236 FLEAS, FLUKES AND CUCKOOS 



act as locomotor organs and also help to chum up the nutritive medium 

 in which the organism lives. 



Bacteria usually multiply by binary fission. The cell lengthens to 

 almost twice its normal size, a septum is formed and it then splits trans- 

 versely into two halves. After division the cells sometimes adhere to one 

 another, forming aggregations or chains. Some bacteria produce spores, 

 a process which might be described as transformation rather than re- 

 production, since no increase in numbers is involved. The spore stage is 

 probably a resting phase in which the bacterial cell is more resistant to 

 unfavourable external conditions. Bacillus anthracis, the causative agent 

 of anthrax, can remain alive on fields in the spore stage for several 

 years and resist boiling for ten minutes ! The cyst stage in certain para- 

 sitic Protozoa and worms is somewhat reminiscent of spore formation 

 in bacteria. 



Owing to their minute size it is difficult to describe these organisms 

 adequately by morphological characters alone and many bacteria 

 must be distinguished by their biochemical and physiological functions 

 and the symptoms they produce in the host. Reactions to dyes and the 

 body fluids of various animals are also of great importance for the pur- 

 pose of identification. Their classification, Topley points out, is based 

 largely on chemical structure rather than the gross architecture of the 

 cell. Most free-living bacteria are saprophytes, but many commensals 

 and most of the symbiotic and parasitic forms obtain their nourishment 

 by decomposing or breaking down living cells or body fluids into a 

 form which they can assimilate and synthesise into protoplasm. The 

 harmful effects on the host are due to the poisonous substances which 

 bacteria produce. These give rise to the various symptoms and lesions 

 which are characteristic of certain diseases. One of the best known 

 groups attacking both birds and mammals is the genus Mycobacterium 

 (family Mycobacteriaceae) . They are so-called "acid-fast" bacteria, 

 that is to say, once stained with carbol-fuchsin they resist decolorisa- 

 tion by mineral acids. They are slender, immobile, rod-shaped 

 organisms which reproduce by simple fission and do not form spores. 

 They are notorious as the causative agent of tuberculosis, although 

 some species are harmless saprophytes and inhabit odd media, such as 

 butter, smegma and the moist surface of timothy grass. Different types 

 of Mycobacterium are responsible for avian and mammalian tuberculosis 

 but birds and man do not infect one another. Cows, however, seem to 

 contract both types in addition to one of their own. Various species of 



