Effects of the resistance plasmid R124 on the level of the OmpF outer membrane protein and on the response of Escherichia coli to environmental agents

Mechanisms of emerging resistance associated with non-antibiotic antimicrobial agents: a state-of-the-art review

Although the development of resistance by microorganisms to antimicrobial drugs has been recognized as a global public health concern, the contribution of various non-antibiotic antimicrobial agents to the development of antimicrobial resistance (AMR) remains largely neglected. The present review discusses various chemical substances and factors other than typical antibiotics, such as preservatives, disinfectants, biocides, heavy metals and improper chemical sterilization that contribute to the development of AMR. Furthermore, it encompasses the mechanisms like co-resistance and co-selection, horizontal gene transfer, changes in the composition and permeability of cell membrane, efflux pumps, transposons, biofilm formation and enzymatic degradation of antimicrobial chemicals which underlie the development of resistance to various non-antibiotic antimicrobial agents. In addition, the review addresses the resistance-associated changes that develops in microorganisms due to these agents, which ultimately contribute to the development of resistance to antibiotics. In order to prevent the indiscriminate use of chemical substances and create novel therapeutic agents to halt resistance development, a more holistic scientific approach might provide diversified views on crucial factors contributing to the persistence and spread of AMR. The review illustrates the common and less explored mechanisms contributing directly or indirectly to the development of AMR by non-antimicrobial agents that are commonly used.

Molecular basis of bacterial outer membrane permeability revisited

Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Potential impact of increased use of biocides in consumer products on prevalence of antibiotic resistance

There has recently been much controversy surrounding the increased use of antibacterial substances in a wide range of consumer products and the possibility that, as with antibiotics, indiscriminate use of biocides might contribute to the overall pattern of susceptibility in the general environment and in the clinic. Such speculation, based on the isolation of resistant mutants from in vitro monoculture experiments, is not reflected by an emergence of biocide-resistant strains in vivo. This review provides a broad coverage of the biocide and resistance literature and evaluates the potential risks, perceived from such laboratory monoculture experiments, against evidence gathered over 50 years of field studies. An explanation for the continued effectiveness of broad-spectrum biocidal agents against the decline in efficacy of therapeutic agents is provided based on the fitness costs of resistance and the ubiquity of naturally occurring substances that possess antibacterial effect. While we conclude from this review of the literature that the incorporation of antibacterial agents into a widening sphere of personal products has had little or no impact on the patterns of microbial susceptibility observed in the environment, the associated risks remain finite. The use of such products should therefore be associated with a clear demonstration of added value either to consumer health or to the product life. Hygienic products should therefore be targeted to applications for which the risks have been established.

Bacterial resistance to disinfectants: present knowledge and future problems

Bacterial resistance to antibiotics is a long-established, widely-studied problem. Increasingly, attention is being directed to the responses of various types of microbes to biocides (antiseptics, disinfectants and preservatives). Different groups of bacteria vary in their susceptibility to biocides, with bacterial spores being the most resistant, followed by mycobacteria, then Gram-negative organisms, with cocci generally being the most sensitive. There are wide divergencies within this general classification. Thus, (i) spores of Bacillus subtilis are less susceptible to biocides than those of Clostridium difficile: (ii) Mycobacterium chelonae strains may show high resistance to glutaraldehyde and M. avium intracellulare is generally less sensitive than M. tuberculosis; (iii) Gram-negative bacteria such as Pseudomonas aeruginosa, Providencia spp and Proteus spp may be difficult to inactivate; (iv) enterococci are less sensitive than staphylococci to biocides and antibiotic-resistant strains of Staphylococcus aureus might show low-level biocide resistance. The mechanisms involved in biocide resistance to biocides are becoming better understood. Intrinsic resistance (intrinsic insusceptibility) is found with bacterial spores, mycobacteria and Gram-negative bacteria. This resistance might, in some instances, be associated with constitutive degradative enzymes but in reality is more closely linked to cellular impermeability. The coats(s) and, to some extent, the cortex in spores, the arabinogalactan and possibly other components of the mycobacterial cell wall and the outer membrane of Gram-negative bacteria limit the concentration of active biocide that can reach the target site(s) in these bacterial cells. A special situation is found with bacteria present in biofilms, which can be considered as being an intrinsic resistance mechanism resulting from physiological (phenotypic) adaptation of cells. Acquired resistance to biocides may arise by cellular mutation or by the acquisition of genetic elements. Plasmid/transposon-mediated resistance to inorganic and organic mercury compounds by hydrolases and reductases has been extensively studied. Plasmid-mediated resistance to some other biocides in Gram-negative bacteria and in staphylococci has been described, but its significance remains uncertain. As to the future, there is a need to establish conclusively whether there is a clear-cut linkage between antibiotic and biocide resistance in non-sporulating bacteria and whether biocides can select for antibiotic resistance. Additionally, the responses to biocides of new and emerging pathogens must be assessed. At the same time, continuing research is necessary to establish further the underlying mechanisms of resistance and to provide more efficient means of bacterial inactivation.

