Single-celled
bacteria communicate with each other using coded messages to coordinate
attacks on their targets. Until now, the diversity of codes employed by
these invading bacteria was thought to be extremely limited. However, a
new report published Dec. 12 in PLoS ONE reveals
bacterial communication by a novel, previously undescribed signal type—and, as is often the case in evolutionary stories, some plants have
evolved a complementary cypher-breaking detection system that intercepts
this bacterial code and uses the information to trigger a robust immune
response, preventing disease.
Over
the last 20 years, researchers have shown that bacteria employ specific
signals to communicate. These signaling molecules, called “bacterial
Esperanto” by Professor Bonnie Bassler, an early pioneer in studies of
bacterial communication, accumulate in the external environment as the
cells grow, and when the concentration reaches a certain threshold
level, the bacteria mobilize concerted, group actions.
Until
now, it was thought that the two major groups of bacteria
(Gram-positive and Gram-negative) use distinctly different types of
communication codes. However, the newly discovered signal, called Ax21
and found in a rice-infecting bacteria, doesn’t fall into either class.
While the previously characterized signals in the bacterial coding
repertoire were all relatively small molecules, Ax21 is a small protein,
which makes it much larger.
Perception
of Ax21 by other bacteria triggers a massive change in their genetic
program, altering the expression of nearly 500 genes, or approximately
10% of the bacteria’s genome. These changes allow the bacteria to
assemble into elaborate protective bunkers, called biofilms, which
render the bacteria resistant to dessication and antibiotic treatment.
Thus, by virtue of communication and communal living, bacteria increase
their chances of survival and proliferation. Ax21 perception also
regulates the production of a virulent arsenal, including “effectors”
that are shot directly into the host to disrupt its defenses and that
initiate motility, allowing the bacteria to colonize new sites for
infection.
Most
rice plants are virtually defenseless against this Ax21-mediated
bacterial attack – except for those plants that carry a particular
immune receptor, called XA21, which detects Ax21. This early detection
gives the plant time to mobilize its defenses and mount an early and
potent defense response. The discovery of a signaling protein from a
Gram-negative bacterium with a dual role in bacterial communication and
in triggering the host innate immune response has not previously been
demonstrated.
The
XA21 receptor belongs to a large and important class of immune
receptors; the discovery of this class in flies and mice earned
Professors Bruce Beutler and Jules Hoffman the 2011 Nobel Prize in
Physiology and Medicine. Today’s report is the first to show that these
receptors can recognize bacterial signaling molecules.
The
authors, led by Professor Pamela Ronald at University of California,
Davis, have also shown that Ax21 is present not only in important plant
pathogens, but also in a human pathogen that infects some hospital
patients. This conservation in both plant and animal pathogens suggests
that Ax21 also serves as a signal in these related microbes.
Furthermore, exploration of bacterial genomes predicts the presence of
an abundance of small secreted proteins similar to Ax21 in many other
bacterial species, suggesting the intriguing possibility that other
species of bacteria also use small proteins to communicate and
coordinate infection.
Control
of Gram-negative bacterial infections in plants and animals remains a
major challenge for the medical profession and for farmers, because
conventional approaches are often not sufficient to eradicate these
infections.
One
major reason for persistence seems to be the capability of most
bacteria to grow within biofilms that protect them from adverse
environmental factors and antibiotics. The knowledge that bacteria use
Ax21 to communicate is expected to lead to new methods for controlling
bacterial diseases.
Small Protein-Mediated Quorum Sensing in a Gram-Negative Bacterium
