The pyogenic potential of the different Streptococcus anginosus group bacterial species: retrospective cohort study

Streptococcus anginosus Group (SAG) bacteria are common causes of pyogenic infections (PIs). We examined the association between SAG species and the presence of a PI through a retrospective, observational, cohort study, between the years 2009 and 2015. All adults with clinically significant SAG infections in one hospital in Israel were assessed for association between SAG species and the presence of a PI defined as an abscess, empyema, or deep/organ space surgical site infection. Risk factors for PI were assessed using multivariate backward stepwise logistic regression analysis. We identified 263 patients with significant SAG infections, 182 (69%) of which were caused by S. anginosus, 45 (17·1%) by S treptococcus constellatus and 36 (13·7%) by S treptococcus intermedius. The mean age of the patients was 56·8 ± 19·1 years. PIs were identified among 160 (60%) of the patients and were mostly non-bacteraemic (147/160, 91·8%), while most non-PI patients had bacteraemia (70/103, 68%). S. anginosus and S. constellatus were associated with a significantly lower incidence of PI than S. intermedius, OR 0·18 (95% CI 0·06-0·53) and 0·14 (0·04-0·48), respectively. Patients with PI were younger and, in general, had less co-morbidities. S. intermedius was associated with pyogenic non-bacteraemic infections, while S. anginosus and S. constellatus were associated with bacteraemia with no abscess or empyema formation. These data may indicate differences in virulence mechanisms of these SAG bacteria.