Fate of Escherichia coli during ensiling of wheat and corn

Abstract

A recombinant Escherichia coli strain carrying a plasmid with an antibiotic resistance marker and expressing the green fluorescent protein was inoculated at a concentration of 3.8 x 10(8) CFU/g into direct-cut wheat (348 g of dry matter kg(-1)), wilted wheat (450 g of dry matter kg(-1)), and corn (375 g of dry matter kg(-1)). The forages were ensiled in mini-silos. The treatments included control (no E. coli added), application of tagged E. coli, and delayed sealing of the inoculated wheat. Three silos per treatment were sampled on predetermined dates, and the numbers of E. coli were determined on Chromocult TBX medium with or without kanamycin. Colonies presumptively identified as E. coli were also tested for fluorescence activity. Addition of E. coli at the time of ensiling resulted in a more rapid decrease in the pH but had almost no effect on the chemical composition of the final silages or their aerobic stability. E. coli disappeared from the silages when the pH decreased below 5.0. It persisted longer in silages of wilted wheat, in which the pH declined more slowly. Control silages of all crops also contained bacteria, presumptively identified as E. coli, that were resistant to the antibiotic, which suggests that some epiphytic strains are naturally resistant to antibiotics.

FIG. 1.

pH change during ensiling of wheat with E. coli. D, direct cut; W, wilted; EC, E. coli; delayed, delayed sealing. The solid lines indicate data for D-wheat, and the dashed lines indicate data for W-wheat. The arrow indicates the time when aerobic exposure was started.

FIG. 2.

Numbers of E. coli from control wheat silages. Bacteria were plated on Chromocult TBX agar with and without kanamycin. D, direct cut; W, wilted; +kan, medium with kanamycin. The solid lines indicate data for D-wheat, and the dashed lines indicate data for W-wheat.

FIG. 3.

Numbers of E. coli from inoculated wheat silages. Bacteria were plated on Chromocult TBX agar with and without kanamycin. D, direct cut; W, wilted; EC, E. coli; delayed, delayed sealing. The solid lines indicate data for D-wheat, and the dashed lines indicate data for W-wheat.

FIG. 4.

pH change during ensiling of corn with E. coli. EC, inoculated with E. coli. The arrow indicates the time when aerobic exposure was started.

FIG. 5.

Numbers of E. coli from corn silages. Bacteria were plated on Chromocult TBX agar with and without kanamycin. C, control; EC, inoculated with E. coli; +kan, medium with kanamycin. The solid lines indicate data for control silages, and the dashed lines indicate data for inoculated silages. The arrow indicates the time when aerobic exposure was started.