An initial approach today to a newly discovered pathogen is to perform whole genome sequencing (WGS). This same approach is relevant for investigations of an understudied high-consequence pathogen such as Burkholderia mallei
, the cause of glanders. This zoonotic bacterium can kill humans and animals 
. It is classified as a Category B biothreat agent and it has been used as a biological warfare agent 
. Its ease of use against civilians and lack of countermeasures has compelled several agencies to rank it very high on their priority lists of biothreat agents
. Its ease of use as a biothreat agent is illustrated by the physician who, even in 1915, grew it out of his Washington DC area home, and distributed it for an attack against US horses destined for Europe as critical transportation in World War I
. Humans were also subject to infection. We remain highly vulnerable to this agent because there is no rapid diagnostic assay, no distinctive diagnostic signs, and the incubation period is short, a few days. In addition there is no vaccine, no infection-induced immunity, and limited reliable in vivo
data on antibiotic efficacy. Overall, our general knowledge of this understudied pathogen and its disease is limited 
. Our desire to decrease our vulnerabilities to this pathogen, as part of the national biodefense efforts, is hampered, in part, by constraints of the Select Agent regulation and the need for BSL-3 facilities. These are appropriately in place for safety reasons, however, the availability of a suitable attenuated surrogate strain would be desirable as it could accelerate B. mallei
related biodefense research in many non-BSL-3 laboratories.
is a Gram-negative non-motile aerobic bacteria with a genome of approximately 6 Mb organized in two circular chromosomes. In addition to infecting humans, B. mallei
can cause acute or chronic fatal contagious zoonotic infections in its natural equine host, such as horses, donkeys, and mules, with a very low infectious dose. Two major potential routes of infection for a biologic attack are aerosol and cutaneous contact. Gastrointestinal ingestion is a common mode of natural infection in equines. The incubation period is typically between 3–6 days but may be longer.
An example of a phenotypic highly virulent strain is B. mallei
strain ATCC 23344 (China 7). It is highly virulent in its natural hosts, equines, in humans, and in mice and hamster models
. This strain of B. mallei
contains an animal type III secretion system (T3SS) gene complex which is essential for virulence
. Genome sequencing (WGS) and analysis of this strain identified a number of other putative virulence factors whose function was supported by comparative genome hybridization and expression profiling of the bacterium in hamster liver in vivo
. Numerous insertion sequence elements that have mediated extensive deletions and rearrangements of the genome relative to the B. pseudomallei
genome were found. As part of our interest to study mechanisms responsible for virulence we performed WGS and comparative genomic analysis on strains of B. mallei
in which we had closely linked information from an actual clinical infection in its natural host or a suitable animal model rather than using an archived strain lacking this information. We intended to compare the gene content of phenotypic virulent to avirulent strains to identify candidate virulence genes. One serendipitous event and an unexpected finding led to what may be a fortuitous observation. The serendipitous event occurred when five of six equids were infected, but not made ill, with what was a previously pathogenic strain of B. mallei
, designated as SAVP1. At the time of inoculation this strain was still believed to be pathogenic. A sixth equid, a donkey, developed clinical signs only after a massive exposure. The strain appeared avirulent even when administered in escalating doses. The other unexpected finding came while performing comparative genomic analysis between SAVP1 and the strain of B. mallei
that was currently behaving as virulent in humans and horses. This finding is both informational and fortuitous because it may open the door for SAVP1 to be classified as suitably avirulent for exclusion from Select Agent lists
thereby expanding our biodefense research efforts.