A Comparison Between Medical Grade Honey and Table Honeys in Relation to Antimicrobial Efficacy

Rose A. Cooper, PhD; Leighton Jenkins, BSc

Disclosures

Wounds. 2009;21(2) 

In This Article

Results

Using the bioassay, antibacterial activity was detected in 9 of the honey samples, with total activity ranging from 4.2% to 18.8% (w/v) phenol equivalent, and non-peroxide activity varying from 4.8% to 18.6% (w/v) phenol equivalent (Table 3).

Five of the active honey samples (lavender, eucalyptus, tropical honey, and 2 heather samples) had no detectable non-peroxide activity, suggesting that they were hydrogen peroxide generating honeys. The remaining 4 active samples possessed non-peroxide activity, of which 1 sample was marketed as buckwheat and 3 samples were manuka (Leptospermum) honey. Nonperoxide activity has not been reported in buckwheat honey before and it is not known whether this particular sample is representative of all buckwheat honeys. It is possible that it was not a unifloral honey, but rather derived from nectar collected from several floral sources.

The labeling on the packaging of the manuka (Leptospermum) honeys (7, 15, and 19) claimed non-peroxide activity equivalent to 10%, 10%, and 18% (w/v) phenol equivalent, respectively. However, only sample 19 (the MGH) conformed to the label; the others possessed lower activity than stated (6.1 and 5.5 for samples 7 and 15, respectively).

The capacity of antimicrobial agents to inhibit pathogenic organisms has long been evaluated in vitro using serial dilution in broth to determine the lowest concentration able to prevent growth (MIC) and the lowest concentration to prevent the survival of viable bacteria (MBC). Here we tested two laboratory reference cultures (S aureus NCTC 6571 and E coli NCTC 10418) and 4 clinical isolates from chronic wounds (MRSA, S pyogenes, S epidermidis, and P aeruginosa). Artificial honey was included because its inhibitory action was confined to its sugar content and allowed a second way to identify inactive honeys. Samples 1, 3, 4, and 16 gave MIC values similar to artificial honey (Table 4), hence, 15 samples demonstrated varying levels of activity; sample 19 possessed the most effective inhibition.

Demonstrating the survival of live bacterial cells in tubes after the MIC test, one can deduce whether an antimicrobial agent is bacteriostatic (prevents growth) or bactericidal (prevents survival). If the ratio of MBC to MIC is ≤ 4, then a bactericidal mode of action is indicated. In most cases, bactericidal action was found. Exceptions occurred with the lower-potency honeys (1, 3, 4, 6, 8, 9, 15, 16, 17, and artificial honey). The tests were limited because the maximum achievable honey concentration in the test was 1 g/mL. Again, MGH demonstrated greater activity and was bactericidal in 5 of the 6 test organisms. The MIC and MBC test results confirm that Gram-positive bacteria are more sensitive to honeys than Gram-negative bacteria and that the most susceptible test species was S pyogenes.

When the MIC and MBC were determined in the presence of catalase, only non-peroxide honeys (7, 15, 18, and 19) retained antibacterial activity (Table 6 and Table 7). The MGH possessed the greatest extent of bactericidal action against the test organisms, while the peroxide honeys demonstrated potency similar to the artificial honey (Table 5).

A wide variety of mesophilic aerobic bacteria were recovered from the 18 culinary honeys (Table 8). Many of these microorganisms are commonly associated with soil and are not normally considered pathogens. Some, however, may colonize chronic wounds such as Clostridium ramosum (honeys 6 and 14) and Staphylococcus warneri (honey 17). Bacillus species were most frequently recovered and were found in 14 honey samples.

Coliforms and salmonellae were not detected in the enrichment experiments with MacConkey broth and selenite broth. Although the numbers of anaerobes were shown to be low by plating onto blood agar, enrichment in Robertson's cooked meat medium demonstrated their presence in 18 samples. Fusobacterium species was found in 9 samples and 3 samples contained clostridia. It was not possible to identify all of the anaerobic isolates with the BBL kit because it is designed for use with clinical, rather than environmental, bacteria. No organisms were detected in sample 19 (MGH). This was expected since the sample was irradiated and microorganisms should not have survived the sterilization process.

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