Spider silk does not have antimicrobial properties despite the findings of many studies. A researcher finds that previous studies have major methodological shortcomings.
For many years, researchers have asked whether spider silk has antimicrobial properties, and this could make sense.
For example, spiders use silk to protect eggs, and antimicrobial activity would protect the eggs from microbial pathogens. Antimicrobial agents in silk could also better conserve the food that the spiders snare and wrap in their silk.
Unfortunately, a new study reviewing the available studies and replicating many of the experiments of other studies, shows that spider silk does not have antimicrobial properties. The researchers also carried out their own controlled experiments using standardised direct contact or disc diffusion assays and fluorescence microscopy.
The conclusion is clear: previous studies had methodological shortcomings.
“This is disappointing, because spider silk as an antibiotic, and the exciting uses emanating from this, would have been much more interesting. Investigating the nature of the antibiotic effect, how it originated and whether it is based on some form of symbiosis would also be interesting. However, spider silk has no inherent antimicrobial properties, and unfortunately this indicates something far more serious: that much of the published research on this topic has been based on flawed empirical data and methods,” explains a researcher behind the study, Trine Bilde, Professor and Centre Leader, Department of Biology, Aarhus University.
The research has been published in iScience.
Reviewing other studies
Trine Bilde and colleagues Andreas Schramm, Simon Fruergaard, Marie Lund and Thomas Vosegaard first reviewed other studies, some of which indicated that spider silk has antimicrobial properties.
These previous studies often placed some spider silk on agar plates with growth medium and a bacterial culture to determine whether a ring forms around the silk to prevent the bacteria from growing close to the silk, which would indicate antimicrobial activity.
Trine Bilde and her colleagues followed the methods of the other studies but could not replicate the results even though they used silk from the same species.
The researchers also tested whether silk from other spiders had antimicrobial properties, but the result was the same.
Too many shortcomings in previous experiments
After several futile experiments with spider silk, the researchers reviewed the protocols used in previous studies.
In their current experiments, these researchers had used the gold standard of appropriate controls and ensured that their samples were not contaminated by using sterile silk. Another important point was testing the solvents used for any antimicrobial effect.
However, the researchers’ review indicated that many of the protocols had shortcomings that increased the risk of contaminating the samples.
Several studies did not have the appropriate controls, which would have revealed sources of error in the experimental set-up.
Finally, many previous experiments had used solvents that may have had antimicrobial effects. This indicates that the effect found previously can probably be attributed to the antimicrobial effect of the solvent and not the spider silk. The researchers tested the solvents used and found that several had bactericidal effects.
“After many long days in the laboratory, we were unable to replicate the results of other studies. This leads us to conclude that spider silk does not have any inherent antimicrobial properties. However, we did not test for possible antimicrobial activity against all known bacteria. However, for standard bacteria, spider silk has no antimicrobial effect,” says Trine Bilde.
Major shortcomings in studies
These results are a major disappointment, because the vision of carrying out more research on possible antimicrobial properties of spider silk is definitely over.
Tine Bilde is also disappointed with the general quality of the published studies within this field.
“This is sad, because we want results that you can trust and refer to in all published scientific studies. This is generally true, but our results indicate that this is not always the case,” concludes Trine Bilde.