2 BACTERIOLOGY 



can thrive whether it is absent or not and are called 

 facultative anaerobes ; while those micro-organisms whose 

 growth can make no progress in the presence of the gas 

 are denominated obligate anaerobes. 



Micro-organisms belong to the following different 

 classes — viz. bacteria, moulds, yeasts, algce, and protozoa. 



Bacteria, scMzomycetes, or fission-fungi, are colourless 

 cells of a glassy transparency, possessing an enveloping 

 membrane with protoplasmic contents but no nucleus, 

 and having a length which amounts in general to a few 

 thousandths, and a breadth of some ten-thousandths, of a 

 millimetre. The interior of the bacterial cell has usually a 

 homogeneous appearance, but sometimes shows oil-like 

 granules. Bacteria are distinguished according to their 

 form as cocci, bacilli, and spirilla. 



Cocci are globular in shape, and are found either singly 

 or united in groups. If they lie singly they are called mono- 

 cocci; if grouped in masses, staphylococci; if the elements 

 are joined in pairs and fours we distinguish respectively, 

 according to the number, diplococci and tetracocci ; if each 

 eight is so united as to resemble a bale of goods, they are 

 named sarcince ; and if they are strung together in chains, 

 streptococci. 



Bacilli are minute straight rods, the smallest discovered 

 up to the present time being the influenza bacillus. Their 

 ends are sometimes sharply cut across, sometimes rounded 

 off, and the rodlets themselves are in some cases thin, in 

 others stout and thick, in others again swollen in the centre, 

 and so forth. 



Spirilla are spirally curved rods, and are subdivided into 

 comma-bacilli or I'ibrios, spi^-illa in the more restricted 

 sense, and spiroclicetce. The vibrios usually form strings of 

 cells which strongly resemble spirOla ; the spirochstse are 

 distinguished by their flexibility. 



