First chromosomal restriction map of Actinobacillus pleuropneumoniae and localization of putative virulence-associated genes

An Actinobacillus pleuropneumoniae arcA deletion mutant is attenuated and deficient in biofilm formation

Actinobacillus pleuropneumoniae is a facultative anaerobic pathogen of the porcine respiratory tract requiring anaerobic metabolic activity for persistence on lung epithelium. The ArcAB two-component system facilitating metabolic adaptation to anaerobicity was investigated with regard to its impact on virulence and colonization of the porcine respiratory tract. Using pig infection experiments we demonstrate that deletion of arcA renders A. pleuropneumoniae significantly attenuated in acute infection and reduced long-term survival on unaltered lung epithelium as well as in sequesters. Contrary to its role in enterobacteria, the deletion of arcA in A. pleuropneumoniae does not affect growth and survival under anaerobic conditions. Instead, other than the parent strain A. pleuropneumoniae DeltaarcA does not show autoaggregation under anaerobic conditions and is deficient in biofilm formation. It is hypothesized that the lack of these functions is, at least in part, responsible for the reduction of virulence.

ISApl1, a novel insertion element of Actinobacillus pleuropneumoniae, prevents ApxIV-based serological detection of serotype 7 strain AP76

Actinobacillus pleuropneumoniae, a gram-negative rod of the Pasteurellaceae family, causes pleuropneumonia in pigs. Establishing A. pleuropneumoniae free herds is difficult due to the occurrence of persistently infected animals. The ApxIV toxin is expressed by A. pleuropneumoniae in vivo and an ELISA based on the toxin is used to detect infection and to differentiate between infected and vaccinated animals. In this study, we have identified a 1070bp insertion element of the IS30 family, designated ISApl1, in the A. pleuropneumoniae serotype 7 strain AP76. ISApl1 contains a 924bp ORF encoding a transposase, which is flanked by 27bp inverted repeats showing six mismatches. We investigated the occurrence of ISApl1 in other A. pleuropneumoniae strains, and its possible interference with virulence associated factors. Four insertion sites were identified in AP76: within the apxIVA toxin ORF, within a putative autotransporter adhesin ORF, upstream of a capsular polysaccharide biosynthesis gene cluster, and downstream of a beta-lactamase gene. ISApl1 is also present in some serotype 7 field isolates, but not in reference or field strains of other serotypes. In A. pleuropneumoniae AP76, the transposase gene is transcribed in vitro. The insertion in the apxIVA toxin gene remains stable after animal passage. Since this insertion should disrupt toxin expression, we tested 7 pigs infected with AP76 at day 21 post-infection. All were negative in the ApxIV ELISA but four out of seven were positive in an ApxII toxin ELISA. These results show that insertion elements can affect the detection of A. pleuropneumoniae infected animals.

Development of a DIVA subunit vaccine against Actinobacillus pleuropneumoniae infection

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia which leads to high economic losses in the swine industry worldwide. Vaccination against this pathogen is hampered by the occurrence of 15 serotypes, and commonly used whole cell bacterin vaccines are not sufficiently cross-serotype protective. In addition, for generating and maintaining specified pathogen-free herds it is desirable to use DIVA (differentiating infected from vaccinated animals) vaccines. Based on a detergent wash extraction of outer membrane associated proteins and secreted proteins we developed a DIVA vaccine using the immunogenic ApxII toxin which is present in 13 of the 15 A. pleuropneumoniae serotypes as the DIVA antigen. The apxIIA gene was deleted in one strain each of serotypes 1, 2, and 5 using a single-step transconjugation system, and equal parts of detergent washes from these strains served as the vaccine antigen. After intramuscular immunisation all pigs developed a strong humoral immune response to the vaccine antigen and showed no reactivity in an ApxIIA ELISA. Upon challenge all pigs were completely protected from clinical symptoms in trials with a homologous (serotype 2) as well as with a heterologous strain (serotype 9); in addition, colonisation of the challenge strain was clearly reduced but not abolished completely. As a result of the highly efficient protection, however, immunised pigs did not develop antibodies to the DIVA-antigen at levels detectable by ELISA but only by a more sensitive Western blotting approach, thereby demonstrating the challenge in developing appropriate marker vaccines for the livestock industry.

Use of an Actinobacillus pleuropneumoniae multiple mutant as a vaccine that allows differentiation of vaccinated and infected animals

Vaccination against Actinobacillus pleuropneumoniae is hampered by the lack of vaccines inducing reliable cross-serotype protection. In contrast, pigs surviving natural infection are at least partially protected from clinical symptoms upon reinfection with any serotype. Thus, we set out to construct an attenuated A. pleuropneumoniae live vaccine allowing the differentiation of vaccinated from infected animals (the DIVA concept) by successively deleting virulence-associated genes. Based on an A. pleuropneumoniae serotype 2 prototype live negative marker vaccine (W. Tonpitak, N. Baltes, I. Hennig-Pauka, and G.-F. Gerlach, Infect. Immun. 70:7120-7125, 2002), genes encoding three enzymes involved in anaerobic respiration and the ferric uptake regulator Fur were deleted, resulting in a highly attenuated sixfold mutant; this mutant was still able to colonize the lower respiratory tract and induced a detectable immune response. Upon a single aerosol application, this mutant provided significant protection from clinical symptoms upon heterologous infection with an antigenically distinct A. pleuropneumoniae serotype 9 challenge strain and allowed the serological discrimination between infected and vaccinated groups.

Differential expression of non-cytoplasmic Actinobacillus pleuropneumoniae proteins induced by addition of bronchoalveolar lavage fluid

Actinobacillus (A.) pleuropneumoniae is the causative agent of a porcine pleuropneumonia occurring worldwide. In order to identify novel non-cytoplasmic putative virulence-associated proteins, we prepared fractions enriched in surface-associated proteins for differential proteome analysis by two-dimensional (2D) gel electrophoresis and quadrupole time-of-flight mass spectrometry (Q-Tof MS). Bacteria grown under standard culture conditions were compared to an ex vivo model based on the addition of bronchoalveolar lavage fluid (BALF) to the culture media. Twelve proteins were found to be upregulated upon induction with BALF, among them a superoxide dismutase, a parvulin-like peptidy-prolyl isomerase, a polynucleotide phosphorylase and the highly immunogenic lipoprotein OmlA. Four of the proteins upregulated by BALF were additionally constitutively expressed by an isogenic A. pleuropneumoniae fur deletion mutant and could be identified by Q-Tof MS as the heat shock protein GroES, a putative dipeptide transporter, a putative metal ion transporter and a conserved protein of unknown function. In silico analysis of the putative promoter regions of the encoding genes revealed putative Fur boxes upstream of two genes, one of which encodes part of a putative metal ion transporter. An isogenic mutant with a deletion in this protein was constructed and designated as A. pleuropneumoniae Deltafui. Analysis of the mutant in an aerosol infection model revealed symptoms indistinguishable from those seen upon infection with wild type A. pleuropneumoniae. This result implies that not all proteins upregulated by BALF are directly involved in A. pleuropneumoniae virulence.

Real-time quantitative reverse transcription-PCR analysis of expression stability of Actinobacillus pleuropneumoniae housekeeping genes during in vitro growth under iron-depleted conditions

The aims of the present investigation were to develop and test a sensitive and reproducible method for the study of gene expression in the porcine lung pathogen Actinobacillus pleuropneumoniae by real-time quantitative reverse transcription (RT)-PCR and to evaluate a number of suitable internal controls, as such controls have not been defined yet for this bacterium. Bacterial gene expression was studied during in vitro exponential and early stationary growth in medium with and without sufficient iron, respectively. First, the stability of expression of five genes, the glyA, tpiA, pykA, recF, and rhoAP genes involved in basic housekeeping, was evaluated on the basis of the mean pairwise variation. All the housekeeping genes included were stably expressed under the conditions investigated and consequently were included in the normalization procedure. Next, the geometric mean of the internal control genes was used to correct five genes of interest. These genes were three genes involved in iron acquisition (tbpA, exbB, and fhuD), the heat shock protein gene groEL, and a putative quorum-sensing gene (luxS). The level of tbpA, exbB, and fhuD expression in A. pleuropneumoniae showed significant up-regulation under iron-restricted conditions compared to bacteria grown in medium with sufficient iron. The observed expression patterns of the genes of interest were consistent with previous observations. This study therefore lends further support to the use of real-time quantitative RT-PCR, with the glyA, tpiA, pykA, recF, and rhoAP genes as internal controls, for future similar gene expression studies in A. pleuropneumoniae.

Deletion of the anaerobic regulator HlyX causes reduced colonization and persistence of Actinobacillus pleuropneumoniae in the porcine respiratory tract

Actinobacillus pleuropneumoniae, the etiological agent of porcine pleuropneumonia, is able to persist on respiratory epithelia, in tonsils, and in the anaerobic environment of encapsulated lung sequesters. We have demonstrated previously that putative HlyX-regulated genes, coding for dimethyl sulfoxide (DMSO) reductase and aspartate ammonia lyase, are upregulated during infection and that deletions in these genes result in attenuation of the organism. The study presented here investigates the role of HlyX, the fumarate nitrate reductase regulator (FNR) homologue of A. pleuropneumoniae. By constructing an isogenic A. pleuropneumoniae hlyX mutant, the HlyX protein is shown to be responsible for upregulated expression of both DMSO reductase and aspartate ammonia lyase (AspA) under anaerobic conditions. In a challenge experiment the A. pleuropneumoniae hlyX mutant is shown to be highly attenuated, unable to persist in healthy lung epithelium and tonsils, and impaired in survival inside sequestered lung tissue. Further, using an A. pleuropneumoniae strain carrying the luxAB genes as transcriptional fusion to aspA on the chromosome, the airway antioxidant glutathione was identified as one factor potentially responsible for inducing HlyX-dependent gene expression of A. pleuropneumoniae in epithelial lining fluid.

Rapid and accurate identification of human isolates of Pasteurella and related species by sequencing the sodA gene

The identification of Pasteurella and related bacteria remains a challenge. Here, a 449- to 473-bp fragment (sodA(int)) internal to the sodA gene, encoding the manganese-dependent superoxide dismutase, was amplified and sequenced with a single pair of degenerate primers from the type strains of Pasteurella (18 strains), Gallibacterium (1 strain), and Mannheimia (5 strains) species. The sodA(int)-based phylogenetic tree was in general agreement with that inferred from the analysis of the corresponding 16S rRNA gene sequences, with members of the Pasteurella sensu stricto cluster (Pasteurella multocida, Pasteurella canis, Pasteurella dagmatis, and Pasteurella stomatis) forming a monophyletic group and Gallibacterium and Mannheimia being independent monophyletic genera. However, the sodA(int) sequences showed a markedly higher divergence than the corresponding 16S rRNA genes, confirming that sodA is a potent target to differentiate related species. Thirty-three independent human clinical isolates phenotypically assigned to 13 Pasteurella species by a reference laboratory were successfully identified by comparing their sodA(int) sequences to those of the type species. In the course of this work, we identified the first Gallibacterium anatis isolate ever reported from a human clinical specimen. The sodA(int) sequences of the clinical isolates displayed less than 2.5% divergence from those of the corresponding type strains, except for the Pasteurella pneumotropica isolates, which were closely related to each other (> 98% sodA(int) sequence identity) but shared only 92% sodA(int) identity with the type strain. The method described here provides a rapid and accurate tool for species identification of Pasteurella isolates when access to a sequencing facility is available.

Enzymes involved in anaerobic respiration appear to play a role in Actinobacillus pleuropneumoniae virulence

Actinobacillus pleuropneumoniae, the etiological agent of porcine pleuropneumonia, is able to survive on respiratory epithelia, in tonsils, and in the anaerobic environment of encapsulated sequesters. It was previously demonstrated that a deletion of the anaerobic dimethyl sulfoxide reductase gene (dmsA) results in attenuation in acute disease (N. Baltes, S. Kyaw, I. Hennig-Pauka, and G. F. Gerlach, Infect. Immun. 71:6784-6792, 2003). In the present study, using two-dimensional polyacrylamide gel electrophoresis and quadrupole time-of-flight mass spectrometry, we identified an aspartate ammonia-lyase (AspA) which is upregulated upon induction with bronchoalveolar lavage fluid (BALF). This enzyme is involved in the production of fumarate, an alternative electron acceptor under anaerobic conditions. The coding gene (aspA) was cloned and shown to be present in all A. pleuropneumoniae serotype reference strains. The transcriptional start point was identified downstream of a putative FNR binding motif, and BALF-dependent activation of aspA was confirmed by construction of an isogenic A. pleuropneumoniae mutant carrying a chromosomal aspA::luxAB transcriptional fusion. Two aspA deletion mutants, A. pleuropneumoniae DeltaaspA and A. pleuropneumoniae DeltaaspADeltadmsA, were constructed, both showing reduced growth under anaerobic conditions in vitro. Pigs challenged with either of the two mutants in an aerosol infection model showed a lower lung lesion score than that of the A. pleuropneumoniae wild-type (wt) controls. Pigs challenged with A. pleuropneumoniae DeltaaspADeltadmsA had a significantly lower clinical score, and this mutant was rarely reisolated from unaltered lung tissue; in contrast, A. pleuropneumoniae DeltaaspA and the A. pleuropneumoniae wt were consistently reisolated in high numbers. These results suggest that enzymes involved in anaerobic respiration are necessary for the pathogen's ability to persist on respiratory tract epithelium and play an important role in A. pleuropneumoniae pathogenesis.

Revised definition of Actinobacillus sensu stricto isolated from animals. A review with special emphasis on diagnosis

The taxonomy of the members of the genus Actinobacillus associated with animals has been reviewed with focus on classification and identification including molecular based characterization, typing and identification. Out of the 22 species or species like taxa reported as Actinobacillus, 19 are associated with animals. When classified on the basis of 16S rRNA sequence based phylogenetic analysis, DNA-DNA hybridizations and phenotypic analysis, Actinobacillus sensu stricto is restricted to include A. lignieresii, A. pleuropneumoniae, A. equuli subsp. equuli, A. equuli subsp. haemolyticus (taxon 11 of Bisgaard), A. hominis, A. suis, A. ureae, A. arthritidis (taxon 9 of Bisgaard), Actinobacillus genomospecies 1 and 2 and the taxa 8 and 26 of Bisgaard. The remaining 11 species of Actinobacillus are unrelated to A. sensu stricto and should consequently be grouped with other genera or be renamed as new genera depending on new data. Identification of members of Actinobacillus at species level is possible through phenotypic characterization combined with information on host of isolation. PCR tests are available for specific detection of A. pleuropneumoniae. Only A. pleuropneumoniae is presently considered as a primary pathogen. Based on different types of RTX genes it is possible to PCR type A. pleuropneumoniae to serotype level. PCR might also be used for the specific detection of A. equuli subsp. haemolyticus. Epidemiological investigations and surveillance have so far included serotyping, multilocus enzyme electrophoresis (MLEE), ribotyping and restriction fragment length profiling.

