Innate, or nonspecific, immunity refers to the basic resistance to disease that all plants and animals possess. The innate immune system is the first line of defense against microbial invasions. One component of the innate immune system is the host-defense antimicrobial peptides. These peptides are active against a broad range of pathogenic microorganisms including Gram-positive and Gram-negative bacteria, fungi, yeast and viruses. The antimicrobial peptides are a diverse group of peptides but are usually short (10-60 amino acids in length), amphipathic and can be either cationic or anionic. These antimicrobial peptides can be further classified according to their secondary structure: alpha helices, beta sheet, extended helices and loops. The toxicity of antimicrobial peptides to bacteria has not yet been fully elucidated but it is believed that these peptides act by disrupting the bacterial cell membrane.

The fact that antimicrobial peptides are active against a wide range of pathogenic microorganisms makes them attractive candidates for possible use as antibiotics ( Nature Biotechnology, 24(12):1551, 2006, Antimicrobial Agents and Chemotherapy, 45(10):2838, 2001), bio-preservatives ( Australian J. Agricultural Res., 55:69, 2004), as agents in wound healing ( J Invest. Dermatol., 120:379, 2003, J Invest. Dermatol., 117:91, 2001), as anticancer agents (Mol. Cancer Ther. 2005; 4: 612-624, J. Natl Cancer Inst. 2004; 96: 1015-1022) or to enhance disease resistance in aquaculture ( Applied and Environmental Microbiology, 66(5):1928, 2000).

Antimicrobial peptides are a diverse group of peptides that are subgrouped according to their amino acid composition and structure:

One subgroup contains anionic peptides and is present in pulmonary surfactant and airway epithelial cells. These peptides are short (7 amino acids) and are rich in glutamic and aspartic acid residues. They are active against Gram-negative and Gram-positive bacteria and require zinc as a cofactor. Included in this group are the surfactant associated anionic peptides (SAAP) found in sheep and cattle (Australian J Agricultural Res, 55: 69-75, 2004).

A second subgroup of peptides (<40 amino acids in length) is linear, cationic, alpha helical and lack cysteine residues. These peptides show antibacterial activity against Gram-negative and Gram-positive bacteria. This group includes Ovispirins (Ov-1, Ov-2, Ov-3), Pleurocidin, LL-37, Cecropin, Melittin and SMAP29. LL-37 is a member of the cathelicidin family and is the only cathelicidin identified in humans. In addition to LL-37 being bactericidal against a wide variety of microorganisms, it also has been shown that LL-37 is found in inflammatory cells and is upregulated in some inflamed tissues such as the skin in psoriasis. LL-37 is produced in atherosclerotic lesions where it may function as an immune modulator (Arterioscler. Thromb Vasc Biol, 26:1551-1557, 2006). LL-37 is also produced in eccrine appendages and processed into fragments (LL-8-37, LL-18-37) that show enhanced antimicrobial activity against Staphylococcus aureus and Candida albicans. The fragments show minimal hemolytic activity against human erythrocytes (J Immunology, 172: 3070-3077, 2004). SMAP29 (Sheep Myeloid Antimicrobial Peptide) is also a cathelicidin peptide composed of 29 amino acids. SMAP29 is a very potent, broad spectrum antimicrobial peptide. It is active against Chlamydia pneumoniae, Pasteurella multocida, Pseudomonas aeruginosa, Staphylococcus aureus and E. coli.( Antimicrobial Agents and Chemotherapy, 45: 2838-2844, 2001, Australian J Agricultral Res 55: 69-75, 2004, Antimicrobial Agents and Chemotherapy, 49: 1201-1202, 2005, Vet Microbial, 119: 76-81, 2007). Pleurocidin has been found in flounder and has been shown to exert activity against Gram-positve and Gram-negative bacteria (Applied and Environmental Microbiology, 66: 1928-1932, 2000).

Melittin, a 26 amino acid peptide, is the main component of Bee Venom. It is a powerful anti-inflammatory peptide that is said to be 100 times more potent than hydrocortisone. Melittin has been shown to inhibit neutrophil superoxide and hydrogen peroxide production. A recent study showed the antiarthritic effects of melittin. Melittin prevented LPS induced transcriptional and DNA binding activity of NF-kB (Arthitis and Rheumatism, 50: 3504-3515, 2004). Melittin exhibits an in vitro inhibitory effect on Borrelia burgdorferi, the bacteria that causes Lyme disease (Clin Infect Dis, 25 Suppl 1, S48-51, 1997). Parasin I is an antimicrobial peptide derived from histone H2A in the catfish , Parasilurus asotus (FEBS Letters, 437: 258-262, 1998).