Copper resistance mechanisms in bacteria and fungi

Copper is both an essential micronutrient and a toxic heavy metal for most living cells. The presence of high concentrations of cupric ions in the environment promotes the selection of microorganisms possessing genetic determinants for copper resistance. Several examples of chromosomal and plasmid copper-resistance systems in bacteria have been reported, and the mechanisms of resistance have started to be understood at the molecular level. Bacterial mechanisms of copper resistance are related to reduced copper transport, enhanced efflux of cupric ions, or copper complexation by cell components. Copper tolerance in fungi has also been ascribed to diverse mechanisms involving trapping of the metal by cell-wall components, altered uptake of copper, extracellular chelation or precipitation by secreted metabolites, and intracellular complexing by metallothioneins and phytochelatins; only the metallothionein chelation mechanism has been approached with molecular detail.

Restriction endonucleases R.EcoKI and R.EcoR124I are probably located in different environments within the bacterial cell

We describe the phenomenon of a transient state of R124I restriction deficiency after long-term storage of the E. coli[pCP1005] strain at 4 degrees C, or after growth of the culture in synthetic M9 medium with the nonmutagenic solvent dimethyl sulfoxide. The unusual high reversion from the R+ 124 to the R- 124 phenotype was observed only in E. coli strain transformed with the high-copy number plasmid pCP1005 carrying EcoR124I hsdR, M and S genes cloned, but not with strains carrying the natural conjugative plasmid R124. The effect of both treatments on the expression of EcoR124I phenotype in relation to the possible location of R.EcoR124I restriction endonuclease in E. coli is discussed.

The PhoE porin and transmission of the chemical stimulus for induction of acid resistance (acid habituation) in Escherichia coli

Escherichia coli K12 becomes resistant to killing by acid (habituates to acid) in a few minutes at pH 5.0. Habituation involves protein synthesis-dependent and -independent stages; both must occur at an habituating pH. The habituation sensor does not detect increased delta pH (or decreased delta psi) nor an increased difference between pHo and periplasmic pH but probably detects a fall in either external or periplasmic pH. Phosphate ions inhibit habituation, at any stage, probably by interfering with outer membrane passage of hydrogen ions. Most outer membrane components tested are not required for habituation but phoE deletion mutants habituated poorly and are acid-resistant. Strains derepressed for phoE, in contrast, showed increased acid sensitivity. These and other results suggest that habituation involves hydrogen ions or protonated carriers crossing the outer membrane preferentially via the PhoE pore, a process inhibited by phosphate and other anions. Stimulation by phosphate of the poor growth of E. coli at pH 5.0 is in accord with the above. Acetate did not enhance acid killing of pH 5.0 cells, suggesting that their resistance does not depend on maintaining pHi near to neutrality at an acidic pHo level.

Decreased DNA damage by acid and increased repair of acid-damaged DNA in acid-habituated Escherichia coli

A study of the conjugal transfer of ColV,I-K94 tn 10 from acid-treated donors suggested that acid-habituated recipients repair acid-damaged plasmid DNA better than those that are not habituated. The presence of an increased repair activity for acid-damaged DNA in habituated cells was confirmed by isolating pBR322 from acid-treated organisms; habituated cells produced more transformants when transformed by it than did non-habituated ones. Additionally, agarose gel electrophoretic studies of pBR322 DNA isolated from acid-damaged cells and tests of its transforming activity both indicated that plasmid DNA in habituated cells is less damaged by extreme acidity than is that in non-habituated organisms.

