A targeted mutagenesis system for Actinobacillus pleuropneumoniae

Optimal regimens based on PK/PD cutoff evaluation of ceftiofur against Actinobacillus pleuropneumoniae in swine

Background

Actinobacillus pleuropneumoniae formerly known as Haemophilus pleuropneumoniae, can cause pleuropneumoniae in pigs, which lead to significant mortality. Ceftiofur was the first cephalosporin antibiotic used in animals, which was effective against gram-negative and gram-positive bacterium. This study aimed to formulate a rational dosage strategy and review the preceding recommended dosage based on PK/PD modeling and Establish Clinical breakpoint of ceftiofur against Actinobacillus pleuropneumoniae based on the pharmacodynamic-pharmacokinetic cutoff.

Results

The epidemiologic cutoff value was 0.125 μg/mL. The results of the pharmacodynamic study showed that the MICs of BW39 were 0.5 μg/mL and 1 μg/mL in vitro and ex-vivo, respectively. The minimal bactericidal concentrations (MBCs) under in vitro and ex vivo conditions were both 1 μg/mL. The time-killing profiles of ceftiofur against BW39 were time-dependent with a partly concentration-dependent pattern. Based on the inhibitory sigmoid E_max model, the AUC_{24 h}/MIC values for the bacteriostatic, bactericidal, and elimination effects in serum were 45.73, 63.83, and 69.04 h for healthy pigs separately. According to the Monte Carlo simulation, the CO_PD was calculated as 2 μg/mL, and the optimized dosage regimen of ceftiofur against Actinobacillus pleuropneumoniae to achieve bacteriostatic, bactericidal, and elimination effects over 24 h was 2.13, 2.97, and 3.42 mg/kg for the 50% target attainment rate (TAR) and 2.47, 3.21, and 3.70 mg/kg for the 90% TAR respectively.

Conclusions

In conclusion, we reveal the EOFF and PK/PD cutoff values of ceftiofur against A. pleuropneumoniae in piglets. However, with the paucity of clinical data for ceftiofur to establish a clinical cutoff against A. pleuropneumoniae, the PK/PD cutoff value of 2 μg/mL will be recommended as surrogate. According to the PK/PD data and the MIC distribution in China, the single bactericidal dose was 3.21 mg/kg for the 90% target, which would be more able to cure Actinobacillus pleuropneumoniae and avoid the emergence of resistance for clinical ceftiofur use in piglet.

Branched-chain amino acids are required for the survival and virulence of Actinobacillus pleuropneumoniae in swine

In Actinobacillus pleuropneumoniae, which causes porcine pleuropneumonia, ilvI was identified as an in vivo-induced (ivi) gene and encodes the enzyme acetohydroxyacid synthase (AHAS) required for branched-chain amino acid (BCAA) biosynthesis. ilvI and 7 of 32 additional ivi promoters were upregulated in vitro when grown in chemically defined medium (CDM) lacking BCAA. Based on these observations, we hypothesized that BCAA would be found at limiting concentrations in pulmonary secretions and that A. pleuropneumoniae mutants unable to synthesize BCAA would be attenuated in a porcine infection model. Quantitation of free amino acids in porcine pulmonary epithelial lining fluid showed concentrations of BCAA ranging from 8 to 30 micromol/liter, which is 10 to 17% of the concentration in plasma. The expression of both ilvI and lrp, a global regulator that is required for ilvI expression, was strongly upregulated in CDM containing concentrations of BCAA similar to those found in pulmonary secretions. Deletion-disruption mutants of ilvI and lrp were both auxotrophic for BCAA in CDM and attenuated compared to wild-type A. pleuropneumoniae in competitive index experiments in a pig infection model. Wild-type A. pleuropneumoniae grew in CDM+BCAA but not in CDM-BCAA in the presence of sulfonylurea AHAS inhibitors. These results clearly demonstrate that BCAA availability is limited in the lungs and support the hypothesis that A. pleuropneumoniae, and potentially other pulmonary pathogens, uses limitation of BCAA as a cue to regulate the expression of genes required for survival and virulence. These results further suggest a potential role for AHAS inhibitors as antimicrobial agents against pulmonary pathogens.

