Bacteriovory of enteric bacteria by soil isolated amoeba depends on both temperature and salinity

Recycling of wastewater for agricultural irrigation is an important means for sustainable reuse practices, but a major concern of such practice is the introduction of pathogens and antibiotic determinants. Previous attempts to mitigate contaminants focused on water treatment, yet the growing appreciation of the soil microbial food web brings into focus its potential to mitigate pathogens introduced by recycled effluent. We hypothesized that effluent-introduced bacteria serve as prey to soil-free living amoeba (FLA), but predation is interfered with by environmental conditions such as temperature and/or salinity. To test our hypothesis, we first demonstrated that an enteric bacterium model, Escherichia coli, is preyed on by soil FLA. We then isolated 12 FLA from three soil types (loamy-sand, loam, and clay) and followed the predation dynamics in three prey types: E. coli, Serratia marsescens, and Enterococcus mundtii. Predation was simulated in an unstructured environment at various temperatures (20, 25, 30 °C), mimicking changing climatic conditions and salinities (2, 10, 30 mM NaCl), mimicking the relative ionic strength and conductivity applied by irrigation effluent. We found that the predation efficiency of the amoeba strains increased significantly increased (p < 0.01) at both low temperature (20 °C) and high salinity (30 mM NaCl) regardless of the types of prey. We suggest that increased salinity applied by effluent irrigation may benefit micropredators, while high temperatures may increase resistance to predation. Our results demonstrate the importance of understanding the effects of environmental conditions on soil trophic interactions and their potential benefits for agricultural practices.