Identification of a second hemolysin (HlyII) in Actinobacillus pleuropneumoniae serotype 1 and expression of the gene in Escherichia coli

Channel formation by RTX-toxins of pathogenic bacteria: Basis of their biological activity

The pore-forming cytolysins of the RTX-toxin (Repeats in ToXin) family are a relatively small fraction of a steadily increasing family of proteins that contain several functionally important glycine-rich and aspartate containing nonapeptide repeats. These cytolysins produced by a variety of Gram-negative bacteria form ion-permeable channels in erythrocytes and other eukaryotic cells. Hemolytic and cytolytic RTX-toxins represent pathogenicity factors of the toxin-producing bacteria and are very often important key factors in pathogenesis of the bacteria. Channel formation by RTX-toxins lead to the dissipation of ionic gradients and membrane potential across the cytoplasmic membrane of target cells, which results in cell death. Here we discuss channel formation and channel properties of some of the best known RTX-toxins, such as α-hemolysin (HlyA) of Escherichia coli and the uropathogenic EHEC strains, the adenylate cyclase toxin (ACT, CyaA) of Bordetella pertussis and the RTX-toxins (ApxI, ApxII and ApxIII) produced by different strains of Actinobacillus pleuropneumoniae. The channels formed by these RTX-toxins in lipid bilayers share some common properties such as cation selectivity and voltage-dependence. Furthermore the channels are transient and show frequent switching between different ion-conducting states. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Induction of protective immune responses against challenge of Actinobacillus pleuropneumoniae by oral administration with Saccharomyces cerevisiae expressing Apx toxins in pigs

Actinobacillus pleuropneumoniae is a causative agent of porcine pleuropneumonia, a highly contagious endemic disease of pigs worldwide, inducing significant economic losses worldwide. Apx toxins, which are correlated with the virulence of A. pleuropneumoniae, were expressed in Saccharomyces cerevisiae and its possible use as an oral vaccine has been confirmed in our previous studies using a murine model. The present study was undertaken to test the hypothesis that oral immunization using S. cerevisiae expressing either ApxI or ApxII could protect pigs against A. pleuropneumoniae as an effective way of inducing both mucosal and systemic immune responses. The surface-displayed ApxIIA#5 expressing S. cerevisiae was selected as an oral vaccine candidate by finding on induction of higher immune responses in mice after oral vaccination. The surface-displayed ApxIIA#5 expressing S. cerevisiae and the ApxIA expressing S. cerevisiae were developed to serve as an oral vaccine in pigs. The vaccinated pigs showed higher specific IgG- and IgA-related antibody activities than the non-treated control and vector control pigs. Additionally, the induced immune responses were found to protect pigs infected with A. pleuropneumoniae according to the analysis of clinical signs and the gross and microscopic pulmonary lesions. These results suggested that the surface-displayed ApxIIA#5 and ApxIA in S. cerevisiae might be a potential oral vaccine to protect pigs against porcine pleuropneumonia. Thus the present study is expected to contribute to the development of a live oral vaccine against porcine pleuropneumonia as an alternative to current conventional vaccines.

An immunosorbent assay based on the recombinant ApxIa, ApxIIa, and ApxIIIa toxins of Actinobacillus pleuropneumoniae and its application to field sera

Actinobacillus pleuropneumoniae is the etiologic agent of porcine pleuropneumonia, a highly contagious pulmonary disease in pigs with major economic losses for pig producers worldwide. Whereas A. pleuropneumoniae isolates are divided into 15 serotypes, the isolates secrete 4 types of exotoxins (ApxI, ApxII, ApxIII, and ApxIV), which are known as major virulence factors. In the current study, the ApxIA, ApxIIA, and ApxIIIA genes were amplified and their recombinant proteins expressed in Escherichia coli M15 cells. The antigenicity of each recombinant protein was demonstrated by Western blot and enzyme-linked immunosorbent assay (ELISA) using sera from pigs vaccinated with a subunit vaccine. When ELISAs using the recombinant antigens were optimized and then applied to sera from 320 randomized pigs in Korea, an observed increase in seroprevalence was found among sows in comparison with weaned piglets and growing pigs, indicating an age-dependent seroprevalence. The results obtained in the study suggest that the developed ELISAs may be useful for A. pleuropneumoniae vaccination strategy as a screening tool for pig herds as well as for detection of specific antibodies to Apx exotoxins.

Renaturation and purification of ApxII toxin of Actinobacillus pleuropneumoniae

ApxII toxin is the only Apx toxin that is produced by Actinobacillus pleuropneumoniae serotype 7. In order to determine whether the recombinant ApxII that derived from Escherichia coli (E. coli) expression is faithful to the natural ApxII so that can be used as additional component in vaccine preparation, the structure gene apxIIA of ApxII toxin was expressed in E. coli with prokaryotic expression vector pGEX-6p-1 (formed pGEX-6p-A). pGZRS-C which is A. pleuropneumoniae-E. coli shuttle vector pGZRS-38 expressing the post-transcriptional activation gene apxII C was co-expressed with pGEX-6p-A. The expression product of rApxII A formed inclusion. The inclusion protein was oxidized, refolded and restored hemolytic activity after denaturation, renaturation and purification. The result indicated that E. coli expressed recombinant ApxII toxin has good fidelity, which makes it possible to produce this valuable antigen for vaccine preparation or diagnosis.

Development and use of a multiplex polymerase chain reaction assay based on Apx toxin genes for genotyping of Actinobacillus pleuropneumoniae isolates

Actinobacillus pleuropneumoniae (A. pleuropneumoniae) is the etiological agent of a porcine pleuropneumonia that threatens the global swine industry. The major pathogenic toxins of A. pleuropneumoniae include ApxI, ApxII, ApxIII, and ApxIV, which are serotype or serovar specific. Several techniques have been developed for the identification and typing of A. pleuropneumoniae. Serological assays are used to identify and serotype A. pleuropneumoniae, but factors such as cross-reactivity limit their specificity. Labor, time, and the requirement for specific antibodies are also drawbacks of these assays. Multistep polymerase chain reaction (PCR) techniques based on apx genes have been reported for the identification and typing of A. pleuropneumoniae. This study developed multiplex PCR for the identification and genotyping of A. pleuropneumoniae based on apx genes. This multiplex PCR technique was successful in differentiating 11 of 15 reference serotypes. Five different primer sets were used to amplify the 4 apx genes from each serotype in a single-step reaction. The multiplex PCR reported in this study was further used in genotyping 51 field isolates of A. pleuropneumoniae from different regions of Korea. The concomitant amplification of all 4 apx genes makes multiplex PCR more specific and convenient for the diagnosis and genotyping of A. pleuropneumoniae.

Use of recombinant ApxIV in serodiagnosis of Actinobacillus pleuropneumoniae infections, development and prevalidation of the ApxIV ELISA

Actinobacillus pleuropneumoniae is the etiological agent of porcine pleuropneumonia, which causes worldwide severe losses in pig farming. The virulence of the 15 serotypes of A. pleuropneumoniae is mainly determined by the three major RTX toxins ApxI, ApxII and ApxIII, which are secreted by the different serotypes in various combinations. A fourth RTX toxin, ApxIV, is produced by all 15 serotypes only during infection of pigs, but not under in vitro conditions. Pigs infected with A. pleuropneumoniae show specific antibodies directed against ApxIV. In contrast, antibodies against the other three toxins ApxI, ApxII and ApxIII are also found in pigs free of A. pleuropneumoniae. The antibodies to the three latter might result from other, less pathogenic Actinobacillus species such as A. rossii and A. suis. We used a recombinant protein based on the N'-terminal part of ApxIV to serologically detect A. pleuropneumoniae infections in pigs by immunoblot analysis. The analysis of sera of experimentally infected pigs revealed that ApxIV-immunoblots detected A. pleuropneumoniae infections in the second to third week post infection. We developed an indirect ELISA based on the purified recombinant N'-terminal moiety of ApxIV. The analysis of sera from pigs that were experimentally or naturally infected by A. pleuropneumoniae, and of sera of pigs that were free of A. pleuropneumoniae, revealed that the ELISA had a specificity of 100% and a sensitivity of 93.8%. The pre-validation study of the ApxIV-ELISA revealed that the latter was able to detect A. pleuropneumoniae-positive herds, even when clinical and pathological signs of porcine pleuropneumonia were not evident. Pigs vaccinated with a subunit vaccine Porcilis App were serologically negative in the ApxIV-ELISA.

Non-pathogenic Actinobacillus isolates antigenically and biochemically similar to Actinobacillus pleuropneumoniae: a novel species?

Two unusual Actinobacillus isolates were recovered from pigs with no clinical signs, no lesions and no history of swine pleuropneumonia. Two representative strains (9953L55 and 0347) analyzed in this study were initially biochemically and antigenically identified as A. pleuropneumoniae serotypes 1 and 9, respectively, by traditional identification methods. Both strains presented, however, negative results with three A. pleuropneumoniae-specific PCR tests and revealed in particular the absence of the apxIV toxin genes. However, both strains produced and secreted ApxII toxin although they only harbored the toxin genes apxIICA, which is an uncommon feature for any of the known A. pleuropneumoniae serotypes. Upon experimental inoculation of pigs, these strains proved to be totally non-pathogenic. Animals infected with one of the strains produced antibodies that cross-react with A. pleuropneumoniae serotypes 1-9-11-specific LC-LPS ELISA. Phylogenetic analysis based on 16S rRNA gene sequence analysis revealed that these strains form a separate phylogenetic group that is distinct from other Actinobacillus species and is particularly different from A. pleuropneumoniae.

