Plant Antimicrobials and the Food Industry: Plant Peptides

Plant antimicrobial peptides, or PAMPs, are protein compounds produced by plants to defend against microbial attack¹. They are positively charged, amphiphilic molecules found throughout the plant, existing in leaves, stem, stalk and other tissues. Plants release PAMPs in response to microbial infection and infestation, or produce them constitutively as part of the overall pathogen defense mechanism.

PAMP activity against microbes is mostly through cell membrane disruption, though some also work by inhibiting compounds such as proteinases and other enzymes that pathogens need for survival. PAMPs can bind to cell surface receptors and, as they aggregate, form pores or other defects that stop the cell membrane from functioning correctly. Microbial death arises from an influx of excess ions or leakage of intracellular contents, among other mechanisms.

Researchers have identified several types of PAMP, grouping them into eight families according to structure and amino acid sequence similarities. Most contain between 10 to 50 amino acid residues, and are cysteine-rich with alpha helical formation, ranging between 2 and 9 kDa in size.

Thionins

Thionins, discovered in 1942, were the first PAMPs to be identified. They show activity against yeasts, bacteria and fungi, apparently by binding to the cell membrane in sufficient numbers to cause damage, thereby allowing intracellular contents to leak out. Thionins from wheat inhibit two species of Listeria, L. monocytogenes and L. ivanovii, in vitro, with activity varying according to temperature.

One of the best-known plant thionins is Viscotoxin, a type V thionin found in European mistletoe (Viscum album). Its action is well studied since it is effective against tumor cells, causing membrane damage that leads to destabilization and lipid bi-layer disruption. Thionins are found in wheat, millet and other grains, in addition to mistletoe.

Plant Defensins

Plant defensins show activity against fungi and bacteria (Gram-positive and -negative), with no lethal effect in plant or animal cells. This makes them particularly interesting as biopreservative agents in foods. So far, most defensins have been isolated from seeds but they are also found in other plant structures such as pods, tubers and bark. In medicine, they also show anti-cancer activity as well as acting against viruses, while other types of defensin show insecticidal activity. IbAMP1, a defensin isolated from Impatiens balsamina is bactericidal against Shiga toxin-producing Escherichia coli in vitro.

Lipid Transfer Proteins

Lipid transfer proteins (LTPs) act through the microbial cell membrane, occasionally synergistically with defensins, to stop pathogen growth. They bind a number of lipid molecules and show antimicrobial actions against fungi predominantly, though some are effective against bacteria. Food producers should be cautious however, since certain LTPs act as plant allergens in latex, nuts, fruits and vegetables.

Myrosinase-binding Proteins

Myrosinase-binding proteins act through ionophore creation in microbial membranes. They have characterized in Brassica and Arabidopsis species as involved in plant defenses against pathogens and insects.

Hevein- and Knottin-like Peptides

Hevein- and knottin-like peptides have been isolated mainly from seeds, where they show antifungal and antibacterial (Gram-positive) activity in vitro. They prevent fungal growth by binding to chitin on hyphae, which leads to the structure disintegrating. WjAMP1 from Wasabia japonica shows activity against E. coli among other bacteria. As with LTPs, food producers should be aware that hevein is a known allergen, so care is needed when using this PAMP as a biopreservative.

Snakins

Snakins are antimicrobials isolated from potatoes, showing activity against fungi and bacteria (Gram-positive and -negative). Researchers have not yet elucidated the exact mechanism for the antimicrobial effect but snakins do show activity against Listeria species in vitro.

Cyclotides

Cyclotides possess a cyclical structure, Mobius or bracelet, and show activity against bacteria including Staphylococcus aureus and E. coli, insects, viruses and enzymes. The mechanism of action is as yet unknown but may be through membrane disruption. Kalata B1, which was the first cyclotide identified, is active against nematodes and insects, as well as showing inhibition of human immunodeficiency virus (HIV). Other cyclotides show activity against animal cells, causing hemolysis in red blood cells and uterine stimulation as an ecbolic.

Peptides from Hydrolysates

Hydrolysates formed by enzymatic or acidic treatment of plant material contain various PAMPs with useful antimicrobial activity. This is especially so for hydrolysates of certain beans, with antimicrobial activity against S. aureus and Shigella flexneri shown by extracts from Phaesolus varieties.

Although researchers have not fully characterized many of these antimicrobial PAMPs, this class of potential food additive clearly shows promise for use in food safety protocols in the future. Moreover, compared to common fungal- and bacterially-derived antibiotics, it does seem to be more difficult for microbes to develop resistance to PAMPs. Researchers think this is probably due to the general mode of action shown by most PAMPs in that they primarily target the cell membrane.

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Reference

1. Hintz, T. et al. (2015) “The Use of Plant Antimicrobial Compounds for Food Preservation“, BioMed Research International 2015, Article ID 246264 http://dx.doi.org/10.1155/2015/246264

Further Reading

Nawrot, R. et al. (2014) “Plant antimicrobial peptides“, Folia Microbiologica 59 (pp.181–196) DOI 10.1007/s12223-013-0280-4

Salas, C.E. et al. (2015) “Biologically Active and Antimicrobial Peptides from Plants“, BioMed Research International 2015, Article ID 102129 http://dx.doi.org/10.1155/2015/102129