42 OUTLINES OF BACTEEIOLOGY 



are of an acid nature, which act more injuriously on some bacteria 

 than on others. Again, changes in the gaseous constitution of the 

 environment, especially in the amount of oxygen present, affect dif- 

 ferent bacteria in different ways. To these may be added changes 

 in the temperature, in the degree of moisture and other factors. 

 Hence we may assert that where bacterial growth is taking place, 

 the environment is continually changing. In the early days of 

 Bacteriology many mistakes occurred, because the conditions of 

 bacterial growth, and especially the relations of the growth to the 

 environment, were only imperfectly understood. It was frequently 

 asserted that any particular species assumed several forms in the course 

 of its life-history, that, for example, a bacillus, under certain conditions, 

 assumed the coccus-shape, and under other conditions even a spiral- 

 form. From what has just been stated it will be seen that the investi 

 gators who made these statements had not worked with pure cultures, 

 and that the apparent change from one shape to another was really 

 due to the predominance first of one organism and then of another of 

 a different shape. 



A good example is given us by one of Lister's experiments. Ordinary 

 milk was allowed to become sour spontaneously. A drop of the sour 

 milk was introduced into boiled milk, another into sterilised beef 

 extract, and a third into sterilised urine. After growth had taken place 

 inoculations were made from each into Pasteur's nutrient solution, then 

 from this to urine, and finally back again into milk. Assuming that 

 the souring of the milk had been effected by one organism, Lister 

 concluded that the various forms found in the resulting cultures were 

 all descendants of one species, which had undergone strange changes 

 in form as a result of having been grown in different media. The 

 explanation is seen when it is remembered that the sour milk from 

 which he started contained several organisms, and in each medium a 

 different one became predominant. These results caused Lister to give 

 the name of Bacterium lactis to what he thought was one species. 

 This mistake went so far on the Continent that one investigator 

 declared that all bacteria were modifications of one species which he 

 called Coccobacteria septica. 



It does not always happen that two organisms feeding on the same 

 medium are so antagonistic that one has to give way. A good example 

 of this is Bacillus coli communis. Although normally present in the 

 alimentary canal, it produces no evil effects; in conjunction however 

 with other organisms it is able to set up acute intestinal irritation 

 and produce various changes of an inflammatory nature in the body. 



