Specific capture and detection of Staphylococcus aureus with high-affinity modified aptamers to cell surface components.

Slow off-rate modified aptamer (SOMAmer) reagents were generated to several Staphylococcus aureus cell surface-associated proteins via SELEX with multiple modified DNA libraries using purified recombinant or native proteins. High-affinity binding agents with sub-nanomolar Kd 's were obtained for staphylococcal protein A (SpA), clumping factors (ClfA, ClfB), fibronectin-binding proteins (FnbA, FnbB) and iron-regulated surface determinants (Isd). Further screening revealed several SOMAmers that specifically bound to Staph. aureus cells from all strains that were tested, but not to other staphylococci or other bacteria. SpA and ClfA SOMAmers proved useful for the selective capture and enrichment of Staph. aureus cells, as shown by culture and PCR, leading to improved limits of detection and efficient removal of PCR inhibitors. Detection of Staph. aureus cells was enhanced by several orders of magnitude when the bacterial cell surface was coated with SOMAmers followed by qPCR of the SOMAmers. Furthermore, fluorescence-labelled SpA SOMAmers demonstrated their utility as direct detection agents in flow cytometry. Significance and impact of the study: Monitoring for microbial contamination of food, water, nonsterile products or the environment is typically based on culture, PCR or antibodies. Aptamers that bind with high specificity and affinity to well-conserved cell surface epitopes represent a promising novel type of reagents to detect bacterial cells without the need for culture or cell lysis, including for the capture and enrichment of bacteria present at low cell densities and for the direct detection via qPCR or fluorescent staining.

Autoinducer-mediated regulation of rhamnolipid biosurfactant synthesis in Pseudomonas aeruginosa.

The opportunistic human pathogen Pseudomonas aeruginosa produces a variety of virulence factors, including exotoxin A, elastase, alkaline protease, alginate, phospholipases, and extracellular rhamnolipids. The previously characterized rhlABR gene cluster encodes a regulatory protein (RhlR) and a rhamnosyltransferase (RhlAB), both of which are required for rhamnolipid synthesis. Another gene, rhII, has now been identified downstream of the rhlABR gene cluster. The putative RhlI protein shares significant sequence similarity with bacterial autoinducer synthetases of the LuxI type. A P. aeruginosa rhlI mutant strain carrying a disrupted rhlI gene was unable to produce rhamnolipids and lacked rhamnosyltransferase activity. Rhamnolipid synthesis was restored by introducing a wild-type rhlI gene into such strains or, alternatively, by adding either the cell-free spent supernatant from a P. aeruginosa wild-type strain or synthetic N-acylhomoserine lactones. Half-maximal induction of rhamnolipid synthesis in the rhlI mutant strain required 0.5 microM N-butyrylhomoserine lactone or 10 microM N-(3-oxohexanoyl)homoserine lactone. The P. aeruginosa rhlA promoter was active in the heterologous host Pseudomonas putida when both the rhlR and rhlI genes were present or when the rhlR gene alone was supplied together with synthetic N-acylhomoserine lactones. The RhlR-RhlI regulatory system was found to be essential for the production of elastase as well, and cross-communication between the RhlR-RhlI rhamnolipid regulatory system and the LasR-LasI elastase regulatory system was demonstrated.

Gene repression by the ferric uptake regulator in Pseudomonas aeruginosa: cycle selection of iron-regulated genes.

The expression of at least 24 distinct genes of Pseudomonas aeruginosa PAO1 is under direct control of the "ferric uptake regulator" (Fur). Novel targets of the Fur protein were isolated in a powerful SELEX (systematic evolution of ligands by exponential enrichment)-like cycle selection consisting of in vitro DNA-Fur interaction, binding to anti-Fur antibody, purification on protein G, and PCR amplification. DNA fragments obtained after at least three exponential enrichment cycles were cloned and subjected to DNA mobility-shift assays and DNase I footprint analyses to verify the specific interaction with the Fur protein in vitro. Iron-dependent expression of the corresponding genes in vivo was monitored by RNase protection analysis. In total, 20 different DNA fragments were identified which represent actual Pseudomonas iron-regulated genes (PIGs). While four PIGs are identical to already known genes (pfeR, pvdS, tonB, and fumC, respectively), 16 PIGs represent previously unknown genes. Homology studies of the putative proteins encoded by the PIGs allowed us to speculate about their possible function. Two PIG products were highly similar to siderophore receptors from various species, and three PIG products were significantly homologous to alternative sigma factors. Furthermore, homologs of the Escherichia coli ORF1-tolQ, nuoA, stringent starvation protein Ssp, and of a two-component regulatory system similar to the Pseudomonas syringae LemA sensor kinase were identified. The putative gene products of seven additional PIGs did not show significant homologies to any known proteins. The PIGs were mapped on the P.aeruginosa chromosome. Their possible role in iron metabolism and virulence of P. aeruginosa is discussed.

