Levy, Stuart B. (Institut du Radium, Paris, France). Isologous interference with ultraviolet and X-ray irradiated bacteriophage T2. J. Bacteriol. 87:1330–1338. 1964.—Qualitative and quantitative analysis of the interference capacity of an irradiated T2 bacteriophage was made with ultraviolet and X-ray irradiation. Two different effects were found to explain the total interference picture in the ultraviolet-irradiated system: exclusion and depression. Exclusion is the absolute inhibition of infectious phage growth in the bacterial host. Depression is the diminution of burst size in instances where the infectious phage has not been excluded. Both effects were seen when the infectious phage was added after the addition of ultraviolet-irradiated phage. Doses between 1,600 and 2,200 ergs/mm2 (survivals, ca. 10−7) showed the greatest exclusion effect (70%). Exclusion was lost between 6,500 and 7,500 ergs/mm2. The depression effect was highest (90%) at lower doses (survivals, ca. 10−6), falling off as the dose range went above 1,600 ergs/mm2 or survivals of 10−7. Depression was lost at 3,000 ergs/mm2. X-ray irradiation (both direct and indirect) to survivals less than 10−2 showed no interference capacity in the phage irradiated. Indirect X-ray irradiation to survivals between 5 and 10% showed 50% exclusion, but no depression.
MarR is the dedicated autorepressor of the marRAB operon found in seven genera of the Enterobacteraceae. The MarA transcriptional regulator directly activates numerous genes involved in multidrug resistance and other environmental responses. MarR is inactivated by certain phenolic ligands, such as salicylate, by an unknown mechanism. Our recent work has shown that several amino acid residues of Escherichia coli MarR affecting ligand binding are located between the dimerization and DNA-binding domains. To further characterize the ligand-binding region of MarR, we have now examined seven point mutants generated by random mutagenesis and eleven site-directed alanine replacement mutants for inactivation by three ligands: salicylate, 2,4-dinitrophenol, and plumbagin. Inactivation of MarR was quantitated in intact cells by loss of MarR-mediated repression of a chromosomal mar-lacZ transcriptional fusion. The results showed that most of the residues important for ligand effectiveness lay in the α1 and α2 helices of MarR, between the putative DNA-binding domain and the dimerization domain of MarR, reinforcing our earlier findings. Moreover, the three ligands had different, but overlapping, sets of residues impacting their effects on MarR.
ligand; repression; induction; mutagenesis; marRAB operon; error-prone PCR
Previous data from our laboratory suggest a relationship between increased pmrAB expression and virulence in an Escherichia coli mouse infection model of pyelonephritis. Competitive infections with wild type and pmrAB mutants showed that disruption of pmrAB caused decreased persistence of E. coli within the mouse kidney. These results were confirmed with plasmid-mediated complementation of the pmrAB mutant. Additionally, increased expression of pmrAB from this complementing plasmid in a previously attenuated marA-rob-soxS triple mutant displayed increased bacterial persistence in the infection when compared with the triple mutant alone. These findings suggest a role for this two-component regulatory system in the virulence of E. coli in a murine pyelonephritis model.
E. coli; pmrAB; kidney; infection
Pseudomonas species can exhibit phenotypic variation resulting from gacS or gacA mutation. P. fluorescens Pf0-1 is a gacA mutant and exhibits pleiotropic changes following the introduction of a functional allele. GacA enhances biofilm development while reducing dissemination in soil, suggesting that alternative Gac phenotypes enable Pseudomonas sp. to exploit varied environments.
The marRAB operon is conserved in seven genera of enteric bacteria (Escherichia, Shigella, Klebsiella, Enterobacter, Salmonella, Cronobacter, and Citrobacter). MarA is a transcriptional regulator affecting many genes involved in resistance to stresses, and MarR is an autorepressor of the operon, but a role for the marB gene has been unclear. A recent work reported that deletion of marB causes resistance to certain stresses and increases the amount of marA transcript. We show here that the small (216 bp) marB gene encodes a protein, not an sRNA, since two different stop codons within the predicted open reading frame of marB prevented plasmid-borne marB from complementing ΔmarB::Kan. The ΔmarB::Kan mutation did not increase the stability of the marA transcript, suggesting that MarB does not destabilize the marA transcript but rather reduces its rate of transcription. Placing the putative signal sequence of MarB upstream of signal-sequence-less alkaline phosphatase guided the phosphatase to its normal periplasmic location. We conclude that MarB is a small periplasmic protein that represses the marRAB promoter by an indirect mechanism, possibly involving a signal to one of the cytoplasmic regulators of that promoter.
