Perturbation of a Human Gut Ecosystem by Silver Chloride Colloids

Abstract

The bactericidal and fungicidal properties of silver (Ag) and Ag salt colloids make them popular choices for a variety of applications including water purification, bio-medical therapies for burns, epilepsy, mental illness, gastroenteritis, syphilis and gonorrhea, amongst other diseases. In addition, individuals deliberately drink colloidal Ag and Ag salts in the belief that this self- therapy will be beneficial. Despite this widespread usage, very little is known about the impact of this metal on human gut consortia. Here we investigated the impact of Ag chloride colloids, at several concentrations, on a defined anaerobic and facultative anaerobic gut bacterial community developed from the collected stool of a healthy donor. Defined microbial ecosystem therapeutic- 1 (MET-1) consortia were exposed to different concentrations of colloidal Ag (25-200 mg/L) for 48 hrs and compared to unamended cultures. Carbon dioxide and nitrogen gases recovered from the headspace of anaerobic cultures were analyzed by an Agilent Technologies 7890B Gas Chromatograph. Fatty acid methyl ester profiles were extracted following the MIDI Sherlock Microbial Identification System protocol and detected by gas chromatography. Colloidal AgCl toxicity was also monitored by a suite of DNA analyses including polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE) and 16S ribosomal RNA gene fragment 454-pyrosequencing. Colloidal AgCl had an overall deleterious impact on MET-1 bacterial abundance and their metabolic activities as evidenced by the 20 - 78% reduction in CO2 respiration and significant (p<0.01) changes in fatty acid profiles, including a greater reduction (15 - 100%) in most Gram negative signatures compared to controls. Multidimensional scaling and cluster analysis produced from PCR-DGGE profile-based DNA analysis, as well as gene sequencing also demonstrated the negative impact of colloidal Ag on MET-1, resulting in a shift in the community structure, including the apparent elimination of some beneficial species. Together, this research suggests that deliberate or inadvertent colloidal Ag ingestion could have a potential negative impact on our overall digestive health.