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 anaérobic species 
grow even with the admission of air if only other 
aérobic 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 
