Uptake and retention of metals by cell walls of Bacillus subtilis

Abstract

Isolated walls of B. subtilis, prepared to avoid metal contamination other than by the growth medium, were incubated in dilute metal solutions, separated by membrane filtration (0.22 .mu.m) and monitored by atomic absorption to give uptake data for 18 metals. Substantial amounts of Mg2+, Fe3+, Cu2+, Na+ and K+ (amounts which were often visible as electron scattering in thin sections), intermediate amounts of Mn2+, Zn2+, Ca2+, Au3+ and Ni2+ (the higher atomic-numbered elements also visible as electron scattering), and small amounts of Hg2+, Sr2+, Pb2+ and Ag+ were taken into the wall. Some (Li+, Ba2+, Co2+ and Al3+) were not absorbed. Most metals which had atomic numbers > 11 and which were detected by EM appeared to diffusely stain thin sections of the wall. Mg2+ partitioned into the central region, and these sections of walls resisted Ru red staining, which was not true for the other metals. Areas of the walls also acted as nucleation sites for the growth of microscopic elemental Au crystals when incubated in solutions of AuCl3. Retention or displacement of the metals was estimated by a chromatographic method using the walls cross-linked by the carbodiimide reaction to adipic hydrazide agarose beads (which did not take up metal but reduced the metal binding capacity of the walls by .apprx. 1%) packed in a column. When a series of 12 metal solutions was passed through the column, Mg2+, Ca2+, Fe3+ and Ni2+ were strongly bound to the walls and were detected by atomic absorption and by their electron-scattering power in thin sections, but other metals were displaced or replaced. Partial lysozyme digestion of the walls (causing a 28% loss of a [3H]diaminopimelic acid label) greatly diminished the Mg2+ retention but not that of Ca2+, Fe3+ or Ni2+, indicating that there are select sites for various cations.