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. x 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. x 1000 diam. Gram's stain.]

_Streptococcus Pyogenes._--This organism also varies greatly in its
virulence; in some instances--for example in erysipelas--it causes a
sharp attack of acute spreading inflammation, which soon subsides
without showing any tendency to end in suppuration; under other
conditions it gives rise to a generalised infection which rapidly proves
fatal. The streptococcus has less capacity of liquefying the tissues
than the staphylococcus, so that pus formation takes place more slowly.
At the same time its products are very potent in destroying the tissues
in their vicinity, and so interfering with the exudation of leucocytes
which would otherwise exercise their protective influence. Streptococci
invade the lymph spaces, a