Plasmid-based resistance

We have reviewed the best-known prokaryotic plasmid-based resistance systems from the view of metal ion homeostasis. Plasmid-based systems are, with few exceptions, inducible and are regulated by activators (mercury and copper) or repressors (cadmium) in different cases. The metal-binding motifs used by these systems are frequently localized (e.g., Cys-Xaa-Xaa-Cys, Glu-His-His, and His-Xaa-His), but occasionally coordinating cysteine residues on different subunits interact (as in the MerR regulatory protein and mercuric reductase enzyme). 

Moreover, the use of resistant strains is of interest. Slama et al. [21] described a heavy metal resistant BOD-sensor using Alcaligenes eufrop/jus, which contains plasmids encoding resistance to nickel, copper, cadmium, and zinc. An arsenic resistant BOD-sensor based on Pseudomonas putida was developed by Ohki et al. [35]. Another interesting possibility is the elimination of heavy metals by covering the BOD-sensor with a poly(4-vinylpyridine)-coated polycarbonate membrane [36] or polyfsodium styrene sulfonate) [62]. 

Antibiotic Resistance. Figure 1 According to Bush, Jacoby and Medeiros [2] four molecular classes of (3-lactamases can be discriminated based upon biochemical and molecular features. Classes 1, 2, and 4 included serine-proteases, while metallo enzymes are included in class 3. The substrate spectrum varies between different subclasses and the corresponding genes can be part of an R-plasmid leading to a wider distribution or are encoded chromosomally in cells of specific species. 

For 38 outbreaks of confirmed mode or source (Table II), multiply resistant salmonella were involved in 33% of the community based outbreaks, 40% of the nosocomial, and 75% of the outbreaks involving both community and hospital. In addition, it has been estimated that each culture-documented case among human beings may represent as many as 100 undocumented cases (10). Agency participation for these cases was made at the request of the local health officials and therefore the survey was not random. However, the resistant strains displayed a fatality rate 21 times that of antibiotic sensitive strains. O Brien et al. (10) found that characterization of plasmid DNA from antibiotic resistant salmonella suggests extensive commingling of human and animal bacteria. 

Plasmids can be introduced into bacteria after the bacteria is treated so that cells are temporarily permeable to small DNA molecules. This process is known as transformation. The bacteria that have been successfully transformed can be selected based on the new phenotype they received from the plasmid, such as resistance to antibiotics. 

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