Bioremediation of Heavy Metal-Contaminated Soils Using Engineered Bacterial Consortia

Abstract

Soil contamination by heavy metals -- principally cadmium, lead, chromium, arsenic and mercury -- persists at
agriculturally and ecologically significant levels across large areas affected by historical mining, smelting and
industrial discharge, and conventional physicochemical remediation (excavation, soil washing, chemical
immobilisation) is frequently too costly or too disruptive for large-area or agricultural application. Microbial
bioremediation using rationally engineered, multi-strain bacterial consortia has emerged as a potentially scalable
alternative, combining complementary functions -- metal sequestration, biofilm-mediated soil retention, and
plant-growth promotion for combined phytoremediation -- that a single engineered strain typically cannot provide
alone. We conducted a systematic review of studies published between January 2010 and December 2022
reporting quantitative heavy-metal removal or immobilisation efficiency achieved by an engineered (genetically
modified or rationally assembled non-native) bacterial consortium in soil or soil-slurry systems, screening 486
records and including 58 studies in the final synthesis. Median reported removal or immobilisation efficiency
across all included studies was 68.4% (interquartile range 52.1-81.7%) over treatment durations ranging from 14
to 180 days, with substantial variation by both target metal and consortium design strategy. Consortia combining a
metal-resistance-gene-overexpressing strain (most often carrying merA for mercury or czc-family efflux systems
for cadmium, zinc and cobalt) with a biofilm-forming support strain achieved the highest median efficiency
(76.8%), significantly outperforming single-engineered-strain treatments (54.2%, P<0.001) and consortia lacking a
dedicated biofilm or soil-retention component (58.9%, P=0.003). Mercury and cadmium showed the highest
median removal efficiencies among the five metals reviewed, while chromium and arsenic showed the lowest and
the widest variability, plausibly reflecting the more complex, redox-state-dependent bioavailability of these two
metals. We identify consortium design principles associated with higher reported efficiency, highlight substantial
inconsistency in how studies report and standardise removal efficiency that complicates direct cross-study
comparison, and outline a set of reporting recommendations intended to make future engineered-consortium
bioremediation literature more comparable and more directly translatable to field application.