Population structure and characterization of viridans group streptococci (VGS) isolated from the upper respiratory tract of patients in the community

A study was undertaken to examine the population structure of viridans group streptococci (VGS) isolated the upper respiratory tract of adult and paediatric patients within the community. VGS are common commensal bacterial inhabitants of the upper respiratory tract and valuable sentinel reporters of underlying antibiotic resistance (AR). Laboratory examination of the colonising VGS species may provide a valuable ecological description of the species isolated from the upper respiratory tract and their antibiotic susceptibility, including an estimation of the AR reservoir in this population. Freshly obtained nasal and oropharyngeal swabs from 84 patients were examined by selective conventional culture on Mitis-Salivarius agar and yielded 363 isolates of VGS. Sequence analyses of the rpnB and 16-23S rRNA ITS genes identified these isolates to belong to 10 species of VGS and included S. anginosus, S. australis, S. constellatus, S. infantis, S. mitis, S. oralis, S. parasanguinis, S. salivarius, S. sanguinis and S. vestibularis. The most frequent VGS organisms isolated was S. salivarius (282/363; 78.0%), followed by S. sanguinis (23/363; 6.3%), S. parasanguinis (21/363; 5.8%), S. mitis (18/363; 5.0%), S. anginosus (5/363; 1.4%), S. vestibularis (5/363; 1.4%), S. australis (3/363; 0.8%), S. oralis (3/363; 0.8%), S. infantis (1/363; 0.3%) and S. constellatus (1/363; 0.3%). All patients examined carried at least one VGS organism, where there were 17 combination patterns of carriage of the 10 species of VGS species isolated, where 54.2%, 37.3%, 7.2% and 1.2% of patients harboured one, two, three and four different VGS species, respectively. Antibiotic susceptibility was determined by standard disk diffusion assay testing against four classes of antibiotics, including the b-lactams [cefotaxime, cefuroxime], the tetracyclines [doxycycline], the fluoroquinolones [levofloxacin] and the macrolides [erythromycin]. Overall, there was no resistance to levofloxacin and cefuroxime, with limited resistance to cefotaxime (3.3%) and doxycycline (9.8%). Antibiotic resistance was highest in erythromycin, where 40.9% of isolates were resistant. S. vestibularis was the most antibiotic resistance of all VGS species examined (S. vestibularis v S. salivarius p=0.011), followed by S. anginosis. S. salivarius was the most antibiotic susceptible VGS species examined. Overall, given their infrequency in causing infection, relatively few studies to date have attempted to examine their ecology in their preferred body niche, namely the upper respiratory tract. However, knowing their prevalence is becoming increasingly important in relation to their ability to exclude significant respiratory pathogens, including Streptococcus pneumoniae. In conclusion, these data indicate that VGS colonisation of the upper respiratory tract in individuals within the community is dominated mainly with relatively antibiotic susceptible S. salivarius.