Bacterial cell surfaces possess negative electrostatic charge due to ionized phosphoryl and carboxylate substituents on outer cell envelope macromolecules which are exposed to the extracellular environment. Several findings have suggested that antimicrobial agents can cause increase or decrease in hydrophobicity or charge of the surface of some bacteria such as S. aureus. The aim of this research was to see whether changes in surface electrostatic charge (SEC) of S. aureus which exposed to various concentration of antibiotic (tetracycline) can be used as criteria for sensitivity or resistant to that antibiotic. For this, minimum inhibitory concentration of tetracycline was determined using microdilution methods and according to MIC, SEC of S. aureus was measured by conductometery. Results showed that on MIC of 0.782 μg/ml conductivity remained constant in the range of 1460-1490 microsiemens while in the absence of antibiotic increased up to 1900 µS. Experiment was repeated with wide range of antibiotic concentrations (0.039-10 μg/ml) in different time intervals (15 min, 105 min, 195 min, 285 min and 24 h). Conductivity did not change significantly in a defined antibiotic concentration in different times sampling. However higher concentration of antibiotics had more effect on SEC of S. aureus decreased from 2070 µS in 0.156 μg/ml to 1391 and 810 µS in 5 and 10 μg/ml respectively after 195 min exposure to antibiotics. Controls were routinely examined in parallel for all tests and comparison made by statistical analysis and also in the form of graphs. In conclusion, the results demonstrate that alterations in SEC can be considered as a rapid and sensitive means of measuring the effect of antimicrobials on bacteria. However more data has to be collected on the effect of different antibiotics on SEC of other bacteria to reach a comprehensive and logical implications for application of such tool.

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