A small cadmium resistance plasmid isolated from Staphylococcus aureus

A cadmium resistance plasmid, pXU5, in Staphylococcus aureus, strain ATCC25923

A 25.9-kb plasmid, pXU5, encoding high level cadmium resistance was isolated from Staphylococcus aureus strain ATCC25923. A labelled cadA probe from plasmid pI258 hybridised to a 2.3-kb EcoRI fragment of pXU5. pXU5 was incompatible with an S. aureus incompatibility group 1 plasmid.

Plasmid-borne cadmium resistance genes in Listeria monocytogenes are similar to cadA and cadC of Staphylococcus aureus and are induced by cadmium

pLm74 is the smallest known plasmid in Listeria monocytogenes. It confers resistance to the toxic divalent cation cadmium. It contains a 3.1-kb EcoRI fragment which hybridizes with the cadAC genes of plasmid pI258 of Staphylococcus aureus. When introduced into cadmium-sensitive L. monocytogenes or Bacillus subtilis strains, this fragment conferred cadmium resistance. The DNA sequence of the 3.1-kb EcoRI fragment contains two open reading frames, cadA and cadC. The deduced amino acid sequences are similar to those of the cad operon of plasmid pI258 of S. aureus, known to prevent accumulation of Cd2+ in the bacteria by an ATPase efflux mechanism. The cadmium resistance determinant of L. monocytogenes does not confer zinc resistance, in contrast to the cadAC determinant of S. aureus, suggesting that the two resistance mechanisms are slightly different. Slot blot DNA-RNA hybridization analysis showed cadmium-inducible synthesis of L. monocytogenes cadAC RNA.

Use of DNA probes and plasmid capture in a search for new interesting environmental genes

Adaptation to a stressed environment leads to organisms bearing DNA, encoding defense mechanisms. These mechanisms can be heavy metal resistance, catabolism of organic xenobiotics or stress reactions. Genes responsible for these mechanisms can be used for monitoring changing environments and therefore it can be important to store such bacteria in a bank. DNA-probing will be presented by the use of DNA fragments (of Alcaligenes eutrophus) coding for heavy metal resistance or xenobiotic degradation. Some strains do not grow on petri dishes and accordingly cannot be isolated from soils. In order to isolate plasmids from such strains, coding for heavy metal resistances or xenobiotic degradations, an exogenous plasmid isolation method was developed. In this method, the endogenous population is conjugated with Pseudomonas or Alcaligenes strains bearing a retrotransfer plasmid like RP4. In that way new plasmids from various sources including non-culturable strains could be obtained. With these methods, a large number of specimens adapted to stressed situations can be isolated or constructed (in the case of the exogenous plasmid isolation method). They form a source of interesting genetic material that can be used to restore polluted areas in natural areas, if necessary with the aid of genetic engineering (in vitro or in vivo techniques). Full knowledge of such bacteria and their resistance mechanisms or degradation pathways, can lead to new constructions able to attack recalcitrant mixtures of different organics and to resist heavy metals.

Resistance to cadmium, cobalt, zinc, and nickel in microbes

The divalent cations of cobalt, zinc, and nickel are essential nutrients for bacteria, required as trace elements at nanomolar concentrations. However, at micro- or millimolar concentrations, Co2+, Zn2+, and Ni2+ (and "bad ions" without nutritional roles such as Cd2+) are toxic. These cations are transported into the cell by constitutively expressed divalent cation uptake systems of broad specificity, i.e., basically Mg2+ transport systems. Therefore, in case of a heavy metal stress, uptake of the toxic ions cannot be reduced by a simple down-regulation of the transport activity. As a response to the resulting metal toxicity, metal resistance determinants evolved which are mostly plasmid-encoded in bacteria. In contrast to that of the cation Hg2+, chemical reduction of Co2+, Zn2+, Ni2+, and Cd2+ by the cell is not possible or sensible. Therefore, other than mutations limiting the ion range of the uptake system, only two basic mechanisms of resistance to these ions are possible (and were developed by evolution): intracellular complexation of the toxic metal ion is mainly used in eucaryotes; the cadmium-binding components are phytochelatins in plant and yeast cells and metallothioneins in animals, plants, and yeasts. In contrast, reduced accumulation based on an active efflux of the cation is the primary mechanism developed in procaryotes and perhaps in Saccharomyces cerevisiae. All bacterial cation efflux systems characterized to date are plasmid-encoded and inducible but differ in energy-coupling and in the number and types of proteins involved in metal transport and in regulation. In the gram-positive multiple-metal-resistant bacterium Staphylococcus aureus, Cd2+ (and probably Zn2+) efflux is catalyzed by the membrane-bound CadA protein, a P-type ATPase. However, a second protein (CadC) is required for full resistance and a third one (CadR) is hypothesized for regulation of the resistance determinant. The czc determinant from the gram-negative multiple-metal-resistant bacterium Alcaligenes eutrophus encodes proteins required for Co2+, Zn2+, and Cd2+ efflux (CzcA, CzcB, and CzcC) and regulation of the czc determinant (CzcD). In the current working model CzcA works as a cation-proton antiporter, CzcB as a cation-binding subunit, and CzcC as a modifier protein required to change the substrate specificity of the system from Zn2+ only to Co2+, Zn2+, and Cd2+.

