CHAPTER I 



BACTERIA 



CLASSIFICATION STRUCTURE COMPOSITION 



AMONGST the lowest forms of living things organisms exist which 

 are too small to be seen without the aid of a powerful micro- 

 scope and of such simple structure that a single cell suffices for 

 all their vital activities. Amongst these unicellular organisms 

 the divergence between animal and plant melts away, and forms 

 are found which possess minor characteristics of both groups. 

 Prior to the seventeenth century little was known of these 

 minute living cells. Conjectures had been made as to the possi- 

 bility of their existence and the role they might be supposed to 

 play in various natural processes and in disease, but they were 

 merely conjectures and no record of trustworthy systematic in- 

 vestigations exists. 



Kircher in 1659 observed their presence in putrid meat and 

 milk ; later, in 1683, Anton van Leeuwenhoeck, a Dutch micros- 

 copist, recording his observations upon tartar scraped from the 

 teeth and mixed with water, wrote, " With the greatest astonish- 

 ment I saw distributed everywhere through the material I was 

 examining ' animalcules ' of the most microscopic size which 

 moved themselves about very energetically." Leeuwenhoeck 

 supplemented his observations with drawings, both of which are 

 remarkably clear and accurate. 



