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In late-stage AIDS patients, a number of opportunistic pathogens can cause serious illness. The most common of these is disseminated Mycobacterium avium complex (MAC) disease, which is estimated to occur in about 60% of AIDS patients and is the result of an infection by Mycobacterium avium. It is not known where or how patients become infected with M avium, but since the bacteria grows in many environments such as water, soil, and animals, people are likely exposed to it through consumption of contaminated food or drink.
Within the past few years, microbiology has begun to realize the significance of bacterial communities called biofilms in the environment. Generally defined, biofilms are aggregates of bacteria consisting of single or multiple species attached to a surface exposed to a continuous flow of nutrients. According to Dr. John W. Costerton, a microbiologist at the Center for Biofilm Engineering, most bacteria found in the environment are primarily in biofilms and not as single-cell organisms, so it is likely that the M. avium bacilli contaminating food or drink are present in the form of biofilms.
What is the significance of bacterial biofilms? The fact that biofilms exist in the environment is not particularly interesting but that these biofilm organisms possess vastly different properties from their single-cell cousins have scientists bursting with curiosity. Each bacterium forming these complex structures appear to have its own special niche within the biological community, much like the cells of a multicellular organism. Another interesting aspect of these formations, and one that may pose a threat in the clinical environment, is their increased resistance to antibiotics. Biofilms have been found growing in hospitals on catheters inserted into the blood vessels of patients for drug administration.
Little is known as to why bacteria form biofilms. Some experts think that biofilm development is the partly caused by the nutritional status of the environment. A study conducted by Drs. Roberto Kolter and George A. O’Toole at Harvard Medical School shows that biofilm formation in the bacteria Pseudomonas fluorescens strain WCS365 progresses at different rates depending on available nourishment. This study also found that the ClpP protein, a protein used to regulate other proteins, is present in greater quantities in biofilms of P. fluorescens, suggesting its involvement in the formation process.
Many studies have focused mainly on the physiological factors of biofilm development, but few have examined the involvement of genetics aspects. The tuberculosis and pathogenesis group at the Centers for Disease Control and Prevention in Atlanta, Georgia has undertaken a new study to understand the genetic factors that lead to biofilm formation and maintenance. The group, led by Dr. Frederick D. Quinn of the National Center for Infectious Diseases division, proposes to construct a genetic library for the examination of gene expression within biofilms. Genetic libraries are approximate representations of all the genes of an organism. These libraries take an enormous effort to assemble, but can be applied to study many genetic factors of an organism. Construction of such a library involves fusing each gene of the organism to a reporter gene, which is a gene that produces a readily identifiable product. In this study, the library will be constructed from the genome of M. avium strain A5 with luciferase, whose product is a protein that gives off light in the presence of an energy source, as the reporter gene. The library will be used to make biofilms from which the genes active during biofilm formation can be observed. Comparison of these genes to the ones active when bacteria are in their single-cell form will reveal the gene activity specifically related to biofilm development.
The CDC research group reported that it is currently in the process of building the genetic library. “This task is especially difficult because we are dealing with an extremely stubborn [bacteria] when it comes to genetics,” says Pei-Hsiu Huang, a member of the research group. “And that is the easy part. Once we have the library, we have to identify which genes are active and each of their functions. We expect this project to be a two to three year effort.”
Though results are still a long time coming, Dr. W. Edward Swords, a microbiologist in the CDC group, believes that the genes identified will provide the necessary information to further our understanding of biofilm formation in M. avium and may ultimately provide clues about the pathogenesis of disseminated MAC disease.
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