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Small Things Considered
The Microbe Blog (at http://www.smallthingsconsidered.us)
A Bacterial Mouth
by Mark Martin, guest author
http://schaechter.asmblog.org/
schaechter/2007/08/a-bacterial-mou.html
This tale begins with Elio’s Talmudic
Question #3: Why are no bacterial pathogens known to use a Bdellovibrio-type of
mechanism to penetrate into eukaryotic
cells? Since I have done a little work with
predatory prokaryotes, this subject particularly interests me. We are only now
starting to understand how Bdellovibrio
obtains entry into that enigmatic compartment of gram-negative cells, the
periplasm. Type IV pili have been implicated by a recent publication from Liz
Sockett’s group in the United Kingdom.
The periplasm is quite a unique place, full
of osmotic challenges and diverse solutes,
with a gel-like consistency.
Eukaryotic cells do not possess a
periplasm, so the simplest way to answer
Elio’s question may be to state that there
is no need for a Bdellovibrio-like mechanism. Instead, pathogens and symbionts
hijack the mechanisms that eukaryotic
cells use to internalize materials from
their surroundings: phagocytosis and
pinocytosis. So I would turn Elio’s ques-
tion around with a not-quite Talmudic
reply: why do prokaryotes not have
phagocytosis or pinocytosis? This lead us
to a seemingly bizarre question: can a
bacterium have a “mouth?”
Many bacteria consume long polymers,
such as cellulose or chitin. One strategy
used by gliding bacteria of the Cytophaga/
Flavobacteria group is to attach themselves to these polymers and secrete enzymes that release monomers or small
oligomers. The fragments are then transported into the bacterial cytoplasm and
further metabolized. One problem with
this approach is that the importable fragments are available to other microbes, not
just the organisms doing the “work.” Diffusion works against these polymer-degrading organisms, as well.
This prompted me to recall a presentation I attended several years ago at
ASMCUE by Julian Davies of the University of British Columbia. Julian told
us about, of all things, a “bacterial
mouth.” A gram-negative bacterium,
Sphingomonas sp. strain A1, does something a bit unexpected. In response to
the presence of alginate, it synthesizes
an alginate lyase to depolymerize the
polymer. But the cell also synthesizes a
“pit” on its surface, approximately 0.02
Talmudic Question of the Month*
What if all the bacteria and archaea on Earth decided to go on strike and stop their
metabolism all at once? Which of the global cycles of matter would be affected
first? How long would it take for life as we know it to come to a stop?
Answers? Add a comment online to Talmudic Question #4, December 19, 2006.
http://schaechter.asmblog.org/schaechter/2006/12/talmudic_quesit.html
*We use this term to denote questions whose answers cannot be found by a Google search.
264 • Microbe / Volume 3, Number 5, 2008
Elio Schaechter and
Merry Youle, fellow
writer and editor
to 0.1 μm in diameter. Only one pit is
synthesized per cell, and it appears to be
lined with ABC-type transporters specific for alginate-derived oligomers and
monomers. This presents a solution to
how a polymer can be broken down in
an exclusive fashion: using a pit or
proto-mouth linked to polymer degrading enzymes and an ABC transport
system. The pit appears to concentrate
the released oligomers, further reducing
loss by diffusion.
If bacteria can have a “mouth,” do they
then “swallow?” Prokaryotes, it appears,
don’t have much of a vacuolar system.
However, this discussion of a pit and
proto-mouth leads to the possibility of
evolution toward something like phagocytosis and pinocytosis. We may yet find
processes that resemble phagocytosis
among the prokaryotes. We should start
looking, perhaps, among the cell wall-less
bacteria and archaea, which could conceivably form vacuoles by “engulfing”
particles from the environment, much as
eukaryotic amoebae do. In a nutrient-rich
gut, or a hot spring, or within a complex
biofilm, I believe that some uniquely weird
and wonderful prokaryotes are waiting to
be discovered. Perhaps even ones with a
“mouth” and “teeth!”
Mark Martin is assistant professor in the
Department of Biology, University of Puget
Sound.
Evans, K. J., C. Lambert, and R. E. Sockett.
2007. Predation by Bdellovibrio bacteriovorus
HD100 requires type IV pili. J. Bacteriol.
189:4850–4859.
Momma, K., M. Okamoto, Y. Mishima, S.
Mori, W. Hashimoto, and K. Murata. 2000.
A Novel Bacterial ATP-Binding Cassette
Transporter System That Allows Uptake of
Macromolecules. J. Bacteriol. 182:3998–
4004.