138 BACTERIOLOGY. 



bacillus fluorescens putidus grows well when inoculated 

 between streaks of staphylococcus, but the latter micro- 

 coccus will not grow at all when inoculated between 

 cultures of the bacillus putidus, the growth of the 

 staphylococcus remaining scanty when the two species 

 are inoculated simultaneously. Again, when gelatin 

 or agar plates are made from two different species of 

 bacteria inoculated into the same tubes while liquid it 

 may be observed that only one of the two grows. A 

 third method of making this experiment is to simulta- 

 neously inoculate the same liquid medium with two 

 species, and then examine them later, both microscop- 

 ically and in thin plates; not infrequently the one spe- 

 cies may take precedence of the other, which it finally 

 overcomes entirely. The practical application of this 

 is to make only very thin plates for the estimation of 

 the number of bacteria or the isolation of pure cultures. 



Finally, bacteria may oppose one another as antago- 

 nists in the animal body. As Emmerich has shown, 

 animals infected with anthrax may often be cured by a 

 secondary infection with the streptococcus. 



The symbiotic or co-operative action of bacteria is of 

 still greater importance, of which the following examples 

 may be given: 



1. Some bacteria thrive better in association with 

 other species than alone. Certain anaerobic species 

 grow even with the admission of air if only other 

 aerobic species are present (tetanus). 



2. Certain chemical effects, as, for instance, the de- 

 composition of nitrates to gaseous nitrogen, cannot be 

 produced by many bacteria alone, but only when two 

 are associated. 



3. In like manner it is observed that in a series of 



