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.

Background and Aim: Uropathogenic Escherichia coli (UPEC) is commonly involved in urinary tract infections (UTIs), which are generally treated with antibiotics. However, the emergence of multidrug-resistant (MDR) strains of UPEC has made the treatment difficult. There is thus a need to continuously assess their sensitivity to antibiotics. This study aimed to determine the antibiotic resistance patterns and MDR phenotypes of UPEC strains isolated from children diagnosed with UTIs at the Russian Children's Clinical Hospital in Moscow, Russia. Materials and Methods: Kirby–Bauer's disc diffusion method was used to study the sensitivity to antibiotics of 106 UPEC isolates from urine specimens from children (aged from 9 months to 18 years old) diagnosed with UTIs. The results were interpreted in accordance with the Clinical and Laboratory Standards Institute guidelines and the correlations of variables with the degree to which each antibiotic inhibited the UPEC strains in terms of diameter on the disc were determined using Spearman's rank correlation test. A t-test and principal component analysis were performed to visualize the correlations of the susceptibility of UPEC to antibiotics with the age and sex of the patients. Statistical significance was set at p≤0.05. Results: Among the 106 UPEC strains tested, none (0%) showed resistance to fosfomycin (FO), while 84 (79.2%) were resistant (R) to at least one antibiotic. The highest rates of resistance were observed to amoxicillin (69.8%), ampicillin (62.3%), cefazolin (39.6%), trimethoprim (TR) (37.7%), ceftriaxone (34.9%), and tetracycline (33.0%). Interestingly, 22 (20.8%) strains were R to imipenem. UPEC isolates from males aged 1-6 years were more R to antibiotics than those from the other groups, with the exception of TR, to which UPEC isolates from females aged 13-18 years old were less sensitive (S). The multidrug-resistance (MDR) index ranged between 0.00 and 0.75 and we found that more than a quarter of UPEC (31/106) had an MDR index ≥0.5 and only 22 (20.7%) strains were S to all antibiotics tested (MDR index=0). Finally, Spearman's rank correlation test showed that, with the exception of FO, there were correlations between the inhibition diameters of all other antibiotics. Conclusion: FO is the only antibiotic to which all UPECs were S and may be suggested as the first line of treatment for UPEC. Further research is needed to continue monitoring antibiotic resistance and to investigate the genetic features associated with such resistance observed in this study.