Microbial Update International; 2/1/2004

L. monocytogenes is a facultative anaerobic Gram-positive rod-shaped bacterium that causes the foodborne illness listeriosis. Annually, there is an average of 2,500 reported cases of listeriosis in the United States, resulting in 500 deaths.

It is challenging to devise food preservation and treatment measures for controlling and preventing listeriosis. Recent studies indicate that herbs, spices and plant-based essential oils (the aromatic kind) can inhibit the growth of Gram-positive and Gram-negative foodborne bacteria, yeasts and molds.

In research at Alabama A&M University, the pathogenicity of L. monocytogenes ATCC 43256 in the presence of allspice (Pimenta dioica) essential oil (AEO) was studied. The objective was to determine the impact of the minimum inhibitory concentration of AEO on the cell death rate of L. monocytogenes ATCC 43256.

Researchers subcultured aliquots (1 mL) of L. monocytogenes ATCC 43256 in a Listeria enrichment broth (106 cfu per mL) for 24 hours in a 24-well microtiter plate. A 100% pure sample of AEO was diluted to 10 separate concentrations, ranging from 0% to 10% in increments of 1 mL; 0% = the controls.

A 100 [micro]L sample of each concentration was inoculated, in duplicate, into each well containing L. monocytogenes. These were incubated for 6 hours. Following the incubation period, scientists removed a 100 [micro]L sample from each well and plated the samples on Oxford media after 2 hours, 4 hours and 6 hours. The plates were incubated for 48 hours. Investigators determined total viable counts.

The results showed that L. monocytogenes in the control samples grew exponentially: 2 hours = 0 cfu per mL; 4 hours = 6.1 x [10.sup.2] cfu per mL ; and 6 hours = 1.2 x [10.sup.3] cfu per mL. The AEO-treated samples at all concentrations showed no growth. AEO completely inhibited the growth of L. monocytogenes ATCC 43256 at 2 hours, 4 hours and 6 hours of incubation.

These results are significant with regard to the safety of food products. They show that plant-based essential oils could be useful in inhibiting the survival and growth of L. monocytogenes.