6. Bacteria.—Normal urine is free from bacteria in the bladder, but becomes contaminated in passing through the urethra. Various non-pathogenic bacteria, notably Micrococcus ureæ (Fig. 54), are always present in decomposing urine. In suppurations of the urinary tract pus-producing organisms may be found. In many infectious diseases the specific germs may be eliminated in the urine without producing any local lesion. Typhoid bacilli have been known to persist for months and even years after the attack.
Bacteria produce a cloudiness which will not clear upon filtration. They are easily seen with the one-sixth objective in the routine microscopic examination. Ordinarily, no attempt is made to identify any but the tubercle bacillus and the gonococcus.
Tubercle bacilli are nearly always present in the urine when tuberculosis exists in any part of the urinary tract, but are often difficult to find, especially when the urine contains little or no pus.
Detection of Tubercle Bacilli in Urine.—The urine should be obtained by catheter after careful cleansing of the parts.
(1) Centrifugalize thoroughly, after dissolving any sediment of urates or phosphates by gentle heat or acetic acid. Pour off the supernatant fluid, add water, and centrifugalize again. Addition of one or two volumes of alcohol will favor centrifugalization by lowering the specific gravity.
(2) Make thin smears of the sediment, adding a little egg-albumen if necessary to make the smear adhere to the glass; dry, and fix in the usual way.
(3) Stain with carbol-fuchsin, steaming, for at least three minutes.
(4) Wash in water, and then in 20 per cent. nitric acid until only a faint pink color remains.
(5) Wash in water.
(6) Soak in alcohol fifteen minutes or longer. This decolorizes the smegma bacillus ([p. 35]), which is often present in the urine, and might easily be mistaken for the tubercle bacillus. It is unlikely, however, to be present in catheterized specimens. It is always safest to soak the smear in alcohol for several hours or over night, since some strains of the smegma bacillus are very resistant.