Analysis of gene order data supports vertical inheritance of the leukotoxin operon and genome rearrangements in the 5' flanking region in genus Mannheimia.
ABSTRACT: BACKGROUND: The Mannheimia subclades belong to the same bacterial genus, but have taken divergent paths toward their distinct lifestyles. For example, M. haemolytica + M. glucosida are potential pathogens of the respiratory tract in the mammalian suborder Ruminantia, whereas M. ruminalis, the supposed sister group, lives as a commensal in the ovine rumen. We have tested the hypothesis that vertical inheritance of the leukotoxin (lktCABD) operon has occurred from the last common ancestor of genus Mannheimia to any ancestor of the diverging subclades by exploring gene order data. RESULTS: We examined the gene order in the 5' flanking region of the leukotoxin operon and found that the 5' flanking gene strings, hslVU-lapB-artJ-lktC and xylAB-lktC, are peculiar to M. haemolytica + M. glucosida and M. granulomatis, respectively, whereas the gene string hslVU-lapB-lktC is present in M. ruminalis, the supposed sister group of M. haemolytica + M. glucosida, and in the most ancient subclade M. varigena. In M. granulomatis, we found remnants of the gene string hslVU-lapB-lktC in the xylB-lktC intergenic region. CONCLUSION: These observations indicate that the gene string hslVU-lapB-lktC is more ancient than the hslVU-lapB-artJ-lktC and xylAB-lktC gene strings. The presence of (remnants of) the ancient gene string hslVU-lapB-lktC among any subclades within genus Mannheimia supports that it has been vertically inherited from the last common ancestor of genus Mannheimia to any ancestor of the diverging subclades, thus reaffirming the hypothesis of vertical inheritance of the leukotoxin operon. The presence of individual 5' flanking regions in M. haemolytica + M. glucosida and M. granulomatis reflects later genome rearrangements within each subclade. The evolution of the novel 5' flanking region in M. haemolytica + M. glucosida resulted in transcriptional coupling between the divergently arranged artJ and lkt promoters. We propose that the chimeric promoter have led to high level expression of the leukotoxin operon which could explain the increased potential of certain M. haemolytica + M. glucosida strains to cause a particular type of infection.
Project description:The mosaic structure and molecular evolution of the leukotoxin operon (lktCABD) was investigated by nucleotide sequence comparison of the lktC, lktB, and lktD genes in 23 Mannheimia (Pasteurella) haemolytica, 6 Mannheimia glucosida, and 4 Pasteurella trehalosi strains. Sequence variation in the lktA gene has been described previously (R. L. Davies et al., J. Bacteriol. 183:1394-1404, 2001). The leukotoxin operon of M. haemolytica has a complex mosaic structure and has been derived by extensive inter- and intraspecies horizontal DNA transfer and intragenic recombination events. However, the pattern of recombination varies throughout the operon and among the different evolutionary lineages of M. haemolytica. The lktA and lktB genes have the most complex mosaic structures with segments derived from up to four different sources, including M. glucosida and P. trehalosi. In contrast, the lktD gene is highly conserved in M. haemolytica. The lktC, lktA, and lktB genes of strains representing the major ovine lineages contain recombinant segments derived from bovine or bovine-like serotype A2 strains. These findings support the previous conclusion that host switching of bovine A2 strains from cattle to sheep has played a major role in the evolution of the leukotoxin operon in ovine strains of M. haemolytica. Homologous segments of donor and recipient alleles are identical, or nearly identical, indicating that the recombinational exchanges occurred relatively recent in evolutionary terms. The 5' and 3' ends of the operon are highly conserved in M. haemolytica, which suggests that multiple horizontal exchanges of the complete operon have occurred by a common mechanism such as transduction. Although the lktA and lktB genes both have complex mosaic structures and high nucleotide substitution rates, the amino acid diversity of LktB is significantly lower than that of LktA due to a higher degree of evolutionary constraint against amino acid replacement. The recombinational exchanges within the leukotoxin operon have had greatest effect on LktA and probably provide an adaptive advantage against the host antibody response by generating novel antigenic variation at surface-exposed sites.
Project description:Trans-tracheal aspirations from 56 apparently healthy calves and 34 calves with clinical signs of pneumonia were collected in six different herds during September and November 2002. The 90 samples were cultivated and investigated by PCR tests targeting the species Histophilus somni, Mannheimia haemolytica, Pasteurella multocida, Mycoplasma bovis, Mycoplasma dispar, and Mycoplasma bovirhinis. A PCR test amplifying the lktC-artJ intergenic region was evaluated and shown to be specific for the two species M. haemolytica and Mannheimia glucosida. All 90 aspirations were also analyzed for bovine respiratory syncytial virus (BRSV), parainfluenza-3 virus, and bovine corona virus by antigen ELISA. Surprisingly, 63% of the apparently healthy calves harbored potentially pathogenic bacteria in the lower respiratory tract, 60% of these samples contained either pure cultures or many pathogenic bacteria in mixed culture. Among diseased calves, all samples showed growth of pathogenic bacteria in the lower respiratory tract. All of these were classified as pure culture or many pathogenic bacteria in mixed culture. A higher percentage of the samples were positive for all bacterial species in the group of diseased animals compared to the clinically healthy animals, however this difference was only significant for M. dispar and M. bovirhinis. M. bovis was not detected in any of the samples. BRSV was detected in diseased calves in two herds but not in the clinically healthy animals. Among the diseased calves in these two herds a significant increase in haptoglobin and serum amyloid A levels was observed compared to the healthy calves. The results indicate that haptoglobin might be the best choice for detecting disease under field conditions. For H. somni and M. haemolytica, a higher percentage of the samples were found positive by PCR than by cultivation, whereas the opposite result was found for P. multocida. Detection of P. multocida by PCR or cultivation was found to be significantly associated with the disease status of the calves. For H. somni a similar association with disease status was only observed for cultivation and not for PCR.
Project description:BACKGROUND: The Mannheimia species encompass a wide variety of bacterial lifestyles, including opportunistic pathogens and commensals of the ruminant respiratory tract, commensals of the ovine rumen, and pathogens of the ruminant integument. Here we present a scenario for the evolution of the leukotoxin promoter among representatives of the five species within genus Mannheimia. We also consider how the evolution of the leukotoxin operon fits with the evolution and maintenance of virulence. RESULTS: The alignment of the intergenic regions upstream of the leukotoxin genes showed significant sequence and positional conservation over a 225-bp stretch immediately proximal to the transcriptional start site of the lktC gene among all Mannheimia strains. However, in the course of the Mannheimia genome evolution, the acquisition of individual noncoding regions upstream of the conserved promoter region has occurred. The rate of evolution estimated branch by branch suggests that the conserved promoter may be affected to different extents by the types of natural selection that potentially operate in regulatory regions. Tandem repeats upstream of the core promoter were confined to M. haemolytica with a strong association between the sequence of the repeat units, the number of repeat units per promoter, and the phylogenetic history of this species. CONCLUSION: The mode of evolution of the intergenic regions upstream of the leukotoxin genes appears to be highly dependent on the lifestyle of the bacterium. Transition from avirulence to virulence has occurred at least once in M. haemolytica with some evolutionary success of bovine serotype A1/A6 strains. Our analysis suggests that changes in cis-regulatory systems have contributed to the derived virulence phenotype by allowing phase-variable expression of the leukotoxin protein. We propose models for how phase shifting and the associated virulence could facilitate transmission to the nasopharynx of new hosts.