Apx toxins in Pasteurellaceae species from animals

Pasteurellaceae species particularly of porcine origin which are closely related to Actinobacillus pleuropneumoniae were analyzed for the presence of analogues to the major A. pleuropneumoniae RTX toxin genes, apxICABD, apxIICA and apxIIICABD and for their expression. Actinobacillus suis contains both apxICABD(var.suis) and apxIICA(var. suis) operons and was shown to produce ApxI and ApxII toxin. Actinobacillus rossii contained the operons apxIICA(var.rossii) and apxIIICABD(var.rossii). However, only the toxin ApxII and not ApxIII could be detected in cultures of A. rossii. The Apx toxins found in A. suis and A. rossi may play a role in virulence of these pathogens. Actinobacillus lignieresii, which was included since it is phylogenetically very closely related to A. pleuropneumoniae, was found to contain a full apxICABD(var.lign.) operon which however lacks the -35 and -10 boxes in the promoter sequences. As expected from these results, no expression of ApxI was detected in A. lignieresii grown under standard culture conditions. Actinobacillus seminis, Actinobacillus equuli, Pasteurella aerogenes, Pasteurella multocida, Haemophilus parasuis, and also Mannheimia (Pasteurella) haemolytica, which is known to secrete leukotoxin, were all shown to be devoid of any of the apx toxin genes and did not produce ApxI, ApxII or ApxIII toxin proteins. However, proteins of slightly lower molecular mass than ApxI, ApxII and ApxIII which showed limited cross-reactions with monospecific, polyclonal anti-ApxI, anti-ApxII and anti-ApxIII were detected on immunoblot analysis of A. equuli, A. seminis and P. aerogenes. The presence of Apx toxins and proteins that imunologically cross react with Apx toxins in porcine Actinobacillus species other than A. pleuropneumoniae can be expected to interfere with serodiagnosis of porcine pleuropneumonia.

Actinobacillus species and their role in animal disease

Actinobacillus species are Gram-negative bacteria responsible for several quite distinct disease conditions of animals. The natural habitat of the organisms is primarily the upper respiratory tract and oral cavity. A. lignieresii is the cause of actinomycosis (wooden tongue) in cattle: a sporadic, insidiously-developing granulomatous infection. In sharp contrast is A. pleuropneumoniae which is responsible for a rapidly spreading often fatal pneumonia, common among intensively reared pigs. Detailed investigation of this organism has provided a much clearer picture of the bacterial factors involved in causing disease. A. equuli similarly causes a potent septicaemia in the neonatal foal; growing apparently unrestricted once infection occurs. Other members of the genus induce characteristic pathogenesis in their preferred host, with one, A. actinomycetemcomitans, being a cause of human periodontal disease. This article reviews recent understanding of the taxonomy and bacteriology of the organisms, and the aetiology, pathogenicity, diagnosis and control of animal disease caused by Actinobacillus species.

Vaccination and protection of pigs against pleuropneumonia with a vaccine strain of Actinobacillus pleuropneumoniae produced by site-specific mutagenesis of the ApxII operon

The production of toxin (Apx)-neutralizing antibodies during infection plays a major role in the induction of protective immunity to Actinobacillus pleuropneumoniae reinfection. In the present study, the gene encoding the ApxII-activating protein, apxIIC, was insertionally inactivated on the chromosome of a serovar 7 strain, HS93. Expression of the structural toxin, ApxIIA, and of the two genes required for its secretion, apxIB and apxID, still occurs in this strain. The resulting mutant strain, HS93C- Ampr, was found to secrete the unactivated toxin. Pigs vaccinated with live HS93C- Ampr via the intranasal route were protected against a cross-serovar challenge with a virulent serovar 1 strain of A. pleuropneumoniae. This is the first reported vaccine strain of A. pleuropneumoniae which can be delivered live to pigs and offers cross-serovar protection against porcine pleuropneumonia.

Cloning and mutagenesis of a serotype-specific DNA region involved in encapsulation and virulence of Actinobacillus pleuropneumoniae serotype 5a: concomitant expression of serotype 5a and 1 capsular polysaccharides in recombinant A. pleuropneumoniae serotype 1

