The secreted hemolysins of Proteus mirabilis, Proteus vulgaris, and Morganella morganii are genetically related to each other and to the alpha-hemolysin of Escherichia coli

Haemolytic activity of Campylobacter pylori

The haemolytic activity of several clinical and reference strains of Campylobacter pylori was determined using cell-free preparations of broth-grown organisms and human, horse, guinea pig, rabbit and sheep erythrocytes. Significant levels of haemolysis were produced only when the cell-free preparations were concentrated tenfold. However, three of 14 strains still gave haemolysis values of less than 50% when tested with guinea pig erythrocytes. Significant haemolytic activity could not be demonstrated with preparations derived from bacteria cultured on solid medium. Irrespective of the strain examined or the type of preparation, strong urease activity was demonstrable. Purified urease had no effect on erythrocytes, and thiourea failed to inhibit the haemolytic activity of Campylobacter pylori cell-free preparations. It was concluded that ureolytic activity was not implicated in the lysis of erythrocytes, either by direct action or via the generation of ammonia. Furthermore, the haemolytic activity produced by Campylobacter pylori was found to be due to a secreted factor, possibly a haemolysin.

Potent leukocidal action of Escherichia coli hemolysin mediated by permeabilization of target cell membranes

The contribution of Escherichia coli hemolysin (ECH) to bacterial virulence has been considered mainly in context with its hemolytic properties. We here report that this prevalent bacterial cytolysin is the most potent leukocidin known to date. Very low concentrations (approximately 1 ng/ml) of ECH evoke membrane permeability defects in PMN (2-10 x 10(6) cells/ml) leading to an efflux of cellular ATP and influx of propidium iodide. The attacked cells do not appear to repair the membrane lesions. Human serum albumin, high density and low density lipoprotein, and IgG together protect erythrocytes and platelets against attack by even high doses (5-25 micrograms/ml) of ECH. In contrast, PMN are still permeabilized by ECH at low doses (50-250 ng/ml) in the presence of these plasma inactivators. Thus, PMN become preferred targets for attack by ECH in human blood and protein-rich body fluids. Kinetic studies demonstrate that membrane permeabilization is a rapid process, ATP-release commencing within seconds after application of toxin to leukocytes. It is estimated that membrane permeabilization ensues upon binding of approximately 300 molecules ECH/PMN. This process is paralleled by granule exocytosis, and by loss of phagocytic killing capacity of the cells. The recognition that ECH directly counteracts a major immune defence mechanism of the human organism through its attack on granulocytes under physiological conditions sheds new light on its possible role and potential importance as a virulence factor of E. coli.

Quantitative study of the binding and hemolytic efficiency of Escherichia coli hemolysin

Mono- and polyclonal antibodies were used to construct a sandwich enzyme-linked immunosorbent assay that permitted quantitation of Escherichia coli hemolysin in soluble and membrane-bound forms. Toxin concentrations of 4 to 14 micrograms/ml were measured in culture supernatants of E. coli LE 2001 at times of peak hemolytic activity. Quantitative studies on the binding of E. coli hemolysin to rabbit erythrocytes were conducted at 0 and 37 degrees C. At 37 degrees C, 85 to 95% of bindable toxin was cell bound after 60 min, and no saturability of binding was observed in the studied range of concentrations, which resulted in deposition of approximately 100 to 50,000 toxin molecules per cell. Binding was slower and less effective at 0 degrees C; however, hemolysis did occur at low temperature. The number of cell-bound toxin molecules required to generate a hemolytic lesion within 60 min was estimated to be approximately 100 molecules per cell at 37 degrees C and 800 to 1,000 molecules per cell at 0 degrees C. Upon prolonged incubation (5 to 20 h, 37 degrees C), the number of molecules evoking a functional lesion decreased to approximately 5 to 20 per cell. These results are compatible with the concept that E. coli hemolysin first adsorbs to the cell surface, with membrane insertion and pore formation following in a second step that may be temporally dissociated from that of binding. The data support the pore concept of toxin action by showing that attachment of a low and finite number of toxin molecules to an erythrocyte will ultimately generate a cytolytic lesion.