Project description:The Pasteurella haemolytica leukotoxin gene cluster (lktCABD) is homologous to the Escherichia coli hemolysin locus (hlyCABD). Since the cloned leukotoxin (LktA) is not secreted from E. coli cells, a heteroplasmid complementation system was developed that permits secretion of the leukotoxin from cells expressing the hemolysin transport proteins HlyB and HlyD. We observed that the secreted leukotoxin protein had weak hemolytic activity when activated by either the HlyC or LktC proteins and that LktC expressed in E. coli could confer weak hemolytic activity upon hemolysin. Thus, it appears that the accessory proteins of the leukotoxin and hemolysin gene clusters are functionally similar, although their expression in E. coli is not equivalent. Northern (RNA) blot analysis of the P. haemolytica leukotoxin gene cluster revealed a major 3.5-kilobase transcript that includes the lktC and lktA genes. The start site for this transcript mapped to a cytosine residue 30 nucleotides upstream from the putative start of lktC; a similar initiation site was observed in E. coli, although adjacent cytosine and adenine residues were also utilized. The 3.5-kilobase transcript terminated near the rho-independent terminator structure between lktA and lktB, but transcription may continue, via antitermination or de novo transcription initiation, into the downstream lktB and lktD genes. We propose that the lack of LktB and LktD function in E. coli is a result, at least in part, of poor lktBD transcription and suggest that a P. haemolytica-specific regulator is required for optimal expression of the leukotoxin genes.
Project description:Mannheimia glucosida, M. haemolytica, and M. ruminalis were isolated from cases of acute mastitis in ewes. M. glucosida was found to be a common cause of clinical mastitis in sheep. Selected phenotypic tests in addition to genotyping were needed to definitively identify Mannheimia species causing ovine mastitis.
Project description:The OmpA (or heat-modifiable) protein is a major structural component of the outer membranes of gram-negative bacteria. The protein contains eight membrane-traversing beta-strands and four surface-exposed loops. The genetic diversity and molecular evolution of OmpA were investigated in 31 Mannheimia (Pasteurella) haemolytica, 6 Mannheimia glucosida, and 4 Pasteurella trehalosi strains by comparative nucleotide sequence analysis. The OmpA proteins of M. haemolytica and M. glucosida contain four hypervariable domains located at the distal ends of the surface-exposed loops. The hypervariable domains of OmpA proteins from bovine and ovine M. haemolytica isolates are very different but are highly conserved among strains from each of these two host species. Fourteen different alleles representing four distinct phylogenetic classes, classes I to IV, were identified in M. haemolytica and M. glucosida. Class I, II, and IV alleles were associated with bovine M. haemolytica, ovine M. haemolytica, and M. glucosida strains, respectively, whereas class III alleles were present in certain M. haemolytica and M. glucosida isolates. Class I and II alleles were associated with divergent lineages of bovine and ovine M. haemolytica strains, respectively, indicating a history of horizontal DNA transfer and assortative (entire gene) recombination. Class III alleles have mosaic structures and were derived by horizontal DNA transfer and intragenic recombination. Our findings suggest that OmpA is under strong selective pressure from the host species and that it plays an important role in host adaptation. It is proposed that the OmpA protein of M. haemolytica acts as a ligand and is involved in binding to specific host cell receptor molecules in cattle and sheep. P. trehalosi expresses two OmpA homologs that are encoded by different tandemly arranged ompA genes. The P. trehalosi ompA genes are highly diverged from those of M. haemolytica and M. glucosida, and evidence is presented to suggest that at least one of these genes was acquired by horizontal DNA transfer.
Project description:The molecular evolution of the leukotoxin structural gene (lktA) of Mannheimia (Pasteurella) haemolytica was investigated by nucleotide sequence comparison of lktA in 31 bovine and ovine strains representing the various evolutionary lineages and serotypes of the species. Eight major allelic variants (1.4 to 15.7% nucleotide divergence) were identified; these have mosaic structures of varying degrees of complexity reflecting a history of horizontal gene transfer and extensive intragenic recombination. The presence of identical alleles in strains of different genetic backgrounds suggests that assortative (entire gene) recombination has also contributed to strain diversification in M. haemolytica. Five allelic variants occur only in ovine strains and consist of recombinant segments derived from as many as four different sources. Four of these alleles consist of DNA (52.8 to 96.7%) derived from the lktA gene of the two related species Mannheimia glucosida and Pasteurella trehalosi, and four contain recombinant segments derived from an allele that is associated exclusively with bovine or bovine-like serotype A2 strains. The two major lineages of ovine serotype A2 strains possess lktA alleles that have very different evolutionary histories and encode divergent leukotoxins (5.3% amino acid divergence), but both contain segments derived from the bovine allele. Homologous segments of donor and recipient alleles are identical or nearly identical, indicating that the recombination events are relatively recent and probably postdate the domestication of cattle and sheep. Our findings suggest that host switching of bovine strains from cattle to sheep, together with inter- and intraspecies recombinational exchanges, has played an important role in generating leukotoxin diversity in ovine strains. In contrast, there is limited allelic diversity of lktA in bovine strains, suggesting that transmission of strains from sheep to cattle has been less important in leukotoxin evolution.