transcript stability; small protein; regulation
The Escherichia coli regulator MarR represses the multiple-antibiotic resistance operon marRAB and responds to phenolic compounds, including sodium salicylate, which inhibit its activity. Crystals obtained in the presence of a high concentration of salicylate indicated two possible salicylate sites, SAL-A and SAL-B. However, it was unclear whether these sites were physiologically significant or were simply a result of the crystallization conditions. A study carried out on MarR homologue MTH313 suggested the presence of a salicylate binding site buried at the interface between the dimerization and the DNA-binding domains. Interestingly, the authors of the study indicated a similar pocket conserved in the MarR structure. Since no mutagenesis analysis had been performed to test which amino acids were essential in salicylate binding, we examined the role of residues that could potentially interact with salicylate. We demonstrated that mutations in residues shown as interacting with salicylate at SAL-A and SAL-B in the MarR-salicylate structure had no effect on salicylate binding, indicating that these sites were not the physiological regulatory sites. However, some of these residues (P57, R86, M74, and R77) were important for DNA binding. Furthermore, mutations in residues R16, D26, and K44 significantly reduced binding to both salicylate and 2,4-dinitrophenol, while a mutation in residue H19 impaired the binding to 2,4-dinitrophenol only. These findings indicate, as for MTH313, the presence of a ligand binding pocket located between the dimerization and DNA binding domains.
A carbapenem-resistant clinical isolate of Escherichia coli, which lacked OmpF and OmpC porins, carried a marR mutation and expressed a functional yedS, a normally nontranslated gene. MarR and YedS are described here as having effects on the ability of this strain to resist carbapenems. Additionally, expression of YedS was regulated by the small RNA MicF in a MarA-dependent way. These findings illustrate how broadly bacteria can mutate within a selective clinical setting, in this case, resistance to carbapenems, by altering three porin genes and one regulatory gene.
AdnA in Pseudomonas fluorescens, an ortholog of FleQ in P. aeruginosa, regulates both motility and flagellum-mediated attachment to various surfaces. A whole-genome microarray determined the AdnA transcriptome by comparing the gene expression pattern of wild-type Pf0-1 to that of Pf0-2x (adnA deletion mutant) in broth culture. In the absence of AdnA, expression of 92 genes was decreased, while 11 genes showed increased expression. Analysis of 16 of these genes fused to lacZ confirmed the microarray results. Several genes were further evaluated for their role in motility and biofilm formation. Two genes, Pfl01_1508 and Pfl01_1517, affected motility and had different effects on biofilm formation in Pf0-1. These two genes are predicted to specify proteins similar to the glycosyl transferases FgtA1 and FgtA2, which have been shown to be involved in virulence and motility in P. syringae. Three other genes, Pfl01_1516, Pfl01_1572, and Pfl01_1573, not previously associated with motility and biofilm formation in Pseudomonas had similar effects on biofilm formation in Pf0-1. Deletion of each of these genes led to different motility defects. Our data revealed an additional level of complexity in the control of flagellum function beyond the core genes known to be required and may yield insights into processes important for environmental persistence of P. fluorescens Pf0-1.
Paralogous transcriptional regulators MarA, Rob, and SoxS act individually and together to control expression of more than 80 Escherichia coli genes. Deletion of marA, rob, and soxS from an E. coli clinical isolate prevents persistence beyond 2 days postinfection in a mouse model of pyelonephritis. We used microarray analysis to identify 242 genes differentially expressed between the triple deletion mutant and its parent strain at 2 days postinfection in the kidney. One of these, znuC of the zinc transport system ZnuACB, displayed decreased expression in the triple mutant compared to that in the parental strain, and deletion of znuC from the parental strain reduced persistence. The marA rob soxS triple deletion mutant was less viable in vitro under limited-Zn and Zn-depleted conditions, while disruption of znuC caused a reduction in the growth rates for the parental and triple mutant strains to equally low levels under limited-Zn or Zn-depleted conditions. Complementation of the triple mutant with soxS, but not marA or rob, restored the parental growth rate in Zn-depleted medium, while deletion of only soxS from the parental strain led to low growth in Zn-depleted medium. Both results suggested that SoxS is a major regulator responsible for growth under Zn-depleted conditions. Gel shift experiments failed to show direct binding of SoxS to the znuCB promoter, thus suggesting indirect control of znuCB expression by SoxS. While SoxS expression in the triple mutant fully restored persistence, increased expression of znuACB via a plasmid in this mutant only partially restored wild-type levels of persistence in the kidney. This work implicates SoxS control of znuCB expression as a key factor in persistence of E. coli in murine pyelonephritis.
