Antibacterial effect of black seed oil on Listeria monocytogenes

Abstract

Listeria monocytogenes is a major foodborne pathogen in the United States. Effective methods for reducing L. monocytogenes in foods would reduce the likelihood of foodborne outbreaks of listeriosis, and decrease economic losses to the food industry. Nigella sativa is a herbaceous plant, whose seeds (black seed) have been used as a spice and condiment in foods in the Middle East. The objective of this study was to determine the antibacterial effect of black seed oil on twenty strains of L. monocytogenes by disc diffusion method. A population of 7.0 log CFU of each strain of L. monocytogenes was inoculated on duplicate plates containing antibiotic medium one agar. The plates were allowed to dry at room temperature for 15 min. Three discs (6 mm diameter), each impregnated with 10 μl of black seed oil, vegetable oil (oil control), or gentamicin (positive control) were placed on each inoculated plate. The plates were incubated at 37 °C for 24 h, and were observed for zones of L. monocytogenes growth inhibition. Black seed oil exhibited a strong antibacterial activity against all the strains of L. monocytogenes, yielding a significantly (P<0.01) larger inhibition zone than that of gentamicin. The mean zones of inhibition produced by black seed oil and gentamicin were 31.50 ± 1.0 and 14.80 ± 0.50, respectively. The vegetable oil had no inhibitory effect on L. monocytogenes. Results indicate that black seed oil could potentially be used to inhibit L. monocytogenes, but appropriate applications in foods need to be validated.

Introduction

Listeria monocytogenes has emerged into a highly problematic and fatal foodborne pathogen throughout the world, including the United States. In 1999, Centers for Disease Control and Prevention, Atlanta, Georgia, USA reported an estimated number of 2490 cases of listeriosis in the United States with a mortality rate of ∼25% (Mead et al., 1999). A wide spectrum of foods, including milk, cheese, beef, pork, chicken, seafoods, fruits, and vegetables, have been identified as vehicles of L. monocytogenes in causing listeriosis (Brackett, 1998). In 2002, a large outbreak of listeriosis involving 46 people, 7 deaths, and 3 miscarriages, resulting in a recall of 27.4 million pounds of fresh and frozen ready-to-eat turkey and chicken products was reported in the United States (Anonymous, 2002).

Listeria monocytogenes possesses several characteristics, which enable the pathogen to successfully contaminate, survive and grow in foods, thereby resulting in outbreaks. These traits include an ability to grow at refrigeration temperature, ability to survive in acidic conditions, e.g., pH 4.2, ability to tolerate up to 10% sodium chloride, and the ability to survive in biofilms on equipment in food processing plants and resist superficial cleaning or disinfection treatments (Nickelson, 1999). Effective methods for killing L. monocytogenes in foods would reduce the likelihood of foodborne outbreaks of listeriosis, and decrease economic losses to the food industry. A variety of different chemical and synthetic compounds have been used as antimicrobials to inhibit L. monocytogenes in foods. However, in the past few years, due to concerns regarding the safety of synthetic antimicrobial agents, there has been an increase in consumer demand for naturally processed foods, which has resulted in a huge increase in the use of naturally derived compounds such as plant extracts as antimicrobials in foods (Hao, Brackett, & Doyle, 1998).

Black seed (black cumin, Nigella sativa) is a herbaceous plant growing in Asian and Mediterranean countries. Black seed oil or extract has been used for thousands of years as a spice, condiment, carminative, food preservative, as well as a protective and curative treatment for numerous disorders in traditional and Indian folk medicine (Aboutabl, El-Azzouny, & Hammerschmidt, 1986; El-Sayed, El-Banna, & Fathy, 1996; Merfort et al., 1997; Nadkarni, 1976). The black seeds contain 36–38% fixed oil, with proteins, alkaloids, saponins, and essential oils making up rest of the composition (Burtis & Bucar, 2000). Although black seed extract or oil has been reported to possess antimicrobial activity (Morsi, 2000), antioxidant activity (Burtis & Bucar, 2000), antitumor activity (Worthen, Ghosheh, & Crooks, 1998), and a stimulatory effect on the immune system (Salem & Hossain, 2000), its full potential as an antimicrobial has not been exploited. A literature search did not reveal any published report on the antibacterial effect of black seed oil on L. monocytogenes. Therefore, the objective of the present study was to determine the antimicrobial effect of black seed oil on L. monocytogenes, for potential application as an antimicrobial in foods.