sahar Honarmand Jahromy - Department of Microbiology, Islamic Azad University, Varamin-Pishva Branch, Iran.
fatemeh Noorbakhsh - Department of microbiology,Islamic Azad university ,Varamin- Pishva branch,Iran
Omid Hosseini - Shahid Beheshti University of Medical Sciences, Tehran, Iran
Abolfazl Sajdeh - Department Microbiology, Islamic Azad university, Varamin- Pishva branch,Iran

Background:Staphylococcus aureus is the most common pathogen causing nosocomial infections that their treatment by antibiotics is difficult. Biofilm potential of Staphylococcus aureus is considered to be one of the main reasons for its survival and is influenced by many environmental factors. Objectives:The purpose of this study was to evaluate the effect of pH on biofilm formation of S. aureus and its visualization by atomic force microscope (AFM). Methods: ۱۰۰ Staphylococcus aureus strains were isolated from clinical specimens of patients who were referred to Milad Hospital, Tehran and diagnosed by biochemical tests. A microtiter plate method was used to determine the strength of biofilm formation under acidic and alkaline pH. The effect of pH on biofilm formation was visualized by using AFM. Results:At pH ۷ and ۹ the biofilm formation of Staphylococcus aureus strains was at highest level, ۷۱.۹% and ۷۸.۱% but in pH۳ and ۱۲ was at lowest rate ۳۵% and ۳۵.۴%. There is a significant association between pH and biofilm formation. AFM microscopy analysis of effect of pH ۳ and ۱۲ in S. aureus showed reduction biofilm structures. In PH ۷ and PH ۹ more biofilm and less planktonic cells were observed. Conclusions: The increase or decrease in pH value was involved in decrease of biofilm formation. The AFM was a useful tool for visualization of Staphylococcus aureus biofilm.

pH, biofilms, Staphylococcus aureus, Clinical isolates, Atomic force Microscope