Introduction. Since their discovery, antibiotics have played an important role in health care. However, the increasing emergence of antibiotic resistance among a variety of microbial pathogens stimulates intensive research efforts with the aim to identify alternative therapeutic approaches (1). Macrocyclic complexes have attracted attention because of their pharmacological properties, i.e. toxicity against bacterial and fungal growth. It was reported that complexes of chromium(III) would have a role in the microbiological application to reduce the virulence and antibiotic resistance (2). In the present work we report the biological activities of chromium(III) complexes against Escherichia coli and Staphylococcus aureus.

Materials and methods. Two test bacterial strains viz S.aureus ATCC 29213 and E.coli ATCC 25922 were considered for the determination of Minimum Inhibitory Concentration (MIC) of the synthesized complexes by the photochemistry group of INFIQC-CONICET, Dpto. de Fisicoquímica (3). The MIC of the compounds was evaluated by using the standard tube dilution method on Mueller Hinton broth. Bacterial growth was observed at 18 h of incubation, following the indications of the Clinical and Laboratory Standards Institute (CLSI) (4). The MIC of the compounds were ranged from 0.004 to 2 μg/mL. In addition, the MIC values for the ligands 1, 10-phenanthroline (phen) and dipyrido [3,2-a:2´,3´-c]-phenazine (dppz) was determined. Dilutions of ciprofloxacin (0.008 to 4 μg/mL) were used to compare the activity. The lowest concentration of the compound that prevented bacterial growth was considered to be the MIC.

Results. Two chemically macrocyclic complexes and their ligands were screened for their in vitro antibacterial activity against S.aureus and E.coli. [Cr(phen)2(dppz)]3+ was found to be the most potent. MIC values obtained for the chromium complexes were in the range of those obtained with ciprofloxacin. E.coli was found to be more susceptible to [Cr(phen)2(dppz)]3+ than S.aureus, with a MIC for E.coli of 0.125 μg/mL and a MIC for S.aureus of 0.5 μg/mL while the MIC for E.coli was 0.25 μg/mL and for S.aureus 1 μg/mL for [Cr(phen)3]3+. The phen and dppz ligands did not show activity against the two bacterial strains.

Conclusions. The work provides the first evidence of antimicrobial activity of the synthetized metallomolecules [Cr(phen)3]3+ and [Cr(phen)2(dppz)]3+. The synthesized metal complexes of chromium(III) exhibited higher antimicrobial activities against Escherichia coli and S. aureus than the ligands alone.

Our results indicate that biologically active chromium(III) complexes have biological toxic activity against Gram negative and Gram positive bacteria.