Scientists’ new lab recipe helps in diagnosing glanders

Representative growth of B. mallei. (a) plate view and (b) colony view, on BM agar after 72 hours of incubation at 37 degrees Celsius. Kinoshita et al.
Representative growth of B. mallei. (a) plate view and (b) colony view, on BM agar after 72 hours of incubation at 37 degrees Celsius. Images: Kinoshita et al.

Researchers have taken a step forward in identifying the bacterium behind one of the most dangerous horse diseases, formulating a new growing medium for laboratory use.

The study team investigated Burkholderia mallei, the bacterium responsible for glanders, a highly infectious notifiable zoonotic disease.

Glanders mainly affects horses, donkeys, and mules, but other animals, including camels, bears, wolves,  dogs and humans, are known to be susceptible to infection.

The acute form is typically seen in donkeys and mules, with high fever, respiratory symptoms, and death occurring within a few days.

In horses, chronic cases usually develop and horses may endure for several years. Chronic glanders increases the risk of spread because of the prolonged shedding of B. mallei.

The disease was eradicated from large areas of the Western world in the early 20th century, but cases have occurred in the last 10-20 years in areas where the disease was previously unknown or had been eradicated.

Warnings have also been given by scientists that the growth in international horse transport risks the spread of the dangerous pathogen.

Bacterial isolation is considered the gold standard method for diagnosing glanders.

B. mallei can be grown in the laboratory on most enriched media but grows very slowly, requiring up to 72 hours of incubation.

“One of the problems is that B. mallei is easily overgrown by other bacteria, especially in animal specimens collected from non-sterile sites,” Yuta Kinoshita and his colleagues noted in the journal BMC Veterinary Research.

Indeed, B. mallei is often overgrown by other bacteria even in fresh samples obtained under sterile conditions.

The study team set out to develop an agar-based culture medium for the laboratory diagnosis of glanders.

They created a new agar formulation, named BM agar, which enriched B. mallei growth, but inhibited the growth of other bacteria and fungi.

They found that BM agar could sufficiently grow almost all of the tested B. mallei strains within 72 hours, enabling scientists to confirm the presence of the bacteria.

Out of the 38 B. mallei strains assessed, 34 grew well on BM agar within 48 hours, and three strains grew within 72 hours.

The remaining strain produced colonies after 72 hours of incubation, but there were fewer colonies on BM agar than the other strains.

“Our study has shown that BM agar is a novel selective agar for effectively isolating B. mallei from horse specimens,” they concluded.

“We believe that this new agar can be used to provide sensitive and selective isolation of B. mallei for both diagnosis and future research.”

Their paper provides the nine-ingredient formulation for BM agar.

The full study team comprised Kinoshita, Ashley Cloutier, David Rozak, S.R. Khan, Hidekazu Niwa, Eri Uchida-Fujii, Yoshinari Katayama and Apichai Tuanyok.

A novel selective medium for the isolation of Burkholderia mallei from equine specimens
Yuta Kinoshita, Ashley K. Cloutier, David A. Rozak, Md. S. R. Khan, Hidekazu Niwa, Eri Uchida-Fujii, Yoshinari Katayama and Apichai Tuanyok
BMC Veterinary Research 2019 15:133

The study, published under a Creative Commons License, can be read here

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