The focus of our laboratory for many years has been
to understand why certain bacteria are pathogenic, how they became pathogenic,
and how they interact with their animal host.
We study several model systems:
1. Salmonella typhimurium infection of the mouse.
Our
research on Salmonella infection first concentrated on the bacterial
genes that were essential for bacterial entry into cultured cells. Increasingly, we have shifted our attention
away from cell culture to examine the interaction of these bacteria with the
Peyer’s patch. The Peyer’s patch contains specialized terminally differentiated
epithelial cells called M cells, which appear to be the primary entry point of Salmonella
typhimurium to penetrate the intestinal mucosal barrier to cause infection.
Several labs, including our own, have characterized the bacterial genes
important for this act of cellular penetration. A few years ago our laboratory reported that when Salmonella
entered the Peyer’s patch, they often interacted with resident macrophages and
killed them by a process that was similar to programmed cell death or
apoptosis. In collaboration with Arturo Zychlinsky’s laboratory at NYU, we
showed that a particular Salmonella virulence gene, SipB, targets
caspase 1 in macrophages and that this interaction leads to macrophage cell
death as well as the induction of the cytokines IL-1b and IL-18. More recently, we showed that caspase-1
knockout mice are resistant to oral challenge by Salmonella. It appears
that in the absence of this host factor, there is no inflammatory response by
the host. We currently are trying to understand why the absence of the
caspase-1-induced inflammatory response by the host leads to host resistance to
infection and disease. Also, we are examining the sequence of events that occur
in infected animals following invasion of the Peyer’s patch to understand how
the bacteria eventually enter the mesenteric lymph nodes and enter the spleen
and liver to cause a systemic infection and subsequent death of the host
animal.
Other facets of Salmonella
research in the laboratory include looking at specific Salmonella genes
that are expressed exclusively when these bacteria enter cells. Salmonella
are facultative intracellular parasites, so they grow in ordinary culture in
the laboratory, but in the animal host, they reside and replicate within host
cells. We know that a major cellular target of Salmonella growth and
replication is within macrophages, but more recent data suggests there are
other cellular targets as well.
Recent studies from our laboratory that illustrate
our research on Salmonella include:
Currently, we have expanded
our research tools with Salmonella to include a spotted DNA microarray
in which we can examine the expression of all the known chromosomal and plasmid
genes of both Salmonella typhi and S. typhimurium during
infection of cultured host cells and in infected animals. We also employ both
human and murine DNA microarrays so that we can simultaneously examine the
response of host genes to Salmonella infection.
2. Helicobacter pylori
Helicobacter pylori colonize the stomach of half of the world's population, causing a wide spectrum of disease ranging from asymptomatic gastritis to ulcers to gastric cancer. Although the basis for these diverse clinical outcomes is not understood, more severe disease is associated with strains harboring a pathogenicity island. To characterize the genetic diversity of more and less virulent strains, we examined the genomic content of 15 H. pylori clinical isolates by using a whole genome H. pylori DNA microarray. We found that a full 22% of H. pylori genes are dispensable in one or more strains, thus defining a minimal functional core of 1281 H. pylori genes. While the core genes encode most metabolic and cellular processes, the strain- specific genes include genes unique to H. pylori (restriction modification genes, transposases, and genes encoding cell surface proteins), which may aid the bacteria under specific circumstances during their long-term infection of genetically diverse hosts. We observed distinct patterns of the strain-specific gene distribution along the chromosome, which may result from different mechanisms of gene acquisition and loss. Among the strain-specific genes, we have found a class of candidate virulence genes identified by their coinheritance with the pathogenicity island.
In parallel, we also have examined the response of 26000 human gene sequences found in cultured gastric epithelial cells to H. pylori infection. These results show that a number of gene families are up regulated in response to infection. One set of human genes up regulated are involved with a generalized inflammatory response. Perhaps the most striking human gene response involves gene families involved with the cytoskeleton, extracellular matrix, host cell motility, and tight-junctions. At least early in the infectious process, it appears that the host cell genes involved with averting apoptosis are brought into play. Some human genes appear to be down-regulated with respect to H. pylori infection and these appear to be involved with the host cell replication cycle. That is, infected epithelial cells appear to enter cell cycle arrest.
A number of individual research projects are ongoing as well. In particular we are investigating animal models of H. pylori infection using well-defined mutant strains of bacteria. A major collaborative project with several laboratories involves examining epidemiological well-defined clinical isolates obtained from patients with gastric cancer. We wish to see if there is a particular bacterial phenotype associated with the eventual development of cancer subsequent to H. pylori infection.
Recent studies that illustrate our research into H. pylori pathogenesis include: