From the point of view of the surgeon the most important varieties of
micro-organisms are those that cause inflammation and suppuration--the
_pyogenic bacteria_. This group includes a great many species, and these
are so widely distributed that they are to be met with under all
conditions of everyday life.

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