When very few bacteria are present, culture methods must be resorted to, but such methods do not come within the scope of this work. When considerable numbers are present, they can be detected and often identified in cover-glass smears. Several smears should be made, dried, and fixed as described under Sputum ([p. 32]). One of these should be stained one-fourth to one-half minute with Löffler's methylene-blue, rinsed well with water, dried, mounted, and examined with an oil-immersion lens. This will show all bacteria except the tubercle bacillus, and often no other stain is necessary for their identification. In many cases special stains must be applied.
Gram's method ([p. 40]) is a very useful aid in distinguishing certain bacteria. The more important organisms react to this staining method as follows:
|
GRAM STAINING (Deep purple). |
GRAM DECOLORIZING (Colorless, unless a counterstain is used). |
| Staphylococcus. | Gonococcus. |
| Streptococcus. | Meningococcus. |
| Pneumococcus. | Bacillus of influenza. |
| Bacillus diphtheriæ. | Typhoid bacillus. |
| Bacillus tuberculosis. | Bacillus coli communis. |
| Bacillus of anthrax. | Spirillum of Asiatic cholera. |
| Bacillus of tetanus. | Bacillus pyocyaneus. |
| Bacillus aërogenes capsulatus. | Bacillus of Friedländer. |
| Koch-Weeks bacillus. | |
| Bacillus of Morax-Axenfeld. |
| FIG. 113.—Staphylococcus pyogenes albus from an abscess of the parotid gland (Jakob). |
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 is so long as to extend across several microscopic fields. Streptococci are more virulent than staphylococci, and are less common.
| 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. The bacilli are generally difficult to find in pus, and bacteria-free pus would suggest tuberculosis.