Identification of dimethyl sulfoxide reductase in Actinobacillus pleuropneumoniae and its role in infection

Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, is capable of persisting in oxygen-deprived surroundings, namely, tonsils and sequestered necrotic lung tissue. Utilization of alternative terminal electron acceptors in the absence of oxygen is a common strategy in bacteria under anaerobic growth conditions. In an experiment aimed at identification of genes expressed in vivo, the putative catalytic subunit DmsA of anaerobic dimethyl sulfoxide reductase was identified in an A. pleuropneumoniae serotype 7 strain. The 90-kDa protein exhibits 85% identity to the putative DmsA protein of Haemophilus influenzae, and its expression was found to be upregulated under anaerobic conditions. Analysis of the unfinished A. pleuropneumoniae genome sequence revealed putative open reading frames (ORFs) encoding DmsB and DmsC proteins situated downstream of the dmsA ORF. In order to investigate the role of the A. pleuropneumoniae DmsA protein in virulence, an isogenic deletion mutant, A. pleuropneumoniae DeltadmsA, was constructed and examined in an aerosol infection model. A. pleuropneumoniae DeltadmsA was attenuated in acute disease, which suggests that genes involved in oxidative metabolism under anaerobic conditions might contribute significantly to A. pleuropneumoniae virulence.

Actinobacillus pleuropneumoniae serotype 7 siderophore receptor FhuA is not required for virulence

A ferrichrome receptor, FhuA, was identified in Actinobacillus pleuropneumoniae serotype 7. An isogenic mutant with a deletion in the ferrichrome uptake receptor gene (fhuA) was constructed and examined in an aerosol infection model. The disease caused by the mutant was indistinguishable from disease induced by A. pleuropneumoniae serotype 7 wild-type; an isogenic mutant lacking expression of the exbB gene that is required for the uptake of transferrin-bound iron retained the ability to utilize ferrichrome, thereby indicating that an energy-coupling mechanism involved in ferrichrome transport remains to be identified.

Construction of an Actinobacillus pleuropneumoniae serotype 2 prototype live negative-marker vaccine

Deletions were introduced into the ureC and apxIIA genes of an Actinobacillus pleuropneumoniae serotype 2 strain by homologous recombination and counterselection. The double-mutant contains no foreign DNA, is highly attenuated, protects pigs from homologous challenge upon a single aerosol application, and facilitates the serological discrimination of immunized and infected herds.

Identification of Brachyspira hyodysenteriae-specific DNA fragments using representational difference analysis

Two novel Brachyspira hyodysenteriae-specific DNA fragments, designated as Bh100 and Bh400, were identified using representational difference analysis. To isolate the fragments the combined DNA of the Brachyspira pilosicoli, Brachyspira intermedia, Brachyspira murdochii and Brachyspira innocens reference strains was subtracted from the genome of B. hyodysenteriae strain B204. Both fragments were present in a single copy and mapped to different positions on the genome of B. hyodysenteriae B78(T). Larger fragments encompassing the continuous open reading frames (ORF) of Bh100 and Bh400 were cloned and analysed. Whereas the ORF of 2130 bp encompassing Bh100 did not show homology to any known bacterial protein, Bh400 was part of a putative operon with significant homology to the phosphotransferase system of Bacillus subtilis.

Actinobacillus pleuropneumoniae iron transport and urease activity: effects on bacterial virulence and host immune response

