Plasmid-mediated partial cross-resistance between ampicillin, mecillinam and cefamandole in Salmonella johannesburg and Salmonella typhimurium

Abstract

The in-vitro activity of mecillinam and cefamandole on ampicillin-sensitive and ampicillin-resistant strains of Salmonella johannesburg and Saim. typhimurium was studied by the agar dilution method. All the ampicillin-sensitive salmonella strains were found to be sensitive to mecillinam and cefamandole whereas partial resistance to mecillinam and cefamandole was observed in most ampicillin-resistant strains. There was a close correlation between resistance to ampicillin and resistance to mecillinam and cefamandole. In transfer studies, resistance to all three antibiotics was co-transferred to recipient strains of Escherichia coli and other salmonellae. Spontaneous loss of resistance to ampicillin in these recipient strains, either to ampicillin alone or together with resistance to other antibiotics, was accompanied by a loss of resistance to mecillinam as well as to cefamandole. The concurrent gain or loss of resistance to the above three antibiotics suggests that such resistance is coded on the same gene determinants on the plasmid, and a plasmid-mediated β-lactamase might be responsible.