The efflux pump AcrAB is important in the antibiotic resistance and virulence of several pathogenic bacteria. We report that deletion of the Yersinia pestis AcrAB-TolC homolog leads to increased susceptibility to diverse substrates, including, though unlike in Escherichia coli, the aminoglycosides. Neither is the Y. pestis pump affected by the efflux pump inhibitor phenylalanine-arginine beta-naphthylamide. In mouse plague models, pump deletion does not have a significant effect on tissue colonization.
Omadacycline is a novel first-in-class aminomethylcycline with potent activity against important skin and pneumonia pathogens, including community-acquired methicillin-resistant Staphylococcus aureus (MRSA), β-hemolytic streptococci, penicillin-resistant Streptococcus pneumoniae, Haemophilus influenzae, and Legionella. In this work, the mechanism of action for omadacycline was further elucidated using a variety of models. Functional assays demonstrated that omadacycline is active against strains expressing the two main forms of tetracycline resistance (efflux and ribosomal protection). Macromolecular synthesis experiments confirmed that the primary effect of omadacycline is on bacterial protein synthesis, inhibiting protein synthesis with a potency greater than that of tetracycline. Biophysical studies with isolated ribosomes confirmed that the binding site for omadacycline is similar to that for tetracycline. In addition, unlike tetracycline, omadacycline is active in vitro in the presence of the ribosomal protection protein Tet(O).
Like animals and people, insects can serve as both collectors and disseminators of antibiotic resistance genes, as exquisitely demonstrated by a recent study (B. Tian, N. H. Fadhil, J. E. Powell, W. K. Kwong, and N. A. Moran, mBio 3:e00377-12, doi:10.1128/mBio.00377-12, 2012). Notably, the relatively confined ecosystem of the honeybee gut demonstrates a large propensity for harboring a diverse set of tetracycline resistance genes that reveal the environmental burden resulting from the long-time selective pressures of tetracycline use in the honeybee industry. As in humans and animals, these genes have become established in the native, nonpathogenic flora of the insect gut, adding credence to the concept that commensal floras provide large reservoirs of resistance genes that can readily move into pathogenic species. The homology of these tetracycline resistance determinants with those found in tetracycline-resistant bacteria associated with animals and humans strongly suggests a dissemination of similar or identical genes through shared ecosystems. The emergence of linked coresistances (ampicillin and tetracycline) following single-antibiotic therapy mirrors reports from other studies, namely, that long-term, single-agent therapy will result in resistance to multiple drugs. These results contrast with the marked absence of diverse, single- and multiple-drug resistance genes in wild and domestic bees that are not subjected to such selective pressures. Prospective studies that simultaneously track both resistance genes and antibiotic residues will go far in resolving some of the nagging questions that cloud our understanding of antibiotic resistance dissemination.
Summary: Antimicrobials are valuable therapeutics whose efficacy is seriously compromised by the emergence and spread of antimicrobial resistance. The provision of antibiotics to food animals encompasses a wide variety of nontherapeutic purposes that include growth promotion. The concern over resistance emergence and spread to people by nontherapeutic use of antimicrobials has led to conflicted practices and opinions. Considerable evidence supported the removal of nontherapeutic antimicrobials (NTAs) in Europe, based on the “precautionary principle.” Still, concrete scientific evidence of the favorable versus unfavorable consequences of NTAs is not clear to all stakeholders. Substantial data show elevated antibiotic resistance in bacteria associated with animals fed NTAs and their food products. This resistance spreads to other animals and humans—directly by contact and indirectly via the food chain, water, air, and manured and sludge-fertilized soils. Modern genetic techniques are making advances in deciphering the ecological impact of NTAs, but modeling efforts are thwarted by deficits in key knowledge of microbial and antibiotic loads at each stage of the transmission chain. Still, the substantial and expanding volume of evidence reporting animal-to-human spread of resistant bacteria, including that arising from use of NTAs, supports eliminating NTA use in order to reduce the growing environmental load of resistance genes.
