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Overuse and abuse of antibiotics are largely responsible for the increasing prevalence of multi-drug resistant bacteria. Hence, the reseachers have come up with new class of antibiotics-enzybiotics, with novel mechanism of action against drug-resistant pathogens.
The global health is gripped with numerous challenges. On one side it is facing the dire consequences of the climate change while on the other side it is dreaded with plentiful of epidemics and pandemics. One such most faced challenge is increasing number of bacterial infections that have become resistant to antibiotic regimen, as a result, superbugs like MRSA and NDM-1 have dramatically escalated statistics dreading the public safety. Thus, there is an urgent need to find an alternative way to address the problem in an efficient course of action.
Enzybiotics is a hybrid of enzyme and antibiotic. These are bacterial cell wall degrading enzymes having antibacterial and/or antifungal activity, thus, proving to be a potential subsitute over conventional antibiotics. The term was first coined in 2001 by Nelson et al. They consist of lytic enzymes of class peptidoglycan hydrolases like lysins, bacteriocins, autolysins and lysozymes. The significant characertics of enzybiotics are:
Their novel approach for antibacterial action.
Their ability to kill antibiotic-resistant bacteria, and
Their low probability of developing bacterial resistance.
Mode of Action
Enzybiotics majorly belong to the class peptidoglycan hydrolases. When these enzymes are added exogenously to Gram positive bacteria they cause rapid disintegration of the cell wall as there is no outer membrane present to hinder their action. But, in Gram negative bacteria outer membrane obtructs their way to to the cell wall ,thus, limiting their activity. Enzybiotics have narrow host range, therefore, they selectively target their pathogenic hosts without affecting the surrounding microflora.
Major classes of Enzybiotics
Bacteriocins: Bacteriocins are proteinaceous extracellular substanaces that are produced by both Gram positive and Gram negative species. They are either produced spontaneously or induced by certain chemicals such as mitomycin C. They inhibit the growth of similar or closely related bacterial strains. These are narrow spectrum class of antibiotics. Their lethal activity involves adsorption to the specific receptors on the exterior of specific bacteria, followed by metabolic, biological and morphological changes resulting in the killing of bacteria. Bacteriocins are produced by non-pathogenic bacteria that normally resides in human body. Antibiotic use results in the loss of these harmless and useful bacteria, paving way for the opportunistic pathogens to invade the human body.
Lysins: Lysins, also known as endolysins or murein hydrolases are peptidoglycan degrading enzymes released by bacteriophages that help in lysis of bacterial cell wall at the end of lytic cycle to release the progeny phage particles.They are incresingly being used as antibacterial agent owning to their high efficacy and specificity. They have characterstic lysis or cell-wall binding domain and degrade peptidoglycan with glycosidase, amidase, endopeptidase, or lytic tansglycosylase activities. They are species specific but some broad spectrum lysins have also been reported. They are highly effective against Gram positive bacteria as the outer memebrane is absent as against Gram negative where outer membrane is present.
Lysozymes: Also known as muramidase or N-acetylmuramide glycanhydrolase, globular protein of 129 amino acid residues.They belong to the class glycoside hydrolase that catalyse the hydrolysis of 1,4-β-linkages. They cleave the 1,4-β-linkages between N-acetyl muramic acid and N-acetyl-D-glucosamine in peptidoglycan structure. Lysozymes have been found in secretion like tears, mucus, saliva and human milk. Egg white (albumin) is an abundant source of lysozyme. It significantly contributes to innate immunity of humans . Lysozymes are most powerful natural antibacterial and antiviral subtance. It also exhibits anti-inflammatory, anti-cancer and immunomodulatory activities.
Autolysins: This enzyme hydrolysis the biological component or tissue in which it is produced. These are found in all peptidoglycan containing bacteria. The enzyme functions similar to lysozyme. It cleaves the 1,4-β-linkage between N-acetyl glucosamine and N-acetyl muramic acid. They break down the peptidoglycan matrix and assist the bacterial cell in growth and cell division. They present a promising target for the development of new type of antibiotics.
Defensins and cathelicidins: These are antimicrobial peptides found in the lysosomes of macrophages and polymorphonuclear leukocytes and keratinocytes. They are a part of mammalian innate immunity system that help fight bacterial infections. Cathelicidins obliterate the lipoprotein membranes of microbes enveloped in phagosomes after fusion with lysosomes in macrophages.
Virion-associated peptidoglycan hydrolases (VAPGH): These are phage encoded lytic enzymes that disintegrate the peptidoglycan of the bacterial cell wall during infection. Their mode of action involves the generation of small hole through which the phage tail enters, crosses the cell envelop and releases the phage genetic material at the onset of the infection cycle. VAPGHs are highly specific, thermostable and have high modular organisation. They serve as a potential candidates for the use as enzybiotics.
Various enzybiotics with their host range, source and types are summerised below.
S. pyrogens group C and E
Lambda SA 2-E
P ly G
Hen egg white lysozyme
Hen’s egg white
Gram positive bacteria
Some Potential Applications of Enzybiotics
In Food industry: Enzybiotics have been widely used in food industry as food additives and preservatives like in the production of cheese and wine. The Food and Drug Administration (FDA) has given a nod to the use of enzybiotics to control Listeria monocytogenes in cheese, classifying them as GRAS (generally recognised as safe) in the year 2006 which was later extended to enzybiotic use on all food products in 2007.
In Medical industry: Their novel approach for antibacterial action, ability to kill antibiotic-resistant bacteria and very low possibility to develop bacterial resistance make them potent antibacterial and antifungal agents. They are used in eye drops, toothpastes etc. Their other uses are well documented above.
In Farm industry: In the recent years the misuse and overuse of antibiotics in food producing livestock have raised a serious alarm on increasing number of antibiotic resistant bacteria, thus, escalating the risk of antibiotic resistant infections in humans. Enzybiotics present a novel way to combat the problem. They have been successfully used in treating septicaemia and meningitis in chikens and calves and also used as biocontrol agenets to control the number of Salmonellla in poultry products.
Enzybiotics present a redoubtable cover over antibiotics. Their objectionable side effects have been rarely reported. Therefore, they are very effective to be used as antibacterial and antifungal agents. Researches are being carried out on further potentials of enzybiotics and it is hoped that they present a promosing future to medical community by help combating the antibiotic resistant strains of pathogens.
Ruchi Tiwari, Kuldeep Dhama,Sandip Chakraborty, Sanjay Kapoor. Enzybiotics: New weapon in the army of antimicrobials: A review. Department of Veterinary Microbiology, College of Veterinary Sciences, Uttar Pradesh. Asian Journal of Animal and Veterinary Advances 9(3): 144-163, 2014.
G K Sudhakar, venkatesh Kamath B and Aravind Pai. Enzybiotics: A review. Department of Pharmaceutical Chemistry, Manipal College of Pharmaceutical Sciences, Manipal University, Manipal. International Journal of Pharmacological Research. DOI: 10.7439/ijpr.
M A Daw, F R Falkiner. Bacteriocins: Nature, function and structure. Faculty of Medicine, Alfateh University of Medical Sciences, Tripoli, Libya. Micron December 27(6): 467-479.