Bruce A. McClane, PhD
420 Bridgeside Point II
450 Technology Dr.
Pittsburgh, PA 15219
Our research is focused on understanding bacterial pathogenesis, which remains a major medical problem in both developing and developed countries. The model pathogenic bacterium we study is Clostridium perfringens, a Gram-positive spore-forming anaerobe. This bacterium has a ubiquitous distribution in nature but is also the second most common cause of bacterial food poisoning in the USA, as well as a major cause of human gas gangrene and human/veterinary necrotic enteritis or enterotoxemia.
The hallmark of all C. perfringens infections is the involvement of potent toxins. C. perfringens can produce at least 17 different toxins, although a single strain expresses only portions of this impressive toxin arsenal. Strain to strain variability in toxin production provides C. perfringens with the ability to cause a broad spectrum of infections, ranging from intestinal disease to gas gangrene. The genetic basis for this variability in toxin production is largely attributable to the presence of most toxin genes on large plasmids, which we are describing and characterizing. Thus far, we have found that a single toxin plasmid can carry up to three different toxin genes, while a single C. perfringens isolate can carry up to three different toxin plasmids. Our work has also shown that these toxin plasmids are often conjugative, which allows their spread to other C. perfringens strains. Transfer of toxin plasmids to normal flora strains of C. perfringens in the intestines could amplify infection by conferring upon those normal flora strains the ability to produce intestinally-active toxins. In addition, we have shown that many plasmid-borne toxin genes are associated with insertion sequences that can mobilize the toxin genes; this toxin gene mobilization may allow their subsequent integration onto other plasmids, contributing to toxin plasmid diversity. Alternatively, these mobilized toxin genes have apparently inserted onto the chromosome in a few C. perfringens strains.
Another major emphasis of our work is to understand the contributions of these toxins, alone or in combination, to pathogenicity. This work involves construction of toxin mutants, and complementing strains, to identify which specific toxins play a role in various C. perfringens infections. The pathogenicity of these strains is then tested in animal models. In addition, we are investigating the regulatory systems that control toxin production. Of particular current interest is the Agr-like quorum sensing system, which appears to be important for regulating the production of many C. perfringens toxins.
A final focus of our laboratory is to elucidate, at the molecular level, how individual C. perfringens virulence factors contribute to infection. Traditionally, this work has involved studying the action of individual toxins at the cellular and molecular levels. Examples of topics now being examined include identifying toxin receptors, studying toxin: receptor interactions, and determining toxin structure/function relationships. In addition, our work has recently expanded beyond toxins to identify the role of accessory proteins, such as sialidases, to C. perfringens virulence. In addition, we recently started to examine the adhesion of C. perfringens to host cells, which is an important step in infection.
Archana Shrestha, Postdoctoral Associate
John Freedman, Postdoctoral Associate