Prolonged Exposure to Antimicrobials Induces Changes in Susceptibility to Antibiotics, Biofilm Formation and Pathogenicity in Staphylococcus aureus

Introduction: Frequent exposure to certain biocidal agents such as hypochlorous acid (HOCl), triclosan and benzalkonium chloride (BAC) has been reported to induce significant changes in Staphylococcus aureus. However, very few studies of this type have been conducted with conventional antimicrobials. Aim: The current investigation aimed to explore the phenotypic changes (susceptibility to antibiotics, biofilm formation and relative pathogenicity) that occur in S. aureus after recurrent exposure to antimicrobials. Methods: We compared the effects of long-term exposure to ampicillin, cefazoline, kanamycin and silver nanoparticles (AgNPs) on their susceptibility to antibiotics, biofilm formation, growth rate and pathogenicity in Staphylococcus aureus ATCC 6538. The minimum inhibitory concentrations (MIC) were determined using the microplate mircodilution method and the bacteria were exposed to increasing concentrations of each antimicrobial (MIC/2 to MIC) prepared in the BHIB for 8 days. The sensitivity of bacteria to antibiotics was assessed using the Kirby-Bauer disc diffusion method, the biofilm formation with crystal violet bacterial attachment assay and relative pathogenicity was assessed through a Galleria mellonella waxworm model. Results: The data in this investigation indicate that long-term exposure to antimicrobials may induce several changes in S. aureus. The exposure to ampicillin induced resistance to ceftazidime, tetracycline and ceftriaxone while the susceptibility to ceftazidime decreased in bacteria exposed to cefazolin and Kanamycin. Meanwhile, exposure to AgNPs induced some changes in susceptibility to trimethoprim and ceftazidime without causing resistance. Similarly, the strains exposed to ampicillin and kanamycin grew more rapidly and produced more biofilms than the control strains whereas the strains exposed to the AgNPs produced less biofilms. On G. melonella model, cefazolin seems to have attenuated the pathogenicity while the 3 other strains were more pathogenic than the controls. Conclusion: Long term exposure of S. aureus to antibiotics and AgNPs induces several changes in susceptibility to other antibiotics, growth rate, biofilm formation and pathogenicity; and these changes should be taken into account when choosing antibiotics for treatment of diseases caused by S. aureus.