Identification and expression of the Actinobacillus actinomycetemcomitans leukotoxin gene

The leukotoxin produced by the oral bacterium Actinobacillus actinomycetemcomitans has been implicated in the pathogenesis of juvenile periodontitis. In order to elucidate the structure of the leukotoxin, molecular cloning of the leukotoxin gene was carried out. A DNA library of A. actinomycetemcomitans, strain JP2, was constructed by partial digestion of genomic DNA with Sau3AI and ligation of 0.5 to 5.0 kilobase pair fragments into the Bam HI site of the plasmid vector pENN-vrf. After transformation into E. coli RR1 (lambda cI857), the clones were screened for the production of A. actinomycetemcomitans leukotoxin with polyclonal antibody. Six immunoreactive clones were identified. The clones expressed proteins which ranged from 21-80 kilodaltons, and the clone designated pII-2, producing the largest protein was selected for further study. Antibodies eluted from immobilized pII-2 protein also recognized the native A. actinomycetemcomitans leukotoxin molecule indicating that both molecules shared at least one epitope. DNA sequence analysis demonstrated that there are regions of significant amino acid sequence homology between the cloned A. actinomycetemcomitans leukotoxin and two other cytolysins, Escherichia coli alpha-hemolysin and Pasteurella haemolytica leukotoxin, suggesting that a family of cytolysins may exist which share a common mechanism of killing but vary in their target cell specificity.

Isolation and analysis of the C-terminal signal directing export of Escherichia coli hemolysin protein across both bacterial membranes

We have studied the C-terminal signal which directs the complete export of the 1024-amino-acid hemolysin protein (HlyA) of Escherichia coli across both bacterial membranes into the surrounding medium. Isolation and sequencing of homologous hlyA genes from the related bacteria Proteus vulgaris and Morganella morganii revealed high primary sequence divergence in the three HlyA C-termini and highlighted within the extreme terminal 53 amino acids the conservation of three contiguous sequences, a potential 18-amino-acid amphiphilic alpha-helix, a cluster of charged residues, and a weakly hydrophobic terminal sequence rich in hydroxylated residues. Fusion of the C-terminal 53 amino acid sequence to non-exported truncated Hly A directed wild-type export but export was radically reduced following independent disruption or progressive truncation of the three C-terminal features by in-frame deletion and the introduction of translation stop codons within the 3' hlyA sequence. The data indicate that the HlyA C-terminal export signal comprises multiple components and suggest possible analogies with the mitochondrial import signal. Hemolysis assays and immunoblotting confirmed the intracellular accumulation of non-exported HlyA proteins and supported the view that export proceeds without a periplasmic intermediate. Comparison of cytoplasmic and extracellular forms of an independently exported extreme C-terminal 194 residue peptide showed that the signal was not removed during export.

Cloning, nucleotide sequence, and characterization of genes encoding the secretion function of the Pasteurella haemolytica leukotoxin determinant

The structural gene of the Pasteurella haemolytica leukotoxin determinant is highly homologous to that of the Escherichia coli hemolysin determinant, which also encodes a specialized set of genes involved in the secretion of the hemolysin. In this report, we describe the cloning and nucleotide sequence of the analogous secretion genes from P. haemolytica which make up the remainder of the leukotoxin determinant. The secretion genes were cloned directly from the P. haemolytica chromosome to form the recombinant plasmid pPH5B. By subcloning the secretion genes together with the leukotoxin structural gene, the cloned leukotoxin determinant was reconstructed on a single plasmid, pLKT52, which directs the synthesis of active leukotoxin to the culture supernatant when expressed in E. coli. DNA sequence analysis showed the presence of two secretion genes, designated lktB and lktD in order of their genetic organization, which code for proteins of 79.7 and 54.7 kilodaltons, both of which were detected when pLKT52 was expressed in E. coli minicells. The lktB and lktD genes were found to be highly homologous to the hlyB and hlyD secretion genes of the hemolysin determinant, and the predicted LktB-HlyB and LktD-HlyD proteins were 90.5 and 75.6% homologous. Nucleotide sequence homology between the leukotoxin and hemolysin determinants was limited to the C, A, B, and D coding regions, although the presence of similar transcriptional terminators in the A-B intercistronic region is suggestive of a similar transcriptional organization. On the basis of these data, we hypothesize that the two determinants share a common evolutionary history and are prototypes for a widely disseminated family of virulence factors, the RTX cytotoxins.

