60 OUTLINES OF BACTERIOLOGY 



Bac. typhosus, however, is not a highly resistant form ; but even 

 Bac. coli communis was reduced to 20 per cent, of the original number 

 after 15 minutes' exposure to direct sunlight, and all the organisms 

 were destroyed after 4 hours' exposure. These two organisms, how- 

 ever, do not form spores. Ward has tried the effect of direct sunlight 

 on spores of Bac. arithracis and stated that two to six hours' exposure 

 produced a germicidal effect. 



The effect of light on pigment-producing bacteria must here be 

 noticed. It sometimes happens that these bacteria when cultivated 

 for some time lose their power of producing pigment, which power, 

 however, may unexpectedly return. The conditions affecting this loss 

 of colour are unknown in most cases, but it is known that Bac. lactis 

 erythrogenes invariably loses its power of forming a red colouring 

 matter when strongly illuminated. Again, in the case of the 

 phosphorescent Photobacterium sarcophilum, the power of producing 

 phosphorescence is lost when the organism is allowed to grow in a 

 lighted room. 



While the vast majority of bacteria are thus disastrously affected by 

 light, the purple-bacteria always grow in places exposed to strong 

 sunlight, and it has been claimed that they are partially dependent 

 upon light for their food supply. These bacteria will be dealt with 

 in a later chapter. 



4. MOISTURE. 



Like all living organisms, bacteria require moisture, the amount 

 required varying with different species. For example, Spirillum 

 giganteum will not grow on a very dry medium, and the same is 

 probably true of all spirilla, but Bac. subtilis, and most of the soil- 

 bacteria grow well on very dry nutrient media. Consequently, 

 when drought occurs, the spirilla perish first, and are followed by 

 the vegetative cells of all genera. The spores are naturally the most 

 resistant, for their coats do not permit the water of the enclosed 

 protoplasm to pass through for a very long time, and they have 

 been known to be capable of germination after having been in a 

 dried-up condition for many years. The importance of water will 

 be readily seen when it is considered that more than three-fourths of 

 protoplasm consists of this liquid. 



