Subsequently,
each of these isolates has been tested by the authors in order to
evaluate their resistance and ability to detoxify Cd, Pb and
Hg. Mechanisms of metal resistance and detoxification in microbes
include precipitation of metals as phosphates, carbonates or sulfides
(for Pb); volatilization (for Hg) via methylation or ethylation;
energy-dependent metal efflux systems, intracellular sequestration
with cysteine-rich proteins and physical exclusion or entrapment of
electronegative components in membranes and extracellular polymeric
substances (EPS).
In
order to evaluate Hg volatilization, cells were transferred by the
authors to plates enriched with 10 ppm Hg. The plates were then
covered with a Kodak XAR film and incubated. All of the tested
isolates were capable of volatilizing mercury, as indicated by
fogging of the film covering the plates (reduction of AgCl2
by gaseous Hg); while a negative control did not volatilize Hg. It is
well-known that the ability of volatilizing mercury is conferred by
the highly conserved mer
operon.
Thus, in order to confirm his presence in the isolates, the authors
extracted genomic DNA from the cultures and then amplified and
visualized the merA
region
through PCR and gel electrophoresis. Surprisingly, the analyses
revealed the presence of merA
gene
fragments only in 9 of the BHRM isolates, whereas all of them were
volatilizing Hg. Thus the authors hypothesized the presence of
another unknown non-mer
mediated
mercury volatilization mechanism in two isolates (CH13 and CM10), but
this interesting possibility remains to be confirmed.
To
estimate Cadmium detoxification instead, two isolates (CH07, GP06)
were tested with different concentrations of Cd (as CdCl2)
and metal concentrations were determined by inductively coupled
plasma-atomic emission spectrometry. Results showed that isolate CH07
removed Cd from the medium at a faster rate than GP06. Cd
concentration was reduced from an initial concentration of 100 ppm to
17.4 ppm (75%) by CH07 and to 19.2 ppm (70%) by GP06 in about 72 h.
Several mechanisms of Cd resistance have been already described, but
according to the authors, the observed metal accumulation in
bacterial cell pellets is perhaps consistent with some kind of
microbial bioabsorption of the metal.
Finally,
regarding lead detoxification, 3 BHRM isolates (CH07, GP13 and S3)
were tested by the authors with different concentrations of Pb (as (CH3COO)2Pb)
using an experimental procedure similar to that used for Cd. Results
showed that CH07 was able to reduce Pb concentration from 100 ppm to 1.8 ppm (>98% removal) in 96 h, while isolates
GP13 and S3 removed more than 87% and 88% of Pb respectively from the
growth medium. Scanning Electron Microscopy (SEM) and Energy
Dispersive X-ray Spectroscopy (EDS) analysis showed that Pb was
entrapped in the exopolymeric substance (EPS) in the case of isolate
CH07 (negatively charged EPS in fact, could bind lead and prevent its
entry into the cell), while hydrogen sulfide production suggested
that Pb was most likely precipitated as sulfide by isolates GP13 and
S3. In addition, EDS analysis of the cells showed the concentrations
of Pb and Cd to be as high as 21% and 19%, respectively, while
bacteria grown in medium without heavy metals showed no signals for
either Cd or Pb. Moreover, the morphology of isolate GP13 was
unchanged after growth in medium with 50 ppm Pb, suggesting thus,
that Pb is not toxic to the isolate under the conditions tested.
Various metal-resistance mechanisms has been already
found and characterized at the molecular level in several other bacterial strains. Moreover, it has been shown that bacterial mobile
genetic elements, such as plasmids or transposons, can carry multiple
genes encoding for metal resistance. Thus, exposure to one agent may
select for microorganisms resistant to several toxicants. So, it is
likely that multi-metal resistant BHRM, possess the
genetic components for dealing with many toxic metal ions (including
Hg, Cd, Zn, Sn, Cu, and Pb) and removing them from the surrounding
medium using several high-efficiency processes. Thus, since heavy
metals are non-biodegradable, highly toxic and very common in many
polluted environments, the bacterial isolates analyzed in this study, definitely hold great
promises for the development of new and innovative bioremediation methodologies.
Reference:
De J, Ramaiah N, Vardanyan L (2008)
Detoxification of Toxic Heavy Metals by Marine Bacteria
Highly Resistant to Mercury. Marine
Biotechnol 10 (4): 471-477
3 comments:
Hi Giuseppe,
Very interesting review. Did the authors mention anything about the chemical reaction that makes Hg more volatile via methylation? I find it interesting that bioaccumulation of mercury in humans is a direct result of methylation, and I was wondering what makes the attachment of a methyl group more harmful. Methylation occurs in DNA to prevent the DNA from being transcribed, so I suppose its a form of epigenetic attenuation of DNA transcription.
I read that methylmercury mimics methionine and so it is easily transported across tissues and is quite harmful to humans and perhaps other 'higher' organisms in large doses.
I also found it interesting that these bacteria are able to survive in what is supposedly a toxic environment. I saw a Horizon program sometime ago and there was a scientist who was investigation a bacteria that is able to live in a lake in the US which has high levels of arsenic; another toxic element. They reckon that this bacteria may have evolved separately from the 3 domains of life and might even be considered a new biological entity. Scary! :)
Hi Mario, Thanks for your comment!
Unfortunately the authors didn't say anything more of what i wrote about the reaction involved in mercury volatilization. Although you are in right, it would have been really interesting to go in deeper detail. Especially because, as far as i know, the most toxic forms of mercury are its organic compounds, such as dimethylmercury and methylmercury. So i wonder how is it possible that, in these bacteria, methylation makes mercury more volatile and therefore less toxic, while in most of other organisms, it makes it more dangerous.
Anyway, i guess that your is a good point. Maybe as you said, it's exactly the fact that methylation makes Hg more transportable across membranes, which makes it more volatile. Probably in this way, it is easier for bacteria to elimate it from the cell and thus detoxify themselves.
However, it is indeed really surprising, how they manage to live in such harmful places, where any other "higher" form of life could not survive. But since we saw that bacteria are basically able to live in every remote and hostile environment of our planet, this doesn't surprise me that much anymore!
Hi Giuseppe,
Interesting review there’s actually another interesting study about TBT resistant bacteria that are also resistant to methyl mercury. It’s interesting that they possess two kinds of genes the type mer A mercuric reductase which converts H2+ to H0 the one you mentioned but also mer B organomercurial lyase which is capable of degrading all mercury compounds being responsible for the mercury-carbon bond cleavage so maybe this gene was expressed in those two strains of Bacillus.
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