A New Prospective of Colistin Resistance Profile of Some <i>Staphylococcus Species</i> Isolated from Iraqi Wound Fractured Foot

Sulbactam belongs to β-lactamase inhibitors that are more commonly used for Gram-negative bacteria than Gram-positive bacteria. In this study, nine different species of Staphylococcus (S. aureus, S. epidermidis, S.hominis, S. lentus, S. pseudintermedius, S. lugdunensis, S. haemolyticus, S. sciuri , and S. warneri. were isolated from wounded-fractured feet of Iraqi patients. The spectrum of activity of sulbactam and colistin against all isolates was evaluated. Surprisingly, among S. aureus isolates, 5 (12) isolates were sensitive to colistin, whereas, all bacterial isolates were completely resistant to sulbactam. Here, our study revealed that combining colistin and sulbactam against colistin-sensitive Staphylococcus may not enhance the susceptibility of bacterial species, as the MIC values of the combination remain unchanged and even increased in some species, indicating the potential activity of colistin alone. Although, the MIC results showed there was no combination effect of colistin and sulbactam, a time killing assay was performed resulting in complete inhibition of colistin-sensitive Staphylococcus species, except for S. aureus and S. aureus standard strain (NCTC 13656). To further examine the effect of colistin, a study of colistin's autolysis activity was performed by increasing the membrane's permeability and leakage of intracellular fluids. Leakage of divalent positive charge (Ca2+ and Mg2+) from sensitive colistin-treated isolates was measured by cytochrome C assay, which resulted in a remarkable reduction in cations on the cell surface. To our knowledge, this is the first study to identify the potential activity of colistin against clinically isolated Staphylococcus species other than S. aureus which showed a pattern of sensitivity against colistin alone.