ANTAGONISM OF MICBO-ORGANISMS (SYMBIOSIs) 105 



cedence, and the other may develop later ; or two or more 

 species may develop at the same time ; or the growth ot 

 one species may prevent the growth of another by either 

 {a) exhausting the food material by its rapid growth, or 

 (6) by producing products which retard or prevent the 

 growth of another. The following are some of the results 

 of Freudenreich's investigations. Bacillus pyogenes fcetidus 

 prevents the growth of the Spirillwm cholerce Asiaticce ; 

 Micrococcus roseus prevents the growth of Micrococcus 

 teiragenus. The following organisms cause a change in 

 broth which prevents the growth of other species : Bacillus 

 pyocyaneus, Bacillus phosphorescens, Bacillus prodigiosus, 

 Spirillum cholerce Asiaticce. The metabolic products of 

 some organisms, e.g., Bacillus typhosus, are capable of 

 restraining the growth of the organism itself. Correlative 

 phenomena are the beneficial alterations which are pro- 

 duced in the behaviour of an organism by the presence in 

 the culture medium of other organisms. Thus the presence 

 of streptococci appears to enhance the virulence of the 

 diphtheria bacillus; or attenuated cultures of Bacillus 

 anthracis may reacquire virulence, if injected simul- 

 taneously with a culture of Bacillus prodigiosus. And 

 in some cases bacteria thrive better in the presence of 

 certain others than without them. 



Certain chemical actions cannot be effected by some 

 bacteria alone, while they can be done by two forms in com- 

 bination : for instance, the decomposition of nitrates into 

 gaseous nitrogen. 



It has also been observed that among certain soil bacteria 

 the smgle varieties are harmless, but in combination they 

 are found to be pathogenic to animals. This fact should 

 merit special attention when investigating new or obscure 

 diseases. Some writers have assumed cholera to be pro- 

 duced by the combination of two organisms (diblastic 