Potential use an Actinobacillus pleuropneumoniae double mutant strain ΔapxIICΔapxIVA as live vaccine that allows serological differentiation between vaccinated and infected animals

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia, a highly contagious and often fatal disease. We have previously reported the construction and characterization of a single gene apxIIC deletion mutant HB04C(-) based on A. pleuropneumoniae serovar 7 which produces ApxII toxin and ApxIV. A precisely defined DeltaapxIICDeltaapxIVA double-deletion mutant of A. pleuropneumoniae was constructed based on HB04C(-) by transconjugation and counterselection, and the levels of virulence of the DeltaapxIIC single mutant and DeltaapxIICDeltaapxIVA double mutant were compared in an experimental infection in mice and pigs. The results demonstrated that the DeltaapxIICDeltaapxIVA double mutant strain was less virulent than HB04C(-). Despite attenuation of virulence, the DeltaapxIICDeltaapxIVA double mutant remains immunogenic and conferred a similar level of protective immunity to pigs against challenge with a lethal dose of a heterologous fully virulent standard serovar 1 strain of A. pleuropneumoniae. The results of the virulence study suggest that ApxIV is a critical virulence factor of A. pleuropneumoniae serovar 7 and is able to induce clinical disease, but it not required for efficient vaccination of pigs against A. pleuropneumoniae infection. Two weeks after the booster immunization, animals vaccinated with HB04C(-) were positive in the ApxIVAM-ELISA based on a recombinant GST-fusion protein GST-ApxIVAM as the solid-phase antigen while animals vaccinated with the DeltaapxIICDeltaapxIVA double mutant were negative. These data demonstrate that the double mutant DeltaapxIICDeltaapxIVA can be used as an effective live marker vaccine allowing serological differentiation between vaccinated and infected animals.

Identification of the Actinobacillus pleuropneumoniae leucine-responsive regulatory protein and its involvement in the regulation of in vivo-induced genes

Actinobacillus pleuropneumoniae is a gram-negative bacterial pathogen that causes a severe hemorrhagic pneumonia in swine. We have previously shown that the limitation of branched-chain amino acids (BCAAs) is a cue that induces the expression of a subset of A. pleuropneumoniae genes identified as specifically induced during infection of the natural host animal by using an in vivo expression technology screen. Leucine-responsive regulatory protein (Lrp) is a global regulator and has been shown in Escherichia coli to regulate many genes, including genes involved in BCAA biosynthesis. We hypothesized that A. pleuropneumoniae contains a regulator similar to Lrp and that this protein is involved in the regulation of a subset of genes important during infection and recently shown to have increased expression in the absence of BCAAs. We report the identification of an A. pleuropneumoniae serotype 1 gene encoding a protein with similarity to amino acid sequence and functional domains of other reported Lrp proteins. We further show that purified A. pleuropneumoniae His6-Lrp binds in vitro to the A. pleuropneumoniae promoter regions for ilvI, antisense cps1AB, lrp, and nqr. A genetically defined A. pleuropneumoniae lrp mutant was constructed using an allelic replacement and sucrose counterselection method. Analysis of expression from the ilvI and antisense cps1AB promoters in wild-type, lrp mutant, and complemented lrp mutant strains indicated that Lrp is required for induction of expression of ilvI under BCAA limitation.

Actinobacillus pleuropneumoniae: pathobiology and pathogenesis of infection

Actinobacillus pleuropneumoniae causes porcine pleuropneumonia, a highly contagious disease for which there is no effective vaccine. This review considers how adhesins, iron-acquisition factors, capsule and lipopolysaccharide, RTX cytotoxins and other potential future vaccine components contribute to colonisation, to avoidance of host clearance mechanisms and to damage of host tissues.

