The Surprising Antimicrobial Power of Clay

Sometimes, the most innovative and unlikely treatments come from ancient sources.

As antimicrobial resistance grows in complexity and scale, investigators have been turning to ancient treatments to assess their effectiveness. The use of clay as a medical measure, especially during ancient times, was prolific. Natural clays have been used for thousands of years as a treatment of skin infections. Recent studies have evaluated certain clays that were used and effective in killing several microorganisms. In fact, one study found that the use of French green clay was effective in killing Mycobacterium ulceranus, the causative agent for the necrotizing fasciitis that is Buruli ulcer.

Recently, a new study sought to evaluate the antimicrobial activity of reduced iron clay and its efficacy against pathogenic bacteria in wound infections. The investigators had previously evaluated several clays against planktonic liquid cultures to assess microbial growth over 24 hours. They found that 1 specific iron (Fe2+)-bearing clay from a deposit in Oregon (OMT Blue Clay) appeared to have considerable strength against a range of organisms. The investigators tested this clay and its aqueous leachate (hydrated clay suspicion, which is essentially the water that surrounds/goes through the clay) in 2 forms (bacteria in biofilm and planktonic) against 12 bacterial species that are the most common culprits of nonhealing superficial wound infections, including “Escherichia coli (ATCC 25922), extended-spectrum β-lactamase-producing E coli (ATCC 51446), Pseudomonas aeruginosa (ATCC 27853), Salmonella enterica subspecies enterica serovar Typhimurium (ATCC 14028), Staphylococcus aureus (ATCC 29213), S aureus (USA300 & IDRL-6169), Staphylococcus epidermidis (ATCC 14990), methicillin-resistant S epidermidis (MRSE) (ATCC35948), and methicillin-resistant S aureus (MRSA).” Of all the strains tested, on the leachate against S aureus IDRL-6169 and USA300 were the only ones not affected.

The investigators also tested biofilm growth by assessing baseline growth after incubation versus treated discs after 24 hours. They found that when compared with the controls, the disc-associated bacterial population was considerably reduced when exposed to both clay and leachate. Interestingly, there were 5 species that were found to have statistically significant population reduction when treated with the OMT Blue Clay versus just the leachate: S aureus IDRL-6169 (P = .0369), S aureus USA300 (P = .0495), S epidermidis RP62A (P = .0253), Enterobacter cloacae IDRL-10306 (P = .0495), and E cloacae IDRL-10375 (P = .0495).

The team found that some organisms had higher population densities with just the leachate, while others proliferated under the singular OMT Blue Clay treatment. Overall, the dual utilization of the OMT Blue Clay and the leachate yielded considerable efficacy against microbial biofilms.

Why was this clay effective? The investigators noted that the natural clay has a majority of illite-smectite, which is a group of minerals within the interlayer of the clay. What makes this interlayer so important though is that it essentially acts like a reservoir for heavy metals, which are believed to have antimicrobial effects. The investigators hypothesize that “hydration of the clay results in dissolution of reduced Fe2+ and aluminum (Al3+) from the minerals, which in concert, damage the bacterial membranes.”

This study underscores the potential use of OMT Blue Clay as an antibacterial tool and a considerable force against biofilms. Although many resources have been put toward the future and finding the next innovative antimicrobial treatment, perhaps we need more research into existing and historical practices, which might surprise us in their efficacy.