CADMIUM-SPECIFIC FORMATION OF METAL SULFIDE Q-PARTICLES BY KLEBSIELLA-PNEUMONIAE

Klebsiella pneumoniae overcomes cadmium toxicity through the 'biotrans formation' of cadmium ions into photoactive, nanometre-sized CdS parti cles deposited on the cell surface. The kinetics of particle formation during batch culture growth was monitored by electron microscopy (EM) , energy-dispersive X-ray analysis and electronic absorption spectrosc opy (EAS). During the deceleration phase of bacterial growth, the pres ence of CdS particles on the outer cell wall of K. pneumoniae (greater than or equal to 5 nm in diameter) was detected by EM. The size of th ese electron-dense particles continued to increase throughout the stat ionary phase of growth, with some of the particles reaching a diameter >200 nm. The formation of the extracellular CdS particles contributed to around 3-4% of the total cell biomass. EAS undertaken on these ext racellular 'bio-CdS' particles suggested that the large 'superparticle s' observed by EM, e.g. 200 nm, were aggregates of smaller particles t ermed 'Q-particles', similar to 4 nm in diameter. Metal sulfide partic les were not formed in batch cultures of K. pneumoniae grown in the pr esence of lead, zinc, mercury, copper or silver ions. Growth in the pr esence of lead ions resulted in the formation of extracellular electro n-dense particles containing lead but not sulfide or phosphate. Intrac ellular phosphorus-containing electron-opaque particles were formed du ring growth in the presence of copper and mercury. Intracellular elect ron-dense particles were formed in the presence of zinc ions but these did not contain phosphorus. From these results it was thought that me tal sulfide formation in K. pneumoniae showed some cadmium-specificity . When cadmium and zinc ions were both added to the growth medium, met al sulfide particles were formed that were predominantly composed of c admium, e.g. 48.6% cadmium and 0.04% zinc. Similarly, when cadmium and lead ions were both present during growth only CdS particles formed. in both cases analysis of the cells by EAS confirmed the presence of C dS only. These observations suggest that the mechanism of CdS formatio n is unlikely to occur simply through a cadmium-induced release of hyd rogen sulfide by the cells into the external environment. If hydrogen sulfide production was the mechanism of sulfide formation then metal s ulfide particles containing lead and zinc ions in addition to cadmium ions should have been produced.