The response of Pseudomonas aeruginosa to iron: genetics, biochemistry and virulence.

During the past decade significant progress has been made towards identifying some of the schemes that Pseudomonas aeruginosa uses to obtain iron and towards cataloguing and characterizing many of the genes and gene products that are likely to play a role in these processes. This review will largely recount what we have learned in the past few years about how P. aeruginosa regulates its acquisition, intake and, to some extent, trafficking of iron, and the role of iron acquisition systems in the virulence of this remarkable opportunistic pathogen. More specifically, the genetics, biochemistry and biology of an essential regulator (Ferric uptake regulator - Fur) and a Fur-regulated alternative sigma factor (PvdS), which are central to these processes, will be discussed. These regulatory proteins directly or indirectly regulate a substantial number of other genes encoding proteins with remarkably diverse functions. These genes include: (i) other regulatory genes, (ii) genes involved in basic metabolic processes (e.g. Krebs cycle), (iii) genes required to survive oxidative stress (e.g. superoxide dismutase), (iv) genes necessary for scavenging iron (e.g. siderophores and their cognate receptors) or genes that contribute to the virulence (e.g. exotoxin A) of this opportunistic pathogen. Despite this recent expansion of knowledge about the response of P. aeruginosa to iron, many significant biological issues surrounding iron acquisition still need to be addressed. Virtually nothing is known about which of the distinct iron acquisition mechanisms P. aeruginosa brings to bear on these questions outside the laboratory, whether it be in soil, in a pipeline, on plants or in the lungs of cystic fibrosis patients.

Genetic and physiological characterization of ohr, encoding a protein involved in organic hydroperoxide resistance in Pseudomonas aeruginosa.

The ohr (organic hydroperoxide resistance) gene product of Pseudomonas aeruginosa was essential for optimal resistance to organic hydroperoxides (OHPs) but not to hydrogen peroxide or paraquat. A Deltaohr mutant was hypersusceptible to OHPs in disk inhibition assays and showed enhanced killing by OHPs in liquid culture. The ohr gene product was demonstrated to contribute to the decomposition of OHPs. Transcription of ohr was induced up to 15-fold upon exposure to OHPs, and this induction was independent of OxyR. Somewhat enhanced ohr-lacZ activity was detected in mutant strains affected in ohr, ahpC, and oxyR, and this phenotype correlated with hypersusceptibility to OHPs, suggesting overlapping or compensatory functions of the ohr and ahpC gene products. A single transcriptional start site for ohr was determined, and ohr transcripts were abundant in cells treated with a sublethal dose of OHPs but not in cells treated with paraquat. An 84-bp portion upstream of the ohr mRNA start site was sufficient for ohr induction by OHPs. Thus, the ohr gene appears to encode an antioxidant enzyme that is not part of the OxyR regulon yet is specifically induced by OHPs.

Isolation, characterization, and expression in Escherichia coli of the Pseudomonas aeruginosa rhlAB genes encoding a rhamnosyltransferase involved in rhamnolipid biosurfactant synthesis.

Transposon Tn5-GM-induced mutant strains of Pseudomonas aeruginosa which are unable to produce rhamnolipid biosurfactants and lack rhamnosyltransferase activity have been isolated. The DNA regions flanking the transposon were cloned and used as specific probes for the isolation of the corresponding wild-type genes from a P. aeruginosa wild-type cosmid gene library. Single cosmid clones capable of restoring rhamnolipid synthesis in the mutant strains were isolated and further subcloned and sequenced, resulting in the identification of two genes (rhlAB) which are organized as an operon upstream of the previously identified rhlR regulatory gene. The RhlA protein (32.5 kDa) harbors a putative signal sequence, suggesting that this protein is located in the periplasm, while the RhlB protein (47 kDa) contains at least two putative membrane-spanning domains. The expression of the rhlAB genes was found to be enhanced 20-fold during the stationary phase of growth under conditions of nitrogen limitation, as measured by using rhlA::lacZ fusions. Moreover, the transcriptional activation of the rhlAB genes appears to depend on a functional RhlR regulatory protein. The sequence upstream of the rhlA promoter contains two inverted repeats which define putative binding sites for the RhlR regulator. The controlled expression of the rhlAB genes in Escherichia coli led to the formation of active rhamnosyltransferase. This provides direct evidence for the fact that the rhamnosyltransferase encoding genes have been identified.

