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Antimicrobial Enzymes: Modern Weaponry against Microbes
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The Same Old Story- Drug Resistance and Biofilms

In the age-old war between humans and microbes, we are losing grounds fast. Those so called magic bullets have long since proved unsuccessful due to the evolution of drug resistance in microorganisms. Bacterial biofilms further strengthen their line of defence acting as a protective shield against antimicrobials and host immunity.

A New Treatment Strategy: Antimicrobial Enzymes

Scientists studying new methods of controlling these invisible enemies have focused their attention on a new approach which has demonstrated promising potential thus far-antimicrobial enzymes.

Antimicrobial enzymes are a major component of the immune system of many living organisms that fight against pathogenic microorganisms. These enzymes act through various mechanisms. They may attack the microbial cells by degrading major structural components of the microorganisms or they may induce the production of antimicrobial substances. They may either prevent biofilm formation or disrupt the existing biofilms by degrading the compounds (mainly the exopolysaccharides) that hold the cells together, thus making the individual cells susceptible to antibiotics. Another group of enzymes may interrupt bacterial quorum sensing, thus preventing the cell aggregation and production of virulence compounds.

Antimicrobial enzymes are being currently used in many formulations such as cleaning liquids, polymer substances, ointments and even toothpastes. These preparations may contain a single enzyme, a combination of two or more enzymes or enzymes combined with another antimicrobial agent.

Hydrolysing Enzymes

These enzymes inhibit microbial growth either by directly attacking the major constituents of the cell wall or by degrading the compounds that glue the cells to each other and solid surfaces.

Proteolytic enzymes such as subtilisins, that hydrolyse adhesins--proteins which are essential for bacterial attachment in biofilms--are widely in use as antimicrobial agents. Subtilisins have particularly shown to be effective against species such as Pseudomonas, Bacillus, Streptococcus and, Listeria monocytogenes. Another protein hydrolysing enzyme that has antibacterial capacity is lysostaphin-an enzyme that disrupts Staphylococci cell walls, thus demonstrating immense potential in controlling Staphylococcus species including multi-drug-resistant MRSA which is the main cause of many nosocomial infections.

Polysaccharide-hydrolysing enzymes are also important in controlling microorganisms. Alpha-amylase has been proven to inhibit the formation of biofilms MRSA, Vibrio cholerae and Pseudomonas aeruginosa. This enzyme was also effective in destroying the preformed mature biofilm by disrupting the exopolysaccharide layers in biofilm matrix. Dispersin B is another important glycoside hydrolase enzyme that catalyses the hydrolysis of poly-N-acetylglucosamine, a sticky extracellular polysaccharide which is important in biofilm attachment. Chitinases and beta-glucanases act as antifungal enzymes by degrading chitin and beta-1,3-glucan, the main components in fungal cell wall. Lysozyme, abundantly found in tears, saliva, human milk, and mucus is another antimicrobial enzyme that attacks the cell walls of Gram positive bacteria by hydrolysing the beta-1,4-glucosidic bonds in the peptidoglycan cell wall. Another polysaccharide degrading enzyme, alginate lyase that degrades bacterial alginate polymer has also been successfully used against Pseudomonas aeruginosa.

Apart from these enzymes, DNases are also being used as antibiofilm enzymes. These DNases digest extracellular DNA in the biofilm matrix which are important for the biofilm formation and stability, thus preventing the initiation of biofilms and disrupting the existing ones.

Bacteriophage lysins are another group of enzymes that target the peptidoglycan layer of the bacterial cell walls. Phage lysins are widely used to control many bacterial pathogens including Listeria monocytogenes, Escherichia coli, Streptococcus pyrogenes, Bacillus anthracis and Bacillus cereus.

Oxidative Enzymes

Enzymes such as glucose oxidase, cellobiose dehydrogenase and superoxide dismutase elicit antimicrobial reactions through the production of hydrogen peroxide which is cytotoxic. Still another type of oxidative enzymes, haloperoxidases, oxidise halides or pseudohalides into toxic compounds. Myeloperoxidase, lactoperoxidase and horseradish peroxidase are commonly known members of this group. Lactoperoxidase, found in milk, saliva and other mucosal secretions, oxidises bromide, iodide and thiocyanate ions into powerful bactericides.


Quorum-Quenching Enzymes


Such enzymes including AHL-lactonase, AHL-acylase and paraoxonases interfere with bacterial cell-to-cell communication, i.e. quorum sensing, by degrading AHL (acylhomoserine lactone), the signal molecules. Inhibition of quorum sensing eliminates the ability of the pathogen to produce virulence compounds and initiate biofilms, thus allowing the host defences to eradicate the pathogen.

Pros and Cons

The antimicrobial enzymes possess many advantages over antibiotics and disinfectants. Many of the enzymes are specific for a particular pathogen, therefore do not disturb the normal flora and bacterial resistance to antimicrobial enzymes is very rare. Furthermore,these enzymes are natural, nonreactive, and nontoxic thus they offer a safe alternative to the antibiotics and chemical disinfectants which are currently in use without causing adverse health effects or corrosion of surfaces.

However, the cost of production and purification of these enzymes are comparatively high. Moreover, these enzymes, being proteins, tend to denature at extreme conditions.

Future Potential

Genetic engineering and synthetic biology approaches are being developed to explore the possibilities of overcoming these hindering factors thereby boosting the use this comparatively novel technology in areas such as food production, agriculture, healthcare and medical fields.
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