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Biochemical Society Transactions (2003) Volume 31, part 1
Yersinia pestis and plague
R.W. Titball*1 , J. Hill*, D.G. Lawton† and K.A. Brown†
*Defence Science and Technology Laboratory, Porton Down, Salisbury, Wiltshire SP4 0JQ, U.K., and †Department of Biological Sciences, Centre for Molecular
Microbiology and Infection, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, U.K.
Abstract
Yersinia pestis is the aetiological agent of plague, a disease of humans that has potentially devastating
consequences. Evidence indicates that Y. pestis evolved from Yersinia pseudotuberculosis, an enteric
pathogen that normally causes a relatively mild disease. Although Y. pestis is considered to be an obligate
pathogen, the lifestyle of this organism is surprisingly complex. The bacteria are normally transmitted to
humans from a flea vector, and Y. pestis has a number of mechanisms which allow survival in the flea.
Initially, the bacteria have an intracellular lifestyle in the mammalian host, surviving in macrophages. Later,
the bacteria adopt an extracellular lifestyle. These different interactions with different host cell types are
regulated by a number of systems, which are not well characterized. The availability of the genome sequence
for this pathogen should now allow a systematic dissection of these regulatory systems.
Introduction
Yersinia pestis is the aetiological agent of plague, a disease
which has occurred in three pandemics and which, at least
in Europe, has shaped social structures and even the genetic
makeup of the population [1]. The most notorious of these
outbreaks is the so-called ‘Black Death’ pandemic, which
occurred in Europe between the 14th and 16th centuries.
Even today, at least 2000 cases of plague are reported annually
to the World Health Organization (WHO), and the outbreak of pneumonic plague in Surat, in India, reminded the
world of the potential for this disease to spread rapidly. Most
of the cases of disease in humans that occur nowadays are
bubonic plague, which is usually the consequence of the
bite from a flea that has previously fed on a rodent infected
with Y. pestis. It is this form of the disease that gives rise
to the classical symptom of plague: the swelling of the local
draining lymph node (or bubo), usually in the groin or
armpit. Occasionally, the infection spreads beyond this focus
of infection to the bloodstream. Such cases of septicaemic
plague are difficult to treat and often result in the colonization
of the lungs. A subsequent secondary pneumonia can result
in the dissemination of bacteria by the airborne route,
as a consequence of coughing. It is these airborne bacteria
that pose the greatest risk to human health, because
their inhalation can result in primary pneumonic plague.
Pneumonic plague is notoriously difficult to treat because of
the speed with which the disease develops (typically the
incubation period is 1–3 days), and also because, by the time
individuals are symptomatic, they are often close to death.
The genome sequence of Y. pestis has recently been
determined. This information has provided new insight into
the evolution of this pathogen. In addition, genome sequence
Key words: pathogenesis, pathogen evolution, type III system.
Abbreviation used: Hms, haemin storage.
1
To whom correspondence should be addressed (e-mail [email protected]).
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and other information provides insight into the ways in which
this pathogen rapidly changes its lifestyle to adapt to survival
in fleas and to growth, first in macrophages and later outside
of host cells. The present paper provides an overview of
the evolution of this pathogen and the consequences of these
evolutionary changes for the lifestyle of the pathogen.
Y. pestis evolved from a low-grade
enteric pathogen
All of the available evidence indicates that Y. pestis evolved
between 1500 and 20 000 years ago from Yersinia pseudotuberculosis [2], a pathogen which causes a relatively
mild enteric disease in humans. A range of biotypes of
Y. pseudotuberculosis are known to exist, differing mainly in
their make-up of the lipopolysaccharide O-antigen. There are
several mutations in the O-antigen cluster in Y. pestis and, as
a consequence, an O-antigen is not produced [3]. However,
this cluster shows an overall make-up that is similar to that
found in Y. pseudotuberculosis biotype 1b [3]. On this basis,
Y. pseudotuberculosis 1b has been suggested to be the ancestral
biotype of Y. pestis.