A DNA region involved in Actinobacillus pleuropneumoniae serotype 5 capsular polysaccharide (CP) biosynthesis was identified and characterized by using a probe specific for the cpxD gene involved in CP export. The adjacent serotype 5-specific CP biosynthesis region was cloned from a 5.8-kb BamHI fragment and an 8.0-kb EcoRI fragment of strain J45 genomic DNA. DNA sequence analysis demonstrated that this region contained four complete open reading frames, cps5A, cps5B, cps5C, and cps5D. Cps5A, Cps5B, and Cps5C showed low homology with several bacterial glycosyltransferases involved in the biosynthesis of lipopolysaccharide or CP. However, Cps5D had high homology with KdsA proteins (3-deoxy-D-manno-2-octulosonic acid 8-phosphate synthetase) from other gram-negative bacteria. The G+C content of cps5ABC was substantially lower (28%) than that of cps5D and the rest of the A. pleuropneumoniae chromosome (42%). A 2.1-kb deletion spanning the cloned cps5ABC open reading frames was constructed and transferred into the J45 chromosome by homologous recombination with a kanamycin resistance cassette to produce mutant J45-100. Multiplex PCR confirmed the deletion in this region of J45-100 DNA. J45-100 did not produce intracellular or extracellular CP, indicating that cps5A, cps5B, and/or cps5C were involved in CP biosynthesis. However, biosynthesis of the Apx toxins, lipopolysaccharide, and membrane proteins was unaffected by the mutation. Besides lack of CP biosynthesis, and in contrast to J45, J45-100 grew faster, was sensitive to killing in precolostral calf serum, and was avirulent in pigs at an intratracheal challenge dose three times the 50% lethal dose (LD50) of strain J45. At six times the J45 LD50, J45-100 caused mild to moderate lung lesions but not death. Electroporation of cps5ABC into A. pleuropneumoniae serotype 1 strain 4074 generated strain 4074(pJMLCPS5), which expressed both serotype 1 and serotype 5 CP. However, serotype 1 capsule expression was diminished in 4074(pJMLCPS5) in comparison to 4074. The recombinant strain produced significantly less total CP (serotypes 1 and 5 CP combined) in log phase (P = 0.0012) but significantly more total CP in late stationary phase than 4074 (P < 0.0001). In addition, strain 4074(pJMLCPS5) caused less mortality and bacteremia in pigs and mice following respiratory challenge than strain 4074, indicating that virulence was affected by diminished capsule production. These results emphasize the importance of CP in the serum resistance and virulence of A. pleuropneumoniae.

Generation of targeted nonpolar gene insertions and operon fusions in Pasteurella haemolytica and creation of a strain that produces and secretes inactive leukotoxin

An efficient method for targeted gene inactivation and generation of chromosomal gene fusions in Pasteurella haemolytica has been devised and used to create an lktC::cat operon fusion by allelic exchange at the leukotoxin gene cluster (lktCABD). A copy of the lktC gene was insertionally inactivated by using a nonpolar, promoterless cat cassette and then delivered into P. haemolytica on a shuttle vector. Plasmid incompatibility was used to detect clones where double recombination events had occurred at the chromosomal locus. The insertion in lktC did not affect expression of the downstream genes, and the mutant strain secreted an antigenic proleukotoxin that was neither leukotoxic nor hemolytic. Expression of the lktC gene in trans restored the wild-type phenotype, confirming that LktC is required for activation of the proleukotoxin to the mature leukotoxin. Construction of the lktC::cat operon fusion allowed us to quantitate leukotoxin promoter activity in P. haemolytica and to demonstrate that transcription was maximal during early logarithmic growth phase but was reduced following entry into late logarithmic phase. This allelic exchange system should be useful for future genetic studies in P. haemolytica and could potentially be applied to other members of Haemophilus-Actinobacillus-Pasteurella family, where genetic manipulation is limited.

Channel-forming activity and channel size of the RTX toxins ApxI, ApxII, and ApxIII of Actinobacillus pleuropneumoniae

The determinants of the Actinobacillus pleuropneumoniae RTX toxins ApxI, ApxII, and ApxIII were expressed in an Escherichia coli strain. The toxins were concentrated from the supernatants of cell cultures. The addition of the toxins to the aqueous-phase-bathing lipid bilayer membranes resulted in an increase in the membrane conductance when membranes made of asolectin or phosphatidylethanolamine were used. The toxins were relatively inactive in membranes made of other lipids. The membrane activity (i.e., the number of channels formed at a given Apx concentration) was different for each of the three Apx toxins. That of ApxI, which has the strongest cytotoxic activity, was highest, followed by that of ApxIII and ApxII, which is the least cytotoxic. The conductance increases of ApxIII and ApxII were smaller by factors of 10 and 50, respectively, than that of ApxI under otherwise identical conditions. Single-channel experiments demonstrated that all three Apx toxins formed ion-permeable channels of different conductances. The major open state was approximately the same for the two hemolytic toxins ApxI and ApxII (540 and 620 pS in 0.15 M KCI), whereas the single-channel conductance of the nonhemolytic ApxIII was approximately one-fifth of that of the other two toxins (95 pS). Experiments with different salts suggested that the Apx channels of A. pleuropneumoniae were exclusively cation selective because of negative charges localized at the channel mouth. Analysis of the single-channel data using the Renkin correction factor suggested that the Apx toxins formed aqueous channels with different diameters for the three toxins. Pore-forming properties of the Apx toxins were compared with those of other RTX toxins. All of these toxins have common features and form channels that are transient but have different sizes as judged from the different single-channel conductances.

