122 NITROGEN METABOLISM 



which in these experiments could only be derived from 

 amino-acids after they have been made available by enzymic 

 hydrolysis of the protein. Even if any extracellular enzyme 

 is carried over in the inoculum it is possible that it is diluted 

 in the subculture to such an extent that its activity is no 

 longer significant. If a small amount of peptone, presumably 

 containing some small peptides or amino-acids, was added 

 to the medium, certain proteolytic bacteria, e.g. Pr. vulgaris^ 

 grew rapidly and crystalline egg albumin and serum pro- 

 teins were then readily degraded and utilized [37]. Hence it 

 appears that the formation of extracellular enzymes is de- 

 pendent on the medium containing sources of carbon, 

 nitrogen and energy that can be utilized immediately with- 

 out having to be first broken down into smaller units by 

 extracellular enzymes. In any event, proteoses and peptones 

 are apparently not attacked by members of the Bacteriaceae 

 or by Staph, aureus or Strep, faecalis [33]. 



Specific proteinases of the streptococci and staphylococci 



The culture filtrates of some Lancefield group A strepto- 

 cocci contain an Og-labile papain-like enzyme which apart 

 from hydrolysing fibrin, casein, gelatin and benzoyl-L- 

 arginamide, also attacked the M antigen, one of the antigens 

 used in typing group A streptococci [14]. The M antigen is 

 usually absent in those strains capable of producing this 

 enzyme, though it may be present if the cultures are grown 

 at a low temperature (22° C.). After passage through mice, 

 formation of the enzyme ceased and the organisms became 

 more virulent and possessed the M antigen. The latter two 

 effects are not directly related, since the M antigen is also 

 present in some avirulent strains. Of some clinical impor- 

 tance is the fibrinolytic activity of haemolytic streptococci 

 (groups A and C), staphylococci and gas-gangrene Clostridia. 

 These bacteria produce an enzyme which converts a pre- 

 cursor (plasminogen) in the globulin fraction of human sera 

 into an active enzyme (plasmin) which digests the fibrin of 

 clotted blood. The mode of action of the bacterial enzyme, 

 named streptokinase in the case of streptococci, is compar- 

 able with the activation of chymotrypsinogen by trypsin. 



