problem is especially evident in Salmonella typhimurium, one of the most prevalent foodborne pathogens. While performing in vitro gentamicin protection-based invasion assays, we found that certain isolates of multiresistant S. typhimurium can be "induced" to exhibit new antibiograms. That is, bacteria not only become resistant to a wider range of antibiotics, but they also exhibit quantitative increases in MIC values for antibiotics that were part of their intrinsic antibiograms. The "induction" process involves growing the bacteria to stationary phase in the presence of beta-lactam antibiotics, such as ampicillin, amoxicillin, or ticarcillin. This phenomenon suggests that resistant Salmonella not only have a selective advantage over nonresistant Salmonella, but their resistance phenotypes can be potentiated when an inappropriate antibiotic is used therapeutically.