In an aquatic environment, the biofilm found on solid surfaces is constituted of attached microorganisms and excreted extracellular polymeric substances (EPS. The presence of biofilm in natural and engineered systems may facilitate the retention of pathogenic bacteria, which may pose a serious concern to public health. Understanding the factors that influence the retention of pathogenic bacteria in environmental biofilms is essential to identify infection sources and develop control strategies. Thus, this study investigated the retention of a surrogate pathogenic bacterium, Escherichia coliT, in Pseudomonas aeruginosa biofilms with varying EPS excreting capacities. The quantities of detained E. coliT in biofilms and the biofilm structural characteristics were assessed using confocal laser scanning microscopy paired with image analysis. Meanwhile, the total interaction energy between E. coliT and the individual P. aeruginosa biofilms was computed using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The results indicated the heterogeneity of biofilm, such as surface roughness, aided the temporary retention of E. coliT in the biofilms.
Additionally, the presence of divalent ions in the solution not only greatly enhanced the pathogen retention, but also increased the heterogeneity of biofilm structure to a great extent. Interpretation of the obtained results using the DLVO theory further indicated the surface characteristics of the biofilm induced by EPS production influenced the magnitude of the energy barrier that must be overcome for initial pathogen attachment. The DLVO analysis also showed the increased divalent ion concentration had an even greater impact on energy barrier reduction to facilitate pathogen retention.