MICROBIAL GENOMICS:
First Food-Borne Pathogen Sequenced
Elizabeth Pennisi
Nature lovers used to marvel over avian ingenuity when they saw a
bird peck its way through a foil bottle cap for a sip of milk. The
clever bird sometimes left an unwelcome present in return:
Campylobacter jejuni, a bacterium that causes severe
gastrointestinal upsets in humans. Milk rarely comes in bottles these
days, but Campylobacter has become a major health problem over
the past 20 years, often passing from its natural avian hosts to humans
through undercooked poultry or contaminated water. Now, C. jejuni
has a new--and more auspicious--claim to fame: It's the first food-borne
pathogen whose genome has been sequenced.
At the Microbial Genomes III meeting held earlier this month in
Chantilly, Virginia, microbiologist Brendan Wren of St. Bartholomew's
Hospital in London reported that a team lead by Bart Barrell and Julian
Parkhill at the Sanger Centre in Cambridge, U.K., has determined the
exact order of the 1.64 million bases that make up the pathogen's
genetic code. The sequence has already revealed how C. jejuni
might evade immune system detection, information that might help
researchers develop vaccines to protect against the bacterium, which
last year caused nearly 300,000 cases of food poisoning in the United
States alone.
It is also shedding light on an occasional aftermath of C. jejuni
infections: a temporarily paralyzing neuromuscular disorder called
Guillain-Barré syndrome, thought to be an autoimmune reaction touched
off by the bacterium. What's more, because C. jejuni is a close
relative of Helicobacter pylori, which causes ulcers, comparing
the two genomes should help researchers better understand that pathogen
as well. "The Campylobacter sequence is going to help the field
no end," predicts Richard Alm, a microbiologist at Astra Research Center
in Boston, Massachusetts.
Microbiologists have found C. jejuni difficult to study
because it grows poorly in the lab. But sequencing it proved much less
of a problem, taking less than 16 weeks from start to finish. "It
sequenced like a dream," Wren said, opening up an entirely new view of
the organism.
Not all of C. jejuni's potential genes have been identified
yet, but those that have may solve some puzzles about the organism. For
example, the Sanger group discovered repeated sequences of either
guanine or cytosine bases--anywhere from 7 to 13 copies of each--in 25
of the microbe's genes. Such repeats are not unusual, but in this case
they helped Wren and his colleagues see how the bacteria might evolve to
evade the host immune system.
These repeated sequences are particularly prone to mutation when the
bacteria replicate their DNA before dividing, because in those regions
the strand being synthesized may slip relative to the one being copied,
with the result that bases are lost or gained depending on the direction
of the slippage. And Wren and his colleagues found that the same gene
could contain, say, nine guanines in a row in one sample and 13 in
another, changes that could affect the structure and activity of the
gene's protein product.
These mutations primarily affect genes that help produce
lipopolysacharides, the sugars that coat the surface of C. jejuni.
By frequently altering these genes, C. jejuni may change how
its surface looks to the immune system and may thus avoid recognition by
antibodies made during previous infections, suggests Julian Ketley, a
microbiologist at Leicester University, U.K. The sequence also revealed
what may be another countermeasure in C. jejuni: three
not-quite-identical copies of a gene called NeuB. These genes,
which make proteins that help cause acidic sugars to be added to various
other molecules in a process called sialation, might help disguise
bacterial components so that they look more like those of the host and
are thus harder for the immune system to detect.
Besides shielding the bacterium from an immune response, these
similarities could cause trouble when the immune system does succeed in
recognizing the camouflaged molecules. Wren reported preliminary
experiments suggesting that one NeuB gene may cause a surface
molecule on C. jejuni to look like a ganglioside, a type of
lipid found in high concentrations in the nervous system. That close
resemblance could trick the immune system into attacking nervous tissue
as well as the invading bacteria, perhaps causing Guillain-Barré
syndrome.
So far, however, the genome hasn't provided many other clues about
how the microbe does its dirty work. For example, researchers thought a
toxin similar to that made by the cholera pathogen might be the cause of
the diarrhea and other symptoms caused by C. jejuni. "But there
doesn't seem to be any evidence of that," says Ketley, who has teamed up
with several other U.K. researchers to identify all of C. jejuni's
proteins and their functions as a way of pinning down the source of its
virulence.
In the meantime, Wren and others have begun comparing the C.
jejuni sequence with that of the closely related H. pylori.
Until now, "no one had sequenced different, side-by-side species," Alm
notes. The differences are surprisingly large, he adds: "The genomes are
indistinguishable by size and yet 17% of the genes are specific to
Helicobacter."
Some of the differences appear to be related to the different
lifestyles of the two organisms. For example, H. pylori settles
only in the stomach and has several genes that appear to help it cope
with the stomach's acid environment by coding for enzymes that break
down urea. This "may create an alkaline cloud around the
Helicobacter," Wren explains. C. jejuni, for its part, has
about twice as many genes as H. pylori that are involved in
sensing and initiating coordinated responses involving multiple genes.
These presumably enable the microbe to adjust to a new environment, be
it the gut of a bird, milk in a bottle, or a human intestine.
Over the next few years, Wren and his colleagues will study the role
of the newly identified genes by using DNA microarrays, glass slides
spotted with DNAs representing all of C. jejuni's genes, to see which
genes are active over the course of an infection. In the near future,
Wren predicts, "[C. jejuni] will go from being one of the least
well-studied pathogens to one of the most well-studied ones."
