200 



pathogenicity; chemotherapy 



Formula of Streptomycin 



OH 



H MH — CZ 



/^\ 



HOCH HCOH 



I u 



HOCH C NH 



HO A O H 



L/CH^OH \| 



(Streptidine) 



N- methyl-L-glucosamine 



3. Toxic peptides secreted by bacteria: B. brevis is a 

 spore-bearing soil organism which secretes an antibacterial 

 substance, called tyrothricin. This is a mixture of peptides, 

 the most important being Tyrocidin and Gramicidin. Tyro- 

 cidin is a surface-active substance which kills both Gram- 

 negative and Gram-positive bacteria by dissolving lipoid 

 materials in their cell-walls. Gramicidin is much less toxic 

 and is bacteriostatic towards Gram-positive bacteria; it is 

 thought to act by interfering with the assimilation and meta- 

 boHsm of phosphate. Both tyrocidin and gramicidin are too 

 toxic for general clinical application, although purified grami- 

 cidin can be used locally in wounds. These two peptides 

 proved to be the forerunners of a series of similar substances 

 produced by bacteria, especially those of the genus Bacillus. 

 Of recent years, antibiotics called Polymyxin, Aerosporin, 

 Bacitracin, Subtilin, Bacillin, etc. have been described. They 

 all seem to be polypeptide in nature and contain some un- 

 natural D-amino-acid residues. Some are known to be cyclic 

 polypeptides. They are stable substances and vary consider- 

 ably in their antibacterial properties: bacitracin has a range 

 of activities very similar to that of penicillin, while polymyxin 

 is effective against organisms of the Gram-negative group 

 which are comparatively resistant to penicillin. The clinical 



