CLASSIFICATION BASED ON CHANGES PRODUCED IN BLOOD 567 



reduction with 01 per cent, sodium hydrosulphite ; provided that it is protected from 

 the air, it remains stable in the ice-chest for years. The other, the S lysin, wliich can 

 be extracted readily from streptococci by shaking with serum, is inactivated, Uke the 

 O lysin, by incubation at 37° C. for 2-4 hours, but unlike the O lysin cannot be re-activated 

 by reduction ; it is very sensitive to both heat and acid and can be preserved only by 

 storage at very low temperatures (— 73° C). The O lysin produces haemolysis rapidly 

 and is antigenic in the free state : the S lysin produces haemolysis more slowly and is 

 antigenic only when present in the organisms. Each is neutralized by a separate anti- 

 body. The O lysin is formed by strains belonging to Group A, C (human), and G. The 

 S lysin appears to be formed by strains of all groups, but the type of lysin produced is 

 group-specific, an antiserum to the S lysin of Group A strains faiUng to neutrahze S lysin 

 formed by other groups. Herbert and Todd (1944) have met with strains of streptococci 

 producing only S and others only O hsemolysin. On blood agar the S strains formed 

 /5-h3emolytic colonies on the surface and in the depth, both aerobically and anserobically. 

 The strains on rabbit blood agar — but not on horse blood, which contains O antilysin — ■ 

 formed /J-hsemolytic colonies only in the depth and only under aerobic conditions. The S 

 lysin was purified and found to be a Upo-protein hapten, incapable of stimulating the 

 formation of antibodies. 



The extreme lability of both and S lysins in broth cultures at 37° C. renders 

 the ordinary titration method of assessing their potency unreliable. Special pre- 

 cautions have to be taken to prevent destruction of either lysin before we can 

 assert that any given strain which produces yS-hsemolysis on blood agar plates is 

 incapable of forming a filtrable haemolysin in a fluid medium. 



Apart from the difficulties caused by the lability of the lysins, it may be noted 

 that some strains of Str. pyogenes produce a-bsemolytic colonies on blood agar, 

 or sometimes completely non-hsemoljrtic colonies. 



The first type has been studied by various workers. Fry (1933) showed that certain 

 strains which formed a-haemolytic colonies on aerobic blood agar plates produced typical 

 /?-haemolytic colonies when incubated anaerobically. Fuller and Maxted (1939) showed 

 that two or three factors were concerned in this result. Under aerobic conditions /?-haemo- 

 lytic colonies were formed if all reducing sugar was removed from the blood agar or if 

 catalase was added to the medium. They bring evidence to suggest that in the a-haemo- 

 lytic colonies peroxide is formed before the haemolysin with the result that a green zone 

 is produced. If the formation of the peroxide is prevented by anaerobic incubation or 

 is neutraUzed by catalase, then the haemolysin produces typical ^-haemolytic colonies. 

 The mode of action of the reducing sugar in the medium is not quite clear, but it appears to 

 inhibit the formation of the haemolysin. It may be noted that Fry's strains formed soluble 

 haemolysin in serum broth under aerobic conditions. For practical purposes it is advisable 

 to incubate primary plate cultures anaerobically, or both aerobicaUy and anaerobically. 



The second type has been reported by Coburn and Pauli (1941) and Colebrook and 

 his colleagues (1942). In a ward outbreak of streptococcal respiratory infection, Coburn 

 and PauU found that the causative organism. Type 12, occurred in two forms. One 

 form gave rise to typical haemolytic colonies at 37° C. ; the other gave rise to haemoljrtic 

 colonies only at 22° C. The second variant appeared to be possessed of a higher degree 

 of infectivity than the normal form. Colebrook's observations were made in a surgical 

 ward in which typical /S-haemolytic colonies belonging to Group A Type 12 had been 

 giving rise to septic comphcations. A series of cases was studied in which completely 

 non-haemolytic streptococci, both aerobicaUy and anaerobically, were isolated from the 

 wounds. These organisms proved also to belong to Group A Type 12. They formed 

 no S haemolysin, but most of them, when specially tested, were found to produce haemo- 

 lysin. The frequency with which such strains occur is at present unknown, but they 

 are believed to be uncommon. 



