Self-immobilization of Recombinant Caulobacter crescentus and Its Application in Removal of Cadmium from Water

Abstract

Microbial biofilms can be valuable in many biotechnological applications, including bioremediation. We have previously constructed a recombinant strain of Caulobacter crescentus JS4022/p723-6H by inserting a hexahistidine peptide to a permissive site of the host surface layer (S-layer) protein RsaA. This engineered strain is highly effective in removal of cadmium from water as free cells. In this communication, we examined the biofilms formed by self-immobilized JS4022/p723-6H and evaluated their ability to retrieve cadmium from contaminated water samples. According to light and electron microscopic observations, JS4022/p723-6H cells developed a uniform monolayer biofilm on borosilicate surfaces through their intrinsic appendage, a stalk with an adhesive holdfast. The density of the biofilms reached a maximum after 48 h of incubation and was not affected by exposure to at least 1 ppm cadmium. When 0.4 ppm Cd(II) was added to the growth medium, immobilized JS4022/p723-6H removed 76.9% of the total metal, whereas the control strain only removed 13.5%. When a water sample collected from Lake Erie was spiked with various amounts of CdCl2, immobilized JS4022/p723-6H was able to sequester 44 approximately 51% of the total metal, compared to 37 approximately 42% accumulated by the control strain. By combining two powerful techniques, cell surface display and self-immobilization, we achieved complete separation of dissolved heavy metals from contaminated water in a single step. This study laid down the foundation to cost-effectively construct large-scale bioreactors with high efficiency and specificity to retrieve environmental contaminants from water.

Bioremediation of soluble heavy metals with recombinantCaulobacter crescentus

“…Because of the unique monolayer feature of the biofilms formed by Caulobacter strains, we previously predicted that immobilized JS4022/p723-6H cells would function equally well as free cells. 10 Similar levels of removal were also observed at pH 4.2 (Fig. 2).…”

“…The levels of removal agree precisely to our previous study using biofilms of the strains, where JS4022/p723-6H sequestered 44∼51% of the total spiked cadmium and the control strain retained 37∼42%. 10 This provides direct evidence that self-immobilization of Caulobacter cells to solid surfaces does not interfere with their bioadsorptive function. Because of the unique monolayer feature of the biofilms formed by Caulobacter strains, we previously predicted that immobilized JS4022/p723-6H cells would function equally well as free cells.…”

“…2) or as immobilized biofilms. 10 Its tolerance to high acidity offers an of JS4022/p723-6H. Although we have demonstrated that biofilms of JS4022/ p723-6H formed on borosilicate surfaces are highly competent for sequestration of Cd(II) from water, 10 glass is actually a poor support surface for Caulobacter spp.…”

“…10 Its tolerance to high acidity offers an of JS4022/p723-6H. Although we have demonstrated that biofilms of JS4022/ p723-6H formed on borosilicate surfaces are highly competent for sequestration of Cd(II) from water, 10 glass is actually a poor support surface for Caulobacter spp. Dr. John Smit's group at the University of British Columbia has identified materials that could hold up to nine times as many Caulobacter cells as glass surfaces within an hour (personal communication).…”

“…The biofilms are equally effective at retrieval of cadmium ions from bacterial growth medium as well as water samples collected from Lake Erie. 10 The utilization of the two practical techniques, cell surface display and self-immobilization, has allowed complete separation of heavy metal ions from contaminated water in a single step without the need of a centrifuge or column. This essay supplies additional data and comments regarding a few aspects that are important to the deployment of this promising technology.…”