A third subgroup contains cationic peptides enriched for specific amino acids. This group includes the tryptophan containing peptides, the histidine-rich peptides, the arginine-rich peptide bactenecin, the proline/arginine rich peptides and the trytophan/arginine peptides. Apidaecin has been shown to be active against Gram-negative bacteria while the Histatins have both bactericidal and antifungal activity (Peptides, 27: 2350-2359, 2006). Indolicidin is active against a wide range of microorganisms including Gram-negative and Gram-positive bacteria and human immunodeficiency virus (HIV-1) (J peptide Res, 64: 171-185, 2004). Bactenecin is classified as a loop peptide with one disulfide bridge and is active against Escherichia coli and Staphylococcus aureus. Replacing the proline residues in Indolicidin and the cysteine residues in Bactenecin with alanine residues results in peptides that have potent Gram-positve activiy and are very active against Enterococcus faecalis (Antimicrobial Agents and Chemotherapy, 44: 2086-2092, 2000).

Milk proteins are a source of biologically active peptides. These peptides are encrypted within the primary sequence of milk proteins and released during gastrointestinal digestion or by the processing of food (J Dairy Sci, 76: 301-310, 1993, J Dairy Sci, 88: 2348-2360, 2005). Bovine Lactoferricin is a peptide fragment produced by acid hydrolysis of Lactoferrin from cow’s milk. Lactoferricin consist of 25 amino acids (FKCRRWQWRMKKLGAPSITCVRRAF) with the antimicrobial and cytotoxic activity residing in the N-terminus (FKCRRWQWRM, FKCRRWQWRMKK). Studies have shown that Lactoferricin has anticancer activity both in vitro and in vivo (J. Pept Sci. 2005; 11: 379-389, Mol. Cancer Ther. 2005; 4: 612-624, Jpn J. Cancer Res. 1997; 88: 184-190, J. Natl Cancer Inst. 2004; 96: 1015-1022).

Lactoferricin has activity against Gram-negative bacteria. However lysis of Gram-negative bacteria results in the release of bacterial endotoxin lipopolysaccharide (LPS) which triggers sepsis. Therefore it is important that the antimicrobial peptide possesses both antibacterial and LPS neutralizing properties. Lactoferricin has shown to be both antibacterial and antiendotoxin (J Biol Chem. 2005; 280: 16955-16961).

A recent study has shown that conjugation of long chain fatty acids to the N-terminus of antimicrobial peptides increase both the bactericidal and LPS neutralizing activity by increasing the hydrophobicity of the cationic peptides (Biochem. J. 2004; 378: 93-103). Fatty acid conjugation to lactoferricin may also enhance the antibacterial, anticancer, antiviral and antiendotoxin activity of this peptide.

Bioactive peptides from kappa casein (169 amino acids) are derived from the hydrolyzed C-terminal fragment of kappa casein known as Caseinomacropeptide (106-169). The active form of Caseinomacropeptideis, Kappacin, is phosphorylated and has been identified as a potent antimicrobial (Antimicrobial Agents and Chemotherapy, 45: 2309-2315, 2001, Antimicrobial Agents and Chemotherapy, 49: 2322-2328, 2005). Fragments of Kappacin also have shown antimicrobial activity (106-115, 138-158, pS149) (J Dairy Sci, 88: 2348-2360, 2005).

A fourth subgroup contains both cationic and anionic peptides. These peptides contain cysteine residues, form multiple intermolecular disulphide bonds and are stable beta sheets. This subgroup includes brevinins, protegrin and a family of defensins (alpha-defensins, beta-defensins, theta-defensins). The defensins are cationic antimicrobial peptides that are active against Gram-negative and Gram-positive bacteria, fungi, spirochetes, mycobacteria and the envelope virus HIV-1.

Antimicrobial peptides have the potential to be used as potent antibiotics, biopreservatives and wound healing agents. However more research is needed to understand the mechanism of action of these peptides. Research is needed on how to produce more efficacious antimicrobial peptides, increase the peptide serum half-life, reduce toxicity of these antimicrobial peptides and lower the cost of production.

QCB has been manufacturing and producing antibodies to antimicrobial peptides for over 10 years. Our experience in peptide libraries has enabled us to produce derivatives of peptides in a very cost effective manner. QCB is ready to produce a large variety of peptides for screening assays, modify peptides or synthesize them with D-amino acids for serum half-life studies, produce large scale quantities for animal studies and generate specific antibodies for detection assays. We are ready to partner with you to produce antimicrobial and host-defense peptides. Contact our Technical Sales Department to see how we can support your research needs.