Peptidoglycan, also known as murein, is
a heteropolymer. It is built up by a carbohydrate backbone of alternating
N-acetylglucosamine and N-acetylmuramic acid; in addition, N-acetylmuramic acid
is cross-linked to peptides. Peptidoglycan is an important component of the
cell wall of most bacteria. However, its content is higher in gram-positive
than in gram-negative bacterial walls, as it represents 40% of the total weight
of gram-positive bacterial walls, but only 1%–10% of the total weight of
gram-negative bacterial walls.
Physiological and pathogenetic roles
of peptidoglycan
Peptidoglycan plays a key role in maintaining
the cellular integrity of bacteria without negatively affecting their growth
and division. However, it is also a pyrogen, which means that it can cause
fever if it comes in contact with the bloodstream. The pyrogenic effects of
peptidoglycan can be attributed to its proinflammatory and immunomodulatory
actions and to its ability to elicit immune responses. In terms of its
pyrogenic effects, peptidoglycan resembles endotoxin; however, the pyrogenic
effect of peptidoglycan is less potent than that of endotoxin by several orders
of magnitude.
Risks of peptidoglycan contamination in
pharmaceutical manufacturing
A contamination of parenterally administered
pharmaceutical products with pyrogens, including peptidoglycan, can have severe
health consequences. Even though aseptic techniques are employed in
pharmaceutical manufacturing, there are a number of possible sources of
contamination. They including the air, water, or drainage; the manufacturing
equipment; product containers; and personnel operations.1
Notably, many
sterilization procedures may not be sufficient to fully eliminate pyrogens.
Considering peptidoglycan in particular, it can be preserved as a cellular
component even after heat sterilization. Moreover, if there is bacterial cell
lysis, peptidoglycan polymers can pass through a sterilizing grade filter.2
Aseptic peritonitis caused by peptidoglycan contamination of
icodextrin-containing dialysate
One of the most prominent examples of the risks
associated with peptidoglycan contamination of pharmaceutical products was an
almost 10-fold increase in cases of aseptic peritonitis reported in 2002. This
led to a voluntary recall of several hundred production lots/batches of
icodextrin-containing dialysis solution, with several thousand units in each
batch. A subsequent investigation concluded that peptidoglycan contamination of
the dialysis solution was the cause of the increased risk of aseptic peritonitis.3
Deciding when to test for
peptidoglycan contamination
Since
peptidoglycan exhibits pyrogenic effects at relatively high concentrations,
corresponding to a high pre-sterilization bioburden, it has been suggested that
such a contamination would be detected by the GMP bioburden control programs of
the individual organizations.4 Therefore, the decision whether to perform
specific testing for peptidoglycan is generally made after a risk assessment
performed by each organization. However, endotoxin testing is carried out
routinely for all parenteral pharmaceutical products. Therefore, if
modifications to pyrogen testing are made, it should always be ensured that not
only peptidoglycan but also endotoxin is evaluated.
