566 STREPTOCOCCUS 



in the immediate neighbourhood of the colony. There is no discoloration. UnHke the 

 zones of /^-haemolysis, a considerable extension of the zones may occur during the 24 hours 

 in the ice-box. It is noted that some strains which produce a'-hsemolysis on horse blood 

 agar may produce typical a -haemolysis on rabbit blood agar. 



y. The colonies develop in the blood agar without any change in the surrounding 

 medium. 



The jS-hsemolytic strains of Smith and Brown correspond to Schottmiiller's 

 Str. hcsniolyticus. The a-hsemolytic strains may be regarded as equivalent to his 

 Str. viridans ; though it may be noted that strains are encountered that produce the 

 characteristic green coloration without the formation of a detectable zone of 

 haemolysis. The significance of the a' type of haemolysis is not clear. It seems 

 to be of infrequent occurrence. There seems no good reason for attaching the label 

 y to those streptococci that cause no change in blood media. Strains of this type 

 have sometimes been referred to as " indifferent streptococci." 



Soluble ^-heemolysins. — The terms a- and /^-haemolysis have attained general 

 currency in bacteriological literature and serve a useful purpose. No effort has been 

 made, however, to reconcile this usage with that of the terms " haemolytic " and 

 " non-haemolytic " as applied to streptococci. By a "haemolytic" streptococcus 

 is meant almost always a strain that causes /^-haemolysis on blood agar. By a 

 " non-haemolytic streptococcus " is meant a strain that either produces a-haemolysis, 

 or gives rise to no change at all. Some workers would confine the term " haemolytic 

 streptococcus," or at least the specific name Str. hcemolyticus if that be used, to 

 strains that, in addition to causing jS-haemolysis in blood agar, can be shown to 

 produce a soluble haemolysin, and this is an aspect of the problem that must be 

 discussed in more detail. 



Marmorek ( 1895) showed that cultures of certain strains of streptococci in a fluid medium 

 had the power of lysing added blood corpuscles, and Besredka (1901) obtained haemolytic 

 filtrates from cultures of streptococci in heated rabbit's serum. Braun (1912) reported 

 that all strains of streptococci which produced haemolysis on rabbit blood agar plates gave 

 rise to a filtrable haemolysin when grown in rabbit serum broth. The factors which deter- 

 mine this haemolysin production have been studied by McLeod (1912), Meader and Robin- 

 son (1920) and de Kruif and Ireland (1920). All these observers noted that haemolysin 

 production was absent or minimal in cultures grown in plain broth without the addition 

 of serum. More recently Todd ( 1932) and Todd and Hewitt ( 1932) have shown that potent 

 haemolytic filtrates may be obtained by growing haemolytic streptococci in a medium con- 

 taining yeast extract, or in a special broth medium, sterilized by filtration instead of by 

 autoclaving, and containing dextrose, sodium bicarbonate and sodium phosphate. 



De Kruif and Ireland have carried out careful quantitative studies of the rate of haemo- 

 lysin production, and of its inactivation in the culture medium. They found that the 

 haemolytic titre of the supernatant fluid from their cultures after centrifugalization reached 

 its maximum after 8 hours' incubation at 37° C, and then rapidly declined. In many 

 cases no haemolysin could be detected after 14 hours ; though, when the whole culture was 

 tested instead of the supernatant fluid, some lytic action might persist up to the 24th hour. 

 A haemolytic filtrate is completely inactivated by heating at 55° C. for 30 minutes, and may 

 lose most of its activity when incubated at 37° C. for 2 hours or more (see McLeod 1912). 



It has been shown by Neill and Mallory (1926) that streptolysin, when exposed to air 

 at relatively low temperatures, undergoes an oxidation that is readily reversible by suitable 

 chemical reagents. The haemolysin is active in the reduced form, inactive in the oxidized. 

 At higher temperatures (55° C.) an irreversible inactivation occurs. 



Todd (1934, 1938a, 1939) has demonstrated the production of two kinds of haemolysin. 

 One, the lysin, is oxygen-labUe at ordinary temperatures, but can be re-activated by 



