Identification of a ClpC ATPase required for stress tolerance and in vivo survival of Listeria monocytogenes

We identified a new chromosomal locus involved in the virulence of the facultative intracellular pathogen Listeria monocytogenes. This locus displays the same genetic organization as that of the clpC/mecB locus of Bacillus subtilis. It contains a thermoregulated operon of four genes, whose transcription is upregulated at 42 degrees C. The last gene of this operon is clpC, which encodes a protein of 826 amino acid residues, identified as a ClpC ATPase, sharing a strong peptide sequence identity (78%) with ClpC/MecB of B. subtilis. Tn917 insertions inactivating the entire operon, or only clpC, gave mutants highly susceptible to stress, including iron limitation, elevated temperatures and high osmolarity. The virulence of these mutants was severely impaired in the mouse. A clpC insertional mutant was also restricted in its capacity to grow in bone-marrow-derived macrophages. These results demonstrate that the ClpC ATPase of L. monocytogenes is a general stress protein involved in intracellular growth and in vivo survival of this pathogen in host tissues.

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.

Evaluation of an immunoperoxidase technique using an only biotin-labeled antibody for the demonstration of Actinobacillus pleuropneumoniae in tissue sections

In this report an immunoperoxidase technique (IPB) using an only biotin-labeled antibody is compared to culture isolation method (CIM) for the demonstration of Actinobacillus pleuropneumoniae serotype 1 in tissues of mice inoculated intravenously. The organism was isolated in 86.7% of mice and detected by IPB in 100% of cases. A. pleuropneumoniae antigen was mainly demonstrated in the liver, spleen and lungs, but also in the kidney and brain, being especially located in the cytoplasm of macrophages. Significant differences were observed between the results obtained by both tests (P < 0.001). Cross-reactions occurred by IPB when a rabbit anti-A. pleuropneumoniae serotype 11 serum was tested but never when an anti-serotype 9 serum was used. The immunoperoxidase test here described yielded much better results than CIM and it could be useful in routine diagnosis of A. pleuropneumoniae infections.

Quantifying by monoclonal antibodies of specific IgG, IgM and IgA in the serum of minipigs experimentally infected with Actinobacillus pleuropneumoniae

Fifteen minipigs were infected intratracheally with three different doses (10(8), 10(5) or 5 x 10(3) colony forming units) of the reference strains of serotypes 2 or 4 of Actinobacillus pleuropneumoniae, and three remained as controls. The titre of specific IgG, IgM and IgA in the serum was measured weekly for 15 weeks with an indirect ELISA using monoclonal antibodies specific for each isotype. IgG attained the highest titres, IgM lower and IgA the lowest, being only detected in four animals. Serotype 4 evoked significantly higher titres than serotype 2 (P < or = 0.01). In general the highest IgG titres were attained at four to six weeks after infection. Some of the minipigs were reinfected after seven weeks but this evoked an increased titre in only two instances.

Viability of Actinobacillus pleuropneumoniae in frozen pig lung samples and comparison of different methods of direct diagnosis in fresh samples

A comparative study on different methods of diagnosis of Actinobacillus pleuropneumoniae from both fresh and frozen pig lungs is described. A total of 196 lung tissues with pneumonic lesions were examined for culture isolation on chocolate blood agar, as well as for antigen detection by means of the coagglutination test, the immunodiffusion test and the indirect ELISA. These samples were subsequently frozen for 1 yr and then they were recultured. A. pleuropneumoniae was recovered from fresh lung specimens in 30 cases (15.3%) and from frozen samples in only two cases (0.9%). Such a different degree of isolation demonstrates that long freezing had an adverse effect on the viability of this organism in lung samples. A pleuropneumoniae detection was positive in 134 samples (68.4%) by at least one of the immunological techniques examined. The indirect ELISA was the most sensitive and specific test, with antigen detected in 125 lungs (63.8%). In comparison with the coagglutination and immunodiffusion tests, the sensitivities of the indirect ELISA were 95.8 and 93.7%, and the specificities were 67.0 and 63.4%, respectively.

Cross-reactivity between Actinobacillus pleuropneumoniae serotypes comparing different antigens and serological tests

Several antigens were prepared from suspensions of reference strains of the 12 serotypes of Actinobacillus pleuropneumoniae (saline extract [SE], capsular extract [CE], whole cell suspension [WCS], boiled extract [BE], and autoclaved cell antigen [ACA]). The cross reactions between each antigen and the antisera against reference strains of the other 11 serotypes were compared by using the complement fixation test, ELISA and the indirect haemagglutination test. ACA produced the most cross reactions, which, in some serotypes, took place in all the antisera tested. BE produced fewer cross reactions, but these were more abundant than those obtained with SE, CE or WCS. The least cross reactivity occurred with SE in the indirect haemagglutination test. This test is, therefore, the most reliable method for serotyping field strains of A pleuropneumoniae.