Effect of haptoglobin phenotypes on growth of Streptococcus pyogenes

The haptoglobin (Hp) 2-1 and 2-2 phenotypes have been shown to agglutinate Streptococcus pyogenes carrying the membrane antigen T4. In this study, the growth rate of two strains of Streptococcus pyogenes (T1 and T4) in human serum was compared among haptoglobin phenotypes in vitro. During incubation for 16 hours in serum of different haptoglobin types, only Hp 2-1 and Hp 2-2 sera showed an inhibitory effect on growth, Hp 2-2 being 1.85 times more potent than Hp 2-1. Growth of Streptococcus pyogenes T4 negatively correlated with the serum concentration of Hp 2-1 (r = -0.908) and Hp 2-2 (r = -0.953). Haptoglobin-depleted serum had no inhibitory effect on bacterial growth. Addition of haemoglobin and ferric citrate to the serum accelerated the growth of Streptococcus pyogenes T4 (P <0.05) but not in Hp 2-2 serum. Haptoglobin types 2-1 and 2-2 can be regarded as inhibitors of Streptococcus pyogenes growth in vitro. These data point towards a potential protective role of Hp 2-2 in Streptococcus pyogenes infection in vivo, independently of iron uptake.