The most common pus-producing organisms are _staphylococci_ and _streptococci_. They are both cocci, or spheres, their average diameter being about 1 µ. Staphylococci are commonly grouped in clusters, often compared to bunches of grapes (Fig. 113). There are several varieties, which can be distinguished only in cultures. Streptococci are arranged side by side, forming chains of variable length (Fig. 114). Sometimes there are only three or four individuals in a chain; sometimes a chain {263} is so long as to extend across several microscopic fields. Streptococci are more virulent than staphylococci, and are less common. [Illustration: FIG. 114.--Streptococcus pyogenes from a case of empyema (Jakob).] Should bacteria resembling _pneumococci_ be found, Buerger's method (p. 37) should be tried. When it is inconvenient to stain before the smears have dried, capsules can be shown by the method of Hiss. The dried and fixed smear is covered with a stain composed of 5 c.c. saturated alcoholic solution gentian-violet and 95 c.c. distilled water, and heated until steam rises. The preparation is then washed with 20 per cent. solution of copper sulphate, dried, and mounted in Canada balsam. Pneumococci may give rise to inflammations in many locations. When they form short chains, demonstration of the capsule is necessary to distinguish them from streptococci. If tuberculosis be suspected, the smears should be stained by one of the methods for the _tubercle bacillus_ (pp. 32 and 127), or guinea-pigs may be inoculated. {264} The bacilli are generally difficult to find in pus, and bacteria-free pus would suggest tuberculosis. [Illustration: FIG. 115.--Diplococcus pneumoniæ from ulcer of cornea (obj. one-twelfth oil-immersion) (study through courtesy of Dr. C. A. Oliver) (Boston).] [Illustration: FIG. 116.--Gonococci in urethral pus (McFarland).] _Gonococci_, when typical, can usually be identified with sufficient certainty for clinical purposes in the smear stained with Löffler's methylene-blue. They are coffee-bean-shaped cocci which lie in pairs with their flat surfaces together (Fig. 116). They lie for the most part within pus-cells, an occasional cell being filled with them, while the surrounding cells contain few or none. A few are {265} found outside of the cells. It is not usual to find gonococci when many other bacteria are present, even though the pus is primarily of gonorrheal origin. Whenever the identity of the organism is at all questionable, Gram's method should be tried. In rare instances it may be necessary to resort to cultures. The gonococcus is distinguished by its failure to grow upon ordinary media. Gonococci are generally easily found in pus from untreated acute and subacute gonorrheal inflammations,--conjunctivitis, urethritis, etc.,--but are found with difficulty in pus from chronic inflammations and abscesses, and in urinary sediments. PERITONEAL, PLEURAL, AND PERICARDIAL FLUIDS