Evidence for Vertical Inheritance and Loss of the Leukotoxin Operon in Genus Mannheimia

Polar bear-adapted Ursidibacter maritimus are remarkably conserved after generations in captivity

Most species in the bacterial family of Pasteurellaceae colonize one specific host species. Vertebrates of very different evolutionary descent including fish, turtles, marsupials, eutherians and birds are colonized by different members of Pasteurellaceae. This one-to-one microbial-host species partnership makes Pasteurellaceae species valuable candidates to study biodiversity, bacterial-host co-evolution and host adaptation, and their widespread distribution across vertebrates provide the possibility to collect a wide array of data, where wildlife species are essential. However, obtaining samples from wild animals comes with logistic, technical and ethical challenges, and previous microbiota studies have led to the presumption that captive animals are poor models for microbial studies in wildlife. Here, we show that colonization of polar bears by Ursidibacter maritimus is unaffected by factors related to captivity, reflecting a deep symbiotic bond to the host. We argue that the study of ecological and evolutionary principles in captive wildlife is possible for host-adapted taxa such as those in the Pasteurellaceae family. Moreover, studying captive, often trained animals protects wild populations from the stress associated with obtaining samples.

RTX Toxins of Animal Pathogens and Their Role as Antigens in Vaccines and Diagnostics

Exotoxins play a central role in the pathologies caused by most major bacterial animal pathogens. The large variety of vertebrate and invertebrate hosts in the animal kingdom is reflected by a large variety of bacterial pathogens and toxins. The group of repeats in the structural toxin (RTX) toxins is particularly abundant among bacterial pathogens of animals. Many of these toxins are described as hemolysins due to their capacity to lyse erythrocytes in vitro. Hemolysis by RTX toxins is due to the formation of cation-selective pores in the cell membrane and serves as an important marker for virulence in bacterial diagnostics. However, their physiologic relevant targets are leukocytes expressing β2 integrins, which act as specific receptors for RTX toxins. For various RTX toxins, the binding to the CD18 moiety of β₂ integrins has been shown to be host specific, reflecting the molecular basis of the host range of RTX toxins expressed by bacterial pathogens. Due to the key role of RTX toxins in the pathogenesis of many bacteria, antibodies directed against specific RTX toxins protect against disease, hence, making RTX toxins valuable targets in vaccine research and development. Due to their specificity, several structural genes encoding for RTX toxins have proven to be essential in modern diagnostic applications in veterinary medicine.

Sequence diversity of the leukotoxin (lktA) gene in caprine and ovine strains of Mannheimia haemolytica

Mannheimia haemolytica is the aetiological agent of pneumonic pasteurellosis in small ruminants. The primary virulence factor of the bacterium is a leukotoxin (LktA), which induces apoptosis in susceptible cells via mitochondrial targeting. It has been previously shown that certain lktA alleles are associated either with cattle or sheep. The objective of the present study was to investigate lktA sequence variation among ovine and caprine M haemolytica strains isolated from pneumonic lungs, revealing any potential adaptation for the caprine host, for which there is no available data. Furthermore, we investigated amino acid variation in the N-terminal part of the sequences and its effect on targeting mitochondria. Data analysis showed that the prevalent caprine genotype differed at a single non-synonymous site from a previously described uncommon bovine allele, whereas the ovine sequences represented new, distinct alleles. N-terminal sequence differences did not affect the mitochondrial targeting ability of the isolates; interestingly enough in one case, mitochondrial matrix targeting was indicated rather than membrane association, suggesting an alternative LktA trafficking pattern.

Bronchopneumonia in two dairy calves associated with Mannheimia species cluster V infection

The pathological, bacteriological, and molecular findings of two 3-week-old Holstein calves with bronchopneumonia are presented. Heavy pure growth of a Mannheimia species most closely aligned with the unnamed cluster V strains on the basis of 16S ribosomal RNA sequencing was detected in the lungs of both calves in association with Bovine respiratory syncytial virus infection. While Mannheimia species closely related to cluster V strains have occasionally been reported in association with pneumonia, meningitis, and abortion in cattle, the current report provides a description of the gross and histopathological lesions produced by a cluster V strain of Mannheimia species. Lesions in the lung were found to be typical of those described for Mannheimia haemolytica with the absence of areas of coagulation necrosis rimmed by leukocytes and more pronounced intra-alveolar hemorrhage. Lesions were linked to the presence of leukotoxin A based on phenotypic hemolysis and molecular demonstration of the leukotoxin A gene.

