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Isoleucine and valine metabolism in Escherichia coli. XI. Valine inhibition of the growth of Escherichia coli strain K-12. J. Bacteriol. 83:624-630. 1962.-The inhibition of the growth of Escherichia coli strain K-12 by valine was shown to be due to the sensitivity of the acetohydroxybutyrate-forming system to valine. It was demonstrated that both E. coli strain W, a strain whose growth is unaffected by valine, and a valine-resistant mutant of strain K-12 have acetolactateand acetohydroxybutyrate-forming systems which are less sensitive to valine than that of strain K-12. It was further shown that a-aminobutyrate accumulates

While investigating the enzymatic formation of acetohydroxybutyrate by extracts of Escherichia coli, it became necessary to devise an assay for the quantitation of this early intermediate in isoleucine synthesis. A microbiological assay, based on the reversal by acetohydroxybutyrate (Umbarger, 1958; Umbarger and Brown, 1958) of the inhibitory effect of L-valine on E. coli strain K-12, provided the necessary tool to accomplish the desired quantitations. Although such an assay would ordinarily require that the reaction mixtures be sterilized by filtration because of the lability of acetohydroxybutyrate to heat, it was possible to eliminate sterilization by employing an Oxford cup assay. However, in the initial attempts to apply the cup assay to the determination of acetohydroxybutyrate, it was clear that the measurement of the zone diameters was not sufficiently precise for quantitating growth. For example, the zones resulting from higher concentrations of acetohydroxybutyrate were not only wider but also denser than those from lower concentrations. Often, two zones would differ only slightly in their diameters although simple visual comparison showed clearly that the two zones differed considerably in the amount of growth. In addition, their margins were often not clearly delineated. These difficulties were overcome by measuring the protein of the bacterial cells in the growth zone. Because the method may be useful in other systems it is described in this paper. MATERIALS AND METHODS The assay organism was strain K-12 of E. coli. The mineral medium employed was that of Davis and Mingioli (1950) modified by the omission of citrate. The solid medium (valine agar) contained 1.5 per cent agar and was supple-' This work was supported in part by U. S.

The growth of microorganisms in fermentations where oil had been maintained as the continuous phase was examined to determine whether advantage could be gained from the increased solubility of oxygen in hydrocarbon. Although cell concentrations were highest in the aqueous phase of oil‐continuous systems, due to the large oil fraction, productivities achieved per unit fermenter volume were generally equivalent to those obtained from water‐continuous systems. With the oil‐continuous emulsions, the power requirement for aeration and mixing was less, and phase reversal resulted in a threefold concentration of cells in the aqueous medium, thereby facilitating their recovery.

Whole organisms and soluble extracts of a Pseudornonas sp. were examined for their ability to oxidize suspected intermediates in the oxidation of p-isopropyltoluene. A comparison of the relative activities of glucosegrown and p-isopropyltoluene-grown bacteria and their extracts indicated that p-isopropylbenzyl alcohol, p-isopropylbenzaldehyde and p-isopropylbenzoic acid are intermediates in the oxidation of the hydrocarbon. p-Isopropylbenzoic acid was formed during growth on p-isopropyltoluene and this was also shown to be a product of the oxidation ofp-isopropyltoluene, p-isopropylbenzyl alcohol and p-isopropylbenzaldehyde by soluble extracts prepared from p-isopropyltoluene-grown bacteria. The enzymes which catalyze the oxidation of p-isopropyltoluene were repressed by glucose and induced by growth on either p-isopropyltoluene or p-isopropylbenzoic acid.