Molecular analysis of the Trichosporon cutaneum DSM 70698 argA gene and its use for DNA-mediated transformations.

Genomic clones capable of complementing a previously isolated arginine auxotrophic mutant strain of the filamentous yeast Trichosporon cutaneum DSM 70698 have been identified by DNA-mediated transformation, and a complementing 4,082-bp subfragment was sequenced. This analysis revealed an intact gene (arg4) showing a high degree of homology with the Saccharomyces cerevisiae CPA2 gene encoding the large subunit of carbamoyl-phosphate synthetase (CPS-A). The inferred amino acid sequence of the T. cutaneum argA-encoded protein contains 1,168 residues showing 62% identity with the sequence of the S. cerevisiae CPA2 protein, and the comparison of the two sequences uncovered a putative intron sequence of 81 nucleotides close to the 5' end of the coding region of the T. cutaneum argA gene. The presence of this intron was confirmed by nuclease protection studies and by direct DNA sequence analysis of a cDNA fragment which had been obtained by PCR amplification. The T. cutaneum intron shares the general characteristics of introns found in yeasts and filamentous fungi. A major transcript of around 4 kb was found in Northern (RNA) blots. The T. cutaneum argA coding region was expressed in Escherichia coli under the control of the regulatable tac promoter. A roughly 130-kDa protein which was found to cross-react with an anti-rat CPS antibody in Western blots (immunoblots) was observed. Two putative ATP-binding domains were identified, one in the amino-terminal half of the argA-encoded protein and the other in the carboxy-terminal half. These domains are highly conserved among the known CPS-A sequences from S. cerevisiae, E. coli, and the rat. From these results we conclude that the T. cutaneum argA gene encodes the large subunit of CPS. This is the first gene to be identified and analyzed in the T. cutaneum DSM 70698 strain.

Role of the ferric uptake regulator of Pseudomonas aeruginosa in the regulation of siderophores and exotoxin A expression: purification and activity on iron-regulated promoters.

The cloned Pseudomonas aeruginosa fur (ferric uptake regulator) gene was overexpressed in P. aeruginosa by using a T7 expression system, and the Fur protein (PA-Fur) was purified by using a combination of ion-exchange chromatography and metal affinity chromatography. The DNA binding activity of the PA-Fur protein was confirmed by gel mobility shift assays and DNase I footprints of the synthetic DNA fragment GATAAT GATAATCATTATC, representing a perfect "Fur box". In addition, it was shown that PA-Fur is capable of binding to promoter and operator determinants of the tightly iron-regulated Escherichia coli fepA-fes enterobactin gene system. The activity of PA-Fur on the promoters of iron-regulated genes involved in the production of two siderophores, pyochelin and pyoverdin, and in the expression of exotoxin A was investigated. Data indicating that the promoters of the pchR gene, encoding a transcriptional activator for pyochelin synthesis, and of the pvdS gene, encoding a positive regulator for pyoverdin production, are specifically recognized by Fur-Fe(II) are presented, suggesting that PA-Fur represses expression of pchR and pvdS during growth in an iron-replete environment. However, neither the promoter region of the gene encoding exotoxin A (toxA) nor the promoters of the regAB operon, required for toxA expression, interacted with high concentrations of purified PA-Fur. These data indicate that iron regulation of exotoxin A production involves additional factors which may ultimately be under the control of PA-Fur.

Production of rhamnolipid biosurfactants.

Biosurfactants are of increasing interest due to their broad range of potential applications. A large variety of microbial surfactants is known at present, some of which may be used for specific applications. Towards the large scale industrial production of biosurfactants, the physiology, biochemistry and genetics of biosurfactant synthesis has to be well understood. A fully integrated process has to be developed, allowing high productivities under optimized conditions. In the past few years, we have investigated the molecular biology of rhamnolipid biosynthesis have been partially purified and characterized. The structural and regulatory genes encoding the rhamnolipid synthesis pathway have been isolated and characterized. The knowledge of the complex mechanisms involved in rhamnolipid synthesis facilitates the overproduction of these extracellular compounds. Furthermore, the transfer of the relevant genes into other species allows the production of rhamnolipids in heterologous hosts under controlled conditions. An integrated process for the production of rhamnolipids on an industrial scale has been developed. This process involves continuous cultivation under optimized media and growth conditions and makes use of refined methods of cell recycling, gas exchange and downstream processing, thus allowing high yields and productivities.

