Bacteria which act as agents in the formation of pus are collectively known as pyogenic organisms. These are divided into two groups: A. _The Obligate_; and B. _The Facultative._ _Obligate_ pyogenic organisms are those whose activity is manifested in the direction of pus formation, which seem to produce it if they produce any unpleasant action whatever. On the other hand, the _facultative_ organisms are those which are known occasionally to be active in this direction, and yet which are not always nor necessarily so. The members of group A are fairly well known and catalogued, and are not numerous. On the other hand, there is reason to believe that many organisms may have the occasional effect of producing pus, as it were, by accident or at least in a way not absolutely natural or peculiar to themselves, but still are frequently found when there is no pus present. A suitable list of the facultative organisms, therefore, can hardly be made, and will not be here attempted, the effort being only to mention the more common organisms which play this facultative role. It may be mentioned also that even the adjectives “obligate” and “facultative” are to be accepted with some mental reservation, since staphylococci, for instance, may be met with even in the absence of pus, although nearly all that we know about these organisms implies that pus would be the result of their presence. Furthermore, there are certain other organisms, not, strictly speaking, bacteria, which also have the power of producing either pus or pyoid material. These also will be mentioned in their place. Some of them belong not only to the vegetable, but also to the animal kingdom. =Obligate Pyogenic Organisms.= A. =The Staphylococcus Pyogenes Aureus, Albus, Citreus, the Staphylococcus Epidermidis, etc.=--One of the characteristics of the staphylococci as a group is the powerful peptonizing action which they exert. Moreover, the chemical products of their life changes seem to be more potent in a local than a general way, leading to greater destruction of tissue in their immediate vicinity, with greater inhibition of the chemotactic powers of the leukocytes; that is, with more interference with phagocytosis, by which their progress would be interfered with. Their presence is recognized by a peculiar odor, as of sour paste, which should lead to a prompt change of dressings and disinfection of the wound (by irrigation, spraying with hydrogen dioxide, etc.).