# Plant Extracts for the Control of _Listeria monocytogenes_ in Meat Products
## 4 Discussion  
In the present investigation, the antibacterial capability of plant extracts containing natural compounds derived from garlic and onion was tested against the psychotrophic foodborne pathogen _L. monocytogenes_, which was artificially inoculated in sausages and in cooked ham slices. The antibacterial activities of these natural compounds are attributed to the high content of organo-sulfur compounds, including diallyl sulfide (DAS), diallyl disulfide (DADS), diallyl trisulfide (DATS) and their precursors that destroy the structure and the metabolic process of bacterial cells [40; 41; 42]. Most studies carried out on the antimicrobial activity of natural products against foodborne pathogens have only been performed in cultural substrates, but the structure and composition of foods, as well as conservation conditions, can influence the effectiveness of the antagonistic compounds [15; 43; 44]. [PERSON] et al. [45] reported that higher levels of oregano and nutmeg extracts were needed to contain the growth of _L. monocytogenes_ in roast chicken than in culture medium trials, and according to [PERSON] et al. [46], the antibacterial activity of both clove and thyme extracts added to a culture broth was significantly lower than that obtained in a food matrix, such as hot dogs. Consequently, the real effectiveness of natural antibacterial compounds should be directly evaluated in food substrates. In the tests carried out during sausages manufacturing, we observed a reduction of _Listeria_ viable counts, but the decrease does not differ between treated and untreated samples. Furthermore, during sausage seasoning, a reduction in the pH and aw values was detected, as also observed by [PERSON] for _L. innocua_[47]. These results were in accordance with [PERSON] et al. [48], who reported that heat-dried Chinese-style sausages containing garlic had no significant differences between the samples. The rate of inactivation that naturally occurs during seasoning and is related to several factors (presence of microbial starters, acidity of the substrate, decreasing activity of water, etc. ), and could explain the lack of an evident antibacterial action due to the addition of the plant extracts. Indeed, _L. monocytogenes_ exhibits compensatory mechanisms to adapt to stressful conditions such as low pH, low aw or low temperature, which may explain the increased survival along the seasoning process [49]. The tolerance of this pathogen to the dry-curing process is probably related to its tolerance to salts and low temperatures in the presence of organic material and underlines the importance of controlling _L. monocytogenes_ during the processing of cured products [50]. Meat and meat products are a complex environment of organic materials, and some constituents may even provide microbial cells protection from antimicrobial substances. [PERSON] et al. [51] suggested that fat in foods could protect them from antimicrobial agents by forming a protective layer around bacteria. On the other hand, the test performed on sliced vacuum-packaged cooked ham showed, immediately after the addition of the plant extracts, the decrease of the _L. monocytogenes_ contamination of about 1.00 log cfu/g. _L. monocytogenes_ was sensitive to spice extracts, which was probably due, at least in part, to the fact that those cooked ham slices were leaner than the sausages. These results were in agreement with [PERSON] et al. 2020 [40], who reported the antibacterial activity of garlic and onion essential oils towards _L. monocytogenes_. Other authors reported the antimicrobial activities of garlic and onion against pathogenic bacteria including _Salmonella_ Typhimuriumurium, _Escherichia coli_ O157:H7, _Listeria monocytogenes_, _Staphylococcus aureus_ and _Campylobacter jejuni_[52,53]. The plant extracts do not appear to interfere with the physico-chemical features of the cooked ham, and the pH and aw values recorded did not change during the shelf-life of this product. Considering that, in cooked ham, the presence of _L.