p-Aminobenzoic acid (PABA) exerts three metabolic effects on E. coli: it acts as a normal vitamin at low concentrations, as a source of another vitamin, p-hydroxybenzoic acid (POB), at moderate concentrations, and as a growth inhibitor at high concentrations (150 to 1600 microg./ml.). The inhibition is competitively reversed by POB in 1/100 the concentration of PABA. The inhibition is also reve...