Production of Pseudomonas aeruginosa Rhamnolipid Biosurfactants in Heterologous Hosts.

The high-level production of rhamnolipid biosurfactants is a unique feature of Pseudomonas aeruginosa and is strictly regulated in response to environmental conditions. The final step in rhamnolipid biosynthesis is catalyzed by the rhlAB genes encoding a rhamnosyltransferase. The expression of the cloned rhlAB genes was studied in heterologous hosts, either under the control of the rhlR and rhlI rhamnolipid regulatory elements or under the control of the tac promoter. A recombinant P. fluorescens strain harboring multiple plasmid-encoded copies of the rhamnolipid gene cluster produced rhamnolipids (0.25 g liter(sup-1)) when grown under nitrogen-limiting conditions. The highest yields (0.6 g liter(sup-1)) and productivities (24 mg liter(sup-1) h(sup-1)) were obtained in a recombinant Pseudomonas putida strain, KT2442, harboring promoterless rhlAB genes fused to the tac promoter on a plasmid. Active rhamnosyltransferase was synthesized, but no rhamnolipids were produced, by recombinant Escherichia coli upon induction of rhlAB gene expression.

Genetic transformation of auxotrophic mutants of the filamentous yeast Trichosporon cutaneum using homologous and heterologous marker genes.

A transformation system for the filamentous yeast Trichosporon cutaneum based on auxotrophic markers is presented and techniques for the induction, isolation and characterization of mutants are described. A number of auxotrophic mutants were isolated and characterized by using biosynthetic precursors and/or inhibitors. A mutant unable to grow in the presence of ornithine could be complemented successfully by spheroplast transformation experiments using the cloned Aspergillus nidulans ornithine transcarbamoylase gene (argB gene) as selection marker with an efficiency of 5-100 transformants per microgram of DNA. In these transformants the heterologous argB gene was present in multiple tandem copies and the transforming DNA was found to remain stable after more than 50 generations in non-selective media. The same mutant could be complemented by a T. cutaneum cosmid gene library and a complementary cosmid was subsequently isolated from this library by a sib-selection strategy. This cosmid transformed T. cutaneum spheroblasts with an efficiency of 50-200 colonies per microgram of DNA. Southern blot analyses were consistent with the view that the transforming sequences became stably integrated into the host genome at the homologous site.

The fur-regulated gene encoding the alternative sigma factor PvdS is required for iron-dependent expression of the LysR-type regulator ptxR in Pseudomonas aeruginosa.

We previously identified a novel regulator of the exotoxin A gene (toxA) in Pseudomonas aeruginosa, PtxR, that belongs to the LysR family of prokaryotic regulatory proteins. Preliminary data also suggest that PtxR affects the expression of siderophores in P. aeruginosa. Because toxA expression and siderophore production in this organism are coordinately regulated by the ferric uptake regulator (Fur) and the Fur-regulated alternative sigma factor PvdS, regulation of ptxR itself in the context of these regulators was examined. RNase protection analyses of ptxR transcription revealed that there are two independent transcription initiation sites (T1 and T2). While transcription from the promoter of T1 is constitutive throughout the growth cycle of PAO1, transcription from the second promoter (P2) is negatively affected by iron. Transcription from the P2 promoter is constitutive in a fur mutant under microaerobic conditions but still iron regulated during aerobic growth. High concentrations (>100 nM) of the ferric uptake regulatory protein (Fur) failed to bind to either of the promoter regions of ptxR in either gel mobility shift assays or DNase I footprint experiments. These results indicate that Fur indirectly regulates the iron-dependent expression of ptxR. Iron-regulated transcription of ptxR from the P2 promoter, but not constitutive expression from the P1 promoter, was dependent on the Fur-regulated alternative sigma factor gene pvdS, even under aerobic conditions. Consequently, there are two levels of iron-regulated expression of ptxR. The iron-regulated expression of ptxR under microaerobic conditions from the P2 promoter of ptxR is mediated indirectly by Fur through the iron-regulated expression of pvdS. In contrast, pvdS-mediated iron regulation of ptxR under aerobic conditions is Fur independent.

