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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 ).
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