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Bacterial growth inhibiting effects of dipicolinic acid (DPA), which occurs in spores of probiotic Bacillus spp. strains, was studied with a range of Lactobacillus spp. (13), Enterococcus spp. (14), and E. coli strains (8) in vitro. In vivo effects were studied in a broiler chicken nutrition trial and DPA supplemented feed (10 g/kg). In vitro studies showed that DPA inhibited growth of most Lactobacillus spp. strains to a larger extent than E. coli strains. Enterococci were generally influenced less by DPA. However in each group of bacteria some strains were less inhibited by DPA than others. Intestinal lactic acid bacteria in the jejunum and ileum showed a trend (P < 0.1) for lower counts in birds fed the DPA-supplemented feed on day 14 and 21. Counts of enterobacteria were not significantly influenced by DPA. No differences in colony counts were observed in 28-day-old-animals. Digesta pH was not statistically reduced, but a lower crop pH was recorded throughout the nutrition trial. The study showed that dipicolinic acid, present in probiotic Bacillus spp. spores, is inhibitory for in vitro growth of most lactobacilli and enterobacteria, while growth of enterococci was less inhibited. Intestinal colony counts in young broiler chickens fed a DPA-supplented diet in part mirrored in vitro results.