The leaves of the carob plant contain antibacterial substances that can help in the fight against Listeria, according to new research.
Nadhern Aissani et al tested the methanolic extract of carob leaves’ (MECL) ability to inhibit the growth of a range of microorganisms.
Writing in the American Chemistry Society’s Journal of Agricultural and Food Chemistry they found tests in which extracts of carob leaves were used proved effective in inhibiting the growth of Listeria bacteria growing in laboratory cultures.
Natural compound search
The study was conducted as part of an ongoing search for new natural compounds for food preservation aimed at a partial or total replacement of current antimicrobial chemicals.
The researchers collected leaves of Ceratonia siliqua from Cagliari, Italy in March 2012, and dried them in the absence of light at room temperature. They were then sealed in paper bags, stored at room temperature, and kept in the dark until use.
They found MECL inhibited the growth of Listeria monocytogenes at 28.12 μg/mL by the broth microdilution method.
Listeria growth inhibition was recorded for myricitrin and gallic acid at 450 μg/mL and for (−)-epigallocatechin-3-gallate and isoquercitin, respectively, at 225 and 112.5 μg/mL.
“The effect of this bacteriostatic concentration on the growth of this bacterium revealed a pattern of inhibition characterized by (a) a resumed growth phase, which showed a lower rate of growth if compared with controls; and (b) first a lag and then a stationary phase at a lower bacterium concentration,” said the researchers.
“For the assessment of listerial growth with and without inhibitor (MECL), a suspension of 103 CFU/mL of the bacterium was made in which we added the desired concentration of MECL, and 100 μL was plated in agar…
“This action was repeated in different plates after dilutions every three hours with reference to a control without extract. The bacterium was incubated overnight at 37°C, and then, the number of survived bacteria was counted.”
The experiments were carried out using Escherichia coli, Salmonella typhimurium and Pseudomonas aeruginosa strains for Gram(−), L. monocytogenes and S. aureus strains for Gram(+).
Potential Carob use
Carob, a tree grown in the Mediterranean region, is also known as a substitute for chocolate that does not contain caffeine or theobromine.
The pods, bark, leaves, fruits and gum are used in a variety of industries from textile to cosmetics.
The researchers said the next step is to study the antilisterial activity of carob leaves’ methanol extract in vivo using real substrates such us meat and fish samples.
The latest Centers for Disease Control and Prevention (CDC) data relating to the current Listeria outbreak from Ricotta Salata cheese, reports 20 cases in 13 states which have led to four deaths.
Source: Journal of Agricultural and Food Chemistry
Published online ahead of print, DOI: 10.1021/jf3029623
“Inhibitory Effect of Carob (Ceratonia siliqua) Leaves Methanolic Extract on Listeria monocytogenes”
Authors: Nadhem Aissani, Valentina Coroneo, Sami Fattouch and Pierluigi Caboni