Streptococci
,
Actinomyces
, etc., and the bridging orange-complex bacterium,
i.e.,
Fusobacterium nucleatum
, are susceptible to the bactericidal activity of LL-37 with low
minimum inhibitory concentrations (MICs) in
g/ml (Ji et al, 2007b). Similar results have
Suttichai Krisanaprakornkit and Sakornrat Khongkhunthian
330
also been obtained from another study (Ouhara et al, 2005), which shows the antimicrobial
effects of LL-37 against various gram-positive oral
Streptococci
. In contrast, the red-complex
periodontopathic bacteria, including
Porphyromonas gingivalis
,
Tannerella forsythensis
, and
Treponema denticola
, are more resistant to LL-37 than are other bacteria (Ji et al, 2007b),
suggesting their strong involvement with periodontitis.
Furthermore, LL-37 exerts its candidacidal activity by disrupting the yeast cell
membrane, leading to membrane fragmentation and a release of intracellular contents, such as
adenosine triphosphate (den Hertog et al, 2005). With respect to the antimicrobial activity of
neutrophil
-defensins, oral microorganisms are usually resistant to HNP-1 to HNP-3, even
though a synergistic antimicrobial effect is revealed between HNP-1 and LL-37 against
Escherichia coli
and
Staphylococcus aureus
(Nagaoka et al, 2000).
The antimicrobial activities of hBD-1, hBD-2, and hBD-3 peptides have been tested
against different strains of gram-negative and gram-positive oral bacteria and fungi in several
invitro
studies
.
In brief, it is found that, among thesethree human
-defensins, hBD-3 has the
strongest antibacterial activity against oral
Streptococci
and some periodontal bacteria,
especially all strains of
Fusobacterium nucleatum
, while hBD-1 and hBD-2 are less effective
against both oral gram-positive and gram-negative bacteria (Ouhara et al, 2005). This may be
owing to the strong basic property of hBD-3 due to several positively charged amino acids in
its molecule (Schibli et al, 2002). However, hBD-2 exerts its antimicrobial activity well with
cariogenic bacteria, including
Streptococcus mutans
and
Streptococcus sobrinus
(Nishimura
et al, 2004). Generally, aerobic bacteria are more susceptible to hBD-2 and hBD-3 peptides
than are anaerobic bacteria (Joly et al, 2004).Although the antimicrobial activity of
-
defensins is normally inhibited by high salt concentrations, as shown in other studies
(Goldman et al, 1997; Midorikawa et al, 2003), the antimicrobial activity of hBD-3 against
periodontal and cariogenic bacteria is not much influenced by high salt concentrations
(Ouhara et al, 2005). It can be concluded that, among the antimicrobial peptides of the
defensin and cathelicidin families, hBD-3 and LL-37 exhibit the greatest degrees of
antimicrobial effects against various oral bacteria, especially most aerobic bacteria and some
periodontal bacteria. Although the red-complex periodontopathic bacteria are more resistant
to hBD-3 and LL-37, it is likely that hBD-3 and LL-37 may still play a role in the
pathogenesis of periodontal disease by reducing the number of early colonizing and bridging
bacteria so that the late colonizers, including the red-complex periodontopathic bacteria,
cannot colonize and thrive in dental plaque.
Interestingly, some pathogenic bacteria have evolved other virulence mechanisms that
enable them to resist the activity of antimicrobial peptides. For example, antimicrobial
peptides can be degraded by distinct enzymes secreted from bacterial pathogens, including
SufA, a novel subtilisin-like serine protease of
Finegoldia magna
(Karlsson et al, 2007),
streptopain of
Streptococcus pyogens
, elastase of
Pseudomonas aeruginosa
, gelatinase of
Enterococcus faecalis
(Schmidtchen et al, 2002), and the 50 kDa metalloprotease (ZapA) of
Proteus mirabilis
(Belas et al, 2004). By analogy,
Porphyromonas gingivalis
, one of the red-
complex bacterial triad, can also be resistant to the bactericidal activity of antimicrobial
peptides due to its ability to synthesize a group of enzymes, called gingipains. In fact, it has
been recently demonstrated that the gingipains efficiently degrade several different
antimicrobial peptides, including HNP-1, hBD-1, hBD-2, and hBD-3 (Carlisle et al, 2009).
However, it was formerly shown that the degradation of antimicrobial peptides by gingipains
The Role of Antimicrobial Peptides in Periodontal Disease
331
does not appear to contribute to the resistance of
Porphyromonas gingivalis
to the
antimicrobial action (Bachrach et al, 2008).
The possible alternative mechanisms for the resistance of
Porphyromonas gingivalis
may
be due to the possibility that gingipains secreted from
Porphyromonas gingivalis
may prevent
destruction of its commensal bacteria, i.e.,
Fusobacterium nucleatum
, which is easily
destroyed by antimicrobial peptides. Otherwise, gingipains and proteases released from
Porphyromonas gingivalis
and
Prevotella intermedia
, respectively, may inactivate cystatins,
inhibitors that function against endogenously-derived proteases, such as host cathepsins, etc.
This ultimately releases cathepsins from their tight control by cystatins. The active cathepsins,
including cathepsin B, L, and S in the cysteine protease family, may then proteolytically
degrade antimicrobial peptides, resulting in depletion of antimicrobial activity (Taggart et al,
2003). The gingipains and other virulence factors make
Porphyromonas gingivalis
one of the
critical periodontal pathogens, and antimicrobial peptides may then be regarded as an
important determinant for the ―normal‖ and ―diseased‖ states of periodontium.
As with
Porphyromonas gingivalis
,
Treponema denticola
, another red-complex
periodontal pathogen, is resistant to the antimicrobial activity of human
-defensins, but by
other distinct mechanisms, since
Treponema denticola
does not produce degrading enzymes.
These mechanisms include an efflux pump of defensin peptides that enter the cytoplasm
(Brissette and Lukehart, 2007) and reduction of defensin binding to the microbial surface due
to the lack of LPS (Brissette and Lukehart, 2002). Furthermore,
Treponema denticola
cannot
induce the host innate immune response, i.e., expression of hBD-2 and IL-8, in gingival
epithelial cells (Brissette et al, 2008). The immune tolerant mechanisms of
Treponema
denticola
, including resistance to the antimicrobial effect of antimicrobial peptides and
silencing host innate immunity, may, therefore, partly explain the strong association of
Treponema denticola
with chronic periodontitis.
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