The evolution of Y. pestis appears to have occurred as the
consequence of the loss of several functions associated with
enteric disease, and the acquisition of a range of new functions
that are encoded primarily on two additional plasmids (pFra
and pPcp) [4]. Y. pestis also exploits some of the virulence
mechanisms found in Y. pseudotuberculosis, but to greater
effect. The origins of pFra and pPcp are not clear; however, a
plasmid with a high degree of overall sequence identity with
pFra has been found in some isolates of Salmonella typhi
(the aetiological agent of typhoid fever) from Vietnam [5].
At this time, it is not clear whether these plasmids have been
acquired from a common ancestor (or donor) or whether these
plasmids have been transferred between Y. pseudotuberculosis
and S. typhi.
Signalling the Future
Pathogenesis of plague
Genes that allow the colonization of the flea
The classical insect vector for Y. pestis is the flea, usually
Xenopsylla cheopsis. The flea becomes infected with bacteria
as a consequence of feeding on a rodent infected with
Y. pestis. Within the flea, the blood meal coagulates, blocking
the foregut of the flea and thus preventing digestion of the
blood meal. A number of proteins appear to contribute to
the survival of Y. pestis in the flea. The chromosomally borne
haemin storage (hms) locus encodes a number of membrane
proteins that result in the pigmentation of Y. pestis when
cultured in vitro on agar containing haemin or Congo Red
dye. The ability of bacteria to bind haemin led to initial
speculation that the Hms proteins might form part of an ironacquisition system. However, hms mutants of Y. pestis are
fully virulent in the murine model of disease and the Hms
system does not appear to function as a system for the
acquisition of nutritionally accessible iron [6]. In the flea,
the Hms proteins appear to alter the hydrophobicity of the
bacterial cell, thereby promoting aggregation and clumping
of bacteria within the blood meal [7]. This may be one of
the main mechanisms by which blocking of fleas occurs. The
Y. pestis phospholipase D plays a key role in promoting
survival of bacteria within the flea gut, and a phospholipase
D mutant of Y. pestis was unable to cause flea blocking [8].
The precise role of this enzyme is not known, but it may have
a role in the restructuring of the bacterial cell wall, thereby
protecting bacteria from serum-derived cytotoxins [8].
Genes that allow growth and regulate survival in
the macrophage
The bite of a mammal from a flea that carries Y. pestis results
in the regurgitation of the bolus of Y. pestis cells, which are
embedded in the coagulated blood meal. As a consequence of
the flea bite, it has been suggested that up to 24 000 Y. pestis
cells are delivered intradermally into the new mammalian
host [1]; these bacteria are readily ingested by professional
phagocytes. Bacteria that are ingested by neutrophils appear
to be readily killed, but bacteria within macrophages are
able to survive and proliferate. The determinants which
allow survival and growth in the macrophage are not
known; however, Y. pestis has been shown to possess a
two-component regulatory system, which is closely related
to the PhoP/PhoQ two-component regulatory system in
Salmonella typhimurium (a pathogen of humans that causes a
relatively mild gastrointestinal disease). Like S. typhimurium,
the Y. pestis PhoP/PhoQ system appears to regulate survival
in macrophages, and also like S. typhimurium, extracellular
Mg2+ levels might activate this system. A phoP mutant
of Y. pestis is moderately attenuated in the murine model
of disease, but, more importantly, this mutant shows an
enhanced suceptibility to macrophage killing mechanisms,
and has reduced survival in macrophages [9]. A number of
genes appear to be regulated by the PhoP/PhoQ system
[9], but their identities have not yet been determined,
although some of the gene products are thought to have
a role in the modification of lipo-oligosaccharide [10]. The
ability of Y. pestis to survive in macrophages is critical
to the early pathogenesis of disease, and bacteria within
macrophages appear to be trafficked to the local draining
lymph node. Within these lymph nodes, a massive infiltration
of phagocytic cells occurs, resulting in the formation of a
bubo.