DNA sequence of the Pasteurella haemolytica leukotoxin gene cluster

Bovine serum was used to identify a recombinant phage clone carrying the Pasteurella haemolytica leukotoxin gene. This fragment produced the 102-kD leukotoxin and several smaller P. haemolytica-specific protein antigens in Escherichia coli. An additional contiguous fragment, containing sequences upstream from the leukotoxin gene. Using these clones, we determined the nucleotide sequence of a 7745-bp region that included four open reading frames: an upstream gene, lktC; the leukotoxin gene, lktA; and two downstream genes, lktB, and lktD. The predicted molecular weights of the proteins encoded by these genes were 19.9, 102, 79.6, and 54.7 kD, respectively. These genes and their predicted proteins were similar in organization and in sequence to the corresponding elements of the gene cluster that encodes an E. coli alpha-hemolysin and its activation and secretion functions. Expression of the leukotoxin was enhanced in E. coli, by fusing the gene to the lac promoter. Under these conditions the leukotoxin was not secreted into the medium, as it is in P. haemolytica. However, in the presence of the alpha-hemolysin genes, the leukotoxin was secreted into the medium, demonstrating functional complementation by the hemolysin secretory system.

The repeat domain of Escherichia coli haemolysin (HlyA) is responsible for its Ca2+-dependent binding to erythrocytes

The haemolysin protein (HlyA) of Escherichia coli contains 11 tandemly repeated sequences consisting of 9 amino acids each between amino acids 739 and 849 of HlyA. We removed, by oligonucleotide-directed mutagenesis, different single repeats and combinations of several repeats. The resulting mutant proteins were perfectly stable in E. coli and were secreted with the same efficiency as the wild-type HlyA. HlyA proteins which had lost a single repeat only were still haemolytically active (in the presence of HlyC) but required elevated levels of Ca2+ for activity, as compared to the wild-type haemolysin. Removal of three or more repeats led to the complete loss of the haemolytic activity even in the presence of high Ca2+ concentrations. The mutant haemolysins were unable to compete with the wild-type haemolysin for binding to erythrocytes at low Ca2+ concentrations but could still generate ion-permeable channels in artificial lipid bilayer membranes formed of plant asolectin, even in the complete absence of Ca2+. These data indicate that the repeat domain of haemolysin is responsible for Ca2+-dependent binding of haemolysin to the erythrocyte membrane. A model for the possible functional role of Ca2+ in haemolysis is presented.

Comparison of the haemolysin secretion protein HlyB from Proteus vulgaris and Escherichia coli; site-directed mutagenesis causing impairment of export function

The hlyB secretion genes of Proteus vulgaris and Escherichia coli showed 81% nucleotide homology and similar E. coli-atypical codon usage. The deduced protein sequences differed in 54 of 707 residues and shared a previously unreported sequence which corresponds to the ATP-binding motif characteristic of protein kinases. The motif was also conserved in the HlyB of Morganella morganii. Of 4 oligonucleotide-directed substitutions introduced into the putative E. coli HlyB motif, 2 non-conservative changes caused radical reductions in the export of active haemolysin protein.

