Genetics and Molecular
Biology
of
Bacterial Pathogens
Glenn M. Young, Ph.D.
Assistant Professor, University
of California, Davis
Educational Background
Postdoctoral Fellow, Department
of Molecular Microbiology, Washington University School of Medicine, 1997-1999
Postdoctoral Fellow, Department
of Microbiology and Molecular Genetics, University of California, Los Angeles,
1994-1996
Ph.D. Microbiology, Washington
State University, 1994
B. S. Microbiology, University
of Illinios, Urbana, 1987
Research
- Bacterial pathogens are
a constant public health challenge. They can infect any organ and thus cause
many different symptoms. Although there is a wide range of diseases caused
by different bacterial infections the virulence mechanisms responsible
for disease transmission, invasion and proliferation in a host often have
common molecular themes. My research focuses on fundemental questions
relating to signal transduction, gene regulation and protein secretion in
bacterial pathogens. We study these molecular events because they represent
key processes which are required by pathogenic bacteria that contribute
to virulence and environmental adaptation. Our model organisms are the pathogens
Yersinia enterocolitica and Salmonella typhimurium.
We are also beginning to study other pathogenic bacteria including E.
coli O157:H7.
- Yersinia enterocolitica is an invasive
foodborne pathogen. This bacterium is closely related to Yersinia pestis,
which is the pathogen that causes the plague. Strains of Yersinia enterocolitica
can be devided into high-virulent and low-virulent groups. Our long-term
goal is to understand the function of virulence factors that distinguish
high-virulent from low-virulent strains. Low-virulent Y. enterocolitica generally
cause a mild self-limiting gastrointestinal disease in humans and are generally
not lethal to experimentally infected mice. By contrast, high-virulent
Y. enterocolitica (restricted to serotypes O:4, O:8, O:13a,b, O:20 and
O:21) are commonly associated with acute gastrointestinal syndromes in
humans and are lethal to mice. High-virulent strains of Y. enterocolitica,
but not low-virulent strains, harbor two candidate pathogenicity islands
in the chromosome. One is called HPI (High Pathogenicity Island) and encodes
an iron uptake system that has been extensively studied by others. The other
is YSA and encodes a type III secretion system (TTSS) predicted to target
virulence effectors into eukaryotic cells. It remains particularly under-studied
and is the focus of our current work. The chromosomally encoded Ysa TTSS
has been shown to secrete a set of proteins called Ysps. Its functions are
independent of the plasmid-encoded Ysc TTSS, which is common to all pathogenic
Yersinia species. Studies with both animal and cellular models
of infection indicate the Ysa TTSS is important for pathogenesis.
Ysa TTSS mutants of Y. enterocolitica O:8 are less virulent for orally infected
mice by LD50 analysis and exhibit a survival defect in competition assays
with virulent Y. enterocolitica O:8. At the cellular level, macrophages
are thought to play an important role in limiting Y. enterocolitica
infections. My lab has shown that cultured macrophages respond differently
to infection by Ysa TTSS mutants compared to wild type bacteria. Under conditions
where the Ysa TTSS is functional, wild type Y. enterocolitica O:8 down-regulates
the release of the pro-inflammatory cytokine TNF-alpha by macrophages and
is cytotoxic to these cells. In contrast, Ysa TTSS mutants lack the ability
to down-regulatethe release of TNF-alpha and are not cytotoxic.
- Several of our research projects are focused
on extending these results to understand how the Ysa TTSS contributes to
pathogenesis at the molecular level. Moreover, this research will provide
insight on why some pathogenic bacteria have multiple TTSSs. While numerous
gram-negative pathogenic bacteria maintain a type III secretion system (TTSS),
a few species are known to maintain two contact-dependent TTSSs. More specifically,
this research will explore why high-virulent Y. enterocolitica maintain two
contact-dependent TTSSs, each of which transports a defined set of proteins
and each of which is under separate control.
Current Projects
- Project 1: Characterize Ysps and evaluate
which Ysps are targeted into eukaryotic cells by the Ysa TTSS. Identifying
potential effector proteins translocated into eukaryotic cells by the Ysa
TTSS will provide valuable information concerning the role this protein secretion
pathway plays in bacterial-host interactions. Our preliminary studies have
already resulted in the definitive identification of three Ysps and bioinformatic
analysis has lead to the discovery of genes that likely encode four additional
Ysps. The goal of this project is to complete the identification of the
remaining Ysps and to use biochemical and cellular-based assays to determine
which Ysps are delivered into eukaryotic cells by the Ysa TTSS.
- Project 2: Define regulatory networks
that coordinate expression of ysa genes. Determining how Y. enterocolitica
integrates regulatory cues to control the Ysa TTSS is important if we are
to ultimately understand how factors involved in virulence are coordinated
and will enhance our goal to identify additional Ysps. The YSA locus contains
genes predicted to encode two putative transcriptional regulatory systems.
