Listeria bacteria yield
clues to workings of other deadly intracellular pathogens, UC
Berkeley scientists report
Robert Sanders, Media Relations
FOR RELEASE UNTIL 2 P.M. EST (11 A.M. PST) THURSDAY, NOV.
2, TO COINCIDE WITH PUBLICATION IN THE JOURNAL SCIENCE
- Many deadly microbes have learned that the key to launching
an infection is not to kill your host - at least not too quickly.
at the University of California, Berkeley, have discovered
how one microbe, Listeria monocytogenes, is able to manage
In a paper
in this week's issue of Science, Daniel A. Portnoy, professor
of molecular and cell biology in the campus's College of Letters
& Science and professor of infectious diseases in the School
of Public Health, along with post-doctoral fellow Amy L. Decatur,
describe the trick these bacteria use to live comfortably
inside a cell until they're ready to break out and spread
the infection to other cells.
could have implications beyond this one bacteria, which causes
a deadly disease called listeriosis. The world's top three
infectious killers - AIDS, tuberculosis and malaria - all
are caused by pathogens that ensconce themselves snugly inside
cells and live to wreak havoc. Yet, these intracellular pathogens
have been hard to study, Portnoy said.
are no effective vaccines for any of these diseases, in part
because it is difficult to study intracellular pathogens,"
he said. "Listeria is a great model system for studying the
host-pathogen interaction of these intracellular bugs."
is a common but deadly bacterium that in recent years has
made headlines as a contaminant of hot dogs, cheese, cole
slaw and other food stuffs, causing more than two thousand
infections every year and 500 deaths. Though it hits immune-compromised
people the hardest, its overall fatality rate is about 20
bacteria establish an infection by inducing immune system
cells, mostly scavenger cells called phagocytes, to corral
and swallow them, so that they end up encased in a bubble
within the body of the cell. The bacteria would be benign
if they remained isolated in the vacuole, because the cell
can kill them there. But they eventually break out and take
over the host cell's machinery to spread the infection. What
makes Listeria virulent is a pore-forming toxin that allows
the bacteria to break through the wall of the vacuole and
enter the cell's innards, Portnoy said.
A big question
has always been why the toxin, listeriolysin O, doesn't also
rupture and kill the cell, exposing the bacteria to the immune
years ago, a post-doctoral fellow in Portnoy's lab compared
listeriolysin O to a similar pore-forming toxin called perfringolysin
O, from the extracellular bacteria Clostridium perfringens,
which cause gangrene. Sian Jones and Portnoy found that if
they substituted perfringolysin O for Listeria's normal toxin,
the altered bacteria were able to punch their way out of a
vacuole, but then they killed the host cell. This made Listeria
totally avirulent, Portnoy said, because the immune system
efficiently mopped up the exposed bacteria.
and Decatur compared the genetic sequences of the two toxins
and found that listeriolysin O contains an extra bit of protein
that looks just like a tag found in a range of organisms from
yeast to humans, and which often tells the cell a protein
is trash and should be chopped up and recycled. The tag is
referred to as a PEST sequence, signifying the four amino
acids characteristic of the tags.
bacteria apparently stole the tag and placed it on the toxin
so that the host cell's clean-up crew recognizes it and targets
it for destruction before it has a chance to make pinholes
in the cell membrane.
great example of how bacteria have taken advantage of the
host's biology to enhance their pathogenicity," Portnoy said.
scientists elegantly demonstrated how critical this PEST sequence
is to the virulence of Listeria. When they mutated the PEST
tag so the cell no longer recognized it, the mutant bacteria
quickly killed off the host cells. The mutant Listeria proved
10,000 times less virulent in mice than the wild Listeria
bacteria. Apparently, immune system cells eliminated the mutant
bacteria once they killed off their host cell.
to survive, the pathogen must maintain a protected niche within
the host cell," Decatur said. "To achieve this, the toxin
has co-opted the cell's own machinery, sprouting a tag that
says, 'Please get rid of me.'"
and his colleagues have discovered the role played by another
important protein in the Listeria lifecycle. Once the bacteria
break free of their protective vacuoles, they take over the
cell machinery and do something amazing. They generate comet-like
tails that push them around the cell like a speedboat. Eventually,
they slam into the cell membrane and pop from one cell into
the next, spreading the infection.
comet tails have been observed and photographed for more than
a decade (see http://cmgm.stanford.edu/theriot/movies.htm
for movies), two years ago Matthew D. Welch, an assistant
professor of molecular and cell biology at UC Berkeley, and
his colleagues reported details of how the tail is made.
that the tail is produced when a protein on the surface of
the bacteria, ActA, interacts with a complex of host cell
proteins called the Arp2/3 complex. The result is rapid polymerization
of a skeletal protein called actin that piles up and physically
propels the newly formed bacteria around the cell.
Portnoy and graduate student Justin Skoble dissected ActA
even further, and in the August 7 issue of The Journal of
Cell Biology reported details of how different parts of the
protein carry out different functions, all of which are critical
to the pathogen's virulence.
elegant about this is that over millions of years, the bacteria
have evolved individual proteins capable of exploiting complex
processes that control host cell biology," Portnoy said.
was funded by the National Institutes of Health. Portnoy is
one of several hundred UC Berkeley researchers involved with
the campus's Health Sciences Initiative, which draws scientists
from a broad range of fields to tackle today's health problems.