Objectives: The antibacterial effect of the organo-tellurium compound AS101 on the Gram-negative bacterium Enterobacter cloacae is shown in this study for the first time. Methods: The antimicrobial effect of the drug was shown by inhibition of growth, by inhibition of biofilm formation and by its ability to penetrate the bacterial cell and to cause damage and ultrastructural changes. Results: AS101 was found to be a bactericidal drug with MICs and MBCs of 9.4 mg/L. It inhibits bacterial growth

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... nd causes a six orders of magnitude decrease in viability in a protein-rich medium, but not in a protein-poorer medium, unless 2-mercaptoethanol is added. Subinhibitory concentrations inhibit motility and biofilm formation. AS101 enters the bacterium through its porins and causes bacterial damage to Na + /K + pumps and leakage of potassium, phosphorous and sulphur. Ultrastructural changes within the bacterial cell and on its surface demonstrate an incomplete surface with a concavity in the centre that looks like a hole from which aggregates are liberated as well as cell lysis. Conclusions: AS101 has antibacterial activity, which may be useful against E. cloacae and other species of Enterobacteriaceae as a substitute for current antibiotics that have become ineffective due to increasing bacterial resistance.