One system is a potential two-component signal transduction system which
consists of the YsrS sensor kinase and YsrR response regulator. Our preliminary
analysis suggests that YsrRS is functional because a mutation in ysrS
prevents Ysp secretion. The second system, YsaE, belongs to the AraC family
of transcriptional regulators and has not previously been characterized.
Our prelmiary analysis indicates YsaE is invovled in regulating the expression
of genes that encode virulence effectors targeted by the Ysa TTSS.
- Project 3. Examine the role of Ysps and the Ysa TTSS in
pathogenesis using animal and cell culture models of infection. The available
evidence indicates the Ysa TTSS is important for virulence, but a detailed
analysis of how the Ysa TTSS affects pathogenesis remains to be completed.
Using the mouse model of infection, we are examining disease pathogenesis
caused by Y. enterocolitica mutants defective for the function
of the Ysa TTSS. This analysis should provide important clues as to how
the Ysa TTSS contributes to colonization and pathology of specific host
tissues during infection. Cellular-based assays are being used to further
examine the effect of the Ysa TTSS on activities of host cells, including
changes in cytokine production and cytotoxicity.
- Project 4. Defining conserved biochemical mechanisms among
different TTSSs. We know that Yersinia enterocolitica maintains three
different TTSSs. Two of these TTSSs are invovled in contact-dependent delivery
of virulence effectors into targeted host cells. The third TTSS is an integral
part of the flagellar organelle. Recently, we have demonstrated that each
of these TTSSs has the capacity to export a protein called YplA, which is
a virulence factor that affects the host inflammatory response. Recognition
of YplA by each of the three TTSSs indicates there is a conserved mechanism
of substrate recognition. We are defining the secretion signal located in
YplA to understand the mechanistic principles that govern substrate recognition.
Recent Publications
Young, B. M. and G. M. Young. 2002. Evidence for targeting
of Yop effectors by the chromosomally encoded Ysa type III secretion system
of Yersinia enterocolitica. Journal of Bacteriology
184: 5563-5571.
Petersen, S. and G. M. Young. 2002. Essential role for cyclic AMP and
its receptor protein in Yersinia enterocolitica virulence. Infection
and Immunity 70: 3665-3672.
Young, B. M. and G. M. Young. 2002. YplA is exported by the Ysc, Ysa
and flagellar type III secretion systems of Yersinia enterocolitica.
Journal of Bacteriology 184: 1324-1334.
Solnik, J. and G. M. Young. 2001. Bacterial Pathogenicity Islands
and Infectious Disease. Pages 111-122. In Horizontal Gene Transfer.
Edited by M. Syvanen and C. I. Kado. 2nd ed. London; New York : Chapman
& Hall.
Hatic II, S. O., W. L. Picking, B. M. Young, G. M. Young and W. P.
Picking. 2001. Purification of two active derivatives of recombinant YplA,
a secreted phospholipase from Yersinia enterocolitica. Biochemistry
Biophysics Research Communications 292: 463-467.
Nelson, K. M., G. M. Young and V. L. Miller. 2001. Identification of
a locus involved in systemic dissemination of Yersinia enterocolitica.
Infection and Immunity 69: 6201- 6208.
Heusipp, G., G. M. Young and V. L. Miller. 2001. HreP, an in vivo expressed
protease of Yersinia enterocolitica, is a new member of the family
of subtilisin/kexin-like proteases. Journal of Bacteriology 183:
3556-3563.
Young, G. M., Badger, J. L.
and V. L. Miller 2000. Motility is required to initiate host cells invasion
by Yersinia enterocolitica. Infection and Immunity 268:
4323-4326
Schmeil, D. H., Young, G. M. and
V. L. Miler 2000. The Yersinia enterocolitica phospholipase gene yplAis
part of the flagellar regulon. J.Bacteriol. 182: 2314-2320
Young, G. M., D. H. Schmeil and
V.L. Miller (1999) A new pathway for the secretion of virulence factors
by bacteria: The flagellar export apparatus functions as a protein secretion
system. PNAS USA 96: 6456-6461
Young, G. M., M. J. Smith, S.
A. Minnich and V. L. Miller (1999) The Yersinia enterocolitica motility
master regulatory operon, flhDC, is required for flagellin production,
swimming motility and swarming motility. J.Bacteriol. 181:
2823-2833
Young, G. M. and V. L. Miller
(1997) Identification of novel chromosomal loci affecting Yersinia enterocolitica
pathogenesis. Mol. Microbiol. 25: 319-328
Young, G. M., D. Amid and V. L.
Miller (1996) A bifunctional urease enhances survival of pathogenic Yersinia
enterocolitica and Morganella morganii. J. Bacteriol. 178:
6487-6495
Contact Information
Glenn M. Young, Ph.D.
University of California,
Davis
Microbiology Graduate Group
Biochemistry and Molecular Biology Graduate Group
Food Science Graduate Group
FS&T, Cruess Hall
UC-Davis
Davis, CA 95616-8598
Tel: 530-754-5292
Fax: 530-752-4759
Email:gmyoung@ucdavis.edu