Burkholderia pseudomallei and B. mallei are Gram-negative bacterial pathogens that cause melioidosis in humans and glanders in horses, respectively. Both bacteria are classified as category B select agents in the United States. Due to strict select-agent regulations, the number of antibiotic selection markers approved for use in these bacteria is greatly limited. Approved markers for B. pseudomallei include genes encoding resistance to kanamycin (Km), gentamicin (Gm), and zeocin (Zeo); however, wild type B. pseudomallei is intrinsically resistant to these antibiotics. Selection markers for B. mallei are limited to Km and Zeo resistance genes. Additionally, there are few well developed counter-selection markers for use in Burkholderia. The use of SacB as a counter-selection method has been of limited success due to the presence of endogenous sacBC genes in the genomes of B. pseudomallei and B. mallei. These impediments have greatly hampered the genetic manipulation of B. pseudomallei and B. mallei and currently few reliable tools for the genetic manipulation of Burkholderia exist. To expand the repertoire of genetic tools for use in Burkholderia, we developed the suicide plasmid pMo130, which allows for the compliant genetic manipulation of the select agents B. pseudomallei and B. mallei using allelic exchange. pMo130 harbors an aphA gene which allows for Km selection, the reporter gene xylE, which allows for reliable visual detection of Burkholderia transformants, and carries a modified sacB gene that allows for the resolution of co-integrants. We employed this system to generate multiple unmarked and in-frame mutants in B. pseudomallei, and one mutant in B. mallei. This vector significantly expands the number of available tools that are select-agent compliant for the genetic manipulation of B. pseudomallei and B. mallei.

The Gram-negative bacterium Burkholderia pseudomallei is the etiological agent of melioidosis and is remarkably resistant to most classes of antibacterials. Even after months of treatment with antibacterials that are relatively effective in vitro, there is a high rate of treatment failure, indicating that this pathogen alters its patterns of antibacterial susceptibility in response to cues encountered in the host. The pathology of melioidosis indicates that B. pseudomallei encounters host microenvironments that limit aerobic respiration, including the lack of oxygen found in abscesses and in the presence of nitric oxide produced by macrophages. We investigated whether B. pseudomallei could survive in a nonreplicating, oxygen-deprived state and determined if this physiological state was tolerant of conventional antibacterials. B. pseudomallei survived initial anaerobiosis, especially under moderately acidic conditions similar to those found in abscesses. Microarray expression profiling indicated a major shift in the physiological state of hypoxic B. pseudomallei, including induction of a variety of typical anaerobic-environment-responsive genes and genes that appear specific to anaerobic B. pseudomallei. Interestingly, anaerobic B. pseudomallei was unaffected by antibacterials typically used in therapy. However, it was exquisitely sensitive to drugs used against anaerobic pathogens. After several weeks of anaerobic culture, a significant loss of viability was observed. However, a stable subpopulation that maintained complete viability for at least 1 year was established. Thus, during the course of human infection, if a minor subpopulation of bacteria inhabited an oxygen-restricted environment, it might be indifferent to traditional therapy but susceptible to antibiotics frequently used to treat anaerobic infections.