Effect of Haptoglobin Phenotypes on Growth ofStreptococcus pyogenes

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Abstract
The haptoglobin (Hp) 2–1 and 2–2 phenotypes have been shown to agglutinateStreptococcus pyogenescarrying the membrane antigen T4. In this study, the growth rate of two strains ofStreptococcus pyogenes(T1 and T4) in human serum was compared among haptoglobin phenotypesin 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 ofStreptococcus pyogenesT4 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 ofStreptococcus pyogenesT4 (P < 0.05) but not in Hp 2–2 serum. Haptoglobin types 2–1 and 2–2 can be regarded as inhibitors ofStreptococcus pyogenesgrowthin vitro. These data point towards a potential protective role of Hp 2–2 inStreptococcus pyogenesinfectionin vivo, independently of iron uptake.