Influence of cadmium on resistance to antibiotics in salmonellæ isolated from pigs

Repeated cultivations (4 passages) of salmonellæ (18 strains) resistant to cadmium, streptomycin and β-lactam antibiotics in Müller-Hinton Broth (MHB) and Mac-Conkey Broth (MCB) without and with CdSO₄ (2, 20 and 100 mg/L) showed a higher toxic effect of cadmium in MCB. The strains survived at CdSO₄ 100 mg/L in MHB for four transfers, in MCB only a single transfer. In dependence on the medium used and amount of metal added, the increase of resistance to antibiotics was different. In MHB, the same levels of resistance to carbenicillin and streptomycin were induced by CdSO₄ (20 and 100 mg/L), in MCB it was by 2 and 20 mg/L. Simultaneous stop of the growth of a control cultureS. typhimurium with chromosomal resistance to streptomycin, isolates with and without plasmid in MCB which contained CdSO₄ 100 mg/L, and the results of conjugal transfer of resistance suggest that changes of resistance to antibiotics were not medated by determinants of resistance to antibiotics. The binding of cadmium to outer membrane protein can cause a decreased permeability to these antibiotics as a resistance mechanism.