Biofilm Formation and Cell Invasion among Environmentally Persistent Escherichia coli Isolates from South Africa Watersheds

Introduction: The increasing scarcity of freshwater around the world, the concomitant loss of aquatic biological diversity, and the ongoing introduction of invasive species highlight the need for expanding investigations into the effects of altering aquatic habitats on water quality, food safety, and disease transmission. In recent years the use of Escherichia coli as in indicator organism has been challenged in the light of various reports that many strains of this bacterium have the ability to survive and proliferate in the external environment, outside of the gastrointestinal tract of the host. 

Purpose: This study seeks to clarify whether E. coli populations in South Africa aquatic environments are structured according to the habitat, and if so, whether these bacteria still have the ability to circulate through and impact human or animal hosts. 

Methods: Environmentally persistent E. coli strains (n = 122) were isolated from seven dams in the Gauteng region of South Africa by sampling water, sediment, and aquatic plant surface habitats. Phenotypic analyses of motility, growth kinetics, and biofilm formation were used to investigate bacterial capacity to survive and proliferate. Invasion and adherence assays were then performed in mammalian cell culture to measure their infection capacity in vitro.

Results: Among six E. coli populations classified as strong biofilm producers (3 fold increase capacity for biofilm formation over a reference strain DH5a), four were isolated from the surface of aquatic plants. Biofilm production strongly correlated with increased motility and reduced growth rates, suggesting that environmental transition from free living existence to biofilm lifestyle requires a unique growth mode specialized for long-term colonization of surfaces. E. coli strains were able to attach and invade cultured mammalian cells, thus having potential to invade into the host cell, induce an inflammatory response, and subsequently, cause disease. 

Significance: A greater understanding of E. coli biofilm processes in various aquatic habitats should lead to novel, effective control strategies for control of environmentally persistent biofilms and a resulting improvement in water management as well as human and ecosystem health.