The role of RTX toxins in host specificity of animal pathogenic Pasteurellaceae

RTX toxins are bacterial pore-forming toxins that are particularly abundant among pathogenic species of Pasteurellaceae, in which they play a major role in virulence. RTX toxins of several primary pathogens of the family of Pasteurellaceae are directly involved in causing necrotic lesions in the target organs. Many RTX toxins are known as haemolysins because they lyse erythrocytes in vitro, an effect that is non-specific, but which serves as a useful marker in bacteriological identification and as an easily measurable signal in vitro in experimental studies. More recent studies have shown that the specific targets of most RTX toxins are leukocytes, with RTX toxins binding to the corresponding β-subunit (CD18) of β2 integrins and then exerting cytotoxic activity. After uptake by the target cell, at sub-lytic concentrations, some RTX toxins are transported to mitochondria and induce apoptosis. For several RTX toxins the binding to CD18 has been shown to be host specific and this seems to be the basis for the host range specificity of these RTX toxins. Observations on two very closely related species of the Pasteurellaceae family, Actinobacillus suis, a porcine pathogen particularly affecting suckling pigs, and Actinobacillus equuli subsp. haemolytica, which causes pyosepticaemia in new-born foals (sleepy foal disease), have revealed that they express different RTX toxins, named ApxI/II and Aqx, respectively. These RTX toxins are specifically cytotoxic for porcine and equine leukocytes, respectively. Furthermore, the ApxI and Aqx toxins of these species, when expressed in an isogenetic background in Escherichia coli, are specifically cytotoxic for leukocytes of their respective hosts. These data indicate the determinative role of RTX toxins in host specificity of pathogenic species of Pasteurellaceae.

Mannheimia species associated with ovine mastitis

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.

A multiplex polymerase chain reaction assay for the identification of Mannheimia haemolytica, Mannheimia glucosida and Mannheimia ruminalis

The objective of this study was to design a multiplex PCR assay to identify Mannheimia haemolytica, Mannheimia glucosida and Mannheimia ruminalis. The multiplex PCR included primer sets HP, amplifying a DNA region from an unknown hypothetical protein, Lkt and Lkt2, amplifying different regions of the leukotoxinD gene, and 16S to amplify universal bacterial sequences of the 16S rRNA gene. Based on positive amplification, isolates were delineated as M. haemolytica (HP, Lkt, 16S), M. glucosida (HP, Lkt, Lkt2, 16S), or M. ruminalis (HP, 16S). The validity of the assay was examined against 22 reference strains within the family Pasteurellaceae and 17 field isolates (nasal) that had been collected previously from feedlot cattle and tentatively identified as M. haemolytica based on morphology and substrate utilization. Additionally, 200 feedlot cattle were screened for M. haemolytica using multiplex PCR. Forty-four isolates from 25 animals were identified as M. haemolytica. The PCR assay positively identified all M. haemolytica, as confirmed by phenotypic tests and clustering based upon cellular fatty acid methyl ester (FAME) profiles. Selected nasal isolates that exhibited evidence of haemolysis, but were M. haemolytica-negative based on PCR, were also confirmed negative by phenotypic and FAME analyses. The multiplex PCR assay required no additional phenotypic tests for confirmation of M. haemolytica, within the group of bacteria tested.

Analysis of gene order data supports vertical inheritance of the leukotoxin operon and genome rearrangements in the 5' flanking region in genus Mannheimia

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.

Occurrence of haemolytic Mannheimia spp. in apparently healthy sheep in Norway

Background

The occurrence of Mannheimia species in healthy sheep has only been investigated to a very limited extend since the genus and its five named species were established. The aim of the present study was to evaluate the occurrence of haemolytic Mannheimia species in apparently healthy sheep originating from four sheep flocks in South-Western Norway.

Methods

Typical β-haemolytic Pasteurellaceae were isolated from nasal swabs and subsequently subjected to bacteriological examination. A total of 57 Mannheimia isolates were obtained in pure culture. All isolates were genotyped by amplified fragment length polymorphisms (AFLP) analysis and compared to six reference strains. The 16S rRNA gene sequences of two isolates were also determined.

Results

β-haemolytic Mannheimia species were isolated from 24% to 64% of the sheep in the four flocks. A total of 26 haemolytic M. ruminalis-like strains were isolated among which, a considerable genetic diversity was found. Eighteen M. glucosida isolates were obtained from three flocks, whereas M. haemolytica was only isolated from two flocks, 16 of them being from only one of the flocks.

Conclusion

We demonstrate that a relatively high number of apparently healthy sheep in Norway seem to carry the potentially pathogenic M. haemolytica and M. glucosida in the upper respiratory tract. An unexpectedly high number of haemolytic M. ruminalis-like organisms were also obtained in all four flocks. The usually non-haemolytic M. ruminalis are typically isolated from healthy ruminants. The significance of β-haemolytic M. ruminalis-like organisms is unknown and should be investigated in a future study.