Cloning and heterologous expression of a gene encoding an alkane-induced extracellular protein involved in alkane assimilation from Pseudomonas aeruginosa.

Pseudomonas aeruginosa PG201 produces a 16-kDa extracellular protein in media containing n-hexadecane as a carbon source but not in media containing glycerol or glucose. This protein was purified, and the N-terminal amino acid sequence was determined. The amino acid composition of the protein was found to be very similar to that of the so-called protein-like activator for n-alkane oxidation (PA) from P. aeruginosa S7B1. This extracellular protein was previously characterized (K. Hisatsuka, T. Nakahara, Y. Minoda, and K. Yamada, Agric. Biol. Chem. 41:445-450, 1977) and found to stimulate the growth of P. aeruginosa on n-hexadecane and to possess emulsifying activity. To study the role(s) of the PA protein and to make it accessible for possible future applications, we have cloned the PA-encoding (pra) gene and determined its nucleotide sequence. This analysis revealed a protein-coding region of 162 amino acids, with the first 25 residues being reminiscent of those of a typical bacterial signal sequence. The pra gene was inactivated by insertional mutagenesis, and the resulting strain was found to lack extracellular PA protein and to be retarded in its growth in n-hexadecane-containing media. These results are consistent with the growth stimulatory role of the PA protein. The pra gene was expressed in Escherichia coli, and substantial amounts of the recombinant protein were found in the extracellular growth medium. The recombinant protein was purified by metal chelate affinity chromatography. The ability to produce secreted PA protein by E. coli provides a simple and safe means to analyze its function(s) in alkane assimilation in the future.

Isolation and characterization of a regulatory gene affecting rhamnolipid biosurfactant synthesis in Pseudomonas aeruginosa.

A mutant strain (65E12) of Pseudomonas aeruginosa that is unable to produce rhamnolipid biosurfactants and lacks rhamnosyltransferase activity was genetically complemented by using a P. aeruginosa PG201 wild-type gene library. A single complementing cosmid was isolated on the basis of surface tension measurements of subcultures of the transconjugants by using a sib selection strategy. The subcloning of the complementing cosmid clone yielded a 2-kb fragment capable of restoring rhamnolipid biosynthesis, rhamnosyltransferase activity, and utilization of hexadecane as a C source in mutant 65E12. The nucleotide sequence of the complementing 2-kb fragment was determined, and a single open reading frame (rhlR) of 723 bp specifying a putative 28-kDa protein (RhlR) was identified. Sequence homologies between the RhlR protein and some regulatory proteins such as LasR of P. aeruginosa, LuxR of Vibrio fischeri, RhiR of Rhizobium leguminosarum, and the putative activator 28-kDa UvrC of Escherichia coli suggest that the RhlR protein is a transcriptional activator. A putative target promoter which is regulated by the RhlR protein has been identified 2.5 kb upstream of the rhlR gene. Multiple plasmid-based rhlR gene copies had a stimulating effect on the growth of the P. aeruginosa wild-type strain in hexadecane-containing minimal medium, on rhamnolipid production, and on the production of pyocyanin chromophores. Disruption of the P. aeruginosa wild-type rhlR locus led to rhamnolipid-deficient mutant strains, thus confirming directly that this gene is necessary for rhamnolipid biosynthesis. Additionally, such PG201::'rhlR' mutant strains lacked elastase activity, indicating that the RhlR protein is a pleiotropic regulator.

Isolation and characterization of a Pseudomonas aeruginosa gene, ptxR, which positively regulates exotoxin A production.