Actinobacillus pleuropneumoniae, a porcine respiratory tract pathogen, has been shown to express transferrin-binding proteins and urease during infection. Both activities have been associated with virulence; however, their functional role for infection has not yet been elucidated. We used two isogenic A. pleuropneumoniae single mutants (DeltaexbB and DeltaureC) and a newly constructed A. pleuropneumoniae double (DeltaureC DeltaexbB) mutant in aerosol infection experiments. Neither the A. pleuropneumoniae DeltaexbB mutant nor the double DeltaureC DeltaexbB mutant was able to colonize sufficiently long to initiate a detectable humoral immune response. These results imply that the ability to utilize transferrin-bound iron is required for multiplication and persistence of A. pleuropneumoniae in the porcine respiratory tract. The A. pleuropneumoniae DeltaureC mutant and the parent strain both caused infections that were indistinguishable from one another in the acute phase of disease; however, 3 weeks postinfection the A. pleuropneumoniae DeltaureC mutant, in contrast to the parent strain, could not be isolated from healthy lung tissue. In addition, the local immune response-as assessed by fluorescence-activated cell sorter and enzyme-linked immunosorbent spot analyses-revealed a significantly higher number of A. pleuropneumoniae-specific B cells in the bronchoalveolar lavage fluid (BALF) of pigs infected with the A. pleuropneumoniae DeltaureC mutant than in the BALF of those infected with the parent strain. These results imply that A. pleuropneumoniae urease activity may cause sufficient impairment of the local immune response to slightly improve the persistence of the urease-positive A. pleuropneumoniae parent strain.

The molecular biology of pasteurella multocida

Pasteurella multocida is an important veterinary and opportunistic human pathogen. The species is diverse and complex with respect to antigenic variation, host predeliction and pathogenesis. Certain serological types are the aetiologic agents of severe pasteurellosis, such as fowl cholera in domestic and wild birds, bovine haemorrhagic septicaemia and porcine atrophic rhinitis. The recent application of molecular methods such as the polymerase chain reaction, restriction endonuclease analysis, ribotyping, pulsed-field gel electrophoresis, gene cloning, characterisation and recombinant protein expression, mutagenesis, plasmid and bacteriophage analysis and genomic mapping, have greatly increased our understanding of P. multocida and has provided researchers with a number of molecular tools to study pathogenesis and epidemiology at a molecular level.

Actinobacillus pleuropneumoniae iron transport: a set of exbBD genes is transcriptionally linked to the tbpB gene and required for utilization of transferrin-bound iron

Upon iron restriction, Actinobacillus pleuropneumoniae has been shown to express the transferrin-binding proteins TbpB and TbpA, both of which have been implied to be important virulence factors. In order to identify additional iron-regulated proteins, we cloned and analyzed the region upstream of the transferrin-binding protein genes in an A. pleuropneumoniae serotype 7 strain. We located immediately upstream of the tbpB gene two open reading frames which were 43% homologous to the neisserial ExbBD protein genes. By raising specific antibodies, we showed that ExbB is expressed under iron-limiting growth conditions only, and RT-PCR analysis revealed that the exbBD genes and the tbpB gene are transcribed on a single polycistronic mRNA. By constructing an isogenic and nonpolar exbBD mutant, we showed that the exbBD genes are required by A. pleuropneumoniae for utilization of transferrin-bound iron. Using PCR and Western blotting, we showed that the genetic organization found in A. pleuropneumoniae serotype 7 is similar in all 12 A. pleuropneumoniae serotype reference strains.

A single-step transconjugation system for the introduction of unmarked deletions into Actinobacillus pleuropneumoniae serotype 7 using a sucrose sensitivity marker

Research on the porcine respiratory tract pathogen Actinobacillus pleuropneumoniae requires the availability of improved genetic tools. Therefore, using the sacB gene of Bacillus subtilis, we developed a sucrose-based counterselection system that allows rapid curing of an Escherichia coli-A. pleuropneumoniae shuttle vector as well as the introduction of unmarked mutations into the A. pleuropneumoniae chromosome. A cassette containing the Tn903 kanamycin resistance determinant (km(r)) and the sacB gene expressed from the A. pleuropneumoniae omlA promoter was introduced by homologous recombination into the ureC gene of A. pleuropneumoniae. The resultant stable plasmid cointegrates were kanamycin-resistant, sucrose-sensitive, and urease-positive. A simple counterselection on sucrose-containing agar plates without an additional transconjugation step allowed the efficient isolation of urease-negative A. pleuropneumoniae mutants that had lost the km(r)-sacB cassette.