Habituation to acid in Escherichia coli: conditions for habituation and its effects on plasmid transfer

Induction of acid resistance (habituation) in Escherichia coli at pH 5.0 took ca 5 min in broth at 37 degrees C and 30-60 min in minimal medium. Induction occurred at a range of pH values from 4.0 to 6.0; it was dependent on continuing protein and RNA synthesis but substantial acid resistance appeared in the presence of nalidixic acid. Acid resistance was long-lasting; organisms grown at pH 5.0 retained most of their resistance after 2 h growth at pH 7.0. Organisms grown at pH 5.0 showed increased synthesis of a number of cytoplasmic proteins compared with the level in cells grown at pH 7.0. DNA repair-deficient strains carrying recA, uvrA or polA1 mutations were more acid-sensitive than the repair-proficient parents but were able to habituate at pH 5.0. Organisms grown at pH 5.0 transferred the ColV plasmid much more effectively at acid pH than did those grown at pH 7.0 and habituated recipients appeared better able to repair incoming acid-damaged plasmid DNA than did those that were non-habituated. Induction of acid resistance at pH 5.0 may be significant for the survival of organisms exposed to periodic discharges of acid effluent in the aquatic environment and habituation may also allow plasmid transfer and repair of acid-damaged plasmid DNA during or after such exposure.

Metal resistance and accumulation in bacteria

Recent research on the ecology, physiology and genetics of metal resistance and accumulation in bacteria has significantly increased the basic understanding of microbiology in these areas. Research has clearly demonstrated the versatility of bacteria to cope with toxic metal ions. For example, certain strains of bacteria can efficiently efflux toxic ions such as cadmium, that normally exert an inhibitory effect on bacteria. Some bacteria such as Escherichia coli and Staphylococcus sp. can volatilize mercury via enzymatic transformations. It is also noteworthy that many of these resistance mechanisms are encoded on plasmids or transposons. By expanding the knowledge on metal-resistance and accumulation mechanisms in bacteria, it may be possible to utilize certain strains to recover precious metals such as gold and silver, or alternatively remove toxic metal ions from environments or products where their presence is undesirable.

Virulence plasmid-associated adhesion of Escherichia coli and its significance for chlorine resistance

Introduction of the ColV, I-K94 virulence plasmid into strains of Escherichia coli led (for four out of five strains tested) to a marked increase in the ability of organisms to adhere to glass beads. For strain 1829, the plasmid led to increased attachment to other materials including sand, agar, agarose, chitin and cellulose. The increased adhesion to glass beads was due to the presence of the plasmid and not to its introduction into a variant with altered adhesive properties. The plasmid-encoded VmpA protein did not appear to be necessary for the ColV, I-K94-promoted adhesion but adhesion was absolutely dependent on the presence of derepressed levels of transfer components in the ColV+ strains and partially dependent on the presence of colicin components. The extent of the plasmid-promoted adhesion was greatest for organisms grown at 30 degrees, 37 degrees or 42 decrees C and adhesion was almost abolished by growth at 21 degrees or 25 degrees C; this finding is in accord with transfer and colicin components being involved in adhesion. Of several other plasmids tested for their effects on adhesion, those with derepressed transfer properties showed a marked effect as did the RI resistance plasmid. Because of the ease of handling glass bead-attached organisms, such preparations were used as a model for studying the relevance of attachment to the resistance of E. coli to chlorination in the water purification process. Organisms of 1829 ColV, I-K94, attached to glass beads, were more resistant to damage and killing by chlorine than were unattached organisms. Three findings suggest that such chlorine resistance may be significant for survival during water chlorination. Firstly, ColV, I-K94+ bacteria became attached if incubated in sewage effluent with glass beads at 20 degrees C. Secondly, ColV+ organisms already attached to glass beads maintained their attachment during 24 h incubation in effluent at 20 degrees C and thirdly such effluent incubated organisms remained chlorine resistant provided that they retained their attachment.