Proteus mirabilis urease: genetic organization, regulation, and expression of structural genes

Proteus mirabilis, a cause of serious urinary tract infection, produces urease, an important virulence factor for this species. The enzyme hydrolyzes urea to CO2 and NH3, which initiates struvite or apatite stone formation. Genes encoding urease were localized on a P. mirabilis chromosomal DNA gene bank clone in Escherichia coli by deletion analysis, subcloning, Bal31 nuclease digestion, transposon Tn5 mutagenesis, and in vitro transcription-translation. A region of DNA between 4.0 and 5.4 kilobases (kb) in length was necessary for urease activity and was located within an 18.5-kb EcoRI fragment. The operon was induced by urea and encoded a multimeric, cytoplasmic enzyme comprising subunit polypeptides of 8,000, 10,000, and 73,000 daltons that were encoded by a single polycistronic mRNA and transcribed in that order. Seventeen urease-negative transposon insertions were isolated that synthesized either none of the structural subunit polypeptides, the 8,000-dalton polypeptide alone, or both the 8,000- and 10,000-dalton subunit polypeptides. The molecular weight of the native enzyme was estimated to be 212,000 by Superose-6 chromatography. Homologous sequences encoding the urease of Providencia stuartii synthesized subunit polypeptides of similar sizes and showed a similar genetic arrangement. However, restriction maps of the operons from the two species were distinct, indicating significant divergence.

Identification of the promoters directing in vivo expression of hemolysin genes in Proteus vulgaris and Escherichia coli

The hemolytic activity of Escherichia coli and Proteus vulgaris is determined by common contiguous genes encoding synthesis (hly C, hly A) and specific secretion (hly B, hly D) of active hemolysin. Nevertheless, the hly C-proximal DNA sequences directing production of the homologous hemolysins by the recombinant DNAs P. vulgaris pVU763-709 and E. coli pANN202-312 showed no extensive homology. Primer extension and S1 nuclease protection were used to define in the two sequences the 5' termini of hly transcripts synthesized in vivo and thus to infer the active hly promoters sequences. The E. coli hly C upstream region contained three separate promotors directing in vivo hly transcription, while the corresponding transcription of the P. vulgaris hly operon originated from a single distinct promotor, the -35 and -10 sequences of which formed part of an inverted repeat sequence. Elevated hemolytic activity caused by upstream Tn5 insertions in pVU763-709 resulted from increased transcription from this promotor.

Transcriptional organization of the Escherichia coli hemolysin genes

The transcriptional organization of the Escherichia coli hemolysin genes (hlyCABD) encoded by pSF4000 was examined. The use of different hemolysin gene-specific radiolabeled probes in blots containing isolated in vivo RNA revealed 4.0-kilobase hlyCA and 8.0-kilobase hlyCABD transcripts. The treatment of cells with rifampin just before RNA isolation showed the half-lives of these mRNAs to be 10.2 and 4.4 min, respectively. The 5' ends of the hly transcripts were 462 and 464 nucleotides from the putative initiation codon of hlyC based on a primer extension method of RNA mapping. Deletion analysis of pSF4000 combined with quantification of the hemolysin structural protein HlyA by immunoblotting confirmed that major control of HlyA expression occurs within a 168-base-pair PstI fragment located 433 base pairs upstream of the start of hlyC. A second recombinant plasmid, pANN202-312, encoding an E. coli hemolysin of different origin expressed 6-fold less total HlyA and 50-fold less extracellular HlyA than pSF4000 in identical cell backgrounds. The pANN202-312 recombinant had a different hly promoter, with the hly mRNA beginning 264 nucleotides upstream from the start of hlyC. We showed by RNA blotting that cells harboring pANN202-312 compared with pSF4000 have similar steady-state levels of the hlyCA transcript but they lack a consistently detectable hlyCABD transcript. We propose that one reason for the disparate levels of extracellular hemolysin produced by hemolytic E. coli is dissimilar levels of mRNA encoding in part the transport genes hlyB and hlyD.