Genes that allow growth and replication outside
of host cells
During growth in macrophages and trafficking to the lymph
node, another key regulatory system modulates the properties
of the bacteria. Bacteria within the flea are incubated at
temperatures below 37◦ C. The exposure to temperatures
of around 37◦ C in the mammalian host results in the upregulation of a range of virulence factors. Within the bubo,
and by an unknown mechanism, the bacteria appear to escape
from infected macrophages and to adopt an extracellular
lifestyle. Some of the virulence determinants responsible
for this temperature-dependent lifestyle switch have been
identified. A polypeptide destined to form a surface capsule
(F1-antigen) is produced in large amounts. This polypeptide
is exported on to the cell surface where it appears to autoassemble into fibrillar-like structures [11]. Mutants of Y. pestis
that are unable to produce F1-antigen show an enhanced
susceptibilty to phagocytosis by macrophages [12].
Possibly, the key virulence mechanism that allows the
bacteria to resist further phagocytosis is the type III secretion
system (Figure 1). The type III secretion system is upregulated at 37◦ C, i.e. within the mammalian host. This system
allows bacteria that are in contact with host cells to inject a
range of so-called effector proteins (or effector Yersinia outer
proteins, or Yops) into the host cell via the secretory apparatus
[13,14]. The functions of the effector Yops fall broadly into
two groups: YopH and YopT target the cytoskeleton by
dephosphosphorylating paxillin/focal adhesion kinase and
by modifying RhoA GTPase respectively [13–15]. YopE
also disrupts the cytoskeleton by an unknown mechanism.
In contrast, YopJ (alternatively known as YopP) downregulates the inflammatory response and induces apoptosis
in macrophages [13,14].
Although the type III system in Y. pestis is thought to be
activated on contact with host cells, and at 37◦ C, the molecular
events which transduce the cell-contact signal are not known.
However, some of the events which occur in the bacterium
on activation of the type III system are understood. In the
repressed state, the type III secretory apparatus system is
thought to be ‘blocked’ on the outside by the YopN protein,
and on the inside of the bacterial cell by the protein LcrG.
Before contact with the host cell, the expression of Yops is
repressed by the accumulation of LcrQ within the bacterium.
Induction of the type III system by contact with a eukaryotic
host cell triggers a cascade of events. In one model, YopN
is then released from the cell surface [16]. Although the
secretory system is still partially blocked by LcrG, there
is sufficient release of LcrQ to allow the induction of Yop
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Biochemical Society Transactions (2003) Volume 31, part 1
Figure 1 Type III system of Y. pestis, shown after contact with a host cell, activation, and delivery of effector Yops into the host cell
Eukaryotic
cell
Yersinia
cytosol
expression. One of the first Yops to be produced is LcrV.
LcrV is thought to bind with high affinity to LcrG [17], which
otherwise blocks the secretory apparatus. Titration of LcrG
by LcrV completely unblocks secretion. The VirF protein,
which is now produced, serves as a positive activator of Yop
expression. Next, the various Yops that are required for pore
formation in the host cell (YopB and YopD) are produced,
and the effector Yops are translocated into the eukaryotic
host cell. Our recent results suggest that the role of LcrV
in the regulation of the type III system is more complex
than was originally thought, and that LcrV might interact
with other components of the type III system in a regulatory
capacity [17].
Clearly, LcrV within the bacterial cell is important for
the activation of the type III system. LcrV is also found
on the cell surface [18] and is also exported from the bacterial
cell. The function of surface-located LcrV is not known.
However, soluble LcrV appears to act on host cells to repress
the production of the pro-inflammatory cytokines such as
tumour necrosis factor α and interferon γ [13].
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Conclusions
Although Y. pestis is considered to be an obligate pathogen, it
shows a remarkable ability to survive and grow in a diversity
of host environments. These interactions with different host
cells are regulated by a number of mechanisms, which are only
now being characterized at a molecular level. The availability
of the genome sequence of this pathogen will now allow these
complex regulatory networks to be characterized, for example
using microarray technology. In the future, we might also
expect further insights into genes which regulate or play a
direct role in the pathogenesis of pneumonic plague.
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Received 18 July 2002
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