Martha H. Mulks, Ph.D.

mulks@msu.edu

Research

Research in my laboratory is focused on the study of bacterial pathogenesis: how bacteria cause disease in humans and animals. At this time, the major focus of our research is Actinobacillus pleuropneumoniae (APP), the causative agent of porcine pleuropneumonia, a severe and often fatal respiratory disease of swine. This organism is of interest to us for two reasons: it is a pathogen of significant economic importance in the United States and worldwide, and development of improved vaccines and diagnostic tools would be of benefit to the swine industry; and it is an excellent model for the study of the pathogenesis of pneumonia caused by similar organisms in humans and other animals of veterinary importance.

A major goal of our research on APP is to identify genes encoding proteins involved in pathogenesis, either as virulence factors, regulatory molecules, or components of pathways necessary for survival in the host animal. We have developed a genetic system, termed an in vivo expression technology (IVET) system, that allows us to identify APP genes that are specifically turned on in the host animal during infection and are turned off in vitro on standard laboratory media. Using this IVET system, we have identified over 50 such genes, and are currently studying the role of these genes in the disease process. An example of one of these in vivo induced genes is ohr, which encodes an organohydroperoxide reductase that we postulate may help APP survive oxidative stress produced by neutrophils in the infected lung. We hope that this research will be broadly applicable to understanding the pathogenesis of respiratory diseases in other animals of veterinary importance as well as in humans.

As part of this research, we have developed genetic tools and techniques to facilitate construction of defined mutants of APP, and have used these techniques to construct biochemical mutants of APP that show promise as live avirulent vaccines. For example, we have established that the ability to synthesize riboflavin is critical for survival of APP within the swine lung, and that riboflavin-requiring mutants of APP are fully attenuated. We have tested these mutants as live vaccines against APP, with good success, and are currently studying whether riboflavin-requiring mutants of related species of Pasteurellaceae also show promise as live vaccines. This work builds on our previous establishment of an experimental model of APP infection in swine and development and testing of improved modified bacterin and subunit vaccines against APP. We have also identified a metabolic gene from Haemophilus ducreyi, nadV, that provides APP with the ability to grow without added NAD and can be used as a selectable marker for the construction of mutants without the use of an antibiotic resistance marker. This is critical in the construction of mutant strains for use as live vaccines.