Iron and virulence in the family Enterobacteriaceae

The ability of bacterial pathogens to acquire iron in the host is an essential component of the disease process. Pathogenic Enterobacteriaceae spp. may either scavenge host iron sources such as heme or induce high-affinity iron-transport systems to remove iron from host proteins. The ease with which iron is acquired from the host will be at least partially determined by the iron status of the host at the time of infection. In response to infection, mammalian hosts reduce serum iron levels and withhold iron from the invading microorganisms. Thus the competition for iron is an active process which influences the outcome of a host-bacterial interaction.

Extensive homology between the leukotoxin of Pasteurella haemolytica A1 and the alpha-hemolysin of Escherichia coli

The 19.8- and 101.9-kilodalton leukotoxin proteins of Pasteurella haemolytica (LKTC and LKTA, respectively) share extensive homology with the HLYC and HLYA alpha-hemolysin proteins of Escherichia coli. The leukotoxin LKTA protein cross-reacts with hemolysin-specific antisera in Western blot (immunoblot) analysis, indicating that it shares epitopes with the alpha-hemolysin HLYA protein. Both LKTA and HLYA contain a conserved hydrophobic region, as well as a set of tandemly repeated domains. These features have been implicated in the lytic function of the alpha-hemolysin.

Identification of two different hemolysin determinants in uropathogenic Proteus isolates

DNA sequences similar to those of the Escherichia coli hemolysin genes were detected among uropathogenic isolates of Proteus vulgaris and Morganella morganii by using the Southern blotting technique and hly gene-specific DNA probe. Immunoblotting revealed that among the hemolytic P. vulgaris and M. morganii isolates there was expressed a polypeptide species similar in molecular size (110 kilodaltons) and antigenicity to Escherichia coli HlyA. A plasmid-mediated P. vulgaris hemolysin determinant identified by Southern blotting analysis was molecularly cloned, and the recombinant plasmid (pWPV100) was characterized by restriction endonuclease fragment mapping. A second recombinant library of genomic DNA prepared from a hemolytic, urinary tract isolate of Proteus mirabilis was constructed in E. coli. A 5.5-kilobase XhoI fragment encoding an extracellular hemolytic activity was molecularly cloned (pWPM100), and this plasmid was subjected to transposon-mediated mutagenesis with TnphoA. The P. mirabilis hemolytic phenotype was determined to be encoded by a polypeptide species (HpmA) with an estimated molecular size of 140 kilodaltons based on minicell polypeptide analysis of pWPM100 and its mutant derivatives. Southern blotting analysis with a HpmA-specific DNA probe revealed that this novel determinant is commonly found in both Proteus species but is not present in hemolytic isolates of M. morganii, E. coli, Citrobacter freundii, and Serratia marcescens.

Nucleotide sequence of the leukotoxin genes of Pasteurella haemolytica A1

A 4.4-kilobase-pair DNA fragment coding for the leukotoxin of Pasteurella haemolytica A1 has been isolated, and its nucleotide sequence has been determined. Two open reading frames, designated lktC and lktA, coding for proteins of 19.8 and 101.9 kilodaltons, respectively, were identified. Expression of the two genes in minicell-labeling experiments resulted in the production of the predicted proteins LKTC and LKTA. By using an antiserum against the soluble antigens of P. haemolytica A1 in Western blot (immunoblot) analysis of total cellular proteins from the Escherichia coli clones, LKTA was identified as an additional antigenic protein. Results from subcloning of the DNA fragment suggested that expression from both lktC and lktA is required for leukotoxin activity, indicating that the leukotoxin of P. haemolytica A1 is encoded by two genes. A comparison of the organization and the DNA sequence of the leukotoxin genes with those of the E. coli alpha-hemolysin genes showed a significant degree of homology between the two loci. This analysis suggested that the leukotoxin genes of P. haemolytica A1 and the E. coli alpha-hemolysin genes may have evolved from a common ancestor and that the two toxins may share similar activities or functional domains or both.