Many conditions favor the bacterial attack.  The patient’s tissues may have an inherited peculiarity, which renders it easy for the bacteria to find a good soil for development; an old injury or inflammation may render the tissues less resistant than usual; the point, at which inoculation has occurred may have certain anatomical peculiarities which make it a good place in which bacteria may multiply; the blood may have undergone certain chemical changes which render it better soil than usual for the rapid growth of these parasitic plants.

The number of bacteria originally present makes a difference also.  It is readily understood that the tissues and white blood cells would find it more difficult to repel the invasion of an army of a million microbes than the attack of a squad of ten similar fungi.  I have said that the experimenter can weaken and augment the virulence of bacteria by manipulating their surroundings in the laboratory.  It is probable that such a change occurs in nature.  If so, some bacteria are more virulent than others of the same species; some less virulent.  A few of the less virulent disposition would be more readily killed by the white cells and tissues than would a larger number of the more virulent ones.  At other times the danger from microbic infection is greater because there are two species introduced at the same time; and these two multiply more vigorously when together than when separated.  There are, in fact, two allied hosts trying to destroy the blood cells and tissues.  This occurs when the bacteria of putrefaction and the bacteria of suppuration are introduced into the tissues at the same time.  The former cause sapraemia and septicaemia, the latter cause suppuration.  The bacteria of tuberculosis are said to act more viciously if accompanied by the bacteria of putrefaction.  Osteomyelitis is of greater severity, it is believed, if due to a mixed infection with both the white and golden grape-coccus of suppuration.

I have previously mentioned that the bacteria of malignant pustule are powerless to do harm when the germs of erysipelas are present in the tissues and blood.  This is an example of the way in which one species of bacteria may actually aid the white cells, or leucocytes, and the tissues in repelling an invasion of disease-producing microbes.

Having occupied a portion of the time allotted to me in giving a crude and hurried account of the characteristics of bacteria, let me conclude my address by discussing the relation of bacteria to the diseases most frequently met with by the surgeon.

Mechanical irritations produce a very temporary and slight inflammation, which rapidly subsides, because of the tendency of nature to restore the parts to health.  Severe injuries, therefore, will soon become healed and cured if no germs enter the wound.

Suppuration of operative and accidental wounds was, until recently, supposed to be essential.  We now know, however, that wounds will not suppurate if kept perfectly free from one of the dozen forms of bacteria that are known to give rise to the formation of pus.