ANTIBIOTICS 165 



gro^\'th of all pathogenic organisms tested ; 0-2 to 0-3 

 fig. per ml. was adequate to inhibit hsemol^^ic streptococci 

 and Type III pneumococci. It is also inhibitory to A. 

 niger, P. italicum and Bhizopus. ^ It is lethal to mice 

 and rabbits in doses of 45 to 65 mg. per kg. of body 

 Aveight. Less than the lethal dose causes kidney lesions 

 and hsematuria. 



Gramicidin. — In 1939 Dubos isolated from soil a spore 

 bearing bacillus, which has since been named B. hrevis. 

 Autolysates of the organism contained a soluble substance, 

 tyrothricin, capable of lysing living Gram -positive cocci. 

 The substance is non- volatile, does not dialyse through 

 coUodion and is heat stable. It is very stable to alkali 

 but not to acid. 0-02 /xg. per ml. inhibits the growi;h of 

 pneumococci, whilst staphylococci and streptococci are 

 not quite so sensitive. Gram-negative organisms are not 

 affected even by large amounts of the substance. The 

 intraperitoneal injection of 2 mg. of the extract protected 

 mice against infection with pneumococcci. Tyrothricin 

 is not to be confused with the enzyme, produced adap- 

 tively by some soil bacilli, which destroys the capsule of 

 Type III pneumococci only, rendering them susceptible 

 to phagocytosis. Tyrothricin inhibits the fibrinol}i}ic 

 action of [B-haemolytic streptococcal filtrates and the 

 coagulase activity of staphylococal filtrates. Tyrothricin 

 has been shown to be a mixture. By extraction of the 

 crude material with equal parts of acetone and ether, 

 evaporation to dryness and extraction with warm acetone 

 a crystalline substance, gramicidin, is obtained on cooling 

 the acetone solution. The residue insoluble in the 

 acetone-ether mixture is taken up in boiling ethanol and 

 acidified with hydrochloric acid when a crystalline 

 material, tyi'ocidine hydrochloride, separates out (see 

 p. 182). Tyrothricin contains about 10 to 20 per cent, 

 of gramicidin and 40 to 60 per cent, of tyrocidine. 

 Gramicidin is a neutral substance which is specifically 