Cloning of pMOL28-encoded nickel resistance genes and expression of the genes in Alcaligenes eutrophus and Pseudomonas spp

The 163-kilobase-pair (kb) plasmid pMOL28, which determines inducible resistance to nickel, cobalt, chromate, and mercury salts in its native host Alcaligenes eutrophus CH34, was transferred to a derivative of A. eutrophus H16 and subjected to cloning procedures. After Tn5 transposon mutagenesis, restriction endonuclease analysis, and DNA-DNA hybridization, two DNA fragments, a 9.5-kb KpnI fragment and a 13.5-kb HindIII fragment (HKI), were isolated. HKI contained EK1, the KpnI fragment, as a subfragment flanked on both sides by short regions. Both fragments were ligated into the suicide vector pSUP202, the broad-host-range vector pVK101, and pUC19. Both fragments restored a nickel-sensitive Tn5 mutant to full nickel and cobalt resistance. The hybrid plasmid pVK101::HKI expressed full nickel resistance in all nickel-sensitive derivatives, either pMOL28-deficient or -defective, of the native host CH34. The hybrid plasmid pVK101::HKI also conferred nickel and cobalt resistance to A. eutrophus strains H16 and JMP222, Alcaligenes hydrogenophilus, Pseudomonas putida, and Pseudomonas oleovorans, but to a lower level of resistance. In all transconjugants the metal resistances coded by pVK101::HKI were expressed constitutively rather than inducibly. The hybrid plasmid metal resistance was not expressed in Escherichia coli. DNA sequences responsible for nickel resistance in newly isolated strains showed homology to the cloned pMOL28-encoded nickel and cobalt resistance determinant.

Genetic and physical analysis of plasmid genes expressing inducible resistance of tellurite in Escherichia coli

A large (greater than 250 kb) conjugative plasmid, pMER610, specifying resistance to tellurium and mercury was isolated from an Alcaligenes strain and transferred by conjugation to Escherichia coli AB1157. The acquisition of pMER610 by AB1157 increased the resistance to both telurite and tellurate by 100-fold. Expression of tellurite resistance by pMER610 and the cloned Ter determinant was inducible by prior exposure to tellurite at levels sub-toxic to the sensitive AB1157. Physical analysis of the cloned Ter fragment located the resistance determinant to a 3.55 kb region. Insertion of Tn 1000 (gamma delta) into this region produced two classes of sensitive mutations, fully sensitive and intermediate or hyposensitive, which map in adjacent regions and form two complementation groups. Maxicell analysis identified four polypeptides (15.5, 22, 23 and 41 kDa) expressed by the Ter clone. The 23 kDa polypeptide may not be required for resistance since tellurium-sensitive gamma delta insertion mutations were not detected in the 23 kDa coding region.

Antimicrobial resistance of Staphylococcus aureus: genetic basis

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Molecular cloning of copper resistance genes from Pseudomonas syringae pv. tomato

A cosmid library of copper-resistant (Cur) Pseudomonas syringae pv. tomato PT23 plasmid DNA was constructed and mobilized into the copper-sensitive recipient P. syringae pv. syringae PS61. One resultant cosmid clone, pCOP1 (46 kilobases), conferred copper resistance. The PT23 Cur gene(s) was located on pCOP1 by subcloning PstI restriction endonuclease fragments of pCOP1 in the broad-host-range vector pRK404. A subclone containing a 4.4-kilobase PstI fragment conferred Cur on PS61. The Cur gene(s) was further located by insertional inactivation with Tn5. A subcloned fragment internal to the Cur determinant on pCOP2 was probed to plasmid and chromosomal DNA of four copper-resistant and three copper-sensitive strains of P. syringae pv. tomato. The probe hybridized to plasmids in resistant strains, but showed no detectable homology to copper-sensitive strains.

Molecular cloning and analysis of Staphylococcus aureus chromosomal aminoglycoside resistance genes

Most of the aminoglycoside resistant Staphylococcus aureus strains isolated in France are resistant to all the antibiotics belonging to this family. Two aminoglycoside-modifying enzymes were detected in the wild-type strains studied: an APH3'III and an AAC6'-APH2". These strains also carry two types of streptomycin resistance: high-level resistance due to chromosomal mutation(s) affecting ribosome affinity and low-level resistance, the mechanism of which was not characterized. All the aminoglycoside resistance genes were located on the chromosome. DNA fragments of 1.5 and 1.95 kb carrying the aphA and aacA genes, respectively, were isolated, by cloning, from the cellular DNA of a clinical isolate. When these genes were introduced into Escherichia coli and Bacillus subtilis strains, the enzymes synthesized were indistinguishable from those produced by the S. aureus strains. When the cellular DNAs of wild-type and resistant strains were hybridized with the cloned fragments, sequences homologous to the fragment carrying the aphA gene were found to be located at the same chromosomal site, while those hybridizing with the fragment carrying the aacA gene were at different chromosomal sites.

Aminocyclitol resistance in Staphylococcus aureus: presence of plasmids and aminocyclitol-modifying enzymes

Aminocyclitol resistance in Staphylococcus aureus has been investigated by the analysis of the plasmids and aminocyclitol-modifying enzymes present in several clinical staphylococcal isolates. All of the strains tested were resistant to a broad range of aminocyclitols and contained large plasmids which encoded a variety of aminocyclitol-modifying enzymes in addition to other antibiotic resistances. All strains expressed multiple aminocyclitol-modifying enzymes. The plasmids present in these strains appear to be related by virtue of their similar restriction endonuclease digestion patterns. The plasmids are related and differ by the gain or loss of small DNA segments, one of which encodes erythromycin and kanamycin resistance.