FORM AND STRUCTURE OF MICROBES 57 



Certain bacteria such as the pneumococcus and the 

 pneumobacillus are surrounded by a sort of case or capsule. 

 One capsule may enclose several bacteria : when a large mass 

 of bacteria is included in one enormous capsule what is called 

 a zooglcea is formed. Bacteria which appear encapsulated in 

 the albuminous fluids of the infected body do not always 

 possess them in artificial culture. The capsule appears to be 

 a defensive secretion. The anthrax bacillus defends itself 

 against the toxic action of the serum of the rat by surrounding 

 itself with a thick sheath made up of a sort of mucus which 

 fixes and renders harmless the toxin. In the blood of lizards, 

 which are very resistant to anthrax, the bacillus surrounds 

 itself with a thick mucous envelope. The streptococci 

 occasionally acquire in the body of resistant guinea-pigs a 



Fig. 26. — Streptococci surrounded by a sheath of mucus. 



similar defensive covering. The streptococci of chronic 

 infections (otitis and nasal infections) are frequently encap- 

 sulated. The sarcinae are not usually pathogenic but encap- 

 sulated sarcinse have been described which were pathogenic 

 for laboratory animals. 



Bacteria multiply by transverse divisions, Pasteuria rantosa, 

 described by Metchnikoff, being an exception. One bacterium 

 put on a suitable medium produces after a certain time two, 

 these two producing four, and so on ; it is easy to imagine to 

 what enormous numbers this geometrical progression may lead. 

 In nature and in artificial culture media, multiplication i§ 



