In vivo metabolism of 2,2'-diaminopimelic acid from gram-positive and gram-negative bacterial cells by ruminal microorganisms and ruminants and its use as a marker of bacterial biomass

Abstract

Cells of Bacillus megaterium GW1 and Escherichia coli W7-M5 were specifically radiolabeled with 2,2'-diamino[G-3H]pimelic acid ([3H]DAP) as models of gram-positive and gram-negative bacteria, respectively. Two experiments were conducted to study the in vivo metabolism of 2,2'-diaminopimelic acid (DAP) in sheep. In experiment 1, cells of [3H]DAP-labeled B. megaterium GW1 were infused into the rumen of one sheep and the radiolabel was traced within microbial samples, digesta, and the whole animal. Bacterially bound [3H]DAP was extensively metabolized, primarily (up to 70% after 8 h) via decarboxylation to [3H]lysine by both ruminal protozoa and ruminal bacteria. Recovery of infused radiolabel in urine and feces was low (42% after 96 h) and perhaps indicative of further metabolism by the host animal. In experiment 2, [3H]DAP-labeled B. megaterium GW1 was infused into the rumens of three sheep and [3H]DAP-labeled E. coli W7-M5 was infused into the rumen of another sheep. The radioactivity contents of these mutant bacteria were insufficient to use as tracers, but the metabolism of DAP was monitored in the total, free, and peptidyl forms. Free DAP, as a proportion of total DAP in duodenal digesta, varied from 0 to 9.5%, whereas peptidyl DAP accounted for 8.3 to 99.2%. These data reflect the extensive metabolism of bacterially bound DAP within the gastrointestinal tracts of ruminant animals and serve as a serious caution to the uncritical use of DAP as a marker of bacterial biomass in the digesta of these animals.