Background & Objective: Staphylococcus aureus, especially methicillin-resistant S. aureus (MRSA), represent serious nosocomial and community infections. Biofilm formation as an important virulence factor may be affected by sub-inhibitory levels of antibiotics. Few studies examined the effects of all therapeutic antimicrobial agents on clinical S.aureus. The current study aimed at observing the inducing and reducing effects of antibiotics, commonly used to treat staphylococcal infections on the production of staphylococcal biofilm. Methods: Four MRSA (1ATCC and 3 clinical) and 1 methicillin-susceptible Staphylococcus aureus (MSSA) strains with biofilm forming ability, evaluated by the Congo red agar (CRA) plate test, were employed. Biofilm formation was measured by crystal violet microtiter plate assay. Cefazolin, rifampicin, vancomycin, oxacillin, clindamycin, cotrimoxazole, minocycline, linezolid, azithromycin, and clarithromycin were added to wells ranging from 0.06to 128 µg/mL (1× to 1/1024 MIC dependent on the MIC value of each strain). Results: The current study showed that azithromycin and vancomycin had a significant inducing effect on biofilm formation. In contrast, linezolid, cefazolin, and clarithromycin, and in the second place, clindamycin and minocycline could inhibit the level of biofilm production in the sub-minimal inhibitory concentrations. Conclusion: The findings demonstrated that the biofilm formation as an important virulence factor may be affected by the subinhibitory levels of antibiotics.

Background: Bacterial burn infections are one of the serious infections and since their treatment with antibiotics is very difficult, infection control in such wounds is very important. Therefore, methods should be sought to enhance the effects of antibiotics and also reduce the resistance to them. It seems that probiotics have antimicrobial features and are effective in wound healing. Objectives: The current study aimed at evaluating and comparing the antimicrobial effects of antibiotics and probiotics, as well as their combination on bacterial burn infections. Methods: Thirty clinically resistant Pseudomonas aeruginosa species isolated from hospitalized patients with burn wounds were collected. Antibacterial activity of five antibiotics and 11 probiotic strains, and also their combination were evaluated by disk diffusion method. One-way ANOVA and Tukey post hoc test were used for data analysis. Results: It was found that the effect of inhibitory zone in combination use of tetracycline + a probiotic strain was more than using the antibiotic and probiotic alone. Also, the current study found that among the probiotics, Lactobacillus plantarum 299v had the highest effect, although not significant, on resistant P. aeruginosa. The current study explained that the inhibitory effect of L. plantarum 299v was significantly higher than that of ciprofloxacin (P = 0.009). In addition, antibacterial activity of gentamicin + L. salivarius (ES1) was significantly higher compared with that of gentamicin + L. routeri (P = 0.01). Conclusions: The current study explained that probiotics had a useful potential inhibitory effect on the growth of the pathogens. The study showed that in most cases, inhibitory zones of probiotics were greater than those of antibiotics as well as combination of antibiotic + probiotic. But, there was an exception in tetracycline, which had synergistic effect with probiotics. Conversely, in the cases of imipenem and chloramphenicol, addition of probiotics had antagonistic effects. Further studies are needed to describe different results. Therefore, it seems that the type of antibiotics and probiotics are important to create the synergistic or antagonistic effects.