Exotoxin A production in Pseudomonas aeruginosa is a complicated and highly regulated process that involves several genes. In this report, we describe the isolation of a new toxA regulatory gene (ptxR) which affects exotoxin A production in P. aeruginosa. In an iron-deficient medium, the presence of a plasmid carrying ptxR in P. aeruginosa PAO1 resulted in a four-to fivefold increase in exotoxin A synthesis. No effect was observed on the levels of elastase, phospholipase C, exoenzyme S, and alkaline protease. Using subcloning and complementation experiments, ptxR was localized to a 2.1 kb Kpnl-Bg/II fragment. Nucleotide sequence analysis revealed the presence of an open reading frame which encodes a 34.97 kDa protein (PtxR). The size of the predicted PtxR compares closely with the 34 kDa PtxR that was synthesized in Escherichia coli using the T7 expression system. The deduced amino acid sequence of PtxR is homologous to that of several members of the LysR family of transcriptional activators. The amino-terminus region of PtxR contains a putative helix-turn-helix DNA-binding motif. Specific ptxR-deletion mutants in P. aeruginosa strains PAO1 and PA103 were constructed. In comparison with their parent strains, both mutants showed a significant reduction in the level of exotoxin A activity. However, upon extensive subculturing, the level of exotoxin A produced by the PAO1::ptxR mutant was similar to that of PAO1. Transcriptional studies, using both toxA-lacZ fusion and RNA analysis, confirmed that ptxR increases toxA and regA transcription. These results suggest that ptxR regulates (through regA) exotoxin A production at the transcriptional level.

Exotoxin A production in Pseudomonas aeruginosa requires the iron-regulated pvdS gene encoding an alternative sigma factor.

Exotoxin A (ETA) is secreted by Pseudomonas aeruginosa under iron-limiting growth conditions. The ETA structural gene, toxA, is regulated at the transcriptional level by the gene products of the regAB operon. The expression of both toxA and regAB is repressed under iron-replete conditions, suggesting a role for the ferric uptake regulator (Fur) in regulation of ETA synthesis; however, the Fur protein does not interact directly with the toxA or the regAB promoters. Evidence is presented that the iron control of ETA synthesis is mediated by a Fur-regulated alternative sigma factor, PvdS, which had initially been identified as a positive activator for the production of the siderophore pyoverdin. In a delta pvdS deletion mutant, ETA was produced at low levels of less than 5% compared to wild type, but still in response to iron starvation, and introduction of a functional pvdS gene on a plasmid fully restored wild-type levels and normal iron regulation of ETA synthesis. Therefore, a functional pvdS locus is essential for ETA production. Neither toxA nor regAB mRNA was detectable in a delta pvdS mutant. Overexpression of pvdS from the tac promoter on a plasmid resulted in a high-level and iron-independent production of ETA in wild-type PAO1, in the delta pvdS strain, but not in a delta regA strain as a host. These findings suggest that PvdS is required for the activation of the regAB promoters. The transcription of regAB and toxA after induction of the P tac-pvdS gene was monitored in cells grown in high-iron medium. While both regAB and toxA were highly expressed during all growth phases under microaerobic conditions, toxA transcripts were detected only during the exponential but not the early stationary phase of growth under aerobic conditions. These results suggest that a second regulatory mechanism besides the Fur-PvdS system is involved in iron regulation of ETA production.

Role of the Pseudomonas aeruginosa oxyR-recG operon in oxidative stress defense and DNA repair: OxyR-dependent regulation of katB-ankB, ahpB, and ahpC-ahpF.

Pseudomonas aeruginosa possesses an extensive armament of genes involved in oxidative stress defense, including katB-ankB, ahpB, and ahpC-ahpF. Transcription of these genes was regulated in response to H(2)O(2), paraquat, or organic peroxides. Expression of katB-lacZ and the observed KatB catalase levels in P. aeruginosa PAO1 were induced up to 250-fold after exposure to oxidative stress-generating compounds. Also, ahpB-lacZ and ahpC-lacZ expression was 90- and 3-fold higher, respectively, upon exposure to paraquat. The dose- and time-response curves revealed that 1 microM paraquat was sufficient for half-maximal activation of each reporter fusion within 5 min of exposure. Expression of these genes was not observed in a DeltaoxyR mutant, indicating that OxyR was essential for this response. The transcriptional start sites of katB-ankB, ahpB, and ahpC-ahpF were mapped, putative OxyR-binding sites were identified upstream of the -35 promoter elements, and direct binding of purified OxyR protein to these target promoters was demonstrated. The oxyR mutant was hypersusceptible to oxidative stress-generating agents, including H(2)O(2) and paraquat, in spite of total KatA catalase activity being comparable to that of the wild type. The oxyR phenotype was fully complemented by a plasmid containing the oxyR gene, while any of the katB, ahpB, or ahpCF genes alone resulted in only marginal complementation. Increased katB-lacZ expression and higher KatB catalase levels were detected in a DeltaahpCF background compared to wild-type bacteria, suggesting a compensatory function for KatB in the absence of AhpCF. In P. aeruginosa, oxyR is located upstream of recG, encoding a putative DNA repair enzyme. oxyR-lacZ and recG-lacZ reporter activities and oxyR-recG mRNA analysis showed that oxyR and recG are organized in an operon and expressed constitutively with regard to oxidative stress from a single promoter upstream of oxyR. Mutants affected in recG but not oxyR were dramatically impaired in DNA damage repair as measured by sensitivity to UV irradiation. In conclusion, we present evidence that the oxyR-recG locus is essential for oxidative stress defense and for DNA repair.

