Bizarro World: Bacteria Eat Antibiotics, Clay Kills Bacteria

Suggesting a twisted version of rock-paper-scissors, 2 recent reports indicate some funky flips in the microscopic food chain.

Last week, investigators at Harvard reported that hundreds of soil bacteria were not only resistant to a range of antibiotics, they could actually subsist on them. The investigators were specifically able to culture clonal bacterial isolates from 11 different soil samples (taken from farms, urban areas, or isolated regions) that could use anywhere from 13 to 17 of the 18 antibiotics tested as their sole carbon source.

These antibiotics included a wide range of compounds—for example, aminoglycosides, penicillins, fluoroquinolones, and sulfonamides. The most accommodating of the antibiotics were chloramphenicol, carbenicillin, penicillin G, vancomycin, ciprofloxicin, and mafenide, which fed bacterial clones from all 11 soil samples. The antibiotic-feeding bacteria were also diverse, with 11 major orders represented; the most common were Burkholderiales (41%), Pseudomonadales (24%), Enterbacteriales (13%), Actinomycetales (7%), Rhizobiales (7%), and Spingobacteriales (6%). Given the diversity of the drug-chewing bacteria and their close relation to some pathogenic organisms, the authors speculate that human pathogens could acquire antibiotic-resistance genes from these wide-ranging, soil-living, über germs.

And yesterday at the meeting of the American Chemical Society in New Orleans, researchers from Arizona State University indicated that certain clay samples can kill or considerably stifle the growth of important pathogenic bacteria—such as MRSA, E. coli, and Salmonella—in culture. The NIH-funded work was inspired by reports of the healing powers of the Côte d'Ivoire's green vocanic mud on Buruli ulcers. WHO currently recommends treatment for these ulcers, which are caused by Mycobacterium ulcerans, with rifampicin and streptomycin/amikacin followed by surgery.

The ASU investigators concentrated their study on volcanic soils (aka bentonite) collected from around the world but also found that samples from Nevada and Oregon were, to their surprise, also bactericidal. However, they report that they do not know the mechanisms by which various clays may hamper bacterial growth and urge against the current use of clay as medicinal therapy. Their next steps are to isolate the potentially active compounds in the clays and perform preclinical testing. 

MRSA = methicillin-resistant Staphylococcus aureus.