22 VIRUSES, RICKETTSIAE AND BACTERIA 



diphosphopyridine nucleotide and coenzyme A on freezing, the 

 freezing in some way affecting the properties of the membrane 

 around the rickettsiae or mitochondria. It is possible that the 

 rickettsiae are developed as free mitochondria and that the viruses 

 are further simplifications. On the other hand the rickettsiae 

 may indicate a stage in the development of the viruses to bacteria 

 or the three groups could be quite unrelated. 



We have insufficient evidence as yet to come to any firm con- 

 clusion concerning the origin and affinity of the rickettsiae. 



Bacteria 



We are no wiser when we come to consider the status of the 

 bacteria. Within recent years there has been a considerable 

 increase in our knowledge of the structure of the bacterial cell 

 (Spooner and Stocker 1956; Zinsser 1957). Thus Robinow in 1946 

 suggested that there were certain components within the cells of 

 Escherichia coli that behaved like nuclear material during cell 

 division (Fig. 4). Lederberg (1947) showed that a type of crossing 

 over occurred between certain strains of E. coli and that in effect 

 it was possible to draw up a map of the positions of various factors 

 in bacterial metabolism. The conclusion, then, is that certain 

 bacteria show nuclear and sexual (parasexual) behaviour. On the 

 other hand there are many bacteria that do not show these 

 phenomena, their structure and life history being much more 

 simple. 



It is not clear whether the bacteria represent an evolutionary 

 approach to the Protozoa, whether they are a retreat from the 

 Protozoa or whether they are quite unrelated. Perhaps some of the 

 difficulties can be illustrated by considering the autotrophic 

 bacteria (Chemoautotrophic) (Fry and Peel 1954). These 

 bacteria such as the sulphur and iron bacteria are able to metabolise 

 various simple substrates. They raise the question " are these 

 bacteria using a more primitive (earlier developed) system than 

 those found in the heterotrophic and photosynthetic bacteria? ' 

 It is not possible to give a definite answer to this question since 

 our knowledge of the biochemistry of the heterotrophic and 

 chemoautotrophic bacteria is still very incomplete. The chemo- 

 autotrophs can obtain their energy from simple sources such as 

 hydrogen, methane, ammonia, nitrite, hydrogen sulphide or iron 



