ANTIBIOTICS 183 



free two basic amino groups, three amide groups and 

 one carboxyl group or a phenolic OH group. About 

 twenty per cent, ol the amino -acid residues have the 

 ^-configuration. It is of interest to note that the capsules 

 of B. anthracis, B. mesentericus and B. suhtilis are made 

 up of a polypeptide composed of (^-glutamic acid (see 

 p. 338). The presence of such a large proportion of 

 ^-amino -acids in gramicidin and tjrrocidine probably 

 accoimts for their resistance to pepsin, trypsin and 

 papain. 



Tyrocidine has marked bactericidal and l^i^ic action 

 in vitro against Gram -negative as well as Gram -positive 

 organisms. Fifty to 100 jug. affords definite protection 

 to mice infected intraperitoneally with pneumococci. 

 Tyrocidine blocks the oxidative processes of metabolism. 

 It is antagonistic to certain lower fungi such as Achorion 

 schcenlandii, Microsporiu7n gypseum, Trichophyton gyp- 

 seum and Candida albicans. 



Tyrocidine has a strong hsemolytic effect on human 

 and rabbit red blood corpuscles, haemolysis occurring in 

 the presence of 0-005 /xg. of the substance, which is also 

 highly toxic to animals. Tyrocidine loses much of its 

 bactericidal power in the presence of blood, serum or 

 pus. It appears to prevent the loss of activity which 

 tetanus toxin undergoes on heating at 55°C. 



Both tyrocidine and gramicidin can be used chemo- 

 therapeutically by local application, to wounds for 

 example. 



Un-named Antibiotics. — When B. mesentericus, in the 

 smooth phase, is grown in nutrient broth it gives rise to 

 a substance which has a specific bactericidal effect on 

 C. diphtherice at a dilution of 1 in 1250. The toxic 

 effects of C. diphtherice are eliminated when the organism 

 is injected along with the B. mesentericus filtrate into 

 guinea-pigs. 



Aspergillus candidus produces a thermostable sub- 

 stance, similar to citrinin, but which is more powerful 



15 