Characterisation of haemolytic RTX toxins produced by Australian isolates of Actinobacillus pleuropneumoniae

The haemolytic RTX toxins of 27 isolates of Actinobacillus pleuropneumoniae, representing all serovars that have been isolated in Australia, were characterised. The quantity of protein secreted by these isolates into the media was not significantly different between serovars, but haemolytic activity was detected only in the unconcentrated supernatants from cultures of serovar 1 and 5 isolates. Haemolytic activity in supernatants of serovar 2, 3 and 7 isolates was detected only after the supernatants were concentrated. On Southern hybridisation blots, genomic DNA of serovar 1 and 5 isolates contained regions that were similar to the cloned structural genes for ApxI (apxIA) and for ApxII (apxIIA). In contrast, genomic DNA of serovar 2, 3 and 7 isolates only contained regions similar to, if not identical with, the cloned apxIIA gene. The haemolytic activity of the culture supernatant depends on the type or composition of media and adaptability of the bacteria to in-vitro cultivation. Low passage cultures of A pleuropneumoniae, which were characterised by waxy colonies, produced significantly weaker haemolytic activity than A pleuropneumoniae after several passages in vitro.

Development of an efficient PCR method for toxin typing of Actinobacillus pleuropneumoniae strains

A method has been developed which allows the determination of the activator, the structural and the secretion genes of the three toxins ApxI, ApxII and ApxIII in Actinobacillus pleuropneumoniae in only two PCR reactions. The oligonucleotide primers were designed to amplify a significant part of the activator and structural genes apxICA, apxIICA and apxIIICA together in a single PCR reaction giving amplification products which differ in length, in order to be clearly separated by agarose gel electrophoresis. Variations in the apxIA and apxIIIA genes which were found in different serotypes were taken into account in the design of the primers to give a uniform amplification product for both variants of the apxIA and the apxIIIA genes. The secretion genes apxIBD and apxIIIBD are also detected in a single PCR reaction containing two pairs of oligonucleotide primers which yield two differently sized fragments to differentiate between apxIBD and apxIIIBD genes. The reference strains of A. pleuropneumoniae serotypes 1-12 and 104 field strains representing all serotypes obtained from various laboratories worldwide were analysed for their content of apx genes. The two PCR reactions give toxin gene patterns which are characteristic for different groups of serotypes in A. pleuropneumoniae and allow the rapid differentiation of five toxin type groups, group 1 including serotypes 1, 5a, 5b, 9 and 11, group 2 including serotypes 2, 4, 6, 8, group 3 with serotype 3, group 4 with serotype 7 and 12 and group 5 with serotype 10. The method enhances and facilitates differentiation of A. pleuropneumoniae strains for diagnostics and epidemiology and allows the detection of serotypes with atypical toxin patterns.

Virulence in Actinobacillus pleuropneumoniae and RTX toxins

RTX toxins are pore-forming, cytolytic protein toxins that occur widely among pathogenic Gram-negative bacteria. RTX toxins appear to play a direct role in the virulence of Actinobacillus pleuropneumoniae, the etiological agent of porcine pleuropneumonia. This discovery has led to the development of new diagnostic and epidemiological tools, as well as vaccines, that are useful for a broad variety of serotypes.

Molecular investigation of the role of ApxI and ApxII in the virulence of Actinobacillus pleuropneumoniae serotype 5

The extracellular hemolytic toxins (ApxI and ApxII) of Actinobacillus pleuropneumoniae are thought to be important factors in this microorganism's virulence and the pathogenesis of swine pleuropneumonia. Using the polymerase chain reaction, the apxI locus of a non-hemolytic, avirulent mutant of A. pleuropneumoniae serotype 5 (mIT4-H) generated by chemical mutagenesis (Inzana T. J., Todd J., Veit H. P. Microb Pathog 1991; 10: 281-96) was found to contain deletions that affected major parts of the entire apxICABD operon, thus inactivating each gene in the operon. The apxII locus was not affected. Monoclonal antibodies to ApxI and ApxII were used to confirm that ApxI was not synthesized, and that ApxII was synthesized but not secreted from the cell. The apxICABD genes and apxIBD genes were cloned into a broad host range vector to obtain plasmids pJFF800 and pJFF801, respectively. Each recombinant plasmid was electroporated into strain mIT4-H to obtain strain mIT4-H/pJFF800 and strain mIT4-H/pJFF801, respectively. Strain mIT4-H/pJFF800 exported ApxI and ApxII, and produced hemolytic activity comparable to or exceeding that of wild type strain J45. Strain mIT4-H/pJFF801 exported only ApxII and produced weak hemolytic activity. Strain mIT4-H/pJFF800 was virulent in mice, and had an LD50 of about 2 x 10(6) colony forming units. In contrast, mIT4-H/pJFF801 and mIT4-H were essentially avirulent in mice, and LD50s for these strains could not be calculated. Strain mIT4-H/pJFF800 was virulent in pigs and caused lethal pleuropneumonia, whereas parent strain mIT4-H was avirulent. Strain mIT4-H/pJFF801 was also able to induce pleuropneumonia in pigs, although a higher dose was required to induce lesions similar to those caused by mIT4-H/pJFF800. Thus, A. pleuropneumoniae strains that produce ApxI and ApxII require ApxI for full virulence and toxic activity in pigs. However, other factors including ApxII contribute to the virulence of A. pleuropneumoniae in pigs.

