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Close-up of honeycomb cell showing stringy mass of material that is the remains of a bee larva infected and killed by foulbrood. Link to photo information
Honey bee larvae infected with American foulbrood become a stringy mass of material that later dries and carries the spores that may infect other larvae. Click the image for more information about it.

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Gene Helps Explain Foulbrood's Spread Among U.S. Bees

By Alfredo Flores
July 17, 2007

A gene for resistance to tetracycline drugs has been discovered in the microbe that causes the bacterial disease American foulbrood (AFB) in honey bees, according to scientists with the Agricultural Research Service (ARS).

AFB, caused by the spore-forming Paenibacillus larvae bacterium, is so serious that infected colonies must be burned—an extremely costly option for beekeepers.

From the 1950s until very recently, the only AFB treatment approved for use in the United States has been the antibiotic oxytetracycline (OTC), sold under the name Terramycin. But recently there have been reports of P. larvae suddenly developing resistance to Terramycin.

At the ARS Honey Bee Research Unit, Weslaco, Texas, molecular biologists Dan Murray and Katherine Aronstein have discovered a natural plasmid in P. larvae—called pMA67—that contains an OTC-resistance gene. Plasmids are small DNA molecules containing up to several dozen genes that bacteria pass on when they reproduce. This is the first report of any tetracycline-resistance gene being found in any Paenibacillus bacteria.

In tests on 36 P. larvae strains gathered from across the United States and Canada, all 21 OTC-resistant strains were found to possess the pMA67 plasmid, and all 15 OTC-sensitive strains did not.

Two explanations for the rapid spread of OTC resistance are thought to be likely. First, bees from broad geographical areas are brought together when beekeepers rent their hives to agricultural producers for pollination. This provides the opportunity for OTC-resistant bacteria to spread among bees that otherwise wouldn’t encounter them.

Second, it appears, based on its DNA sequence, that plasmid pMA67 has the ability to move into other bacterial cells through a process called plasmid mobilization. In this process, copies of pMA67 are physically transferred to other bacterial cells that they come in contact with.

Read more about the research in the July 2007 issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture’s chief scientific research agency.