Heavy metal-resistant bacteria from Laguna de Bay [Philippines]

Earlier reports of deterioration of heavy metal contamination of Laguna de Bay, the largest fresh-water lake in the Philippines, led to the isolation, characterization and identification of some heavy metal-resistant bacteria from the lake. Twenty-three bacteria isolates exhibiting varying resistances to heavy metals of environmental concern (Pb, Hg, Cd, Cr, Cu, Ni and Zn) were isolated from the sediments of Laguna de Bay. They were grouped under eight genera namely; Bacillus, Enterobacter, Citrobacter, Klebsiella, Shigella, Pseudomonas, Escherichia and Yersinia. Most belong to the family Enterobacteriaceae and are indicative of the polluted state of the lake although LLDA [Laguna Lake Development Authority] reported that the water quality parameters of the lake for 1996 were, in general, within the Class "C" criteria. The bacterial isolates exhibited relatively high levels of resistance to the selected metals viz., Hg, 1 mM; Cu 10 mM; Ni, 20 mM; Cd, 50 mM; and Zn, 100 mM. These isolates could be useful biomonitors of heavy metal pollution in the lake. The resistance markers for nickel and cadmium could also serve as useful markers in ecological studies. Metal uptake values for Hg and Pb are considerably higher for metabolically active cells as opposed to the other metals (Cd, Cr, Cu, Ni, and Zn) where the heat-killed cells gave higher uptake values. This confirms the report (Nies, 1992) that reduced accumulation based on an active efflux of the cation is the primary mechanism developed in prokaryotes. However, even dead cells show metal uptake because they contain a number of aniomic ligand sites and can still participate in binding the metals. The uptake process is very fast, occurring within the first 15 minutes of contact and is very similar in ion-exchange. Thus, both types of metal binding were observed, i.e., 1) metabolically-mediated, and 2) passive biosoption, possibly, to ensure the cell's survival in a toxic environment. Localization studies using XRMA techniques reveal that not much metal is actually taken up intracellularly, but rather is accumulated in the cell surface only. Further studies on the basic biochemical and genetic basis for microbial resistance to heavy metals are essential to fully understand the interactions of microbes with heavy metals in order to follow the pathways and transformations of heavy metals in the environment and to be able to design interventions that will somehow reduce pollution