Now that recombinant microbes are making their mark indrugs and foods, suggested bacterial geneticist Neil H.Mendelson, it's time to assign them industrial chores.
He has put a genetically engineered Bacillus subtilis strain towork crystallizing metals on its cell wall matrix. Last Friday'sissue of Science reported this teaching an old bug new tricksunder the title "Production and Initial Characterization ofBionites: Materials Formed on a Bacterial Backbone."
Mendelson, who teaches molecular and cellular biology at theUniversity of Arizona, coined the word "bionite" to describe, ashe explained to BioWorld, "taking a biological structure, namelybacterial cell-wall polymers, and using them as a template orscaffold on which to grow something inorganic."
Peptidoglycans, the cell's strength-bearing polymers, are rigid,insoluble polysaccharides that form the gel-like armor platingwrapped around the B. subtilis cell, which is some four micronslong by 0.7 thick.
Mendelson's recombinant B. subtilis strain embodies the E. colilacZ gene, and so synthesizes the beta-galactosidase enzyme.After culturing these cells in the form of a web in petri dishesfor 18 hours, he seeded them with small quantities of solublecalcium, copper and iron salts. After these metals hadincubated for an hour or more, they precipitated on the cottonymicrobial webs of cell-surface polymer. Then Mendelsondipped a sterilized hook into the petri dish and lifted out thestart of a continuous mineralized fiber, pulled by a rate-regulated motor at five millimeters per minute. "The way nylonwas drawn out of its polymer melt when nylon was firstdiscovered," was how he described the process.
Hung out to dry, the bacterial cable, which consists of some7,000 to 20,000 untwisted filaments, forms a brittle bone-likerod of metal salts, or bionites, which may be 10 to 15centimeteres long by two millimeters in diameter. What arebionites good for?
Mendelson, who calls himself an "amateur" at industrial usesfor a bacteriological process, nevertheless proposes severalpotential pre-application indications:
-- Burning out the organic phase of iron bionite, or"febactonite," yields a black magnetic material, presumablymagnetite, which is used to coat magnetic recording tape.
-- The calcium "calbactonite" when rehydrated, retains thebeta-gal's genetically engineered enzyme activity, suggestingits use in cell-enzyme immobilization. Also, as it carries calciumcarbonate, it might find a role in bone regeneration, although"whether a hydroxylapatite (bone substance) can be maderemains to be determined."
-- For bioremediation, use bacterial web forms as bindingsubstrates for removing metals, by drawing the products infiber, rather than methods used now. Essentially, put them insolid form and get them out of solution.
Mendelson ascribes the ravenous appetite of B. subtilis formetal ions to the extreme electronegativity of its cell wall. "Cellwalls behave as complex ion exchangers," he explained. "Somebacterial walls have capacities equivalent to those ofcommercial resins, such as Dowex. The bacterium mustneutralize at least 97 percent of the charges in its wall to keepit from blowing up. So it carries positively charged counter-ions, notably metals, to neutralize the negative charges. Iron,copper, calcium or magnesium ions bind in high quantities.
Mendelson sees the man-mediated mineralization of metals ondrawn-out strands of bacterial wall polymer as "pointing theway for using biotechnology to meet goals in material science,of trying to bring the powers of molecular genetics to bear onthe production of materials in the same way these powerfultechniques now serve the chemical and pharmaceuticalindustries."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.