Association of the CAMP phenomenon in Actinobacillus pleuropneumoniae with the RTX toxins ApxI, ApxII and ApxIII

A non-hemolytic mutant of Actinobacillus pleuropneumoniae serotype 5 has a deletion spanning the entire apxI operon. Therefore it does not produce ApxI and is unable to secrete ApxII. This mutant also has lost the co-hemolytic CAMP effect which is characteristic of the species A. pleuropneumoniae. The CAMP effect is restored when the mutant is complemented in trans by the apxIBD genes cloned in a broad host range vector, thus permitting secretion of ApxII, or when the entire apxI operon is cloned in the mutant, thus restoring the original toxin phenotype ApxI+ ApxII+. When the toxins ApxI, ApxII or ApxIII individually are expressed and secreted from E. coli harboring recombinant plasmids containing the genes apxICA and apxIBD or apxIICA and apxIBD or apxIIICABD, respectively, the distinct CAMP phenomenon is produced by the recombinant strains. The CAMP phenomenon is strongest by the recombinant E. coli strain expressing the non-hemolytic ApxIII, somewhat less when ApxI is expressed, and weak when ApxII is expressed. In A. pleuropneumoniae the CAMP phenomenon is also strongest in those serotypes which express ApxIII. The CAMP phenomenon of A. pleuropneumoniae is assumed to be directly caused by any of the RTX-toxins ApxI, ApxII or ApxIII. A previously reported gene from A. pleuropneumoniae, named cfp or hlyX, which provides E. coli strains with a hemolytic character and a CAMP phenomenon, shows high similarity to the E. coli global regulation gene fnr, and which is able to complement a delta fnr mutant.(ABSTRACT TRUNCATED AT 250 WORDS)

The RTX haemolysins ApxI and ApxII are major virulence factors of the swine pathogen Actinobacillus pleuropneumoniae: evidence from mutational analysis

The involvement of the RTX haemolysins (ApxI and ApxII) of the swine pathogen Actinobacillus pleuropneumoniae in virulence was investigated using haemolysin-deficient mutants constructed by a mini-Tn10 mutagenesis procedure. Two types of haemolysin mutant with single insertions of the transposon were obtained from a serotype 1 strain producing both ApxI and ApxII. One presented a complete loss of haemolytic activity because of the absence of ApxI and ApxII production. The other displayed weaker haemolysis than the wild type and produced only ApxII. The chromosomal regions flanking mini-Tn10 were cloned and sequenced. In the non-haemolytic mutant, the transposon had inserted in apxIB, a gene involved in the exportation of ApxI and ApxII toxins. The weakly haemolytic mutant resulted from the disruption of the structural gene for ApxI. Both mutations in the apxI operon were associated with a significant loss of virulence for mice and pigs, demonstrating that haemolysins are involved in A. pleuropneumoniae pathogenicity. The non-haemolytic mutant was apathogenic and the weakly haemolytic mutant retained some virulence for pigs, suggesting that both ApxI and ApxII are needed for full virulence.

Sequence analysis and transcription of the apxI operon (hemolysin I) from Actinobacillus pleuropneumoniae

The DNA sequence of the entire apxI operon from Actinobacillus pleuropneumoniae serotype 1 reference strain 4074 has been determined. This 8292-bp fragment of the chromosomal DNA contains four open reading frames (ORFs) of the strongly hemolytic ApxI toxin. These ORFs correspond to the genes apxIC, apxIA, apxIB and apxID, encoding the activator, the structural toxin protein and the two secretion proteins, respectively. Each of the four ORFs is preceded by a consensus sequence for a putative ribosome-binding site (RBS). The region upstream from apxIC contains several sites that could act as promoters. The transcription start point (tsp) of the apxI operon in A. pleuropneumoniae has been determined by primer extension analysis and was found to be located 133-bp upstream from the translation start codon. The tsp is preceded by sequences matching the -10 and -35 consensus sequence of promoters from Escherichia coli. This is the first promoter identified in A. pleuropneumoniae. The same tsp was used when the expression of apxI was induced by a high concentration of free Ca2+ in the growth medium, as well as when the expression of apxI was not induced by growing the cells in medium depleted of free Ca2+ ions. However, the signal strength of the primer extension was approximately tenfold stronger in Ca(2+)-grown cells. The leader sequence of the transcript is unusually long and very A+U rich (75% A+U).

