NUCLEIC ACIDS 131 



Apart from adaptive variation, a large subject which cannot be adequately 

 discussed here, there is the subject of random mutation. The reproductive rate of 

 bacteria is so rapid — it can be over 100,000 times faster than man — that random 

 mutations are inevitably more commonly met than in the higher animals. Aside 

 from the question of the number of generations, however, bacteria are more susceptible 

 to detectable variations since variation of one cell in a multicellular organism might 

 be missed and have negligible effects, neutralised by other cells in contact with it. 

 In a unicellular organism there is no matrix of orthodox cells to regularise the unruly. 

 Presumably all bacteria in the world had a common progenitor and the variety to-day 

 is mostly the result of random mutation followed by natural selection of the fittest 

 to survive in the relevant conditions. 



The form of bacterial variation leading to drug-fastness and resistance to 

 antibiotics is dealt with in the next chapter on Chemotherapy. 



The change of type of pneumococci associated with enzymic changes of nucleic 

 acids is discussed in the next section. 



Another type of bacterial mutation is produced by exposure of the cells to various 

 damaging agents including X-rays, ultra-violet light and colchicine. The enzyme 

 systems of bacteria may be altered by these agents so that bacteria deficient in one 

 or more enzyme systems may be obtained. This becomes of great use in modern 

 microbiological assay methods (page 133). These changes are probably caused by a 

 damaging effect on the nucleic acids of the cell. 



In the case of nitratase and tetrathionase adaptation Pollock and Wainwright 

 (1948), have shown that the adaptation occurs in the lag phase before growth occurs. 

 Anaerobic growth using nitrate and tetrathionate as hydrogen acceptors cannot 

 occur until the necessary enzymes have been synthesised. It is clear, however, that 

 not all types of bacterial enzymic adaptation can occur in the absence of multiplica- 

 tion. Some enzymic adaptations may require mutation and the selection of mutants, 

 in which case multiplication of the cells is essential. It seems probable that the 

 increase in nucleic acids during the lag phase (Caspersson) is a symptom of protein and 

 enzyme synthesis, but it is not known how the presence of the substrate leads to 

 modification of the nucleo-proteins responsible for the synthesis of the appropriate 

 enzyme system. 



NUCLEIC ACIDS, GRAM STAINING, TYPE TRANSFORMATION 



It is becoming increasingly evident that in the nucleic acids reside many of the 

 secrets of Ufe. Stanley's work in isolating a virus in the form of a crystalline chemical 

 compound, a nucleoprotein, was a milestone in biological research. In these nucleo- 

 proteins we have chemical compounds able to reproduce themselves, to effect their 

 own autosynthesis, when in contact with the tissues of their host. Of course, a 

 bacterium does the same when it infects another organism or is sub-cultured in broth, 

 but in the case of bacteria or higher organisms a host of enzymes is known to be present 

 and to many persons, a half-suspected " vital force " was necessary to give the spark 

 of life. But when a crystalline chemical small enough to be certainly free of hypo- 

 thetical enzymes, yet large enough to be seen in the electron microscope, can synthesise 

 more of itself, a great stimulus is provided for research workers to continue their 

 prying into nature without the fear that just around the corner they may meet the 

 elan vital which may render negative all their efforts. 



