Abstract
In a study on the concentration of tubercle bacilli from urine 1 two standard procedures were used as controls: Petroff's tannic-acid method† and direct centrifugation for one hour. Since the detailed study of newer methods will not appear for some time, and since the results of one of the present technics are definitely improved by simplification, a brief analysis of this procedure is desirable.
Using Breed's counting method, the 2 procedures were analyzed quantitatively for their ability to collect human tubercle bacilli which had been added from “clump-free” suspensions to clear normal urine in predetermined numbers. The results were expressed as “concentration-factors” which indicated the increase in the average number of bacilli per microscopic field following concentration.
The concentration-factors were determined for direct centrifugation, for each of the steps outlined below for the Petroff procedure, and for a procedure which duplicated Petroff s, except for the omission of the tannic acid. Two methods—milk and HCl—for “fixing” the sediments to the glass slides were compared. The results shown in Fig. 1 are average values from 4 experiments in which different concentrations of bacilli were added to the urines. There were no exceptions to the general ratings as illustrated.
The results in the chart illustrate several useful facts: (a) The Petroff method appears to depend on the spontaneous precipitation of urates from acidified, chilled urine rather than on the use of tannic acid. It could be designated more properly as a “urate” concentration-method. (b) The first “urate” (or tannic acid) sediment produces a higher concentration-factor than is obtained by direct centrifugation. (c) The remainder of the Petroff procedure results in a loss of the majority of the bacilli which were collected in the first step by precipitation, (d) Slide losses are reduced by the use of milk as a fixative.
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