Peptidoglycans and Gastrointestinal Functionality in Broilers

PGN in the fragments of dead bacteria may constrain the performance of poultry, possibly by hindering gut functionality and lowering intestinal efficiencies.

Peptidoglycan (PGN) is a polymer in bacterial cell walls that provides rigidity and shape to the cells, whether spherical, rods, spiral or filamentous. PGN is sometimes called the exoskeleton of bacteria for this reason. The polymeric structure of PGN is unique, exclusive to bacterial cells, and is absent in all other organisms. Further, as much as 90% of cell dry weight of gram-positive bacteria can be PGN, in comparison to less than 10% for gram-negative bacteria (Malanovic and Lohner, 2016).

One of the most densely populated microbial habitats is the gastrointestinal tract of animals. Just the intestines of the world’s broiler population account for an estimated 23 megatons of bacteria (Whitman et al., 1998). More than half of human fecal mass is comprised of bacterial biomass, where dead cells make up about one-third of the total mass (Ben-Amor et al., 2005; Stephen and Cummings, 1980).

Dead bacterial biomass in the gut is largely overlooked in research, which focuses narrowly on the living segment of the microbiome despite the fact that both the live and dead fractions likely impact gastrointestinal functionality. Figure 1 illustrates the relative nature of live and dead bacteria, and bacterial fragments, based on data available from studies.

Bacterial Cell Wall Debris

Bacterial debris in the small intestine includes remnants derived from major microbial colonizers such as lactobacilli, streptococci, enterobacteria, fusobacteria and eubacteria, all of which contribute to the accumulation of cell wall debris. Anaerobic species (Eubacterium spp., Clostridium spp., Lactobacillus spp., Fusobacterium spp.) and Bacteroides are important sources of remnants in the cecum (Rehman et al., 2007).

PGN can represent a prominent component of this debris. This heteropolymer consists of a sugar backbone with alternating N-acetylglycosamine and N-acetylmuramic acid components. Muramic acid is the connection point for peptide subunits of four to five amino acids which are linked to other sugar chains by peptide bridges of one to seven amino acids. Unique compositional differences in PGN from different bacteria can provide a ‘fingerprint’ by bacterial class. In general, PGN from gram-negative bacteria are similar across different strains, whereas PGN from gram-positive bacteria can be quite diverse.

To date, over 100 different types of PGN have been described in the literature.

Recently, a novel muramidase added to feed significantly improved 42-day weight gain and the feed conversion ratio (FCR) of broilers (Lichenberg et al., 2017). No changes were detected in total anaerobic bacteria, total aerobic bacteria, enterobacteria and coliforms in the ceca of birds fed this particular muramidase. Populations of Salmonella, Campylobacter and Clostridia were also unaffected (Lichenberg et al., 2017). Because muramidase specifically catalyzes the degradation of PGN – and because live bacterial populations were unaffected with this muramidase – these results suggest that the weight gain and FCR improvements arise from the destruction of PGN in the dead bacteria remnants.

In another study, this novel fungal muramidase improved broiler performance when added to a pelleted corn/SBMbased diet (Yegani et al., 2018). In this 42-day trial, 1,280 Cobb broilers were vaccinated with a commercial coccidiosis vaccine and raised in floor pens. Muramidase supplementation significantly (P < 0.05) improved body weight gain by 5.0% and FCR by 2.5% (Table 1). Mortality was unaffected (P > 0.05) by the treatment.

Ultimately, bird performance was elevated by feeding muramidase, presumably because gut function was enhanced when excess debris was cleared, thus, preventing PGN from causing adverse effects on animal performance (Ward and Cowieson, 2017; Celi, 2017). This suggests that the importance of this bacterial biopolymer has largely been overlooked, and that it may impact bird performance more than previously recognized.

Conclusion

Research has focused largely on the role of live bacteria in the intestinal tract, with little recognition of the impact of dead bacteria. PGN in the fragments of dead bacteria may constrain the performance of poultry, possibly by hindering gut functionality and lowering intestinal efficiencies. Muramidase degrades PGN, and muramidase supplementation in feeds is a novel means to improve weight gain and feed efficiency.

References available upon request.