Decreased vitality and viability of Escherichia coli isolates by adherence to saponite particles

Abstract

This work was aimed at a detailed analysis of the interaction between Gram-negative bacteria of Escherichia coli and synthetic saponite (Sap), with the main focus on the changes in viability (survival) and/or vitality (metabolic activity) that are relevant parameters for evaluating the fitness of different microorganisms. The tested bacterial strains originated from various environmental and clinical sources, representing E. coli with different properties in terms of resistance and biofilm production. This included characteristics related to the cell surface structures (adhesins) that are thought to facilitate the adhesion of bacteria to Sap particles. Sap particles at a concentration of 5 mg/mL demonstrated an antimicrobial effect on all the tested E. coli isolates. The other tested concentrations (2.5 and 1.25 mg/mL) were efficient in a strain-dependent manner. The interaction between bacteria and Sap particles was affected more by viability than vitality, indicating a bacteriostatic mode of action by Sap. To obtain deeper insights into the mode of action of Sap, microscopy evaluations including fluorescence in situ hybridization (FISH) were used. The results of the FISH assay showed the ratio of live/dead E. coli cells after the adhesion to Sap particles for different time periods (4 h, 12 h and 24 h) and revealed that the cells adhered to Sap particles had remained active but lost their ability to multiply. Moreover, after the 18-h incubation, inactive bacterial cells were detected in the inner layers of Sap. Scanning electron microscopy confirmed damage of the cell surface of bacteria firmly bound to Sap particles. From the group of 4 tested genes for the fimA, pap, afa, and aaf adhesins, only the FimA and Pap genes were proven by PCR in some strains and a correlation between changes in viability/vitality and those genes was not observed. It seems that physico-chemical interactions rather than bacterial surface structures play a significant role in the interaction between Sap particles and the bacterial surface.