Therapeutic use of virolysins has several therapeutic advantages including Immunogenicity is not a problem, due to its high potency [μg/l] neutralizing by immune system and allergic responses are not a problem, resistance to virolysins is less due to the mutational change in the bacterium cell wall would causes the disease of the bacterium and rapidly decreases the pathogenicity. Phages encode two types of antimicrobial peptides. The first is Lytic factors which acts like virolysin-holin system that holin is a peptide that oligomerizes with bacterial membrane to form disruptive membrane lesions, allowing virolysins to pass the cell wall [Jin et al., 2012, Parisien et l., 2008]. Example: L lytic factor encoded by MS2/GA, E lytic factor encoded by φX174, A2 lytic factor encoded by RNA Qβ/SP. The second AMP is Phage tailed complexes that act on specific receptors on the surface, inter and break down peptidoglycan then release the phage genome into the infected microbes. Example: phage T4 [Parisien et l., 2008]. Phages used to deliver genes encoding antimicrobials into the host bacteria. filamentous phage genetically modified by replacing a gene [i.e. the gene used to remove the phages from the host bacteria] with restriction enzyme gene so that the phages cannot be removed from host bacterial, but gain the ability to degrade the bacterial genome, used as effective anti-infection agents . Filamentous phages can also used to deliver the therapeutic genes into target mammalian cell. The other lytic phages also induce bacteriolysis by inhibition of peptidoglycan synthesis.Advantages of phage therapy
Phage therapy has a lot of advantages over antimicrobial agent including the mode of action is different to the available antimicrobial agent, abundant in nature, fast replication rate at the site of infection and can be genetically modified to be host-specific viruses during an active microbial infection. Phages also help to transport and deliver antimicrobial agent into the infecting microbes. Phages have low toxicity, effective against multidrug-resistant mutant and developing costs for a phage therapy is less expensive than that of new antimicrobial agent [Carlton, 1999, Fortuna et al., 2008]. Some phage virolysins are potent and rapid acting for the control of bacterial infection on mucous membranes. This rapid effect at low dose decreases the chance to be neutralized by the immune response or causing allergic responses in the hosts. During bacterial lysis, virus particles are released that can infect another micoorganism that are found in the body. This means that the amount of phages increase in number in the presence of the target, a vital phenomenon not observed in antimicrobial agents [Carlton, 19999, Marks and Sharp, 2000].
Barriers of phages therapy development
Quality control and standardization are difficult; Phage therapy needs monitoring because of their immunogenic property and may induce neutralizing antibodies which facilitate treatment failure. Massive bacterial lysis may result the release of endotoxins which may cause toxic shock. Phage resistance bacterial emerges quickly necessitates to use a cocktail of phages to prevent the emergence of resistance [Fortuna et al., 2008]. Phages are very specific, needs the identification of the specific bacteria, not suitable for severe infections. Some phages may encode toxins and lack of pharmacokinetic data .Conversion of lytic phages to lysogenic prophages occurs when phages insert their genome into the bacterial genome. Bacteria containing prophage are immunized against the attack by the corresponding lytic phages [Hermoso et al., 2007]. This is called the lysogenic immunity and leads to acquired resistance and this may also alter the virulence of the bacteria. Phages may carry harmful genes. When a prophages excises its genome from the bacterial genome to enter the replicative lytic cycle, some bacterial genome can be up-taken and become part of the phage genome. Therefore, phages may carry toxins and virulence factors from one bacterium to another [Parisien et l., 2008, Skurnik et al., 2007].
Antimicrobial efficacy of phage therapy
Recent studies have demonstrated phage therapy successfully treated infections caused by resistant bacteria species. Phages were effective for the elimination of food poisoning pathogenic bacteria such as Campylobacter jejuni ,Listeria monocytogenes, and Salmonella spp. In 2006, FDA approved the use of combination of six phages to be sprayed on ready-to-eat meat and poultry to eradicate Listeria monocytogenes. Topical phage therapy: Biophage PA — is in clinical trials against multidrug -resistant P. aeruginosa in chronic otitis [Skurnik et al., 2007, O’Flaherty et al., 2009].
Antimicrobial peptides (AMPs) are the component of innate immune response, which is the main defense system synthesized by Plants, animals and bacteria. AMPs are peptides of less than 100 amino acid residues with a positive charge, due to the presence of multiple lysine and arginine residues and a high portion (≥30%) of hydrophobic residues. The cationic property makes them to interact with anionic cell membrane. AMP are also amphipathic compounds with hydrophobic and hydrophilic moieties segregating into different patches on the surface, which allow them to form pores in the cell membrane, increasing the permeability of the membrane and loss of cell content [Brogden, 2005]. AMP is a promising novel alternatives to antibiotics based on their efficacy, safety and diversity. They have rapid onset, broad spectrum and less prone to develop resistance against them, when compared to antibiotics. But, AMP is unstable and quickly loses their efficacy when entering our bodies, highly toxic on human cells therefore mainly evaluated as topical treatments for local infections [Seo et al., 2012]. Until now, more than 800 AMPs with has been identified from different organisms. In humans, three AMP groups identified and named as: defensins, cathelicidins and histatins which contribute in the innate immune response against pathogenic microorganism like bacteria, fungi and viruses.
In addition, AMP neutralize effects of endotoxin, repair damaged tissue and facilitate wound closure,by promoting wound neovascularization and re-epithelialization of healing skin[De Smet and Contreras , 2005].
