Broad-spectrum in vitro antibacterial activities of clay minerals against antibiotic-susceptible and antibiotic-resistant bacterial pathogens

Objectives: The capacity to properly address the worldwide incidence of infectious diseases lies in the ability to detect, prevent and effectively treat these infections. Therefore, identifying and analysing inhibitory agents are worthwhile endeavours in an era when few new classes of effective antimicrobials have been developed. The use of geological nanomaterials to heal skin infections has been evident since the earliest recorded history, and specific clay minerals may prove valuable in the treatment of bacterial diseases, including infections for which there are no effective antibiotics, such as Buruli ulcer and multidrug-resistant infections. Methods: We have subjected two iron-rich clay minerals, which have previously been used to treat Buruli ulcer patients, to broth culture testing of antibiotic-susceptible and antibiotic-resistant pathogenic bacteria to assess the feasibility of using clay minerals as therapeutic agents. Results: One specific mineral, CsAg02, demonstrated bactericidal activity against pathogenic Escherichia coli, extended-spectrum β-lactamase (ESBL) E. coli, Salmonella enterica serovar Typhimurium, Pseudomonas aeruginosa and Mycobacterium marinum, and a combined bacteriostatic/bactericidal effect against Staphylococcus aureus, penicillin-resistant S. aureus, methicillin-resistant S. aureus (MRSA) and Mycobacterium smegmatis, whereas another mineral with similar structure and bulk crystal chemistry, CsAr02, had no effect on or enhanced bacterial growth. The <0.2 ̄m fraction of CsAg02 and CsAg02 heated to 200 or 550°C retained bactericidal activity, whereas cation-exchanged CsAg02 and CsAg02 heated to 900°C no longer killed E. coli. Conclusions: Our results indicate that specific mineral products have intrinsic, heat-stable antibacterial properties, which could provide an inexpensive treatment against numerous human bacterial infections.