Mechanism of the cytolytic action of Pseudomonas aeruginosa cytotoxin: oligomerization of the cytotoxin on target membranes

Pseudomonas aeruginosa Cytotoxins: Mechanisms of Cytotoxicity and Impact on Inflammatory Responses

Pseudomonas aeruginosa is one of the most virulent opportunistic Gram-negative bacterial pathogens in humans. It causes many acute and chronic infections with morbidity and mortality rates as high as 40%. P. aeruginosa owes its pathogenic versatility to a large arsenal of cell-associated and secreted virulence factors which enable this pathogen to colonize various niches within hosts and protect it from host innate immune defenses. Induction of cytotoxicity in target host cells is a major virulence strategy for P. aeruginosa during the course of infection. P. aeruginosa has invested heavily in this strategy, as manifested by a plethora of cytotoxins that can induce various forms of cell death in target host cells. In this review, we provide an in-depth review of P. aeruginosa cytotoxins based on their mechanisms of cytotoxicity and the possible consequences of their cytotoxicity on host immune responses.

[Genome analysis-based studies on bacterial genetic diversity]

There are a huge number of bacterial species on earth, and a huge intra-species genomic diversity are also observed in many bacteria. The high ability of bacteria to acquire foreign DNA and the presence of various mobile genetic elements contribute the generation of such genomic diversity. During the biochemical and genetic analysis of a Pseudomonas aeruginosa toxin, called cytotoxin, and its converting phage, which I first engaged in my research carrier, I became very interested in the genetic diversity of bacteria and mobile genetic elements such as bacteriophages, and realized the usefulness and power of genome analysis. Since then, I have been involved in genome analyses of various pathogenic bacteria such as enterohemorrhagic Escherichia coli (EHEC), commensal bacteria of human and other animals, and bacteria or bacterial communities in natural environments. I was so lucky that I jumped in this research field at the very begging of genome analyses and experienced a very exciting time of surprisingly rapid advance in genome sequencing technologies which revolutionized a wide range of biology. In this article, I first review the main findings which our group obtained from the genome analyses on the P. aeruginosa cytotoxin converting phage and those on the evolution and genomic diversity of EHEC and related bacteria. The results of our analyses of Rickettsiaceae family genomes, which show surprisingly very low genomic diversity, and genome sequence-based analyses of an intrahospital bacterial outbreak and within-host genomic diversity are also summarized.

Blood components influence cytokine induction by bacterial substances

While some studies clearly demonstrated transfer of cytokine-inducing substances (CIS) through dialysis membranes, other authors were unable to reproduce such a transfer. This inconsistency may have been caused by marked differences in experimental design. We performed a systematic evaluation of cytokine induction in whole blood and from separated peripheral blood mononuclear cells (PBMC) using either purified lipopolysaccharide (LPS) or culture supernatants from various bacterial strains. An in vitro hemodialysis circuit with whole blood in the blood compartment was employed; the dialysate was contaminated with sterile filtrates from Pseudomonas aeruginosa cultures. Addition of plasma samples from the blood side to PBMC readily induced interleukin (IL)-1beta and IL-6 after contamination of the dialysate, while the same samples failed to induce cytokine production in whole blood. In experiments using direct incubation, purified P. aeruginosa LPS induced more IL-1beta and IL-6 in whole blood compared to PBMC. In contrast, bacterial filtrates from Escherichia coli, Enterobacter cloacae and especially P. aeruginosa induced significantly less cytokines in whole blood compared to PBMC. Addition of erythrocytes but not granulocytes decreased cytokine induction by bacterial filtrates as well as by LPS, probably by adsorption of these substances. The addition of 30% plasma increased cytokine induction by LPS but decreased cytokine induction by P. aeruginosas filtrates. Our results suggest that cellular and plasmatic components of whole blood interact with bacterial CIS altering their pyrogenic activity. The detection of CIS depends on the test system used; whole blood cultures are not as sensitive for the detection of CIS from bacterial filtrates as PBMC cultures.

