Inhibition of Major Listeria monocytogenes Serotypes Grown in Media Supplemented with Aqueous Extracts of Gongronema latifolium and Vernonia amygdalina

Aims: To determine the growth inhibitory effect of extracts of Gongronema latifolium and Vernonia amygdalina on major Listeria monocytogenes serotypes.

Study Design: Preliminary analytic observational studies.

Place and Duration of Study: University of Nottingham, United Kingdom; Study was carried out between September 2012 and September 2013.

Methodology: The major serotypes of Listeria monocytogenes serotypes 1/2a, 1/2b, 4b and 1/2c found in evolutionary lineage I and II were grown in media containing aqueous extract of Gongronema latifolium and Vernonia amygdalina obtained from a 250 g/l leaves decoction. The percentage growth inhibition relative to the control was determined by taking viable counts of L. monocytogenes after growth for 24 hours at 30°C in Brain Heart Infusion broth supplemented with 10, 20, and 30% w/v of the extracts.

Results: It was found that there was no significant growth reduction for media supplemented with 10 and 20% (v/v) of the extracts. However significant (p < 0.5) inhibition was observed for cultures of L. monocytogenes serotype 4b grown in media supplemented with 30% (v/v) of extracts of V. amygdalina. The highest growth reduction was 29% and overall, more inhibition was found with extracts of V. amygdalina than extracts of G. latifolium.

Conclusion: Aqueous extracts of G. latifolium and V. amygdalina appeared to inhibit major serotypes of L. monocytogenes but they were not bactericidal. Serotypes 4b maybe more sensitive to the extracts than other serotypes. If further purification of the aqueous extracts studied is carried out, microbial retardants could be developed which may help to delay the growth of L. monocytogenes where ever they are prevalent.