Comparison of antimicrobial susceptibilities and biofilm forming abilities of Enterococcus strains isolated from blood: Three center study

Enterococci are important pathogens that can cause fatal infections such as infective endocarditis and catheter-related bloodstream infections. The combination of beta- lactam/glycopeptide and aminoglycoside is an important treatment option for these infections if synergistic interaction is predicted. A total of 90 enterococcal isolates (45 Enterococcus faecalis and 45 Enterococcus faecium) from 3 different centers obtained from blood culture were evaluated in terms of various demographic characteristics of patients, time to positivity of blood cultures, antibiotic susceptibility profiles, some antibiotic resistance genes, biofilm-forming abilities and the association between biofilm formation and antibiotic resistance. In terms of demographics, E. faecalis isolates were obtained significantly more frequently from males, when compared to females. 27.8% of the isolates were obtained from patients at surgical wards and 72.2% were obtained from patients at medical wards. While oncology (16.7%) and nephrology (13.3%) were the most common depertments at medical wards, cardiovascular surgery (11.1%) and general surgery (8.9%) were the most common surgical wards. Regarding the antibiotic susceptibility profiles, no glycopeptide resistance was observed in E. faecalis isolates, while glycopeptide resistance was observed in 31.1% of E. faecium isolates. High-level gentamicin and streptomycin resistance rates among E. faecalis isolates were 24.4% and 26.7% respectively. These rates were 66.7% and 77.8% among E. faecium isolates. Significant differences were also observed between centers, especially in terms of high-level gentamicin and streptomycin resistance rates. These differences are important in terms of synergy between beta-lactam/glycopeptide and aminoglycoside, which may affect treatment regimen choices. In terms of the investigated resistance genes, a significant homogeneity has been observed. The vanA gene was observed in all of the isolates showing phenotypic glycopeptide resistance. The aac(6')-Ie-aph(2')-Ia gene was found in 92.7% of the isolates showing phenotypic high-level gentamicin resistance and the ant(6')-Ia gene was found in 95.6% of the isolates showing phenotypic high level streptomycin resistance. These findings suggest that mainly aac(6')-Ie-aph(2')-Ia gene is responsible for high-level gentamicin resistance and ant(6')-Ia gene is responsible for high-level streptomycin resistance in enterococci causing bacteremia. Significant differences were observed between E. faecalis and E. faecium isolates in terms of biofilm formation rates (100% and 55.6%, respectively). This difference was also observed regarding the strength of biofilm formation. While 35.6% of E. faecalis isolates were classified as strong biofilm producers, none of the E. faecium isolates produced strong biofilms. It is known that biofilm formation can lead to antimicrobial resistance in various ways. In our study, high-level gentamicin resistance rate among strong biofilm producing E. faecalis isolates (50%) was statistically higher than that among moderate biofilm producing E. faecalis isolates (10%). The differences between antibiotic susceptibilites of biofilm producing and non-biofilm producing E. faecium isolates were not statistically significant. However, the difference between quinupristin-dalfopristin susceptibilities approached the threshold of significance (52% and 80% for biofilm producing and non-biofilm producing E. faecium isolates, respectively) (p=0.051). For E. faecium, although not statistically significant, ciprofloxacin, quinupristin/dalfopristin, and high-level gentamycin resistance rates were higher in isolates that formed moderate biofilms compared to those that formed weak or no biofilms. It is thought that pheromone-mediated conjugation is a special and important gene transfer mechanism in enterococci, especially in E. faecalis, and biofilm formation increases the possibility of the development of this mechanism. In the literature, it has been indicated that plasmids that transfer genes in this way can carry tetracycline, vancomycin and high-level gentamicin resistance genes. The results of our study suggest that some Enterococcus isolates may have acquired resistance through this mechanism—and perhaps through these plasmids—against some antibiotics. Eravacycline is a newly developed fluorocycline derivative antimicrobial that is highly effective against enterococci, including those resistant to linezolid and vancomycin. In a global study, eravacycline resistance in enterococci was observed to be 0.6% and 2.3% for E.faecalis and E. faecium, respectively. It is important that there are no reported eravacycline resistant Enterococcus isolates in the literatüre of our country and we encountered two eravacycline-resistant E. faecium isolates in our study. This situation shows the importance of conducting multi-center research on this subject and investigating resistance mechanisms.