2 AIDS TO BACTERIOLOGY 



to the remainder of the cell (metachromatic granules). 

 These are composed of glycogen, fat, or, according to 

 Dobell, nucleic acid combined with an organic base. They 

 are probably in large part reserve food substance (Jordan). 



The presence of a nucleus in all species is regarded as 

 proved by the work of Clifford Dobell. 



Movement. Many bacteria, particularly bacilli and 

 spirilla, are capable of motion, produced by the little 

 protoplasmic threads (flagella). Ellis has shown that 

 flagella arise from the cell substance and not from the 

 membrane. 



Some organisms, such as the cholera spirillum, have a 

 single flagellum at one end (monotricha), in others there is 

 one at each pole (amphitricha) ; the flagella may assume 

 the form of a tuft at one pole (lophotricha), or they may 

 be scattered round the cell (peritricha). The position of 

 the flagella is always the same in each species; but in 

 those species having more than one flagellum the number 

 is not always constant, and depends on the health of the 

 culture (Ellis). Even these organisms are not always 

 motile, but go through a resting stage. Motility is best 

 seen in young cultures when conditions are favourable 

 for growth. Flagella are not seen when the organism is 

 examined under the microscope in the usual way, but they 

 can be observed in specimens specially stained. This 

 independent movement must not be confused with the 

 motion that solid particles, whether bacteria or not, exhibit 

 when suspended in a fluid medium, known as the ' Brownian 

 movement,' which is variously attributed to electrical 

 disturbances and to surface tension. 



Size. The unit of measurement adopted is the ' micron ' 

 (often erroneously called a micro-millimetre), which is 

 equal to 0-001 millimetre, and is represented by the letter//. 

 The influenza bacillus measures about 0-5 x 0-2 IJL, while 

 the spirochsete of relapsing fever may attain 40^ in length. 

 Great differences in length are not observed among the 

 majority of species, most of the bacilli, for instance, measur- 

 ing about 2 //. Even smaller than the influenza bacillus 

 is the organism causing pleuro-pneumonia in cattle, which 

 is just visible under the highest powers. ' Ultra-micro- 

 scopic ' organisms capable of passing through the filter mass 

 of a porcelain filter cause many diseases (see Chapter 

 XIX.). 



