The nature of the inflammatory and suppurative processes will be considered in detail later; suffice it here to say that they are brought about by the action of one or other of the organisms that we have now to consider. It is found that the _staphylococci_, which cluster into groups, tend to produce localised lesions; while the chain-forms--_streptococci_--give rise to diffuse, spreading conditions. Many varieties of pyogenic bacteria have now been differentiated, the best known being the staphylococcus aureus, the streptococcus, and the bacillus coli communis. [Illustration: FIG. 2.--Staphylococcus aureus in Pus from case of Osteomyelitis. × 1000 diam. Gram's stain.] _Staphylococcus Aureus._--This is the commonest organism found in localised inflammatory and suppurative conditions. It varies greatly in its virulence, and is found in such widely different conditions as skin pustules, boils, carbuncles, and some acute inflammations of bone. As seen by the microscope it occurs in grape-like clusters, fission of the individual cells taking place irregularly (Fig. 2). When grown in artificial media, the colonies assume an orange-yellow colour--hence the name _aureus_. It is of high vitality and resists more prolonged exposure to high temperatures than most non-sporing bacteria. It is capable of lying latent in the tissues for long periods, for example, in the marrow of long bones, and of again becoming active and causing a fresh outbreak of suppuration. This organism is widely distributed: it is found on the skin, in the mouth, and in other situations in the body, and as it is present in the dust of the air and on all objects upon which dust has settled, it is a continual source of infection unless means are taken to exclude it from wounds. The _staphylococcus albus_ is much less common than the aureus, but has the same properties and characters, save that its growth on artificial media assumes a white colour. It is the common cause of stitch abscesses, the skin being its normal habitat. [Illustration: FIG. 3.--Streptococci in Pus from an acute abscess in subcutaneous tissue. × 1000 diam. Gram's stain.]