Protection of mice and swine against infection with Actinobacillus pleuropneumoniae by vaccination

CaCl2 and LiCl cell extracts and a crude hemolysin preparation were isolated from Actinobacillus pleuropneumoniae serotype 1 strain 4074 and tested for protection against A. pleuropneumoniae serotype 1 and 5 in mice. The LiCl cell extract adsorbed on AlPO4 and the crude hemolysin preparation adsorbed on Al(OH)3 showed a highly significant protection (P < 0.01) against both serotypes. Different vaccine preparations were used to immunize pigs by intra-muscular injection at days 0 and 14; the pigs were then challenged at day 21 by intra-tracheal inoculation of 1 x 10(8) colony forming units (CFU) of a serotype 1 strain 4074. A vaccine which combined the LiCl extract and the crude hemolysin preparation adsorbed on Al(OH)3 gave the best protection with no mortality and no sign of morbidity in the vaccinated pigs. In the other experimental groups which included a group immunized with a commercial bacterin, mortality, respiratory disease and extensive pulmonary lesions were noted. This mixture shows good potential as a vaccine against pleuropneumonia in pigs.

ABC transporters: bacterial exporters

The ABC transporters (also called traffic ATPases) make up a large superfamily of proteins which share a common function and a common ATP-binding domain. ABC transporters are classified into three major groups: bacterial importers (the periplasmic permeases), eukaryotic transporters, and bacterial exporters. We present a comprehensive review of the bacterial ABC exporter group, which currently includes over 40 systems. The bacterial ABC exporter systems are functionally subdivided on the basis of the type of substrate that each translocates. We describe three main groups: protein exporters, peptide exporters, and systems that transport nonprotein substrates. Prototype exporters from each group are described in detail to illustrate our current understanding of this protein family. The prototype systems include the alpha-hemolysin, colicin V, and capsular polysaccharide exporters from Escherichia coli, the protease exporter from Erwinia chrysanthemi, and the glucan exporters from Agrobacterium tumefaciens and Rhizobium meliloti. Phylogenetic analysis of the ATP-binding domains from 29 bacterial ABC exporters indicates that the bacterial ABC exporters can be divided into two primary branches. One branch contains the transport systems where the ATP-binding domain and the membrane-spanning domain are present on the same polypeptide, and the other branch contains the systems where these domains are found on separate polypeptides. Differences in substrate specificity do not correlate with evolutionary relatedness. A complete survey of the known and putative bacterial ABC exporters is included at the end of the review.

Expression of the Pasteurella haemolytica leukotoxin is inhibited by a locus that encodes an ATP-binding cassette homolog

Multicopy and single-copy chromosomal fusions between the Pasteurella haemolytica leukotoxin regulatory region and the Escherichia coli beta-galactosidase gene have been constructed. These fusions were used as reporters to identify and isolate regulators of leukotoxin expression from a P. haemolytica cosmid library. A cosmid clone, which inhibited leukotoxin expression from multicopy and single-copy protein fusions, was isolated and found to contain the complete leukotoxin gene cluster plus additional upstream sequences. The locus responsible for inhibition of expression from leukotoxin-beta-galactosidase fusions was mapped within these upstream sequences, by transposon mutagenesis with Tn5, and its DNA sequence was determined. The inhibitory activity was found to be associated with a predicted 440-amino-acid reading frame (lapA) that lies within a four-gene arginine transport locus. LapA is predicted to be the nucleotide-binding component of this transport system and shares homology with the Clp family of proteases.

Transposon mutagenesis in Actinobacillus pleuropneumoniae with a Tn10 derivative

A transposon mutagenesis procedure functional in the gram-negative swine pathogen Actinobacillus pleuropneumoniae was developed for the first time. The technique involved the use of a suicide conjugative plasmid, pLOF/Km, carrying a mini-Tn10 with an isopropyl-beta-D-thiogalactopyranoside (IPTG)-inducible transposase located outside the mobile element (M. Herrero, V. de Lorenzo, and K. N. Timmis, J. Bacteriol. 172:6557-6567, 1990). The plasmid was mobilized from Escherichia coli to A. pleuropneumoniae through the RP4-mediated broad-host-range conjugal transfer functions provided by the chromosome of the donor strain. When IPTG was present in the mating medium, A. pleuropneumoniae CM5 transposon mutants were obtained at a frequency of 10(-5), while no mutants were detected in the absence of IPTG. Since the frequency of conjugal transfer of the RP4 plasmid from E. coli to A. pleuropneumoniae CM5 was found to be as low as 10(-4), the above result indicated that the expression level of the transposase was a critical factor for obtaining a workable efficiency of transposon mutagenesis. The transposon insertions occurred at random, as determined by Southern blotting of chromosomal DNA of randomly selected mutants and by the ability to generate mutants defective for the selected phenotypes. Almost all the mutants analyzed resulted from a single insertion of the Tn10 element. About 1.2% of the mutants resulted from the cointegration of pLOF/Km into the A. pleuropneumoniae chromosome. The applicability of this transposon mutagenesis system was verified on other A. pleuropneumoniae strains of different serotypes. The usefulness of this transposon mutagenesis system in genetic studies of A. pleuropneumoniae is discussed.

