104 PRINCIPLES OF BACTERIOLOGY 



that many species will not grow at all or only sparingly when in close 

 proximity to some other species. This antagonism, however, is often 

 only one-sided in character; for instance, the bacillus fluorescens putidus 

 grows well when inoculated between streaks of staphylococcus, but 

 the latter micrococcus will not grow at all when inoculated between 

 cultures of the bacillus putidus, the growth of the staphylococcus remain- 

 ing scanty when the two species are inoculated simultaneously. Again, 

 when gelatin or agar plates are made from two different species of 

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

 method of making this experiment is to simultaneously inoculate the 

 same liquid medium with two species, and then examine them later, 

 both microscopically and by making plate cultures; not infrequently 

 the one species may take precedence of the other, which it finally 

 overcomes entirely. The practical application of this is to make 

 sufficient dilutions of material when plating for the estimation of the 

 number of bacteria or the isolation of pure cultures. 



Finally, bacteria may oppose one another as antagonists 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. Brueger has recently shown that pneumococci when grown 

 together with a bacillus obtained from the throat, produce very large, 

 succulent colonies. 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 decomposition of 

 nitrates to gaseous nitrogen, cannot be produced by many bacteria 

 alone, but only when two are associated. 



3. Attenuated varieties of bacteria may regain virulence when grown 

 in contact with other bacteria. 



Duration of Life in Pure Water. When bacteria which require much 

 organic food for their development, and these include most of the path- 

 ogenic species, are placed in distilled water they soon die that is, 

 within a few days; even in sterilized well water or surface water their life 

 duration does not usually exceed eight to fourteen days, and they rarely 

 multiply. Instances, however, of much more extended life under certain 

 conditions are recorded. 



Effect of Drying. Want of water affects bacteria in different ways. 

 Upon dried culture media development soon ceases; but in media dried 

 gradually at the room temperature (nutrient agar, gelatin, potato) they 

 live often for a long time, even when there are no spores to account for 

 it. A shrunken residue of such cultures in bouillon has often been 

 found, after a year or more, to yield living bacteria. The question as 

 to how long the non-spore bearing forms are capable of retaining their 

 vitality when dried on a cover-glass or silk threads has been variously 

 answered. We know now that there are many factors which influence 

 the retention of vitality. The following table of the results obtained 



