ACTION OF LIGHT 19 



organisms may have different capacities for producing 

 pigment or toxine and different resistance to external 

 influences. Even different specimens of the same culture 

 may vary if they are subcultured in different conditions or 

 even at different ages of the original culture. The failure 

 to recognise this fact has led to much confusion and error, 

 and it must be constantly borne in mind, as among the 

 most important and fundamental data of bacteriology. 



Eesistancb of the Bacteria to External Influences. 



Bacteria, like other living organisms, are exposed to many 

 outside influences. These we will consider under six heads, 

 namely, Light, Heat, Cold, Desiccation, Electricity, Chemical 

 Agents. The resistance of micro-organisms to these in- 

 fluences is very high, especially in the spore stage. 



(1) Light. — Messrs. Downes and Blount, in a communica- 

 tion to the Koyal Society in 1881, first called attention to 

 the fact that light had an injurious effect upon bacteria, 

 and that cultures may be destroyed by exposure to sunlight. 

 About 1885, Duclaux and others took up this subject, and 

 with various pure cultures of micro-organisms it was found 

 that by exposure to sunlight the spores of various bacteria 

 lose their capacity to germinate. It was also found that 

 cultures lost their power of reproduction in diffused light, 

 and that they also became ' attenuated ' in their pathogenic 

 power. In his address before the International Medical 

 Congress at Berlin, in 1890, Koch stated that the tubercle 

 bacillus was killed by the action of sunlight in a time vary- 

 ing from a few minutes to several hours, depending upon 

 the thickness of the layer exposed. Diffused daylight 

 had the same effect, although a considerably longer time 

 was required ; when placed close to the window, about a 

 week was required. Similar results have been obtained by 

 Janowski (1891) on the typhoid bacillus. 



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