Cloning and characterization of the Actinobacillus pleuropneumoniae-RTX-toxin III (ApxIII) gene

To study the role of Actinobacillus pleuropneumoniae-RTX-toxin III (ApxIII) in the pathogenesis of porcine pleuropneumonia, we cloned and characterized the gene encoding this toxin. For that purpose, we screened an expression library of genomic DNA of serotype 8 with an ApxIII-specific monoclonal antibody and isolated a 425-bp fragment of an immunoreactive clone. Using this fragment as a probe, we identified and cloned an overlapping chromosomal NsiI restriction fragment of 5.0 kbp. Escherichia coli cells that contained this fragment produced a protein similar to ApxIII. Like ApxIII, the protein had a molecular mass of approximately 120 kDa, was recognized by an ApxIII-specific antibody, killed porcine lung macrophages, and was not lytic for sheep erythrocytes. We concluded from these data that the 5.0-kbp NsiI fragment contained the ApxIII-coding gene. Nucleotide sequence analysis of the 5.0-kbp NsiI fragment revealed the presence of two genes, apxIIIC and apxIIIA. These genes coded for proteins ApxIIIC and ApxIIIA, respectively, which were 53 and 50% identical to the prototypic RTX proteins HlyC and HlyA of E. coli. We assumed that the apxIIIA gene coded for the structural RTX toxin and that the apxIIIC gene coded for its activator. In addition, we found that ApxIII could be secreted from E. coli by the heterologous RTX transporter proteins HlyB and HlyD. The deduced amino acid sequence of ApxIIIA was 50% identical to that of ApxIA and 41% identical to that of ApxIIA. We concluded that, beside ApxI and ApxII, ApxIII is the third RTX toxin produced by A. pleuropneumoniae.

Analysis of hemolysin operons in Actinobacillus pleuropneumoniae

Among the twelve different serotypes of Actinobacillus pleuropneumoniae, the causative agent of swine pleuropneumonia, a strongly active hemolysin I (HlyI) is produced by serotypes which are particularly virulent, and less active hemolysin II (HlyII) is produced by all serotypes except type 10. In the serotypes 1, 5a, 5b, 9, 10 and 11, which produce HlyI, the hemolysin (hly) operon consists of a structural hlyIA gene, encoding pre-HlyI, an activator gene, hlyIC, necessary for the activation of pre-Hly to active Hly, and two genes, hlyIB and hlyID, involved in Hly secretion. These genes are clustered in the order, hlyICABD. This is characteristic to RTX toxin (repeats in the structural toxin) operons. The HlyII operons in all serotypes producing HlyII consist only of the pre-HlyII-encoding gene, appA, and its activator gene, appC. The serotypes, which produce HlyII, but not HlyI, contain a truncated HlyI operon, with the promoter, hlyIB and hlyID, and a small segment of the C terminus of hlyIA. This partial HlyI operon might have been formed by deletion of hlyIC and most of hlyIA. In serotype 3, which produces HlyII, but no HlyI, and which releases only minute amounts of this Hly into the growth medium, none of the hlyI genes and consequently no Hly secretion genes were found. The above results postulate that HlyII is secreted via the products of hlyIB and hlyID, and explain the low amount of HlyII secreted by serotype 3. Cloning and analysis of the structural genes encoding pre-HlyI and pre-HlyII among the different serotypes revealed differences in the hlyIA genes which are highly similar in the serologically related serotypes 1, 9 and 11, and differ from the serotypes, 5a, 5b and 10. The hlyIIA genes, in contrast, seem to be conserved in all serotypes.

Functional analysis of the Ca(2+)-regulated hemolysin I operon of Actinobacillus pleuropneumoniae serotype 1

The genetic determinant encoding the synthesis and secretion of hemolysin I (HlyI; gene designation, hlyI) by Actinobacillus pleuropneumoniae serotype 1 4074T was cloned in the lambda vector EMBL4. A 10.2-kb fragment that encoded hemolytic activity in the phage lysate was aligned by Southern blot hybridization to genes hlyC, hlyA, hlyB, and hlyD of the Escherichia coli hemolysin operon, and expression of the A. pleuropneumoniae genes in E. coli revealed that they have the same functions as their E. coli analogs: hlyIC encodes a protein that activates inactive 105-kDa prohemolysin I (encoded by hlyIA) to active hemolysin I, while hlyIB and hlyID are necessary for HlyIA secretion. Northern (RNA) hybridization of A. pleuropneumoniae RNA revealed that the gene cluster is transcribed as two RNA species, a major one of 3.5 kb, corresponding to hlyICA, and a second, minor one of 7.5 kb, corresponding to the whole operon, hlyICABD. The level of hlyI mRNA was substantially higher in A. pleuropneumoniae 4074T cells grown in the presence of Ca2+, supporting the view that the expression of the hlyI determinant is Ca2+ regulated. Parallel RNA hybridization with random gene probes suggested that this Ca2+ regulation is specific for the hlyI determinant.