CHEMICAL COMPOSITION OF BACTERIA, ETC. 65 



added to the medium for that purpose. Such colour 

 production is not, of course, pigment formation in the 

 true sense of the term, but an artificial chemical test 

 applied hy the bacteriologist. 



Growth Substances. — Certain bacteria, yeasts and 

 moulds produce substances which accelerate the gK)^vth 

 either of themselves or of other micro-organisms. Of 

 these " bios " is perhaps the best as well as the oldest 

 known of such substances. Of recent years many such 

 " growth substances " or " gro^\i:h factors " have been 

 discovered. They will be discussed in Chapter IX. 



Vitamins. — Yeast is a rich source of the vitamin-B 

 complex, and it has been claimed that Esch. coli, Bacillus 

 subtilis, B. mycoides, Torula rosea and Oospora lactis 

 can synthesise it. Bacteria and yeasts do not appear 

 to produce vitamins -A, -C or -D to any marked extent, 

 but certain of the lower fungi of the genera Aspergillus 

 and Penicillium produce a strongly reducing substance 

 which gives the chemical reactions of vitamin-C (ascorbic 

 acid). 



Antibiotics. — Many micro-organisms produce sub- 

 stances which have an inhibitory effect on the gro^Hh 

 of other micro-organisms. The name antibiotic has 

 been applied to such substances, among which penicillin 

 is an outstanding example. They comprise compounds 

 of a very wide range of constitution. They will be 

 considered in some detail in Chapter XI. 



The composition of bacteria, yeasts and fungi, as 

 regards their main constituents, is summarised in 

 Table 3. 



So far comparatively little appears to be loiown 

 about the composition of the viruses. Evidence is accu- 

 mulating, however, that there is a range of viruses of 

 increasing complexity from the crystalline proteins of 

 the tobacco mosaic viruses to those like vaccinia and the 

 influenza viruses which have a composition similar to 

 that of bacteria. 



