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Control of campylobacters in extensively reared chickens: an investigation of growth inhibition and inactivation of campylobacters by plant extracts
Project Code: B15008
Kelly, A ; Mueller-Harvey, I; Mackey, B
Campylobacter spp. are responsible for the majority of reported food-borne illnesses in England and Wales each year and chicken meat is thought to be the main vehicle. Whilst biosecurity on intensive farms has resulted in a reduction in campylobacter positive poultry flocks, extensively reared poultry are prone to higher rates of flock infection. Tannins are natural products and have several potentially useful properties which have not yet been exploited because of a lack of research and knowledge. They offer potential in agriculture due to antibacterial and anti-parasitic properties. Furthermore, they have beneficial effects on human health. There is a general misconception that all tannins are toxic although recent research suggests many have therapeutic uses with minimal toxic side-effects. However, as yet there is insufficient knowledge on which types of tannins are most effective in specific situations. This project was undertaken to provide information regarding the potential effectiveness of plant extracts, notably tannins, to inhibit the growth of campylobacter strains initially by the submission of a literature review and subsequently through screening for activity.
Summary of approach
Tannins were extracted from commercially available products as supplied to the leather industry and from whole plant samples. The crude tannin extracts were fractionated further according to molecular weight. These fractions were used in an agar diffusion screening procedure to determine if they had inhibitory activity towards individual C. jejuni or C. coli strains as well as presumptive campylopbacter strains isolated from retail chicken samples. Subsequent analysis to determine the activity of fractions in the presence of mixed strains was also determined (to mimic conditions more likely to be present during rearing and/or processing of poultry). The level of antibacterial activity of fractions with inhibitory activity was determined by assessing the viability of cells suspended in tannin preparations over a period of time.
Outline of the main findings
A number of extracts, both un-fractionated and fractionated, were found to elicit growth inhibitory activity towards known and retail strains of campylobacter and S. aureus during screening. A significant difference was found to exist between the 3 fractions produced from the plant extracts, with the two lower molecular weight fractions eliciting a greater response than the highest molecular weight fraction.
There was also a significant difference between the levels of inhibition displayed by the Gram positive organism S. aureus when compared to the Gram negative campylobacters with a number of fractions tested, which may be of interest when elucidating the mechanism of action. However, there was no significant difference in the inhibition recorded between C. jejuni and C. coli strains.
Extracts showing greatest growth inhibitory activity for campylobacter strains include the B1 and B2 fractions of one willow ascension (Salix viminalis x schwerinii) and one Calliandra calothyrsus ascension (San Ramon). Furthermore, spent oak leaves, copper beech leaves, Velani oak, Pycnogenol, Solid and Mimosa Wattles were found to elicit substantial growth inhibitory activity by agar diffusion. This suggests that the different tannin types from the various sources as well as between the same genus of plant have significantly different activities. Minimal activity was recorded with Leucaena leucocephala and A2 fractions of many extracts in agar diffusion. However, agar diffusion assays were no measure of bactericidal activity for suspension assays.
A degree of variability in the reduction in viability was observed between strains during suspension assays. An approximate 100-fold reduction in viability was recorded for a C. coli strain incubated in the presence of 3 samples, namely un-fractionated Mimosa Wattle and two lower molecular weight fraction of two separate extracts (B2 fraction of S. viminalis x schwerinii and B1 fraction of C. calothyrsus Patulul). However, greater variation in the reduction was recorded by a C. jejuni strain, with between a 5- and 50-fold reduction in viability seen with un-fractionated chestnut and Solid Wattle respectively.
Many of the fractions possessing growth inhibitory activity were from agroindustrial waste products (e.g. willow residues, pressed fruit residues, spent oak leaves) which may have a bearing on the cost effectiveness of tannins in potential studies as they are relatively cheap to obtain.
Interestingly, a small pilot study indicated that stored tannin samples retained some inhibitory activity. This could be of practical relevance but would need further investigation.
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