Cationic antimicrobial peptides (CAMPs), a component of the mammalian immune system, protect the host from bacterial infections. The roles of the Escherichia coli transcriptional regulators MarA, SoxS and Rob in susceptibility to these peptides were examined. Overexpression of marA, either in an antibiotic-resistant marR mutant or from a plasmid, decreased bacterial susceptibility to CAMPs. Overexpression of the soxS gene from a plasmid, which decreased susceptibility to antibiotics, unexpectedly caused no decrease in CAMP susceptibility; instead it produced increased susceptibility to different CAMPs. Deletion or overexpression of rob had little effect on CAMP susceptibility. The marRAB operon was upregulated when E. coli was incubated in sublethal amounts of CAMPs polymyxin B, LL-37 or human β-defensin-1; however, this upregulation required Rob. Deletion of acrAB increased bacterial susceptibility to polymyxin B, LL-37 and human β-defensin-1 peptides. Deletion of tolC yielded an even greater increase in susceptibility to these peptides and also led to increased susceptibility to human α-defensin-2. Inhibition of cellular proton-motive force increased peptide susceptibility for wild-type and acrAB deletion strains; however, it decreased susceptibility of tolC mutants. These findings demonstrate that CAMPs are both inducers of marA-mediated drug resistance through interaction with Rob and also substrates for efflux in E. coli. The three related transcriptional regulators show different effects on bacterial cell susceptibility to CAMPs.
Letters of the English alphabet have heretofore been used to name tetracycline resistance determinants. Since all 26 letters have now been used, a nomenclature employing numerals is recommended for future determinants, and one laboratory has offered to coordinate the assignment of numerals.
Pseudomonas spp. adapt rapidly to environmental fluctuations. Loss or overproduction of polyphosphate reduces the fitness of Pseudomonas fluorescens Pf0-1, indicating the importance of the fine-tuning of polyphosphate production. An antisense RNA was investigated and shown to regulate the polyphosphate kinase gene (ppk) by a posttranscriptional mechanism reducing ppk transcript abundance.
Clumping of bacteria containing R factors derepressed for pilus synthesis interfered with the separation of cells and minicells. Mucoid derivatives of these bacteria provided the means of preventing clumping and allowing a good separation. This method may be generally useful in dealing with autoagglutination of bacterial cultures.
The ability of soil bacteria to successfully compete with a range of other microbial species is crucial for their growth and survival in the nutrient-limited soil environment. In the present work, we studied the behavior and transcriptional responses of soil-inhabiting Pseudomonas fluorescens strain Pf0-1 on nutrient-poor agar to confrontation with strains of three phylogenetically different bacterial genera, that is, Bacillus, Brevundimonas and Pedobacter. Competition for nutrients was apparent as all three bacterial genera had a negative effect on the density of P. fluorescens Pf0-1; this effect was most strong during the interaction with Bacillus. Microarray-based analyses indicated strong differences in the transcriptional responses of Pf0-1 to the different competitors. There was higher similarity in the gene expression response of P. fluorescens Pf0-1 to the Gram-negative bacteria as compared with the Gram-positive strain. The Gram-negative strains did also trigger the production of an unknown broad-spectrum antibiotic in Pf0-1. More detailed analysis indicated that expression of specific Pf0-1 genes involved in signal transduction and secondary metabolite production was strongly affected by the competitors' identity, suggesting that Pf0-1 can distinguish among different competitors and fine-tune its competitive strategies. The results presented here demonstrate that P. fluorescens Pf0-1 shows a species-specific transcriptional and metabolic response to bacterial competitors and provide new leads in the identification of specific cues in bacteria–bacteria interactions and of novel competitive strategies, antimicrobial traits and genes.
bacterial competition; inter-specific interactions; Pseudomonas fluorescens; transcriptional responses; antibiosis
LcrF (VirF), a transcription factor in the multiple adaptational response (MAR) family, regulates expression of the Yersinia type III secretion system (T3SS). Yersinia pseudotuberculosis lcrF-null mutants showed attenuated virulence in tissue culture and animal models of infection. Targeting of LcrF offers a novel, antivirulence strategy for preventing Yersinia infection. A small molecule library was screened for inhibition of LcrF-DNA binding in an in vitro assay. All of the compounds lacked intrinsic antibacterial activity and did not demonstrate toxicity against mammalian cells. A subset of these compounds inhibited T3SS-dependent cytotoxicity of Y. pseudotuberculosis toward macrophages in vitro. In a murine model of Y. pseudotuberculosis pneumonia, two compounds significantly reduced the bacterial burden in the lungs and afforded a dramatic survival advantage. The MAR family of transcription factors is well conserved, with members playing central roles in pathogenesis across bacterial genera; thus, the inhibitors could have broad applicability.