182 BACTERIOLOGICAL CHEMISTRY 



The structure of the streptidine moiety of strepto- 

 mycin makes it tempting to speculate that the antibiotic 

 is active in virtue of its resemblance to the growth factor 



inositol, 



CHOH 



/\ 

 / \ 

 CHOH CHOH 



I I 



CHOH CHOH 



CHOH 



Streptothricin. — A species of Actinomyces, occurring 

 in the soil, A. lavendulce, gives rise to the antibiotic 

 streptothricin, which has a selective action on Gram- 

 negative organisms, for example E. coli and Shigella 

 dysenterice, but also acts on some Gram-positive bacteria, 

 for example B. subtilis and Staph, aureus, moulds and 

 yeasts. Streptothricin is a base and appears to be built 

 up of amino -acid residues, resembling tyrocidine. It is 

 only inhibitory in the undissociated state ; factors such 

 as the presence of salts and ^H values which cause 

 dissociation lower the activity of streptothricin. Bacteria 

 subjected to its action increase in size and tend to form 

 chains. Resistant strains of bacteria can be developed. 

 It has a low toxicity for animals and could probably be 

 used for internal chemotherapy. 



Tyrocidine. — Tyrothricin, isolated from auto'lysates of 

 B. hrevis, is a mixture of gramicidin and tyrocidine 

 (see p. 165). Tyrocidine is the fraction insoluble in 

 acetone -ether mixtures, but soluble in hot ethanol and 

 crystallised out as the hydrochloride. Tyrocidine hydro- 

 chloride has m.p. 237-239°C. (decomposition), and 

 [a] Jf — 102° (c = 1 in 95 per cent, ethanol). Tyrocidine 

 is a polypeptide having a molecular weight about 2500 

 and containing about twenty amino -acid residues in- 

 cluding tryptoj)hane, tyrosine, alanine, phenylalanine 

 and aspartic acid, combined in such a way as to leave 



