Modification of Rappaport-Vassiliadis enrichment broth into a semisolid motility medium (MSRV) provided a sensitive means for detecting Salmonella in contaminated foods. The type of peptone, the concentration of magnesium chloride, the presence of novobiocin and the temperature of incubation were determinants in medium performance. The analytical procedure consists of preenrichment for 20 h, followed by motility enrichment on MSRV for 24 h and, if there is migration, serological tests with the motile culture. The test result is obtained within 48 h from the start of preenrichment. This approach gave 39% more Salmonella-positive samples than enrichment in tetrathionate brilliant green broth with subsequent plating.

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The mutant strain Aerobacter aerogenes 62-1 AC, pab-, accumulates a labile substance, desig­nated Compound A which supports the growth of other pab- mutants. Previous studies seemed to indicate that Compound A might be an intermediate in the conversion of chorismate to 4-amino-benzoate (Altendorf, Bacher, and Lingens, Z. Naturforsch. 24 b, 1602 [1969]). The present experi­ ments show that Compound A is 4 -(D-glucosylamino)-benzoic acid. This substance which is clearly not an intermediate in the biosynthesis of 4-aminobenzoate is formed by reaction of 4-aminobenzoate produced de novo by the mutant with exogenous glucose. Mutant 62-1 AC has the unimpaired capacity to synthesize 4-aminobenzoate as shown by direct enzyme studies. Mutant 62-1 AC re­ quires approximately 200 times more 4jaminobenzoate for growth than other pab" mutants of A. aerogenes with defective 4-aminobenzoate synthetase. The genetic defect of mutant 62-1 AC seems to be located in a metabolic system concerned with the utilization of 4-aminobenzoate rather than its synthesis.

Aerobacter aerogenes mutant 62-1 AC requires high concentrations of 4-aminobenzoate for growth. The mutant accumulates N-glucosyl-4-aminobenzoate and has an intact 4-aminobenzoate synthetase (Bacher, Gilch, Rappold, and Lingens, Z. Naturforsch. 28c, 614 - 617 [1973]). On the other hand the ability of the mutant to synthesize dihydropteroate is markedly reduced. The dihydropteroate synthetase level of mutant 62-1 AC is 1% as compared to the parent strain. Spontaneous revertants of mutant 62-1 AC show wild type levels of dihydropteroate synthetase. We conclude that the requirement for 4-aminobenzoate in mutant 62-1 AC is due to poor utilization of 4-aminobenzoate as a consequence of the low level of dihydropteroate synthetase activity.