The ancient Greeks and Romans are said to have suffered wounds in battle. have treated Spider silk envelopes, believed that the silk had healing properties, as well as being used to treat skin lesions and warts. There have also been reports of people in the Carpathian Mountains using spider webs as bandages, and older doctors sometimes prescribed putting silk cocoons on infected teeth.
This notion that spider silk might have antimicrobial properties – which makes it a kind of “webiccillin” – has been the focus of numerous studies, particularly over the past decade, with conflicting results. Some found evidence of antimicrobial activity, others did not. Now researchers at Aarhus University in Denmark have provided the strongest evidence yet against the rumored healing properties of spider silk a current paper published in iScience magazine. The authors suggest that previous positive results are either the result of bacterial contamination or the use of solvents in the experiments that have antimicrobial properties.
“Spider silk has always been admired and has almost a mythical status”, said co-author Trine Bilde, Biologist at Aarhus University. “It is one of those myths that appears to be” founded “on” belief “rather than strong empirical support.”
Scientists have long known that spider silk is an amazing material, especially one with impressive tensile strength, toughness, and elasticity. For example, it is stronger than steel and tougher than both steel and kevlar. Rigid layers hold the spider silk together and give it strength, and these layers are interspersed with softer areas to allow flexibility to allow the silk to stretch. The silk cannot twist and swing, so it stabilizes the spider when it is hung up and can absorb a lot of energy before it breaks.
The secret lies in the proteins. Spiders have special abdominal glands that secrete a liquid, fiber-filled protein that is similar in structure to keratin (the protein in hooves and hair). The silk quickly hardens (or “polymerizes”) as it is spun. There are seven amino acids that make up spider silk proteins: mainly alanine and glucose, with smaller amounts of glutamine, leucine, arginine, tyrosine, and serine.
Because of its miraculous properties, Polynesian fishermen are known to use spider silk as a fishing line, while some New Guinea tribes construct water-repellent hats from cobwebs. Black widow spider threads were used in telescopic sights. Spider silk also promises for wear-resistant shoes and clothing; strong ropes, nets and parachutes; as strong, tough paper for banknotes; and better bulletproof vests, among others. In 2012, a Japanese scientist even used spider silk Make violin strings.
So it’s not the craziest idea that spider silk could also have antimicrobial properties, much like another popular folk remedy for infection: applying honey-soaked gauze to open wounds. (This honey legend could be scientific. In 2008, University of Wisconsin doctor Jennifer Eddy noticed, that Honey is a useful treatment for diabetic foot ulcers, although she insisted that people with diabetic ulcers should still urgently see a doctor and treat them.)
According to Bilde and her co-authors, spiders usually line their retreats with silk and lay their eggs in a silk sheath. Spider eggs contain high-energy compounds that would be a feast for these pathogens, while the amino acids in the protein-based fibers of spider silk could also provide an attractive substrate for microbes. If there were antimicrobial molecules on the silk, it could protect both the spiders and their eggs from pathogens. This would be particularly beneficial for social spider species that live in large groups and have a weaker immune system due to inbreeding. They would be particularly susceptible to infection, so the intrinsic antimicrobial properties of silk would be beneficial.
Several studies over the past decade seem to have shown that spider silk actually has some antimicrobial effects. These experiments are mostly direct contact assays or diffusion assays in which untreated silk (direct contact) or silk extracts (diffusion) are applied to agar plates inoculated with test bacteria. If a bacteria-free zone (known as a “zone of inhibition”) appears on the plate, it is typically interpreted as an indication of antimicrobial activity.
However, Bilde and her colleagues from Aarhus were skeptical of these claims. They performed similar experiments on silk from seven different species of spider to test the silk’s effectiveness against three types of bacteria. They kept the spiders in the lab at room temperature and ingeniously used a mechanical LEGO setup to harvest dredging silk from immobilized spider species that produce dredging silk. The dredge silk was then shaped into small discs and placed on agar plates.
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