Epidemic population structure of Pseudomonas aeruginosa: evidence for a clone that is pathogenic to the eye and that has a distinct combination of virulence factors

The genetic structure of a population of Pseudomonas aeruginosa, isolated from patients with keratitis, endophthalmitis, and contact lens-associated red eye, contact lens storage cases, urine, ear, blood, lungs, wounds, feces, and the environment was determined by multilocus enzyme electrophoresis. The presence and characteristics of virulence factors were determined by restriction fragment length polymorphism analysis with DNA probes for lasA, lasB, aprA, exoS, exoT, exoU, and ctx and by zymography of staphylolysin, elastase, and alkaline protease. These analyses revealed an epidemic population structure of P. aeruginosa, characterized by frequent recombination in which a particular successful clone may increase, predominate for a time, and then disappear as a result of recombination. Epidemic clones were found among isolates from patients with keratitis. They were characterized by high activity of a hitherto-unrecognized size variant of elastase, high alkaline protease activity, and possession of the exoU gene encoding the cytotoxic exoenzyme U. These virulence determinants are not exclusive traits in strains causing keratitis, as strains with other properties may cause keratitis in the presence of predisposing conditions. There were no uniform patterns of characteristics of isolates from other types of infection; however, all strains from urinary tract infections possessed the exoS gene, all strains from environment and feces and the major part of keratitis and wound isolates exhibited high elastase and alkaline protease activity, and all strains from feces showed high staphylolysin activity, indicating that these virulence factors may be important in the pathogenesis of these infectious diseases.

Oligomerization and structural changes of the pore-forming Pseudomonas aeruginosa cytotoxin

Pseudomonas aeruginosa produces a pathogenic factor, the 29-kDa pore-forming protein cytotoxin. Nonspecific oligomers of cytotoxin up to the hexamer, induced by oxidative crosslinking or detergent micellae, were based on intermolecular disulfide bridges. SDS induced tetramer, hexamer and mainly pentamers that were resistant to reducing conditions, indicating an additional oligomerization mechanism. Functional oligomerization after incubation with different membranes resulted in an oligomer of approximately 145 kDa that was identified as the pentamer by comparison with the SDS-induced oligomers. Covalent modification with diethylpyrocarbonate showed that histidine residues are indispensable for functional pentamerization. Pentamer formation was not influenced by the lipid composition of the liposomes tested, indicating that rising membrane fluidity did not increase oligomerization. The secondary structure of cytotoxin determined by spectroscopy is characterized by approximately 50% beta-sheet, 20% beta-turn, 10% alpha-helix and 20% remaining structure. Contact with detergent micellae or liposomes induced a reorganization of beta-structure associations, as observed by attenuated total reflection-Fourier transform infrared spectroscopy. Electron microscopy and principle component analysis of the cytotoxin monomer demonstrated a tapered molecule of 11 nm in length and a maximum width of 3.5 nm. These results classify the cytotoxin as a pore-forming toxin, rich in antiparallel beta-structure, that needs to oligomerize and inserts into membranes; it is very similar to the Staphylococcus aureus alpha-toxin.

Purification and characterization of procytotoxin of Pseudomonas aeruginosa. Dimer to monomer conversion of protoxin by proteolytic activation

Cytotoxin of Pseudomonas aeruginosa is a cytolytic toxin that forms a pore on the target membrane by oligomerizing into a pentamer. This toxin is produced as an inactive precursor (proCTX) and is converted to an active form by proteolytic cleavage at the C terminus. We purified proCTX to apparent homogeneity and characterized it in a comparison with the active toxin. ProCTX bound to the erythrocyte membrane but did not form an oligomer on the membrane, hence the lack of hemolytic activity in proCTX. Circular dichroic experiments showed that active and proCTX have similar beta-sheet dominant structures. Intrinsic fluorescence analysis indicated that a molecule-buried tryptophan residue(s) of proCTX was exposed to the surface of the molecule as a result of conversion to the active form. In analytical gel filtration, chemical cross-linking, and analytical ultracentrifugation experiments, dimer to monomer conversion occurred with proteolytic activation.

Determination of the amino acid sequence of the plant cytolysin enterolobin

The cytolytic seed protein enterolobin from seeds of Enterolobium contortisiliquum was purified by using FPLC on a Mono Q column giving a single peak in capillary electrophoresis. The complete amino acid sequence of the plant cytolysin was determined by an automated method, yielding a molecular mass of 54,806 Da. Databank searches and sequence alignment demonstrated a high degree of sequence identity and similarity between enterolobin and bacterial aerolysins from Aeromonas hydrophila and A. sobria. Several key residues involved in oligomerization of A. hydrophila aerolysin are conserved in enterolobin. Circular dichroism measurements and structural predictions revealed that enterolobin is very rich in beta sheet, like aerolysin. Light-scattering studies revealed that enterolobin oligomerizes as a hexamer at pH levels below 7.0. NaCl concentrations above 50 mM caused dimerization of enterolobin. Dithiothreitol did not cause oligomerization.