Bacteria synthesize AMP called bacteriocins which has narrow spectrum of antimicrobial activity. E.coli strain of a colicin has an immune gene that help defend it against the colicin it produces, but sensitive E. coli strains do not have the immune gene[Williams and Hergenrother, 2008]. Lantibiotics are synthesized by S.aureus with unusual amino acids such as lantine, and vital for the biological activities. From the different lantibiotics, nisin binds to lipid II, a precursor lipid component of the cell wall and inhibits the peptidoglycan synthesis. Phage-encoded AMP also has similar activity as that of the virolysin-holin system of large lyticphages, i.e. inducing bacteriolysis to allow the release of phage particles into the environment, but with different mechanisms (i.e. nonenzymatic modes). Protein E, lytic factor that inhibit phospho-MurNAc-pentapeptide translocase, a membrane bound enzyme vital for peptidoglycan synthesis, inhibits in a way similar to the antibiotics and viewed as a “protein antibiotic” [Bernhardt et al., 2001].
The Mechanism of AMP
The mode of AMPs depends on the amino acid sequence, membrane lipids and peptide concentration. AMP may act by forming pores across the cell membrane which causes membrane perturbation, disturbance of electrochemical gradient across the membrane, and leakage of cell content. Lipid II-targeting lantibiotics remove lipid II from the cell to block peptidoglycan synthesis.AMP also acts by using ribonuclease [RNase] or deoxyribonuclease [DNase] activities. For example, colicin E9 causes death of E. coli by DNA break down at specific ribonucleolytic cleavage of 16S rRNA [i.e. RNase activity]. Phage tail-like bacteriocins kill bacteria by specific binding to the bacterial receptor, which provokes perforation of the cell membrane [Boix E and Nogués, 2007].
Regarding the specificity, bacteriocins are highly specific and toxic to bacterial strains closely related to the producing strains. But, most eukaryotic AMP has broad-spectrum activity and toxic to both bacterial and animal cells whereas most of the bateriocins do not show toxicity to animals at their effective antimicrobial concentration. Bacteriocins, have high efficacy in killing bacterial species closely related to the producing strains. The streptococcal bacteriocin,tomicide protects in mice from local staphylococcal infection. Nisin mostly used as a food preservative against many strains of bacteria, including drug-resistant strains and food borne pathogens such as Clostridium, botulinum and L. monocytogenes [Boix E and Nogués, 2007, Hancock and Diamond, 2000].
Advantages and disadvantages of AMP therapy
The characteristics that help AMP promising alternatives to antibiotics include broad-spectrum activity (antimicrobial, antiviral, antifungal), rapid and potent activity, low level of induced resistance and vast variety in terms of structures and activity representing a tremendous potential. Major challenge for the therapeutic use of AMP including lack of higher efficiency than antibiotics , Higher costs for drug development, Sensitive to salt, high toxicity, rapid clearance due to degradation restrict their use for in vivo. For this reason, many trials of peptide manly focused on topical use but, prokaryotic AMP is safer and efficient in vivo [Marr et al., 2006].
According to the Food and Agriculture Organization of the United Nations and the WHO probiotics defined as the use of non‐pathogenic live microorganisms in adequate amounts to give a health benefit to the host. These effects include the balancing and restoration of intestinal normal flora, preventing the proliferation of pathogens bacteria, modulation of the immune response, suppressing the synthesis of virulence factors by pathogens, the maintenance and repair of intestinal barrier functions [Alvarez-Olmos and Oberhelman, 20011].
Human intestinal is a home for more than 500 microorganism species. The presence of normal flora is vital for normal immune activity, epithelial function and to defend mucosal surfaces from pathogenic bacteria. The Interactions between intestinal microbes and the host are important because these interactions changes may contribute to the development of diseases. The intestinal normal flora can be changed by administration of antibiotics, prebiotics and probiotics. For microbes to be used as a probiotics it should be non-pathogenic, beneficial for the host, isolated from the same species as its intended host & could survive through git and on storage could survive for a long period of time [Alvarez-Olmos and Oberhelman, 20011, Rolfe, 2000].
The effectiveness of aprobiotic depends on the mechanism by which they act their activity. Several mechanisms proposed including probiotic colonize the human gut and prevent the adherence of the pathogenic bacteria to the host cells by improving the barrier effect of the Intestinal mucosa and secrete gut-protective metabolites (glutamine, arginine, short-chain fatty acids).The adherence of probiotics in human gut enhance biological activities including the release of chemokines and cytokines which stimulate mucosal and systemic host immunity[, Rolfe, 2000, Walker, 2008].Probiotic also act as antimicrobial by producing antimicrobial compound called bacteriocins and substances like organic acids (acetic, lactic and butyric acid) and H2O2.
The DNA of probiotic organisms has shown to inhibit apoptosis of epithelial cells. In addition, probiotics may improve bowel dysmotility, degradation of toxin receptors, and competition for nutrients, antiproliferative effects and modulation of the intestinal microflora [Alvarez-Olmos and Oberhelman, 2001, Walker, 2008].
Many probiotic strains of microbes are effective to treat diarrhea caused by lactose intolerance by producing lactase, which hydrolyses lactose in to glucose and galactose, acute diarrhea from viral and bacterial infections, e.g. rotavirus caused diarrhea, Clostridium difficile gastroenteritis, antibiotic-associated diarrhea, travelers’ diarrhea, diarrhea in tube-fed patients; chemo- or radiotherapy induced diarrhea; Atopic dermatitis inflammatory bowel diseases [ulcerative colitis ,Crohn’s disease]; small bowel bacterial overgrowth; and irritable bowel syndrome with diarrhea, Pouchitis [Ouwehand et al., 2002]. In addition, probiotics also used for intentional colonization of the gut of newborns to inhibit the colonization of pathogenic and multi resistant microorganisms. The most widely used microbes are Lactobacillus and Bifidobacterium species. In addition, nonpathogenic species belong to the class of Saccharomyces, Streptococcus and Lactococcus are also used as probiotics [Shlomai et al., 2014].