Streptolysin S: one of the most potent and elusive of all bacterial toxins

Streptolysin S (SLS) is responsible for the hallmark Phemolytic phenotype seen around colonies of Group A Streptococcus (GAS), also known as Streptococcus pyogenes, when cultured on blood agar. It has proven difficult to characterize due to the difficulty of obtaining purified preparations since additional factors are required for synthesis and a carrier molecule is needed to provide stability in solution. However, attempts at N-terminal sequencing of the peptide proved to be unsuccessful, and the amino acid structure remains a mystery despite more recent genetic information as to the structural gene for SLS. Although the cytolytic capacity of SLS has been well documented, its contribution to the pathogenesis ofGAS disease was not clear until the use of SLS-deficient mutants demonstrated a role in lesion formation and tissue necrosis. Recent work also provides evidence that the SLS RNA transcript might act as a regulator for other GAS determinants, in which case its role could prove to be complex. This chapter summarizes early work and focus on the more recent findings, such as its genetic organization, regulation, and possible role(s) in the pathophysiology of streptococcal infections. The amino acid structure of the SLS peptide has not been elucidated, nor have the theoretical predictions regarding its processing been verified. Whether the sagA transcript alone acts as a regulator or whether SLS may also function as a quorum-sensing molecule deserves further investigation.