EFFECTS OF CHANGES IN Mg⁺⁺ ION CONCENTRATION UPON BACTERIAL INHIBITION BY β‐2‐THIENYLALANINE IN DEFINED MEDIA

Abstract

Summary.Variation in the inhibition of growth of Escherichia coli and Shigella sp. in various media containing p-2-thienylaIanine was attributed to differences in concentrations of Mg+ + ions.Random blood samples from 112 infants were tested for elevated phenylalanine in the phenylketonuria (PKU) screening assay. Magnesium ion levels also affected the results of this assay. At 0 05 g/1 MgSO^, the concentration present in commercial PKU test agar, four false positives were detected, while no readings could be made due to overgrowth of the Bacillus suhtilis test strain when the concentration was increased to O'l g/1.INTRODUCTION.

Comparison of Liquid and Agar‐Solidified Defined Media Regarding the Physiological Mechanism by which β‐2‐Thienylalanine Inhibits Growth of Escherichia, Shigella, and Salmonella Cultures

“…Cultures were maintained on Dorset Egg Medium (13) at room temperature, transferred to peptone water-2% (w/v) tryptone (Oxoid, London) and 0.9% (w/v) NaCl/liter, and incubated at 37 C until growth reached 0.1 A450 [2 x 108 cells/ml, Brown and Lines (2)] as measured with a "Spectronic 20" spectrophotometer (Bausch and Lomb, Rochester, N.Y.). Then, 0.05 ml of a logarithmic phase culture was transferred to a defined diluent, a 1/10 dilution of a mineral salts concentrate previously elaborated (2,3), as adapted from the Vegetative Minimal Medium of Demain (12). Sterile glucose solution, to a concentration of 10 g/liter, was added to the diluent.…”

“…Bacteria were screened for inhibition by /3-2-t by streaking strains in parallel on VMM or Medium VI, placing a strip of filter paper perpendicularly to the streaks, impregnating the paper with 100 pl of a 0.01 M /3-2-t solution, and incubating at 37 C. The assay was performed and bacterial culture reactions were read [inhibition ( +), borderline (+), or non-inhibition (0)] as elaborated previously (2,3).…”

“…In contrast to the findings that /3-24 is generally inhibitory to bacterial growth in liquid defined media, in a plate assay using agar-solidified defined media, no /3-24 inhibition is exhibited by strains of Salmonella, Proteus, Pseudomonas, Citrobacter, or Enterobacter, while only Shigella and Escherichia strains are inhibited (2). The two inhibited genera can be made insensitive to inhibition in the plate assay by adding Mg2+ (0.05 to 0.1 g MgSO4/liter) (3). Increased Mg2+ concentration is not, however, protective for these bacteria in liquid media (unpublished results).…”

“…The two inhibited genera can be made insensitive to inhibition in the plate assay by adding Mg2+ (0.05 to 0.1 g MgSO4/liter) (3). Increased Mg2+ concentration is not, however, protective for these bacteria in liquid media (unpublished results).…”

“…Increased Mg2+ concentration is not, however, protective for these bacteria in liquid media (unpublished results). In addition, B. subtilis can be made less sensitive to (3-24 inhibition by adding Mg2+ to the plate test medium of the Guthrie and Susi (22) PKU Screening Assay, affecting the reading of that test (3), and the same test when used for monitoring blood Phe concentrations from PKU patients (4).…”