Honey has been used as a topical antibiotic since the Egyptians wrote papyrus prescriptions. Now, a Cornell food scientist has identified an antimicrobial compound in a honey that makes it a promising candidate as a natural preservative to prevent food-borne illness and food spoilage.
Randy Worobo, associate professor of food microbiology at the New York State Agricultural Experiment Station in Geneva, and his lab members tested more than 2,000 strains of bacteria from eight types of honey from the United States and New Zealand. One of them stood out.
"In sunflower honey from South Dakota, we identified a strain of Bacillus thuringiensis the biological control known to organic gardeners as 'Bt' — which was effective against common food-borne pathogens including Listeria monocytogenes, the bacteria behind the recent deadly cantaloupe outbreak," said Worobo. "This Bt strain was intriguing, because it had both strong antibacterial and strong antifungal activity."
In analyzing the compounds produced by the bacteria, they found one with strong antibacterial activity that they designated as thurincin H. They recognized it as a bacteriocin, a common class of antimicrobials that bacteria produce to compete against other microbes. But compared with the some 40 known bacteriocins, it is unique: It is coded in the bacterial DNA as a unit containing three identical copies of the same bacteriocin gene.
Their findings were reported in September in Angewandte Chemie International Edition.
"This is the first report of a bacteriocin gene with this type of a triplet tandem repeat, and they are controlled by the same genetic on-switch," said Worobo. "This might partially explain the bacteria's success against other microbes. It may allow the bacteria to rapidly produce large amounts of this compound."
It wasn't until Worobo collaborated with University of Alberta chemistry professor John Vederas that more unusual aspects of thurincin H were discovered. Using an array of techniques that gives a 3-D picture of the molecule, Vederas noticed several unexpected properties.
"After the bacteria make the protein, it undergoes several enzymatic changes that determine the shape and rigidity of the molecule," said Vederas. "Thurincin H forms four links between sulfur molecules and particular carbon molecules, which creates hairpins that are twisted into a helical structure. It's actually the first time anyone has described a peptide with four of these linkages."
The result is a molecule that looks like a rotary hairbrush, with water-repelling residues on the outside. This combination of shape and function may allow thurincin H to infiltrate membranes of other bacteria.
Wolf in sheep's clothing
Like a wolf in sheep's clothing, the compound mimics the structure of the molecules that form bacterial membranes, which also have water-repulsing residues on the outside, but it may disrupt those membranes by forming a rigid pore.
"The protein seems perfectly engineered to kill competing bacteria," said Worobo.
This strain is effective against several strains of Bacillus, Listeria and Carnobacterium that cause food-borne disease or spoilage in food kept at improper temperatures and raw milk.
Now, Worobo and Vederas are exploring these unusual linkages and are working to characterize the mode of action of this unique peptide, with the goal of developing thurincin H as an alternative to synthetic food preservatives.
"Bacteriocins are promising natural food preservatives for the food, livestock and agricultural industries," said Worobo. "Because they come from food-grade microorganisms, they are generally regarded as safe."