The oxygen- and iron-dependent sigma factor pvdS of Pseudomonas aeruginosa is an important virulence factor in experimental infective endocarditis.

In Pseudomonas aeruginosa, pvdS, a key oxygen (O2)-dependent locus, regulates expression of a number of virulence genes, including toxA (which encodes exotoxin A production). To define the in vivo role of differing O2 tensions on pseudomonal virulence, 2 knockout mutants, DeltapvdS and DeltatoxA, were compared with their parental strain, PA01, in rabbit aortic and tricuspid endocarditis models (representing aerobic vs. microaerobic conditions in vivo, respectively). In aortic endocarditis, DeltapvdS densities were significantly less than those of PA01 in vegetations, kidneys, and spleen (P<.01). In contrast, in tricuspid endocarditis, there were no significant differences between DeltapvdS and PA01 tissue densities in these same target tissues. The DeltatoxA mutant proliferated within target tissues to the same extent as the parental strain. Thus, pvdS (but not toxA) appears to be required for optimal virulence of P. aeruginosa, particularly in tissues preferentially exposed to high O2 tensions (e.g., aortic vegetations).

Pseudomonas aeruginosa fur overlaps with a gene encoding a novel outer membrane lipoprotein, OmlA.

A novel outer membrane lipoprotein in Pseudomonas aeruginosa is encoded by the omlA gene, which was identified immediately upstream of the fur (ferric uptake regulator) gene. The omlA and fur genes were divergently transcribed and had overlapping promoter regions. The proximal fur P2 promoter and the omlA promoter shared a 5-bp DNA motif for their -10 promoter elements. The distal fur P1 promoter was located within the omlA coding sequence, and the omlA and fur T1 mRNAs overlapped by 154 nucleotides. Optimal expression of both fur and omlA required roughly 200 bp of DNA upstream of the promoter regions, suggesting the presence of cis-acting transcriptional activation elements located within the omlA and fur genes, respectively. The levels of Fur and OmlA proteins had no influence on omlA or fur expression, excluding any trans-acting cross-regulation between fur and omlA. Expression of omlA was constitutive regardless of growth phase, oxygen tension, iron concentration, pH, and temperature. OmlA contained a signal sequence typical of bacterial lipoproteins, with a cysteine as a putative cleavage and lipid attachment site. Inhibition of signal peptidase II by globomycin resulted in failure to process OmlA, thus giving strong evidence that OmlA is a lipoprotein. Cell fractionation followed by Western blot analysis indicated that all OmlA protein is localized in the outer membrane. Mature OmlA was an acidic (pI = 4. 5) protein of 17.3 kDa and had close to 40% amino acid sequence identity to SmpA (small protein A) of Escherichia coli, Vibrio cholerae, and Haemophilus influenzae, a protein of unknown function. All P. aeruginosa strains tested as well as Pseudomonas fluorescens were found to produce OmlA. A mutant strain with impaired production of OmlA but no change in the expression of the overlapping fur gene was constructed. The omlA mutant was hypersusceptible to anionic detergents such as sodium dodecyl sulfate and deoxycholate, and it showed increased susceptibility to various antibiotics, including nalidixic acid, rifampin, novobiocin, and chloramphenicol. A structural role of OmlA in maintaining the cell envelope integrity is proposed.

Effect of catalase on hydrogen peroxide penetration into Pseudomonas aeruginosa biofilms.

The penetration of hydrogen peroxide into biofilms formed by wild-type and catalase-deficient Pseudomonas aeruginosa strains was measured using microelectrodes. A flowing stream of hydrogen peroxide (50 mM, 1 h) was unable to penetrate or kill wild-type biofilms but did penetrate and partially kill biofilms formed by an isogenic strain in which the katA gene was knocked out. Catalase protects aggregated bacteria by preventing full penetration of hydrogen peroxide into the biofilm.