الفهرس 
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Abstract
A total of 120 Staphylococcus aureus isolates were collected from several Tanta University teaching hospital departments. Antibiotic susceptibility of the recovered isolates was performed. Antibiotics used were penicillin, oxacillin, cefoxitin, azithromycin, chloramphenicol, trimethoprim-sulfamethoxazole, linezolid, levofloxacin, tetracycline, clindamycin, rifampicin, vancomycin, ciprofloxacin, gentamicin, and amikacin. from antibiotic susceptibility testing (AST) multidrug resistanc (MDR) isolates and 34 methicillin-resistant Staphylococcus aureus (MRSA) isolates were identified. The multi-antibiotic resistant (MAR) index calculation of the recovered isolates identified 56 with a MAR index above 0.2. Biofilm and virulence factors were screened by MDR isolates using crystal violet, skimmed milk agar, and blood agar. Out of 70 MDR 21 isolates were strong, 38 isolates were moderate and 11 isolates were weak biofilm forming. Also, there were 19 and 14 that were protease and hemolysin isolates producers respectively. Isolates that produce protease and/or hemolysin were screened for toxic shock syndrome toxin 1 gene (tst) using PCR which revealed its presence in 2 isolates out of 20 tested isolates. Through growth curve construction it was revealed that 1/2 minimum inhibitory concentration (MIC) of oxacillin (OX), ampicillin, (AMP) ceftazidime (CAZ), cefepime (CEF), ceftriaxone (CTR), and cefazoline (CZ) affected bacterial growth while, in the majority of cases, 1/4 and 1/8 MICs have a negligible effect. Treatment of the isolates with sub-MICs of the selected beta-lactams generally resulted in a notable decrease in the production of biofilms which was detected using crystal violet microtitration, phenol sulfuric acid, visual test tube settelling method, and confocal laser scanning microscope. Protease, hemolysin, and coagulase production were also generally reduced; this reduction was observed by skimmed milk agar media, spectrophotometric techniques, and coagulation titer determination. The RT-qPCR analysis revealed that the detected reduction in biofilm and virulence factors production was associated with a reduction in agrA, icaA, coa, and tst gene expression. Elongation, increased size, and changed permeability of the cell wall and cell membrane of OX sub-MIC treated S 33 isolate were detected using scanning and transmission electron microscope. Finally, wound infection rat models were performed to illustrate the effect of sub-MIC on rats’ wound healing, and from our data, it was observed that S 33 infections treated with OX either at 1/4 or 1/8 MIC increased the rate of wound healing. The histological analysis also revealed that the treated wound had higher levels of collagen deposition, granulation tissue, and epidermal regeneration than the untreated one. Conversely, rats given 1/8 MIC of CTR and infected with S 8 showed reduced rat wound healing, as evidenced by decreased granulation tissue, collagen deposition, and epidermal regeneration. In conclusion, the use of sub-MIC of beta-lactams might be potentially helpful as an antivirulence strategy to combat MDR S. aureus infections. Such a strategy exhibits the advantage of decreasing the antibiotic selective pressure and consequently antibiotic resistance. However, further large-scale studies should be conducted to exclude possible undesired results that may occur.