_Staphylococcus pyogenes aureus_—a microorganism producing yellow pus. _Staphylococcus pyogenes citreus_—a microorganism producing lemon-colored pus. _Staphylococcus pyogenes albus_—a microorganism producing white pus. _Streptococcus pyogenes_—a streptococcus producing pus. (Erysipelas for example). _Micrococcus gonorrhea_—bacillus of Neisser, or gonococcus. _Bacillus pyocyaneous_—producing a green pus. _Bacillus coli communis_—producing intestinal conditions. (Appendicitis for example). _Bacillus typhosis_—Eberth’s bacillus; producing typhoid fever. _Bacillus tuberculosis_—Koch’s bacillus; producing tuberculosis. _Bacillus tetani_—Nicolaier’s bacillus; causing tetanus. _Treponema pallidum_, or _spirochaeta pallida_ of Hoffman and Schaudin—a protoza causing syphilis. The first six are known as _pyogenic bacteria_, as they all produce pus; in addition to the above there are many more microorganisms, but from a surgical standpoint those mentioned are the most important. The _staphylococcus pyogenes_ is a spherical coccus of somewhat variable size but averaging about 8 microns; when properly stained it can often be seen to be formed of two separate hemispheres. In pus it is generally found in small heaps containing from two to ten members, but it also occurs singly and in pairs, and even in short chains like the streptococcus, thus rendering diagnosis difficult with the microscope alone. Its cultures are of a yellowish tinge. The aureus type is the most usual cause of abscesses (circumscribed suppurations) and 77% of acute abscesses are due to the staphylococci. The _staphylococcus pyogenes aureus_ is a facultative anaerobic parasite which is widely distributed in nature, and is found in the soil, in the dust of air, in water, in the alimentary canal, under the nails, and in the superficial layers of the skin. It forms the characteristic color only when it grows in air. It is killed in ten minutes by a moist temperature of 58°C. and is instantly killed by boiling water. Carbolic acid (1 to 40) and bichloride of mercury (1 to 2000) are quickly fatal to these cocci. _Staphylococcus pyogenes citreus_, the lemon-colored coccus, is found occasionally in acute circumscribed suppurations, but far more rarely than the other two forms. Its pyogenic power is even weaker than that of the albus. _Staphylococcus pyogenes albus_, the white coccus, acts like the aureus, but is more feeble in power. When this organism is found upon and in the skin, it is called _staphylococcus epidermis albus_, an organism which is the cause of stitch abscesses.