Etiological structure, drug resistance and biofilm forming capabilities of isolates from respiratory system from outpatients

The study is focused on the etiological structure and drug resistance of bacterial strains causing upper respiratory tract infections, which are widely spread amongst children. The aim is to update the data about antimicrobial susceptibility and to establish virulence factors important for infectious process for Staphylococcus spp. isolates from the upper respiratory tract. A total of 711 strains were isolated from outpatients with nasal and oropharyngeal infections at Medical Diagnostic Laboratories „Sinevo-Bulgaria“ LTD in Plovdiv for a two-year period 2019-2020. The results demonstrate that the etiological structure is highly related to the site of infection.  Branhamella catarrhalis was found to dominate in the samples from the oropharynx (40%), while the majority of isolates from the nasal and ear samples were identified as Staphylococcus aureus - 65% and 35% respectively.  S. aureus showed higher drug resistance compared to other isolates, reaching up to 70% to penicillins. Over 20% of the Staphylococcus strains were also resistant to macrolides, fluoroquinolones and aminoglycosides. Methicillin resistance was established for 39% strains. The ability of S. aureus to form biofilms was tested as significant virulence factor by cultivation on unmodified and modified (supplemented with 5% human plasma) tryptic Soy Broth (TSB). The analysis shows that only 16,3% St. aureus were capable to form stable biofilm on TSB. The addition of human plasma increases the number up to 87%. The susceptibility profile of the investigated strains in the present study confirms need always to detect resistance before antibiotic prescriptions from physicians.