SCHIZOMYCETES 



provided with one or more cilia, which propel them through the liquid in 

 which they grow. 



Bacteria do not contain chlorophyll, but are not infrequently coloured 

 as seen in mass, though usually colourless when seen individually under the 

 microscope. 



Some live only on dead organic material (saprophytes), some only on 

 living animals or plants (parasites), while the majority can carry on either 

 a parasitic or saprophytic existence. To the last category belong most of the 

 disease-causing {pathogenic) bacteria. Lastly, there are a few which are 

 capable (like the green plants) of producing their organic food from the carbon 

 dioxide of the atmosphere. 



Owing to their small size, it is often impossible to distinguish different 

 bacteria from one another under the microscope, and it is therefore customary 

 to cultivate them on plates of sterilised jelly (made from gelatine dissolved in 

 a nutrient solution), as it is found that each species shows characteristic 

 features when grown in this way. By mixing a trace of the liquid containing 

 the bacteria with the liquefied jelly, pouring this on sterilised glass plates, 

 which are cooled by means of iced water in order to produce a quick setting 

 of the jelly, and then keeping the plates at the ordinary temperature protected 

 against infection from the atmosphere, it is possible to secure colonies, each 

 developed from a single bacterium or spore. In this way different species can 

 be isolated from a mixture of bacteria and " pure cultures " of each obtained. 

 Bacteria in the vegetative condition are readily destroyed by heat, but 



their spores, generally formed singly within 

 the cells, are exceedingly resistant to high 

 temperatures. It is this resistancy which 

 causes such difficulty in sterilising substances 

 thoroughly for human consumption, i.e. 

 milk, bottled fruits, vegetables, etc. It is 

 well known that in many cases boiling, 

 unless very prolonged, is not sufficient to 

 destroy all bacterial spores. This has led 

 to what is known as the method of " dis- 

 continuous sterilisation." 



To study the life history of a bacterium, 

 the hay bacillus {Bacillus subtilis) is easily 

 procured as a practically pure culture. The 

 spores are very resistant to heat and are 

 universally present in hay. To develop the 

 bacillus it is only necessary to boil some hay 

 in a little water for a few minutes, to pour 

 off the infusion and leave it to itself. The 

 boiling kills practically everything except the 

 spores and these subsequently germinate to 

 form minute rods, motile by means of a number of cilia scattered over their 



Fig. 



2. Bacillus subtilis (Ehr.) Cohn 

 a, Motile bacillus; />, non-motile 

 bacilli, single and in a chain ; 

 c, spores from film e; d, a chain of 

 motile bacilli; e, spores embedded 

 in slimy film, a-d, 1 500/1 ; 

 e, 250/1. (From Fischer, Vor- 

 lesungen uber Bakterien.) 



