Antimicrobial compounds of plants collectively referred as "green chemicals". The majority of antimicrobial compounds are identified as secondary metabolites. Prohibitins are essential oil components with antimicrobial activity. 1389 plants have a potential green chemical sources and 250 new antifungal metabolites. This method of preservation is termed as "Allelopathy". The major concern is the control of microorganisms to increase the shelf life and prevent microorganisms.

Phytoalexins - Phytoalexins are defined as host synthesized, low molecular weight, broad spectrum anti microbial compound whose synthesis from distant precursors is induced in plants in response to microbial infection eg : Pisatin, Phaseolin, Trifolirhizin, Orchinol and Isocumarin.
Organic acids - Citric, malic, succenic, tartaric are commonly found in fruits and vegetables.
Jasmonals - Jasmonic acid and its volatile esterified derivatives, methyl jasmonals are naturally occuring lipid compounds of plant cell membrane.

Phenolic compounds - Plant phenolics encompass a wide range of substances that have an aromatic ring with at least one hydroxyl group like eugenol, quercetin, ametoflavin, ferulic acids,catechol seen in cloves, coffee, olives etc.

Essential oils and there compounds - These are volatile and odoriferous substances contained in certain aromatic compounds

Glucosinolates - These are sulphur containing plant secondary metabolites occurring mainly in cruciferous crops and restricted to other families like caricacea and capparaceae
The antibacterial and antifungal activity of essential oils is well proved. But the evidence for there mode of action is very meager. Only few have attempted to know the mode of action of essential oils. By compiling the attempts we can say in a nut shell that the essential oils act by
1. Causing lysis of cell
2. Reducing protein synthesis and leucine incorporation
3. Reducing growth and development of mycelial and conidia

Antimicrobials from spice extracts

CLOVE : Eugenol the major component of clove oil. Active against organisms like E.coli, Enterobacter sakavaki and Klebsiella pneumonia. Eugenol inhibit the production of alpha-amylase, protease and subtilis. It prevented germination of spores and had greater inhibitory effect on vegetative growth. Germicidal effect against various bacteria and Candida albicans. Growth and aflatoxin B production by Aspergillus flavus is inhibited no phytotoxicity on the host.

GARLIC: Inhibition was presumed to be due to Ajoene, a Sulfur- containing compound present in garlic bulbs though active inhibitory principle is allicin (diallyl disulphide and diallyl trisulphide). Growth of gram positive bacteria such as B.cereus, B.subtilis. Mycobacterium smegmatis and Streptomyces griseus were inhibited at 4-5 micro grams of ajoene per ml.

CINNAMON: Cinnamaldehyde an aromatic aldehyde is known to be the effective inhibitory compound present in extract. Cinnamon oil had the antimicrobial activity on germination of spores and vegetative growth of Clostridium botulinum. Inhibit the Aflatoxin B production.

GINGER: The essential oil extracted from ginger showing varied degrees of inhibition against Lactobacillus acidophillus, B.cereus, Saccharomyces cervisiae, Aspergillus niger and Mycoderma spp.

CHILLI: Capsanthin and capsaicin, the coloring and pungent principles of red chilli influenced the growth and aflatoxin producing potentials of Aspergillus flavus.

BLACK PEPPER: Alcoholic extract of black pepper was quite active in inhibiting the Clostridium botulinium in culture media.

Antimicrobials from vegetables

CARROT: The antimicrobial effect of purified ethanolic extracts of peeled and shredded carrots against a range of food-borne microorganisms. 6-methoxymellein as a common phytoalexin.

CABBAGE: Saurkraut, the fermentation product of cabbage contains sulfur compounds. S-methyl-1-cysteine sulfoxie, sinigrin, dimethyl disulphide dimethyl trisulphide, methyl methanethiosulfinate and allylisothiocyanate. Cabbage extract showed antimicrobial activity against gram negative bacteria. Against fungi, only Aspergillus sp. was inhibited and to others it was less effective.
Antimicrobials from fruits

MANDARIN: The antimicrobial properties of essential oils, terpineol and orange oil in particular varied with various bacteria. Gram positive bacteria were more sensitive to essential oils than gram negative bacteria

OLIVE: Olive fruits naturally contain a variety of simple and complex phenolic compounds, of which caffeic acid was the most effective compound. The commercial pure oleuropein and phenolics extracted from olives inhibited the growth and enterotoxin production production by Staphylococcus aureus, S-6 in broth and model food system.

Antimicrobials from essential oils

THYME - The active compounds in this oil are thymol and P-cymene. Complete inhibition of the growth of Aspergillus species.

TEA TREE OIL - Tea tree oil or the essential oil of Melaleuca alternifolia is a naturally occurring microbial agent. Terpene-4-ol was active against wide range of organisms.

MINT ESSENTIAL OIL - The effect of mint (Mentha piperita) essential oil on Salmonella enteritidis and Listeria monocytogenes in a culture medium. The increased antibacterial activity of essential oil at low pH.

Chenopodium ambrosoides: Essential oil isolated from leaves of Chenopodium ambrosoides is potential source as natural fumigant.

Antimicrobials from medicinal plants: The crude leaf extract of plant Juniper communis showed antibacterial activity where all gram positive and gram negative bacteria tested namely Staphylococcus aureus, Micrococcus, Bacillus Streptococcus, E.coli, Vibrio and Klebsiella were found to be sensitive to a concentration of 250 micro grams/ml of leaf extract.

Antimicrobials from economic plants: Extracts of black tea, green tea or coffee inhibited growth of various bacteria known to cause diarrhoeal disease.

Among the different plant extracts tested for the antimicrobial property quite effective extracts were from cloves, cinnamon, and garlic. Moderately active were from black pepper, turmeric, ginger, mint leaves, coriander and fenugreek extracts and less effective were from tomato, chilly, capsicum, onion and mandarin sources.

As there is lot of evidence for the antifungal, antibacterial and antinematicidal properties of essential oils, they can be used for the control of both animal and plant diseases. These can become the safest control as they are non toxic to humans and animals, biodegradable and non-pollutants. They provide less phytotoxic and more systematic fungicides.

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