INFLUENCE OF EXTERNAL CONDITIONS 15 



to spore formation. Of these, there are at least three of which 

 bacteriologists have knowledge, namely, moisture, oxygen, and a 

 certain temperature. Fluid media forms an excellent nidus for 

 sporulation so long as some oxygen can gain access to the sporulating 

 germs. But many organisms will not sporulate if lying deep in 

 such a medium. In moulds and yeasts oxygen is essential, and for 

 some spore-bearing bacilli a supply of oxygen is a sine qud non (the 

 exceptions are strict anaerobes like B. tetani, B. butyricus, etc.) of 

 sporulation. Prazmowski has pointed out that it is characteristic of 

 these forms that they are non-motile during sporulation. B. tetani, 

 B. lutyricus, and other strict anaerobes continue to remain motile 

 during sporing. Temperature exerts a marked influence on the 

 process.* In the case of B. subtilis, an organism frequently present in 

 milk, spore formation did not occur below 6 C. ; at 18 C. it required 

 two days ; at 22 C. one day ; and at 30 C. only twelve hours, f 



When free in the field of the microscope, spores must be dis- 

 tinguished from fat cells, micrococci, starch cells, some kinds of ova, 

 yeast cells, and other like objects. Spores are detected frequently 

 by their resistance to ordinary stains and the necessity of colouring 

 them by special staining methods. When, however, a spore has 

 taken on the desired colour, it retains it with tenacity. In addition 

 to their shape, size, thickened capsule, and staining characteristics, 

 spores also resist desiccation and heat in a much higher degree than 

 bacilli not bearing spores. It has been suggested that bacteria 

 should be classified according to their method of spore formation. 



The Influence of External Conditions on the 

 Growth of Bacteria 



In the earliest days of the study of micro-organisms it was 

 observed that they mostly congregate where there is suitable food 

 for their nourishment. The reason why fluids such as milk, and 

 dead animal matter such as a carcase, and living tissues such as a 

 man's body, contain many microbes, is because each of these three 

 media is favourable to their growth. Milk affords almost an ideal 

 food and environment for microbes. Its temperature and con- 

 stitution frequently meet their requirements. Dead animal matter, 

 too, yields a rich diet for certain species (saprophytes). In the 

 living tissues bacteria obtain not only nutriment, but a favourable 



* Koch has shown in the case of B. anthracis that at least 16 C. is necessary 

 for spore formation, and at this temperature limited formation of spores did not 

 occur until after seven days. At 21 C. spores had formed after seventy-two hours, 

 at 25 C. after thirty- five to forty hours, and between 30 C. and 40 C. in about 

 twenty-four hours ; the best and strongest cultivations were obtained from 20 

 to 25 C. 



f Fliigge. Micro-organisms. Translation by W. Watson Cheyne, 1890, p. 539. 



