Comparative Study of Presumptive and Confirmative Media for Bacteria of the Coliform Group and for Fecal Streptococci

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Abstract
Confirmation of coliform bacteria by inoculation of secondary tubes of brilliant green lactose bile broth has been found as efficient as the more involved E.M.B. plate method; confirmation of Escherichia coli by inoculation of secondary tubes of buffered lactose broth at 45.5[degree]C is also a simple, practical method. Addition of 0.15% of Bacto bile salts No. 3 to this medium, with a slight modification as to amts. of other ingredients, promises a method of efficiency comparable to the brilliant green lactose broth method for the coliform group. Lauryl sulfate tryptose broth yields many more positives for the coliform group than standard lactose broth and gives comparatively few false presumptive reactions. A new "EC" (E. coli) medium is equally as efficient for the isolation of coliform bacteria as lauryl sulfate tryptose broth and gave no false presumptives on a series of 147 samples of treated and untreated waters. This medium inhibits almost completely fecal streptococci and other Gram-positive bacteria with no apparent inhibition of coliform bacteria. The new "EC" medium can be used equally well for isolation of coliform bacteria at 37[degree]C or for isolation of E. coli at 45.5[degree]C. It can be used either as a primary medium for the growth of E. coli or as a satisfactory secondary medium for the confirmation of E. coli. A new "SF" medium is described which is highly specific at 45.5[degree]C for fecal streptococci. The mere presence of growth and acid in this medium at 45.5[degree]C is confirmatory evidence of fecal streptococci. The medium may be inoculated directly with water, milk, or sewage, or may be inoculated from other primary media, such as standard lactose broth.

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