Microorganisms that colonize the
insect intestine enter the hemocoel together with the nematode (Isaacson and
Webster, 2002; Walsh and Webster, 2003; Gouge and Snyder, 2006). In a commonly
used insect host, Galleria mellonella,
Enterococcus faecalis and Gamma proteobacteria were shown to be predominant
bacterial species in the insect intestine (Jarosz, 1996; Walsh and Webster,
2003). These bacteria were present in the hemocoel soon after nematode invasion
(Isaacson and Webster, 2002; Gouge and Snyder, 2006). More recently, the
tobacco horn worm, Manduca sexta, has
been developed as an insect model (Tabatabai and Forst, 1995; Martens et al., 2003). Because breeding colonies
of M. sexta are susceptible to epidemic
infections, antibiotics are generally added to artificial diets to suppress
bacterial contamination (Hoffman et al.,
1966; Bell and Joachim, 1976). The gut microbial communities of M. sexta had not been analyzed
previously. Moreover, the effect of antibiotics on microbial diversity and the
ability of nematodes to reproduce in M.
sexta had not been studied. To further develop this model system, we
analyzed the microbial community of the midgut in insects raised on diets with
and without antibiotics and assessed the ability of nematodes to reproduce in M. sexta under the different dietary
regimes.
Neonatal bacterial meningitis
caused by E. coli contributes to more
than 50,000 annual deaths worldwide, and it is of serious concern that these
numbers remain significant, despite advances in antimicrobial chemotherapy (de
Louvois et al., 1991; Unhanand et al., 1993; Kim, 2002, 2006). Although
haematogenous spread is known to be a pre-requisite in E. coli meningitis, it is not clear how circulating bacteria cross
the blood-brain barrier to gain entry into the central nervous system (CNS) to
produce disease (reviewed by Kim, 2001, 2002, 2006). Over the last few decades
only a handful of bacterial virulence determinants have been identified as
associated with E. coli pathogenesis
(Kim, 2006).
There are many parallels between
the innate immune responses of mammals and insects, and it is envisaged that
insects could make useful models for the study of disease pathogenesis (Scully
and Bidochka, 2006), especially as insects possess a highly selective
blood-brain barrier exhibiting functional properties comparable with that of
vertebrates (Carlson et al., 2000).
Here, L. migratoria, the African
migratory locust, is used as a model to study E. coli pathogenesis. The aim of the present study was to develop
an Ex vivo locust model specifically
to study invasion of the CNS by E. coli.
To achieve this, ligated head capsules were isolated and injected with high
concentrations of bacteria. In this way, the ability of the locust immune
system to respond to infection was severely impaired, producing an ideal
environment where the progress of bacteria crossing the blood-brain barrier
could be studied.
Insects
such as cockroaches represent a plentiful and untapped potential source of new
antimicrobial drugs prompting us to investigate the antibacterial activity of
their various tissues. According to Khan et
al, (2008) report to have shown that cockroach and locust brain tissues
have powerful antibiotic properties and may serve as potential sources of
antimicrobials in the future. Lee et al,
(2011) and Khan et al, (2008) they
often inhabit environment niches that present a significant threat in terms of
microbial infection and have evolved defenses to counter this threat. It was
interesting that brain lysates but not muscle, fat body or haemolymph of
cockroaches (Periplaneta americana)
and locusts (Locusta migratoria or Schistocerca gregaria) showed remarkable
antimicrobial activity killing more than 90% of different bacteria including Staphylococcus aureus, S. epidermidis, and E. coli. The bactericidal activity was heat-resistant, but
SDS-labile and pronase-sensitive indicating its proteinaceous nature. However,
it is noteworthy that glycopeptide molecules (e.g. Vancomycin) and lipopeptide
molecules (e.g. Daptomycin) are currently used as antibiotics clinically and
there are several antimicrobial compounds undergoing development in phase 2 or
later clinical studies that are lipoglycopeptides in chemical nature. Talbot et al, (2006) reported, insect brain
lysates had no and/or minimal cytotoxic effects on human brain microvascular
endothelial cell cytotoxicity. The potent antimicrobial activity of cockroach
or locust brain lysates is not surprising as their nervous system needs to be
well-protected because if the nervous system goes down, the insect dies, but
they can suffer damage to their peripheral structures. Additionally, the
nervous system has limited cell-mediated immunity; thus, it was hypothesized
that the nervous system possesses molecules with powerful antimicrobial
properties. Clynen et al, (2009), the
biological significance and in vivo
function of most of these neuropeptides remains unknown. Work is currently underway
to characterize further the antibacterial properties of insect brain lysates
and to investigate the spectrum of activity against a panel of wild-type and
resistant human bacterial pathogens. It is hoped that these molecules could
eventually be developed into treatments for bacterial infections that are
increasingly resistant to current drugs. These new antibiotics could
potentially provide alternatives to currently available drugs that may be
effective but have serious and unwanted side effects. Overall, natural products
are often superior to synthetic products as antibacterial drugs because they
display optimal cellular penetration and privileged structures allowing
interaction with the finite structural spaces in protein folds. The discovery
of antimicrobial activity in cockroach/locust brains will stimulate researchÂ
