Biomedical Engineering Reference
In-Depth Information
may have positive or negative effects on PDTC production depending on the PDTC function.
For example, if PDTC was excreted simply for its antimicrobial/antagonistic effect, site
conditions would have to be manipulated to trigger an antimicrobial response (Cortese et al.,
2002
).
PDTC was established as a siderophore through experiments with a PDTC-producing strain
of
P. putida
(DSM3601) (Lewis et al.,
2004
). Those experiments showed that: (1) exogenously
added PDTC 'rescued' a mutant, unable to produce PDTC, from the growth inhibitory effects
of synthetic metal chelators, and (2) an outer membrane protein required for such a rescue
effect and for transport of radiolabeled-Fe:PDTC, is encoded within the '
pdt
gene cluster'
(Leach and Lewis,
2006
). An additional regulation unique to siderophores and superimposed
upon iron regulation is coordination among alternative siderophores produced by an individual
organism. Current explanations for this coordination in pseudomonads are based on the fact
that individual siderophore systems are positively regulated by their own product (i.e., they
show positive autoregulation); under iron-limited conditions, no negative regulatory mechan-
isms are known to repress siderophore gene expression. Therefore, the siderophore that is most
successful at procuring iron becomes the most predominant, because the expression of its
respective biosynthetic and transport component genes is stimulated more than the genes for
less effective siderophores (Poole and McKay,
2003
).
Both organisms known to produce PDTC also produce another siderophore: ferrioxamine E
for strain KC and pyoverdine for
P. putida
(Lewis et al.,
2004
). Coordination was observed for
the PDTC system of
P. putida
as PDTC production decreased in response to added pyoverdine,
while pyoverdine production decreased with added PDTC (Lewis et al.,
2004
). It is not yet clear
what environmental conditions might favor PDTC production over pyoverdine production, but
PDTC had a greater role in allowing growth in the presence of a chelator with greater affinity
for divalent metals, while pyoverdine had a greater role in resistance to a trivalent metal
chelator (Lewis et al.,
2004
). This finding suggests that the concentrations of a range of metals
(Leach and Lewis,
2006
) may affect the physiological 'choice' of the siderophore produced.
A fundamental and critical difference between the two PDTC-producing organisms that
have been characterized to date is that
P. stutzeri
KC is a denitrifier, capable of anaerobic growth
in the presence of nitrate, whereas
P. putida
is an obligate aerobe. Given that nitrate is a highly
soluble electron acceptor and is often already present in aquifers or easily added,
P. stutzeri
KC
would be the preferred choice for
in situ
bioaugmentation and production of PDTC.
9.3 CT TRANSFORMATION BY
P. STUTZERI
KC
AS A NOVEL DECHLORINATION REACTION
The initial description of the CT dechlorination process carried out by
P. stutzeri
KC
(Criddle et al.,
1990
) was of particular interest not because it represented the first of such
biological transformations, but because it presented a potentially novel mechanism of dechlo-
rination promoted by a physiologically versatile organism (a denitrifier). Transformations of
CT by strictly anaerobic organisms and biomolecules had been described (Gantzer and Wackett,
1991
; Vogel et al.,
1987
). Examples of biomolecule-catalyzed processes include those involving
transition metal-containing coenzymes (B
12
, heme, F
430
) that promote reductive mechanisms.
The best described of those processes is 'hydrogenolysis', a process in which chlorine atoms
are sequentially replaced with hydrogen atoms. The finding that strain KC brought about a net
hydrolysis (CT
CO
2
) and that CF was not a product (Criddle et al.,
1990
), indicated that
hydrogenolysis was not involved in that transformation pathway. This property presented the
potential advantage of avoiding the accumulation of partially-dechlorinated products, and
instead achieving complete dechlorination with a single process. The initial mass balance studies
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