To evaluate the antibacterial potential of pioglitazone, a member of the thiazolidinediones class of drugs, against Gram-positive (Streptococcus pneumoniae) and Gram-negative (Escherichia coli and Klebsiella pneumoniae) bacteria.Susceptibility testing was done using the antibiotic disk diffusion method and the minimal inhibitory concentration (MIC) of pioglitazone was measured according to the broth micro incubation standard method.Pioglitazone induced a dose-dependent antibacterial activity in which the optimal concentration was 80 μM. Furthermore, results indicated that while E. coli was sensitive (MIC = 31.25 ± 3.87 mg/L) to pioglitazone-induced cytotoxicity, S. pneumoniae and K. pneumoniae were resistant (MIC = 62.5 ± 3.77 mg/L and MIC = 62.5 ± 4.14 mg/L, respectively). Moreover, pretreatment of bacteria with a suboptimal concentration of pioglitazone (40 μM) before adding amoxicillin, cephalexin, co-trimoxazole, or ciprofloxacin enhanced the antibacterial activity of all agents except co-trimoxazole. This enhancing effect was particularly seen against K. pneumoniae.These results indicate the possibility of a new and potentially important pioglitazone effect and the authors' ongoing studies aim to illustrate the mechanism(s) by which this antibacterial effect is induced.