Understanding cellular responses to toxic agents: a model for mechanism-choice in bacterial metal resistance

Abstract

Bacterial resistances to metals are heterogeneous in both their genetic and biochemical bases. Metal resistance may be chromosomally-, plasmid- or transposon-encoded, and one or more genes may be involved: at the biochemical level at least six different mechanisms are responsible for resistance. Various types of resistance mechanisms can occur singly or in combination and for a particular metal different mechanisms of resistance can occur in the same species. To understand better the diverse responses of bacteria to metal ion challenge we have constructed a qualitative model for the selection of metal resistance in bacteria. How a bacterium becomes resistant to a particular metal depends on the number and location of cellular components sensitive to the specific metal ion. Other important selective factors include the nature of the uptake systems for the metal, the role and interactions of the metal in the normal metabolism of the cell and the availability of plasmid (or transposon) encoded resistance mechanisms. The selection model presented is based on the interaction of these factors and allows predictions to be made about the evolution of metal resistance in bacterial populations. It also allows prediction of the genetic basis and of mechanisms of resistance which are in substantial agreement with those in well-documented populations. The interaction of, and selection for resistance to, toxic substances in addition to metals, such as antibiotics and toxic analogues, involve similar principles to those concerning metals. Potentially, models for selection of resistance to any substance can be derived using this approach.

Practiced Gram negative bacteria from dyeing industry effluents snub metal toxicity to survive

“…UOMGNS616 failed to grow at these concentrations when growth determined by measuring turbidity. It is possible that bacteria acquire resistance against metals by preventing the access of metals to sensitive cellular components or by altering them to reduce the sensitivity [28]. In the present investigation mucoid phenotype was noticed in C. freundii UOMGNS516 and Achromobacter sp.…”

Novel Nickel Resistance Genes from the Rhizosphere Metagenome of Plants Adapted to Acid Mine Drainage

“…In general, six metal resistance mechanisms are postulated: exclusion by permeability barrier, intra-and extracellular sequestration, active transport efflux pumps, enzymatic detoxification, and reduction in the sensitivity of cellular targets to metal ions (43,47). However, most nickel resistance determinants known to date are efflux pumps isolated from cultivable bacteria, and new nickel resistance systems are still being discovered in E. coli (42), Pseudomonas putida S4 (54), Enterobacter spp.…”

Co-contamination of water with chlorinated hydrocarbons and heavy metals: challenges and current bioremediation strategies

“…In an endeavor to safeguard susceptible cellular components, a cell can build up resistance to metals. Metal containing environments have built up selective pressures which eventually lead to the expression of resistance mechanisms to practically all heavy metals (Rouch et al 1995). Numerous mechanisms by which microorganisms resist metal toxicity exist, and five mechanisms have been hypothesized (Bruins et al 2000).…”

“…Examples of such alterations include; prevention of entry of heavy metals by diminishing production of membrane channel proteins (Rouch et al 1995), possible saturation of metal-binding sites in the membrane and periplasm with non-toxic metals (Mergeay 1991), prevention of metals from reaching the surface of the cell by the formation and binding of an extracellular polysaccharide coat (Scott and Palmer 1990). Gram-negative bacteria possess a cell wall that is a more effective barrier to heavy metals than Gram-positive bacteria, as demonstrated by Balestrazzi et al (2009) that tetra-resistant bacterial isolates were more frequent among the Gram-negative bacteria belonging to the genus Pseudomonas.…”

“…Metals generally sequestered are Cd(II), Cu(II), and Zn(II). The two most widespread molecules used for intracellular sequestration are metallothioneins and cysteine-rich proteins (Rouch et al 1995;Silver and Phung 1996). Alcaligenes eutrophus CH34 strains internally sequester metals by forming carbonate precipitates within the cell or extra sequestration outside the cell where heavy metals bind to outer cell membrane proteins (Mahvi and Diels 2004).…”