Virulence potential of Group A streptococci isolated from throat cultures of children from north India

Background & objectives: Rheumatic fever (RF)/rheumatic heart disease (RHD) caused by Group A streptococcus (GAS) are more prevalent in north India as compared to the western world, where invasive diseases are common. This could be due to variation in the virulence of GAS in different geographic locations. Hence, we studied the virulence potential of GAS isolated from the throat of children from north India. Methods: Fifty GAS isolated consecutively, from children with mild pharyngitis (20), severe pharyngitis (24) and asymptomatic pharyngeal carriers (6), were characterized by emm typing and opacity factor (OF). Adherence and internalization of GAS in HEp-2 cells and opsonophagocytosis in convalescent serum samples were studied. Results: Twenty emm types, six sequence types, and one non-typeable GAS were circulating in the community. emm type 74, 11, 68, StI129 and NS292 were most prevalent. Twenty seven (54%) GAS isolates were OF negative. Sixty five per cent of the most prevalent emm types were OF negative indicating their rheumatogenic potential. Adhesion of GAS ranged from 0.1 to 100 per cent. Forty eight per cent of GAS were highly adherent. Invasion of GAS in HEp-2 cells ranged between 0 to 30 per cent. Only 20 per cent isolates exhibited highest invasion. GAS were opsonophagocytosed with highly divergent efficiency ranging from 0 to 91.7 per cent. Nineteen GAS were not opsonophagocytosed and 15 multiplied during the assay. Isolates of the same emm type also varied in their virulence potential. Interpretation & conclusions: GAS isolates from the throat of children from north India belonged to several emm types, majority were OF negative, excellent adherents but poor invaders. This explains why throat infections in these children tend to lead to ARF/RHD rather than invasive diseases. A few isolates exhibiting high invasion efficiency indicate that GAS throat cultures can also lead to invasive diseases.