Biology Reference
In-Depth Information
for
C. jejuni
, where growth has proved to be severely impaired, micro-
aerobic cultures with extremely restricted oxygen transfer (oxygen-limited
conditions) are able to grow in media supplemented with fumarate, nitrate,
nitrite, TMAO or DMSO, indicating alternative pathways for electron
acceptor-dependent energy conservation (
Sellars et al., 2002
). Variability
of oxygen availability during host colonisation can be expected, although
the behaviour of
Campylobacter
in the presence of different electron acceptors
under oxygen-limited conditions
in vivo
remains unexplored.
4.3.
Impact of nitrosative stress upon
Campylobacter
respiration
Campylobacter
is exposed to nitrosative stresses from a variety of sources from
different environmental niches, in addition to NO arising from the action of
NOS (
Section 2
). Production of NO other than the specific host defence
response is likely to come from other nitrogenous species, such as nitrite
on the skin (
Suschek, Schewe, Sies, & Kroncke, 2006
) and in the oral cavity
(
Rausch-Fan & Matejka, 2001
). Indeed, reaction of dietary nitrite with
stomach acid produces NO, and reduction of dietary nitrate to nitrite
(
Olin et al., 2001
) by oral microflora exacerbates this process. Furthermore,
nitrates are used as a preservative on meat, further increasing the exposure of
Campylobacter
to sources of nitrosative stress.
Like most bacteria, aerobic respiration of
Campylobacter
is inhibited by
NO. However,
Campylobacter
has a range of respiratory complexes involved
in anaerobic respiration that can process sources of nitrosative stress:
C. jejuni
NCTC 11168 possesses both periplasmic nitrite reductase (Nrf ) and nitrate
reductase (Nap) (
Pittman & Kelly, 2005; Sellars et al., 2002
). NrfA is a pent-
ahaem cytochrome
c
Nrf, playing a role as the terminal enzyme in the dis-
similatory reduction of nitrite to ammonia (
Pittman & Kelly, 2005; Sellars
et al., 2002
). The
nap
operon in
C. jejuni
is composed of
napAGHBLD
, and
the periplasmic machinery consists of two subunits, NapA and NapB. NapA
(
90 kDa) is the catalytic subunit that reduces nitrate to nitrite and contains
a bis-molybdenum guanosine dinucleoside cofactor and a [4Fe-4S] group,
and NapB (
16 kDa) is a di-haem
c
-type cytochrome (
Butler et al., 2001
).
Even though this operon lacks
napC
, a gene encoding a tetra-haem cyto-
chrome that couples nitrate reduction to quinol oxidation in
E. coli
(
Brondijk, Nilavongse, Filenko, Richardson, & Cole, 2004
),
C. jejuni
does
encode a putative
napC
gene that is probably related to the Nrf system
(reviewed by
Pittman & Kelly, 2005
).