Report of unusual clinical appearance in bacteraemia with nonhaemolytic M-type 58 Streptococcus pyogenes.

ABSTRACT

BACKGROUND & OBJECTIVES: The occurrence of haemolytic colonies on blood agar often provides the starting point for the laboratory diagnosis of pyogenic streptococci, while non-haemolytic variants could pass unrecognised, leading to a failure of diagnosis. We report the details of two epidemiologically unrelated patients with bacteraemia featuring M-type 58 Streptococcus pyogenes, a seemingly rare cause of human infection in the UK, and briefly review previous reports of infection with non-haemolytic strains of this species. METHODS: Case notes of the two patients were reviewed. Isolates obtained from clinical specimens were recovered and identified and cultured on horse blood agar to observe pattern of haemolysis. RESULTS: In the first case, of a 75 year-old man with leukaemia and a retropharyngeal abscess, the isolate was consistently non-haemolytic, probably due to a failure to produce streptolysin S as has been described before in a small number of reports involving various M-types. In the second case, of an 84 year-old woman with dermatitis and septicaemia, the organism was principally beta-haemolytic but with no haemolysis on aerobic culture where the colonies were well spaced, a phenomenon thought to be associated with abundant production of serum opacity factor (OF). INTERPRETATION & CONCLUSION: These cases are a reminder that misleading cultural appearances can occur with S. pyogenes and that OF positive strains can produce poor haemolysis on aerobic culture, or fail even to do so at all.