We find a spot which answers to the description of a colony of tubercle bacilli.  We now take a minute particle from this colony on a wire and convey it to the surface of some hardened blood serum in a test tube.  We plug the tube so that no air germs may drop in, and place it in an incubator at the proper temperature.  After several days, if no contamination be present, a colony of bacilli will appear around the spot where we sowed the spores.  Let us repeat the process.

Take a particle from this colony, and transfer it to another tube.  This is our second culture.  This must be repeated until we are satisfied that we have secured a pure culture.  If this be carried to the twenty-fifth generation, we may be assured that there remains no pus, no ptomaines, nothing but the desired bacilli.

It is a proper material now for inoculation, and if we inoculate some of the lower animals, for instance the monkey, we produce a disease identical with phthisis pulmpnalis.  Bacteria also afford peculiar chemical reactions.  For example, nitric acid will discharge all the color from all bacilli artificially dyed with anilin, except those of tubercle and anthrax.  One species is stained readily with a dye that leaves another unaltered.  Thus we are enabled in the laboratory to determine whether the bacilli found in sputum, for example, are from tubercle or are the bacteria of decomposition.

From what I have said of the tubercle bacillus, it would seem thoroughly demonstrated that it is the cause of tubercle in these animals.  But we must walk cautiously here.  These are not human beings, who know that like results would follow their inoculation.  The animals used by Koch are animals very subject to tubercle.

We must, from the very nature of our environment, be constantly inhaling these germs as we pass through the wards of our hospitals; yes, they are floating in the air of our streets and dwellings.  It becomes necessary then for us to inquire:  If bacteria cause disease, in what manner do they produce it?  The healthy organism is always beset with a multitude of non-pathogenic bacteria.  They occupy the natural cavities, especially the alimentary canal.  They feed on the substances lying in their neighborhood, whether brought into the body or secreted by the tissues.  In so doing they set up chemical changes in their substances.  Where the organs are acting normally these fungi work no mischief.  The products of decomposition thus set up are harmless, or are conveyed out of the body before they begin to be active.

If bacteria develop to an inordinate degree, if the contents of organs are not frequently discharged, fermentative processes may be set up, which result in disease.  Bacteria must always multiply and exist at the expense of the body which they infest, and the more weakened the vital forces become, the more favorable is the soil for their development.