Identification, antimicrobial resistance and molecular characterization of the human emerging pathogen Streptococcus gallolyticus subsp pasteurianus

Introduction Streptococcus bovis group taxonomy has progressively changed and the most common species encountered in human diseases is S. gallolyticus, with the subspecies gallolyticus (SGSP; formerly biotype I) and pasteurianus (SGSP; formerly biotype II/2). Due to specific disease association, an accurate identification of S. bovis isolates is mandatory. This study aimed to retrospectively identify 22 S. bovis clinical isolates based on the new taxonomy, as well as to investigate their phenotypic and genotypic resistance profiles and clonal relationship. Materials and Methods Identification was performed biochemically by Phoenix100 system, by 16S rRNA gene PCR and sequencing, and by two different MALDI TOF MS platforms, Bruker Biotyper software 3.0 and Vitek MS v2.0 system. Antibiotic susceptibility/resistance was determined by conventional methods and PCR [erm(A), erm(B), mef(A), tet(M) and tet(O)]. D-tests were performed to determine the phenotypes of macrolide-lincosamide-streptogramin resistance. The clonal relationship was determined by PFGE after SmaI-enzyme DNA digestion. Results Sixteen S. bovis strains had been isolated in 14 patients from urine (of which 10 from patients with urinary tract infections), three from bile of patients with biliary tract malignancy, two from blood of patients with endocarditis and one from a diabetic ulcer. The most common patients’ underlying disease was diabetes (31.8%). By the automated Phoenix system all isolates were found to belong to S. bovis biotype II; 16S rRNA gene sequencing and MALDI Biotyper properly identified isolates as SGSP. Six isolates were correctly identified to the subspecies level (SGSP) by MALDI Vitek MS, while the remaining 16 isolates showed identification to the species level (S. gallolyticus) with low discrimination at the subspecies level. All isolates were susceptible to penicillin, cefotaxime, vancomycin, meropenem, and chloramphenicol. Erythromycin resistance rate was 31.8%, all erythromycin-resistant isolates being also clindamycin-resistant (phenotype cMLSB) and carrying erm(B). Tetracycline resistance rate was 68.2%; most strains carried tet(O). All erythromycin-resistant isolates were also tetracycline-resistant. Overall, 17 different PFGE types were identified, of which 14 were unique types. Conclusion Both MALDI TOF systems correctly identified the isolates to the species level (S. gallolyticus), although only MALDI Biotyper accurately identified all isolates to the subspecies level (SGSP). SGSP was most frequently associated with urinary tract infections. An association between SGSP infection/colonization and diabetes as underlying disease was detected. High erythromycin and tetracycline resistance rates were observed. Finally, SGSP isolates showed a high genetic variability.