The Mechanism of Plasmid Curing in Bacteria

Abstract

Bacterial plasmids have a major impact on metabolic function. Lactose fermentation of E. coli or hemolysin B transporter expressed by the plasmids that carry these respective genes could be readily obviated by heterocyclic compounds that readily bind to plasmid DNA. These compounds could also reverse the resistance to antibiotics of E. coli, Enterobacter, Proteus, Staphylococcus and Yersinia strains by eliminating plasmids. However, the frequency and extent of this effect was significantly less than might have been expected based on a complex interaction with plasmid DNA. The effects of heterocyclic compounds on the plasmids responsible for the virulence of Yersinia and A. tumefaciens, or on nodulation, nitrogen fixation of Rhizobia accounted for the elimination of 0.1 to 1.0 % of plasmids present in the populations studied. Bacterial plasmids can be eliminated from bacterial species grown as pure or mixed bacterial cultures in the presence of sub-inhibitory concentrations of non-mutagenic heterocyclic compounds. The antiplasmid action of the compounds depends on the chemical structure of amphiphillic compounds having a planar ring system with substitution in the L-molecular region. A symmetrical pi-electron conjugation at the highest occupied molecular orbitals favours the antiplasmid effect. The antiplasmid effect of heterocyclic compounds is expressed differentially in accordance with the structural form of the DNA to which they bind. In this manner "extrachromosomal" plasmid DNA that exists in a superhelical state binds more compound than its linear or open-circular form; and least to the chromosomal DNA of the bacterium, that carries the plasmid. It can also be noted that these compounds are not mutagenic and their antiplasmid effects correlate with the energy of HOMO-orbitals. Plasmid elimination is considered also to take place in ecosystems containing numerous bacterial species. This opens up a new perspective in rational drug design against bacterial plasmids. The inhibition of conjugational transfer of antibiotic resistance plasmid can be exploited to reduce the spread of antibiotic resistance plasmid in the ecosystem. Inhibition of plasmid replication at various stages, as shown in the "rolling circle" model (replication, partition, conjugal transfer) may also be the theoretical basis for the elimination of bacterial virulence in the case of plasmid mediated pathogenicity and antibiotic resistance. The large number of compounds tested for antiplasmid effects provides opportunities for QSAR studies in order to find a correlation between the antiplasmid effect and the supramolecular chemistry of these plasmid curing compounds. Plasmid elimination in vitro provides a method of isolating plasmid free bacteria for biotechnology without any risk of inducing mutations.

Disrupting antibiotic resistance propagation by inhibiting the conjugative DNA relaxase

“…The compounds identified could be used alone or in combination with existing antibiotics to treat recalcitrant bacterial infections. Other antibiotics and natural extracts have been reported to disrupt conjugative DNA transfer and the presence of plasmids within actively dividing bacterial cells (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). In each case, however, the macromolecular target of those compounds was not understood and their mechanism of action has not been determined.…”

“…Small-molecule inhibition of conjugation could prove to be a powerful method for curbing the generation and spread of multidrug-resistant strains. Past studies suggested that various antibiotics, polycyclic chemicals, and crude extracts inhibit conjugation at concentrations less than the antibacterial minimum inhibitory concentration (5-11); however, most of these effects have been attributed to nonconjugation-specific inhibition of bacterial growth or DNA synthesis (12)(13)(14)(15). This study describes a bottom-up approach used to identify the first small molecule inhibitors of conjugative DNA transfer that target an enzyme of the conjugative system.…”

Vancomycin-Resistant Lactococcus lactis 1A-1 Isolated from a Competitive Exclusion Product Transfers Vancomycin Resistance Genes to Staphylococcus aureus

“…To determine if the resistance genes were carried by a plasmid, L. lactis 1A-1 was grown in MRS broth for 15 days in the presence of 10 g/ml acridine orange [12]. Following repeated exposure to the curing agent, the bacteria were diluted in MRS broth and spread on MRS agar plates.…”

Anti Biofilm and Anti Plasmid Activites of Syzygium aromaticum and Kaempheria galanga against Pseudomonas aeruginosa

“…Agarose gel electrophoresis (Fig No:3) has shown relative decrease in plasmids of S aromaticum and K galanga treated extracts in comparison with sodium hypochlorite. Most of the efficient curing agents such as acridine orange, ethidium bromide or sodium dodecyl sulphate are toxic to mammals in one form or another which limits their therapeutic or industrial applications (Spengler et al, 2006). The plasmid curing potential of phytochemical is in accordance with the efficiency of heterocyclic compounds to bind differentially with different structures of DNA.…”