Project description:The leukotoxin produced by Actinobacillus actinomycetemcomitans has been implicated in the etiology of localized juvenile periodontitis. To initiate a genetic analysis into the role of this protein in disease, we have cloned its gene, lktA. We now present the complete nucleotide sequence of the lktA gene from A. actinomycetemcomitans. When the deduced amino acid sequence of the leukotoxin protein was compared with those of other proteins, it was found to be homologous to the leukotoxin from Pasteurella haemolytica and to the alpha-hemolysins from Escherichia coli and Actinobacillus pleuropneumoniae. Each alignment showed at least 42% identity. As in the other organisms, the lktA gene of A. actinomycetemcomitans was linked to another gene, lktC, which is thought to be involved in the activation of the leukotoxin. The predicted LktC protein was related to the leukotoxin/hemolysin C proteins from the other bacteria, since they shared a minimum of 49% amino acid identity. Surprisingly, although actinobacillus species are more closely related to pasteurellae than to members of the family Enterobacteriaciae, LktA and LktC from A. actinomycetemcomitans shared significantly greater sequence identity with the E. coli alpha-hemolysin proteins than with the P. haemolytica leukotoxin proteins. Despite the overall homology to the other leukotoxin/hemolysin proteins, the LktA protein from A. actinomycetemcomitans has several unique properties. Most strikingly, it is a very basic protein with a calculated pI of 9.7; the other toxins have estimated pIs around 6.2. The unusual features of the A. actinomycetemcomitans protein are discussed in light of the different species and target-cell specificities of the hemolysins and the leukotoxins.
Project description:All sixteen serotypes of Pasteurella haemolytica were shown to produce a leukotoxin protein which is immunologically related to the well-characterized serotype 1 leukotoxin. All of the leukotoxins were weakly hemolytic and were able to bind to BL-3 target cells. The leukotoxin determinants were characterized by Southern blot hybridization by use of the previously cloned serotype 1 determinant as the probe, and a number of distinct classes were identified. The leukotoxin determinants from serotypes 2, 3, and 11 were cloned. Nucleotide sequence analysis of the lktC and lktA genes of the serotype 3 and 11 determinants revealed nucleotide substitutions throughout the coding sequences. A comparison of the lktC and lktA genes and deduced proteins of serotypes 1, 3, and 11 showed that they are highly homologous.
Project description:A Pasteurella haemolytica-like organism, a new species of bacterium isolated from piglets with diarrhea, secretes a leukotoxin into the culture media. Western blot (immunoblot) analysis indicated that this leukotoxin cross-reacted with antileukotoxin antibody derived from cattle immunized with P. haemolytica. Five overlapping recombinant bacteriophages carrying the gene for this 105-kDa polypeptide were identified with a DNA probe containing sequences from the P. haemolytica lktCA genes from a P. haemolytica-like organism strain 5943 genomic library. Sequence analysis of a region of the cloned DNA revealed two open reading frames encoding proteins with predicted masses of 19.4 and 101.6 kDa. These genes, which we designate pllktC (P. haemolytica-like organism leukotoxin C gene) and pllktA (A gene), respectively, are similar in sequence to the RTX (repeat of toxin) toxin family. The structure of the 101.6-kDa protein derived from the DNA sequence shows three transmembrane domains in the N-terminal part of the protein, 13 glycine-rich repeat domains in the second half of the protein, and a hydrophobic C-terminal part. pllktC and pllktA are strongly homologous to P. haemolytica lktC and lktA genes. However, this leukotoxin kills both BL-3 and pig leukocytes and is not hemolytic.