Abstract
Quaternary alkylammonium compounds (QAACs) are a group of cationic surfactants which are disinfectants with numerous industrial and agricultural applications and frequently released into the environment. One recent hypothesis is that bacteria present in soil will be protected from acute toxic effects of QAACs in the presence of expandable layer silicates due to interlayer sorption. We therefore studied bacterial growth kinetics with high temporal resolution and determined minimal inhibitory concentrations (MICs) of two QAACs, benzyldimethyldodecylammonium chloride (BAC-C12) and didecyldimethylammonium chlorid (DADMAC-C10), for eight strains of different bacterial taxa (Escherichia coli, Acinetobacter, Enterococcus faecium, Enterococcus faecalis, and Pseudomonas fluorescens) in relation to QAAC sorption to smectite and kaolinite. The MICs of BAC-C12 and DADMAC-C10 were in the absence of smectite and kaolinite in the order of 10 to 30 µg mL−1 and 1.0 to 3.5 µg mL−1 for all strains except the more sensitive Acinetobacter strain. For all tested strains and both tested QAACs, the presence of smectite increased apparent MIC values while kaolinite had no effect on MICs. Sorption curves without bacteria showed that smectite sorbed larger amounts of QAACs than kaolinite. Correcting nominal QAAC concentrations employed in toxicity tests for QAAC sorption using the sorption curves explained well the observed shifts in apparent MICs. Transmission electron microscopy (TEM) demonstrated that the interlayer space of smectite expanded from 13.7 ± 1 Å to 19.9 ± 1.5 Å after addition of BAC-C12. This study provides first evidence that low charge 2:1 expandable layer silicates can play an important role for buffering QAAC toxicity in soils.
Highlights
Quaternary alkylammonium compounds (QAACs) are a group of cationic surfactants which are disinfectants with numerous industrial and agricultural applications and frequently released into the environment
minimal inhibitory concentrations (MICs) values for BAC-C12 and DADMAC-C10 were in a similar range of 10 to 30 μg mL−1 and 1.0 to 3.5 μg mL−1, respectively, for all tested strains according to their compound specific susceptibilities (Table 1)
Lower QAAC concentrations had no effect on the growth kinetics of the strains, while higher concentrations totally inhibited the bacterial growth
Summary
Quaternary alkylammonium compounds (QAACs) are a group of cationic surfactants which are disinfectants with numerous industrial and agricultural applications and frequently released into the environment. Within the group of cationic surfactants, quaternary alkylammonium compounds (QAACs) are a heterogeneous group of organic compounds comprising a quaternary nitrogen atom This nitrogen atom carries one permanent positive charge and at least one alkyl chain r est[3], which together cause their amphiphilic properties used for disinfection and sanitation purposes. Direct application of QAACs with manure, sewage sludge, during irrigation with wastewater or as biocides or adjuvants in agricultural pesticide formulations are the main entry pathways. The concentrations determined in sewage sludge were at least one order of magnitude higher than other antimicrobial active substances with average concentrations often exceeding 100 μg g−1 d.w. The release of large quantities of QAACs into the environment poses a risk for environmental and human health especially with regard to the evolution and spreading of disinfectant and antibiotic resistant bacteria. Plasmid and genome sequence based studies showed that several QAAC resistance genes are located on the same genetic units as antibiotic resistance genes, promoting the co-selection of antibiotic resistance genes in the presence of QAACs15–18
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