INTRODUCTORY 15 



due to the fact that bacteriology as a science has been studied 

 and developed mainly by medical men who have been concerned 

 with the practical bearings of their discoveries, rather than with 

 the scientific classification, a side of the subject which really 

 requires the expert knowledge of the highly -trained botanical 

 and zoological specialist. Until our knowledge of bacteria is 

 much more accurate and intimate, all such classifications must 

 be looked upon as merely provisional. At present we have to 

 depend chiefly upon the shape, size, and method of reproduc~ 

 tion of these organisms ; but it is probable that physiological 

 and functional activities will, in the future, have to be taken 

 more into account. The classification here given is founded 

 mainly upon that of Migula, which is now generally adopted. 



Bacteria are very minute, single-celled, usually colourless and 

 chlorophyll-free, clear, refractile micro-organisms of various 

 shapes and sizes (see Frontispiece). Some are spherical or 

 nearly so, and are termed Cocci, from the Greek word kolckos, 

 signifying a berry. Others are rod-shaped and are therefore 

 called Bacilli or little rods, from the Low Latin word bacillus, 

 the diminutive of baculus, a rod or staff. Some of these are 

 straight or very slightly curved ; but when the curve is more 

 distinctly marked they are known as Comma-bacilli or 

 Vibrios. Where these latter are built up into spiral or 

 wavy chains, the name Spirillum is used, though it is often 

 also applied to the single curved unit ; whilst the non-septate 

 spiral rods which are not built of shorter curved elements are 

 called Spirochaetes. 



A difficulty here meets us from the fact that certain protozoa 

 of the flagellate group (see p. 14) are in some phases of their 

 life-history practically indistinguishable from these spiro- 

 chaetes, for example the very important disease-producing 

 Treponema pallidum (see p. 86), which, though probably a 

 protozoon, is often somewhat loosely referred to as a " Spiro- 

 chaete." 



The bacteria above considered belong to the Lower group 

 of Bacteria, sometimes spoken of as the Haplo-bacteria, and 

 to these a large number of disease-producing bacteria belong. 

 Just above them in the scale come bacteria which tend to 

 grow in threads and which are therefore called the Tricho- 

 mycetes or Tricho-bacteria. They usually possess a common 

 enclosing sheath. Some are unbranched (e.g. Leptothrix 

 group) ; others show branching (e.g. Streptothnx group), and 

 often exhibit the setting aside of special portions of the 

 threads for reproductive purposes, in this respect approxi- 

 mating in some instances to the moulds in character. The 

 classification of the members of this higher group is extremely 



