ENERGY-DISPERSIVE X-RAY-ANALYSIS OF THE EXTRACELLULAR CADMIUM-SULFIDECRYSTALLITES OF KLEBSIELLA-AEROGENES

Klebsiella aerogenes forms electron-dense particles on the cell surfac e in response to the presence of cadmium ions in the growth medium. Th ese particles ranged from 20 to 200 nm in size, and quantitative energ y dispersive X-ray analysis established that they comprise cadmium and sulfur in a 1:1 ratio. This observation leads to the conclusion that the particles are cadmium sulfide crystallites. A combination of atomi c absorption spectroscopy, inductively coupled plasma mass spectrometr y, and acid-labile sulfide analysis revealed that the total intracellu lar and bound extracellular cadmium:sulfur ratio is also 1:1, which su ggests that the bulk of the cadmium is fixed as extracellular cadmium sulfide. The tolerance of K. aerogenes to cadmium ions and the formati on of the cadmium sulfide crystallites were dependent on the buffer co mposition of the growth medium. The addition of cadmium ions to phosph ate-buffered media resulted in cadmium phosphate precipitates that rem ove the potentially toxic cadmium ions from the growth medium. Electro n-dense particles formed on the surfaces of bacteria grown under these conditions were a combination of cadmium sulfide and cadmium phosphat es. The specific bacterial growth rate in the exponential phase of bat ch cultures was not affected by up to 2mM cadmium in Tricine-buffered medium, but formation of cadmium sulfide crystallites was maximal duri ng the stationary phase of batch culture. Cadmium tolerance was much l ower (10 to 150 mu M) in growth media buffered with Tris, Bistris prop ane, Bes, Tes, or Hepes. These results illustrate the importance of co nsidering medium composition when comparing levels of bacterial cadmiu m tolerance.