Biology Reference
In-Depth Information
Exposure to endogenously produced NO has been proposed, particu-
larly in microorganisms that use nitrite as an electron acceptor under anaer-
obic conditions. Indeed, enteric bacteria, such as
E. coli
, produce low
concentrations of intracellular NO when nitrite is reduced to ammonia
(
Corker & Poole, 2003
). Moreover, several bacteria possess NOS enzymes
(reviewed by
Bowman et al., 2011
); bacterial NO production in Gram pos-
itives has been proposed to confer resistance to antibiotics through chemical
modification and alleviation of oxidative stress (
Gusarov, Shatalin,
Starodubtseva, & Nudler, 2009
), although the physiological role for NO
production in Gram negatives is poorly understood.
Resistance to NO and RNS in bacteria has been related to the presence
of haemoglobins (see
Table 4.1
)(
Poole, 2005
). However, other proteins
have also been associated with tolerance to nitrosative stress. For instance,
while the FHb Hmp represents the main mechanism for NO detoxification
under aerobic conditions in
E. coli
(
Gardner, Costantino, et al., 1998
), the
NO sensor NorR positively regulates the expression of the flavorubredoxin
protein (NorV) and its reductase partner (NorW) during NO exposure
under anaerobic conditions, and this system mediates the reduction of
NO to nitrous oxide (N
2
O) (
Gardner & Gardner, 2002; Hutchings,
Mandhana, & Spiro, 2002
).
3. OXYGEN AND REACTIVE OXYGEN SPECIES
IN BIOLOGY
The limited tolerance of microorganisms for oxygen is well known.
Anaerobes and microaerophiles are unable to grow in air-saturated environ-
ments and committed aerobes experience deleterious or even lethal effects in
hypoxic conditions. The stability of the oxygen molecule and its relatively
weak capacity to accept electrons make it an inefficient oxidant of organic
molecules such as amino acids and nucleic acids. However, oxygen readily
interacts with organic radicals and transitions metals. Whereas oxygen is a
relatively weak electron acceptor, superoxide (O
2
), hydrogen peroxide
(H
2
O
2
) and the hydroxide ion (OH
) are much stronger oxidants
(
Imlay, 2003
).
The term oxidative stress has been defined as 'a disturbance in the pro-
oxidant-anti-oxidant balance in favour of the former, leading to potential
damage' (
Sies, 1991
). The rate of O
2
and H
2
O
2
formation dictates the level
of oxidative stress experienced by a microorganism (
Imlay, 2003
). Partially
reduced oxygen species form via the reduction of oxygen by electron
