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Su1MMARY: The method of most probable numbers (MI") was employed in order to determine the number of resistant brucella cells at concentrations of streptomycin of 1,lO and 100pg.lml. The examination showed agreement with the centrifugation method (plating of the sediment of at least 1OOml. of culture together with the required concentration of streptomycin) in a range up to 1000 resistant cells/100 ml. The method showed that there exist differences in the numbers of resistant cells in different strains, and that in cultures originating from the same colony variations occur. The general trend is a rtlative increase of the resistant cells with the age of the culture. Methods for determining the number of streptomycin-resistant cells in bacterial cultures have been described by many authors. Klein & Kimmelman (1946) employed broth cultures of Shigella dysenteriae. Tenfold dilutions up to were assayed in varying concentrations of streptomycin in melted agar, the test requiring at least 42 plates, In another experiment they distributed 40 ml. of broth culture into 400 broth tubes containing 1000 units strepto- mycin/ml. After 48hr., growth appeared in 5 out of 400 tubes. Yegian & Vanderlinde (1948) incubated different sizes of inocula of Mycobacteriurn ranae on glycerol agar with varying quantities of streptomycin; one test required at least 120 plates. English & McCoy (1951) incubated 100 tubes containing 0.3 ml. broth culture of Micrococcus pyogenes and 200 units of streptomycin. One tube out of 100 showed visible growth after 24 hr. They estimated the number of bacteria incubated in 100 tubes at 67 x lo8 and concluded, there- fore, that one resistant cell was present in this total bacterial population. All these methods require large quantities of tubes, agar and plates, and are time and material consuming. We tried, therefore, to simplify this technique by the determination of the most probable number (MPN) of streptomycin resistant micro-organisrps in a culture. This method was proposed by McCrady (1915) for the numerical interpretation of fermentation tube results. For our purposes, the determination of small numbers of streptomycin resistant cells in a large bacterial population, we used the tables published by Hoskins (1934), reprinted in Standard Methods for the Ezamination of Water and Sewage (1936). We expected that with this relatively simple technique it would be possible to detect the presence of streptomycin resistant organisms in brucella cultures at different stages of their incubation.

The Determination of the Most Probable Numbers of Streptomycin-fast Cells in Brucella Cultures and their Variability in Growing and Ageing Cultures