[Cu,Zn]-Superoxide dismutase mutants of the swine pathogen Actinobacillus pleuropneumoniae are unattenuated in infections of the natural host

Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, contains a periplasmic Cu- and Zn-cofactored superoxide dismutase ([Cu,Zn]-SOD, or SodC) which has the potential, realized in other pathogens, to promote bacterial survival during infection by dismutating host-defense-derived superoxide. Here we describe the construction of a site-specific, [Cu,Zn]-SOD-deficient A. pleuropneumoniae serotype 1 mutant and show that although the mutant is highly sensitive to the microbicidal action of superoxide in vitro, it remains fully virulent in experimental pulmonary infection in pigs.

A genetically-defined riboflavin auxotroph of Actinobacillus pleuropneumoniae as a live attenuated vaccine

Actinobacillus pleuropneumoniae is a gram negative pleiomorphic rod that is the causative agent of a severe, highly infectious and often fatal pleuropneumonia in swine. We have previously reported the construction of genetically-defined stable riboflavin auxotrophs by replacement of a portion of the APP riboflavin biosynthetic operon (ribGBAH) with an antibiotic cassette encoding resistance to kanamycin, and have demonstrated that such riboflavin auxotrophs are avirulent. In this study, we evaluated riboflavin auxotrophs of A. pleuropneumoniae for their ability to stimulate protective immunity against pleuropneumonia. An initial challenge experiment demonstrated that intramuscular vaccination with a live attenuated serotype 1A rib mutant, in a vaccine formulation that included a limiting amount of exogenous riboflavin, provided better protection against challenge with virulent A. pleuropneumoniae than either intratracheal immunization or intramuscular immunization with live bacteria in the absence of exogenous riboflavin. Subsequent studies in which the vaccine inoculating dose, concentration of exogenous riboflavin, and serotype of the vaccine strain were varied demonstrated that immunization with live avirulent riboflavin auxotrophs could elicit significant protection against experimental challenge with both homologous and heterologous virulent serotypes of A. pleuropneumoniae.

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.

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.

Cloning and molecular characterization of Cu,Zn superoxide dismutase from Actinobacillus pleuropneumoniae

Copper-zinc superoxide dismutases (Cu,Zn SODs), until recently considered very unusual in bacteria, are now being found in a wide range of gram-negative bacterial species. Here we report the cloning and characterization of sodC, encoding Cu,Zn SOD in Actinobacillus pleuropneumoniae, a major pathogen of pigs and the causative organism of porcine pleuropneumonia. sodC was shown to lie on a monocistronic operon, at the chromosomal locus between the genes asd (encoding aspartate semialdehyde dehydrogenase) and recF. The primary gene product was shown to have an N-terminal peptide extension functioning as a leader peptide, so that the mature Actinobacillus enzyme, like other bacterial examples, is directed to the periplasm, where it is appropriately located to dismutate exogenously generated superoxide. While the role of these secreted bacterial SODs is unknown, we speculate that in A. pleuropneumoniae the enzyme may confer survival advantage by accelerating dismutation of superoxide derived from neutrophils, a central host defense response in the course of porcine infection.

A riboflavin auxotroph of Actinobacillus pleuropneumoniae is attenuated in swine

Actinobacillus pleuropneumoniae is the etiological agent of a highly contagious and often fatal pleuropneumonia in swine. A riboflavin-requiring mutant of A. pleuropneumoniae serotype 1, designated AP233, was constructed by deleting a portion of the riboflavin biosynthetic operon (ribGBAH) and replacing it with a gene cassette encoding kanamycin resistance. The genes affected included both the alpha- and beta-subunits of riboflavin synthase as well as a bifunctional enzyme containing GTP cyclohydrase and 3,4-dihydroxy-2-butanone-4-phosphate synthase activities. AP233 was unable to grow in the absence of exogenous riboflavin but otherwise was phenotypically identical to the parent wild-type strain. Experimental infection studies with pigs demonstrated that the riboflavin-requiring mutant was unable to cause disease, on the basis of mortality, lung pathology, and clinical signs, at dosages as high as 500 times the normal 50% lethal dose for the wild-type parent. This is the first demonstration of the attenuation of A. pleuropneumoniae by introduction of a defined mutation in a metabolic gene and the first demonstration that mutations in the genes required for riboflavin biosynthesis can lead to attenuation in a bacterial pathogen.