264 PURE CULTURES 



It will be realised that the identification of the pathogenic 

 forms (i.e. those causing disease), in particular, is of great im- 

 portance. Owing, however, to the small dimensions of the 

 individuals, it is as a rule impossible to identify the species by 

 microscopic examination alone. Resource is therefore had to 

 the various reactions of Bacteria under conditions of culture. 



Bacteria can be grown in the laboratory on various artificial 

 media, such as extract of meat, decoctions of fruit, etc. Under 

 such conditions, and at temperatures suitable for each species, 

 growth is very rapid, and a single organism soon multiplies to 

 such an extent that the mass of its offspring is visible to the 

 naked eye. In making cultures all the vessels and instruments 

 employed are scrupulously cleaned and freed from live Bacteria 

 (sterilised) , either by heating at high temperatures or by washing 

 with special disinfectants, since even small quantities of dust 

 arfe replete with bacterial spores. 



The different kinds of Bacteria present in any material to 

 be examined can be separated from one another and isolated 

 by using the various culture media mentioned above, with the 

 addition of gelatine or agar-agar (p. 204). A little of the material 

 containing the organisms is well mixed with a considerable 

 quantity of such a medium, liquefied by warming, so that the 

 individual bacilli are widely separated from one another. On 

 allowing the mixture to cool the Bacteria are immobilised ; they 

 soon grow, and each of the widely separated individuals gives 

 rise to a small group (colony), visible to the naked eye, which can 

 be transferred to another lot of culture medium. In this way 

 cultures can be obtained which have arisen from single organisms 

 and consist of one species only (pure cultures] ; in them the form 

 of the Bacteria, their mode of growth, the formation of pigment 

 (usually outside the cells) and other chemical substances, can 

 be studied and the species identified. 



Some Bacteria are entirely parasitic (e.g. the Micrococcus 

 which causes spotted fever) and cannot live apart from their 

 host, whilst some are semi-parasitic (e.g. Bacillus tetani) and can 

 live either in earth, etc., or in the body of a Mammal. Each 

 parasitic species can generally use only one or two species of 

 Mammal as hosts ; children, for example, do not have distemper, 

 and dogs and cats do not have measles and chicken-pox. Simi- 



