Antibacterial and Antibiofilm Screening of New Platinum ( IV ) Complexes with some S-Alkyl Derivatives of Thiosalicylic Acid

This investigation showed influence of 5 new Pt(IV) complexes on 16 strains of bacteria. Antibacterial activity was tested using microdilution method with resazurin while antibiofilm activity was observed by tissue culture plate method and antibiotic doxycycline was used as positive control. The results were expressed as minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and biofilm inhibitory concentration (BIC). The complexes were labeled from C1 to C5. The best result on Gram positive bacteria was obtained with C1 and MIC on Staphylococcus aureus ATCC 25923 was ˂7.81 μg/ml. Bifidobacterium animalis subsp. lactis (probiotic) was sensitive to C2 (MIC at 15.625 μg/ml). The best sensitivity on Gram negative bacteria was observed on Escherichia coli ATCC 25922 with C1, C2, C3 and C4, on Proteus mirabilis ATCC 12453 with C1, and on Pseudomonas aeruginosa with C2, C3 and C5 (all MICs at 250 μg/ml). The tested complexes were more efficient as antibiofilm agents and the best results were obtained with C2 acting against S. aureus and S. aureus ATCC 25923 biofilm. In conclusion, we noticed that the tested compounds exhibited promising properties as antibacterial agents and antibiofilm agents.


Antibacterial and Antibiofilm Screening of New Platinum(IV) Complexes with some S-Alkyl Derivatives of Thiosalicylic Acid Introduction
The interest in determining the influence of new metal complexes on microorganisms is increasing due to the growing pathogenic resistance.New synthesized Pt(IV) complexes were labeled as: C1 for Pt(S-bz-thiosal) 3 , C2 for Pt(Smet-thiosal) 3 , C3 for Pt(S-et-thiosal) 3 , C4 for Pt(S-pr-thiosal) 3 and C5 for Pt(S-buthiosal) 3 .Our goal was in vitro testing of those complexes, in order to obtain the antimicrobial results and for the first time the antibiofilm results of any Pt(IV) complexes.

The synthesis of complexes
S-benzyl derivative of thiosalicylic acid (for C1 and for: C2 S-methyl, C3 S-ethyl, C4 S-propyl, C5 S-butyl) in amount of 0.6 mmol was slowly added to the solution of 0.2 mmol (0.1 g) potassium-hexachloroplatinum(IV) with 10 ml of distilled water.
The reaction mixture was heated on a water bath with stirring for 3 h.During this period, small portions from a solution of LiOH (0.6 mmol with 10 ml of distilled water) were added.The precipitate of the complex was separated by filtration, rinsed with distilled water and dried in air.

Antibacterial activity
The test results of in vitro antimicrobial activity of Pt(IV) complexes are presented in Tables 1 and 2, showing only the strains that exhibited sensitivity.The detected values were in range from less than 7.81 up to more than 1000 μg/ml.For comparison, MIC and MBC values of doxycycline are also listed.Gram positive bacteria showed higher sensitivity than Gram negative bacteria.Significant sensitivity, between Gram positive bacteria in the presence of Pt(IV) complexes, showed Bifidobacterium animalis subsp.lactis, Bacillus subtilis, Staphylococcus aureus and Staphylococcus aureus ATCC 25923.The best result was obtained with C1 and MIC on S. aureus ATCC 25923 was ˂7.81 μg/ml.B. animalis subsp.lactis (probiotic) showed sensitivity with C1 (MIC at 62.5 μg/ml) and better with C2 (MIC at 15.625 μg/ml).MIC for the Gram negative bacteria was in the range from 250 to ˃1000 μg/ml.The best sensitivity showed Escherichia coli ATCC 25922 with C1, C2, C3 and C4, Proteus mirabilis ATCC 12453 with C1 and Pseudomonas aeruginosa with C2, C3 and C5 (all MICs at 250 μg/ml).
Comparing the results obtained for the Pt(IV) complexes with the results of corresponding ligands from which they were synthetized (Radić et al., 2012) it can be concluded that these complexes had better antibacterial activity than the ligands.Hegazy and Gaafar (2012) tested synthetized Pt(IV) complex on 10 pathogenic bacteria and had high efficiency against all the strains, including Salmonella sp., S. aureus and B. subtilis, while the Pt(IV) dithiocarbamate complexes investigated by Manav et al. (2006) were less active against E. coli, B. subtilis and P. aeruginosa.

Antibiofilm activity
The test was performed against 4 strains of bacteria to obtain the in vitro antibiofilm activity of Pt(IV) complexes presented in Table 3.The best results showed C2 acting against S. aureus and S. aureus ATCC 25923 biofilm and it was noticed that obtained values were lower than antibiotic values in this first antibiofilm testing of this kind of complexes.