Culture media, DMSO and efflux affect the antibacterial activity of cisplatin and oxaliplatin

Cisplatin was originally discovered through its antibacterial action and subsequently has found use as a potent broad‐spectrum anticancer agent. This study determines the effect of growth media and solvent on the antibacterial activity of cisplatin and its analogue, oxaliplatin. Escherichia coli MG1655 or MG1655 ΔtolC was treated with the platinum compounds under different conditions and susceptibility was determined. Our results showed that DMSO reduced the activity of cisplatin by fourfold (MIC 12·5 mg l(−1)) compared with 0·9% NaCl‐solubilized cisplatin (MIC 3·12 mg l(−1)) when tested in MOPS. Surprisingly, complete loss of activity was observed in Mueller–Hinton Broth II (MHB II). By supplementing MOPS with individual components of MHB II such as the sulphur‐containing amino acids, l‐cysteine and l‐methionine, individually or in combination reduced activity by ≥8‐fold (MIC ≥25 mg l(−1)). Oxaliplatin was less active against E. coli (MIC 100 mg l(−1)) but exhibited similar inactivation in the presence of DMSO, MHBII or MOPS spiked with l‐cysteine and l‐methionine (MIC ≥400 mg l(−1)). Our data suggest that the antibacterial activity of cisplatin and oxaliplatin is modulated by both choice of solvent and composition of growth media. We demonstrate that this is primarily due to sulphur‐containing amino acids cysteine and methionine, an essential component of the recommended media for testing antimicrobial susceptibility, MHBII.