Antimicrobial activity exhibited by bacteria isolated from honey.

The antimicrobial properties of honey have been known for years. Honey's antimicrobial activity has been attributed to hydrogen peroxide, osmolarity, acidity, aromatic acids and phenolic compounds. Honey is characteristically quite acidic. Its pH is between 3.2 and 4.5, which is low enough to be inhibitory to many pathogens. The optimum pH for growth of these species normally falls between 7.2 and 7.4. The minimum pH values for growth of some common species is: E. coli, 4.3; Salmonella spp., 4.0; P. aeruginosa, 4.4; and S. pyogenes, 4.5. So, in undiluted honey, the acidity is a significant antibacterial factor.

But an additional unidentified antimicrobial compound exists that is only found in select honey samples. Right now, investigators are not sure of the exact compound, but they have postulated that it may be due to the different microbial flora contained in the different honeys.

The antimicrobial compounds contained in honey may be able to inhibit foodborne pathogens and spoilage microorganisms. The objective of researchers at Cornell University was to investigate the nature of the various antimicrobial compounds in honey and to examine the honey microflora for the production of antimicrobial substances.

Five U.S. honey samples from different flora sources and geographical locations were supplied by the National Honey Board: Tarweed, Blackberry, Cottonseed, Buckwheat and Rabbitbush. Investigators tested the samples for the presence of antimicrobial activity. The bacterial microflora of each honey was isolated by plating onto non-selective media (Tryptic Soy Agar) and incubating it at 30 C for 48 hours.

These isolates were screened for production of antimicrobial compounds against B. subtilis ATCC 6633, B. cereus F4552, L. monocytogenes F2-586 1053 and B. fulva H25 using the deferred inhibition assay.

The scientists observed a high incidence of antimicrobial inhibition with the bacterial isolates from all five varieties of honey. A range of 44% to 89% of the bacterial isolates from all of the honeys showed strong antifungal activity. Approximately 30% of bacterial isolates from the honey exhibited antibacterial activity against B. cereus and B. subtilis. L. monocytogenes F2-586 1053 showed higher overall rates of sensitivity, ranging from 11% to 66% of the bacterial isolates from the honey samples.

Additional research is ongoing to purify compounds as well as to examine the range of antimicrobial compounds being produced by the bacteria contained in the different honeys.

Further information. Randy Worobo, Food Microbiology, Cornell University, Department of Food Science and Technology, New York State Agricultural Experiment Station (NYSAES), 630 W. North St., Geneva, NY 14456; phone: [phone]; fax: [phone]; email: [e-mail].