ANTAGONISTIC ACTIVITY OF SELECTED PLANT EXTRACTS AGAINST Listeria monocytogenes ISOLATED FROM RETAILED READY-TO-EAT FOODS

Abstract

Listeriosis is caused by eating foods contaminated with Listeria monocytogenes. Antibiotic resistance in Listeria monocytogenes makes the treatment of listeriosis difficult hence, the need to explore other alternatives which natural plant products could offer. There is a dearth of information on the use of medicinal plants against Listeria monocytogenes. The aim of this study was to determine the antagonistic activity of selected plant extracts against Listeria monocytogenes isolated from ready-to-eat foods.

Four hundred and eleven retailed samples consisting of edible worms (124), salads (85), meat pies (94) and spiced snails (108) were purchased purposively along travellers’ routes covering Ore to Port Harcourt (South South). Listerial isolation was done using Listeria selective agar. The isolates were characterised using standard cultural and molecular methods. Virulence markers such as HlyA, Iap and InlA were used to confirm the identity of Listeria monocytogenes. Effect of pH (2-9), NaCl (2-10%) and temperature (-4 to 70ºC) on growth of Listeria monocytogenes was determined and susceptibility to Augmentin, Cloxacillin, Ofloxacin and Ciprofloxacin was carried out by disk diffusion. Dried pulverised leaves of Vernonia amygdalina (Va), Psidium guajuva (Pg), and Dacryodes edulis (De) were extracted in ethanol and water. Anti-listerial potentials of different concentrations (100, 200, 400, 600, 800 and 1000mg/mL) of the extracts were determined in vitro using agar-well diffusion assay. Eighty-eight mice (18-20g) were randomly distributed into eleven equal groups for in vivo studies. Mice in groups 1 to 10 were orally infected with 1mL Listeria monocytogenes (107cfu/mL), then, aqueous concentrations (0.6, 0.8 and 1.0g/kg) of Va, De and Pg were orally administered as 1mL doses to groups 1, 2 and 3; 4, 5 and 6; and 7, 8 and 9, respectively. Group 10 served as the infected control, while group 11 was the uninoculated control. Histopathological changes in liver and assay of alanine aminotransferase (ALT) to determine the extent of liver damage was carried out using standard methods. Data were analysed using descriptive statistics and ANOVA at α0.01

Two hundred and forty five Listeria species were isolated from edible worms (115), salads (50), meat pies (35) and spiced snails (45) of which detection of the virulence markers confirmed ninety species as Listeria monocytogenes. Optimum growth of Listeria monocytogenes was at pH 8, 2% NaCl and 37°C. Resistance by Listeria monocytogenes to Augmentin, Cloxacillin, Ofloxacin and Ciprofloxacin were 100.0%, 100.0%, 42.2% and 63.6%, respectively. Aqueous and ethanolic extracts of Va had higher in vitro anti-listerial activities against LmEW94 at 100mg/mL (35.0±0.0; 35.0±0.0mm) compared to De (25.0±14.1; 25.0±1.4mm) and Pg (27.0±1.2; 27.0±1.3mm). Mortality (37.5%), apoptotic cells, cytoplasmic vacuolation and significant ALT (84.36±12.01U/L) increase were recorded only in infected but untreated mice (group 10). Liver histology of infected mice treated with 0.8g/kg Va (group 4) revealed mild kupfer cell with normal liver histological characteristics. Reduction in ALT concentration observed in infected mice treated with 0.6g/kg Pg (32.77±8.95U/L) signified improvement in liver health as the extracts had anti-listerial activities.

Extracts of Vernonia amygdalina, Psidium guajuva and Dacryodes edulis had in vitro and in vivo antagonistic activities against virulent strains of Listeria monocytogenes.