WATT PoultryUSA - January 2018 - Cover3
Garbage in the Gut: Peptidoglycans of Bacterial Origin
Pietro Celi | DSM Nutritional Products | Columbia, MD
Optimal gastrointestinal functionality is inﬂuenced by many
factors, including by the bacteria that live in the gut and the
way they interact with the host. In fact, the gastrointestinal
microbiota has proven to modulate the development of the
gastrointestinal tract and improve its morphology, stimulating the
development of the gastrointestinal immune system, promoting
host nutrition and reducing gastrointestinal pathogens. Therefore,
the gastrointestinal microbiota has a signiﬁcant impact on host
or animal welfare (Figure 1).
Naturally occurring bacterial cell death generates PGNs in the
gastrointestinal tract. When an increase in the gastrointestinal
microbiota load occurs, the result is an increase in bacterial cell
debris, hence more PGNs. In an in vitro study, it was observed
that the ratio of intact bacteria to bacterial cell debris was
approximately 3.5 to 1. The ratio decreased to 1.2 to 1 after 24
hours, and dropped to 0.45 to 1 after 96 hours. This demonstrates
that the spontaneous lysis of cultured bacteria occurs over time.
In an in vivo study, the ratio of intact bacterial cells to cell
debris differed signiﬁcantly depending on the location in the
gastrointestinal tract. It suggests that older bacteria populations
are found in segments of the tract where the ratio of intact bacteria
to bacterial debris is at its lowest.
When bacteria and their cell debris (PGNs) accumulate on the
intestinal mucosa they can impair absorption by competing for
nutrients and by causing enterocyte damage resulting in increased
intestinal permeability. Anaerobic overgrowth is most common
during intestinal bacteria overgrowth. Coliforms, staphylococci
and enterococci (Clostridium and Bacteroides spp. predominating)
are the main species of bacteria in the duodenal juice of animals
affected by intestinal bacterial overgrowth. Anaerobic bacteria
and the consequent accumulation of their PGN-rich cell debris,
have a much greater potential to damage the intestinal brush
border and cause malabsorption. Moreover, anaerobes, especially
Bacteroides, are the major cause of bile salt deconjugation which
results in fat malabsorption. Various gut health conditions leading
to malabsorption are often associated with secondary overgrowth
of bacteria in the small intestine.
Figure 1. Functions of the gastrointestinal (GIT) microbiota.
The gastrointestinal microbiota is composed of both grampositive (G+) and gram-negative (G-) bacteria. The bacterial
cell wall of G+ bacteria, in particular, but also of G- bacteria is
a signiﬁcant source of peptidoglycans (PGNs). When fragments
of these cell walls accumulate in the gastrointestinal tract and
become "intestinal garbage," they can decrease gastrointestinal
functionality. Nutrient digestion and absorption are particularly
prone to the adverse impact of this cell debris.
Gastrointestinal Health Problems on the Increase
Incidence of subclinical necrotic enteritis and nonspeciﬁc small
intestinal overgrowth of certain intestinal bacteria (dysbacteriosis)
have been on the rise since antibiotic growth promoters were
banned in the European Union. These health problems typically
occur in 20 to 30 day-old broilers and result in wet litter,
nonspeciﬁc enteritis, poor weight gain, and decreased nutrient
digestibility and absorption.
Bacterial Overgrowth Impacts Nutrient Absorption
An increase in bacterial burden leads to an increase in intestinal
permeability, bacterial translocation and absorption of endotoxins
such as ammonia and D-lactate. When bacteria overgrow in the
intestine, bile acids are deconjugated by bacteria resulting in
malabsorption of fat and fat-soluble vitamins. One product of
deconjugation is lithocholic acid, which is poorly absorbed and
acts as a toxin on the intestinal mucosa.
Imbalances Disrupt Gastrointestinal Functionality
Normal composition of the microbiota of the gastrointestinal tract
plays a fundamental role in the preservation of its integrity and
normal functioning in the animal. Therefore, any disturbance or
alteration of this composition may lead to bacterial overgrowth.
This, in turn, can lead to subsequent profound malabsorption in
the proximal portion of the gastrointestinal tract.
In the intestinal lumen, bacterial cell wall fragments containing
PGNs can be released from commensal bacteria. A variety of
efﬁcient mechanisms are devoted to their disposal to prevent
them from becoming harmful to the organism. Gastrointestinal
functionality can be disrupted as a consequence of the imbalance
between the production and disposal of cellular debris. Therefore,
a prolonged and increased exposure to PGNs in the GIT can
hinder optimim gastrointestinal functionality.
Normal gastrointestinal motility involves a complex, tightly
coordinated series of events designed to move material through
the gastrointestinal tract. Excess bacteria in the small intestine
decreases gastrointestinal motility. Gut motility is crucial for the
prevention of colonization of bacteria in the small intestine as it
sweeps and moves bacteria and the associated cell debris into the
It has been suggested that PGNs are absorbed in the intestinal
mucosa. Therefore, timely disposal of PGNs and other cell debris
may contribute to maintenance of gastrointestinal functionality.
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