Multiple low-level antibiotic resistance in Aeromonas salmonicida

Mutants with multiple low-level antibiotic resistance were isolated from virulent wild-type Aeromonas salmonicida strains exposed to a low concentration of any one of several low-molecular-mass (approximately 635 daltons or less) antibiotics. Multiple resistance was toward beta-lactam compounds (penicillin G, ampicillin, cloxacillin), quinolones (flumequine, oxolinic acid, nalidixic acid), tetracyclines, chloramphenicol, and novobiocin. Susceptibilities of the mutants toward several higher-molecular-mass (greater than 700 daltons) hydrophobic or polycationic antibiotics such as rifampin, erythromycin, polymyxin B, and streptomycin sulfate were not affected. The mutants were obtained at frequencies suggesting point mutations. Outer membrane protein profiles, examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, revealed that all multiple low-level resistant mutants were deficient in a major protein of approximately 38.5 kilodaltons and contained a major protein of approximately 37 kilodaltons which was not present in significant amounts in the wild-type strains. In addition, these mutants lacked exoprotease activity. Furthermore, mutants isolated as deficient in exoprotease were found, with the exception of one avirulent strain, to exhibit multiple low-level antibiotic resistance and the outer membrane protein changes.

A motility lesion in ColV+ Escherichia coli strains and its possible clinical significance

The introduction of the ColV-K30 or ColV,I-K94 plasmid into Escherichia coli strains produced derivatives which had a motility lesion if grown without shaking at 37 degrees C. Although most ColV+ organisms from shaken cultures were motile, 80-90% of free unclumped organisms from static cultures were flagellate but non-motile. This plasmid effect was temperature-dependent with only those ColV+ organisms grown at 37 degrees C being affected; ColV+ organisms grown at 30 degrees C or below were predominantly motile. The motility lesion depended on the presence, in the ColV+ organisms, of transfer and colicin components together but not of the VmpA protein. Aside from the changed motility, there was extensive autoagglutination (clumping) of ColV+ organisms in static cultures, and the two phenomena (clumping and motility lesion) appeared to be governed by the same factors. The Flac plasmid of FI incompatibility group had a slight inhibitory effect on motility of strain 1829 and caused slight clumping, but representative plasmids of groups FII, FIII, FIV, C, H, I, K, M, N, P, W and X had no appreciable effect on either parameter. Non-motile ColV+ organisms regained motility on incubation with buffered detergent solutions, suggesting that an envelope change might be responsible for the altered motility. It can be hypothesized that ColV+ organisms in the intestine would be motile and hence able to reach the intestinal epithelium for invasion but that, once such organisms had reached the tissues and bloodstream, they would be predominantly non-motile and hence might be less susceptible to phagocytosis.

Envelope protein changes, autoagglutination, sensitivity to hydrophobic agents and a conditional division lesion in Escherichia coli strains carrying ColV virulence plasmids

The presence of the virulence plasmids ColV,I-K94 or ColV-K30 in Escherichia coli produces a number of cell membrane and envelope changes. The most striking of these are (1) the presence of the 33K VmpA outer membrane protein and (2) the ColV-associated occurrence of autoagglutination. The VmpA protein is a plasmid-encoded outer membrane protein which is synthesized from a larger precursor. It is distinct from the chromosomally-encoded OmpA protein but resembles it in a few respects. The VmpA protein does not appear to be involved in colicin synthesis or immunity, or in plasmid transfer. This protein was found in 6 out of 8 new ColV+ isolates, but not in 2 ColIa+ strains. ColV-induced autoagglutination occurred for strains grown in static culture at 37 but not at 25 degrees C. Detergents prevented agglutination, as did the presence in a ColV+ strain of a fi+ plasmid, ColB. Autoagglutination may be a virulence phenotype. Associated with the ability of ColV+ bacteria to agglutinate was inhibition of motility. ColV+ bacteria also showed changes in envelope permeability indicated by inhibitor sensitivity and by a ColV-associated suppression of the lac Y lesion. Some ColV,I-K94+ strains showed a mucoid colonial phenotype and this ability to form mucoid colonies was efficiently transferred with ColV but apparently not without it. The mucoid ColV+ strains resembled lon mutants in UV-sensitivity, division behaviour and sensitivity to lambda phage.