Isolation and characterization of mini-Tn10 lipopolysaccharide mutants of Actinobacillus pleuropneumoniae serotype 1

Lipopolysaccharide (LPS) has previously been identified as the major adhesin of Actinobacillus pleuropneumoniae involved in adherence to porcine respiratory tract cells. The purpose of the present study was to isolate and characterize mutants in LPS biosynthesis by using a mini-Tn10 transposon mutagenesis system. Seven mutants appeared to possess a rough LPS (among which two had similar Southern blot profiles) while one mutant (#5.1) expressed the high-molecular-mass LPS, but as visualized by Tricine SDS-PAGE, showed an additional band in the core-lipid A region. The LPS mutants showed sensitivity to pig serum to various degrees, while the parent strain was serum-resistant. Use of piglet frozen tracheal sections indicated that, surprisingly, the rough LPS mutants adhered similarly or in greater numbers than the parent strain. However, the LPS mutant #5.1 adhered significantly less than the parent strain and was also less virulent in pigs. The gene affected by mini-Tn10 in LPS mutant #5.1 is galU, the structural gene for UTP-alpha-D-glucose-1-phosphate uridylyltransferase, involved in LPS core biosynthesis. Complementation analysis confirmed that the phenotypic characteristics of LPS mutant #5.1 are the result of the inactivation of the galU gene. Our data suggest that although the presence of O-antigen does not seem to be essential, an intact core-lipid A region might be required for adherence of A. pleuropneumoniae to porcine respiratory tract cells. To the best of our knowledge, these mutants represent the first isogenic mutants of A. pleuropneumoniae defective in LPS biosynthetic genes.     

Adherence ofPasteurella multocida to porcine upper respiratory tract cells

A model to study the adherence ofPasteurella multocida to porcine upper respiratory tract cells is described. The ability of 27 differentP. multocida isolates to adhere to isolated tracheal epithelial cells was examined. The mean number of adherent bacterial cells was significantly greater (p<0.005) for capsular type A cells than for capsular type D cells. No significant differences were observed between toxigenic and nontoxigenic isolates, or between isolates exhibiting different somatic antigens. However, isolates from pigs without atrophic rhinitis showed only 65% of the adherence of isolates from pigs with atrophic rhinitis. Adherence ofP. multocida to porcine tracheal cells decreased with animal age; adherence to cells from adults was only half of the adherence to cells from newborn animals. The data indicate that, in the present experimental conditions, theP. multocida strains tested possess different abilities to attach to porcine upper respiratory tract cells.     

Mutation in the LPS outer core biosynthesis gene, galU, affects LPS interaction with the RTX toxins ApxI and ApxII and cytolytic activity of Actinobacillus pleuropneumoniae serotype 1

Lipopolysaccharides (LPS) and Apx toxins are major virulence factors of Actinobacillus pleuropneumoniae, a pathogen of the respiratory tract of pigs. Here, we evaluated the effect of LPS core truncation in haemolytic and cytotoxic activities of this microorganism. We previously generated a highly attenuated galU mutant of A. pleuropneumoniae serotype 1 that has an LPS molecule lacking the GalNAc-Gal II-Gal I outer core residues. Our results demonstrate that this mutant exhibits wild-type haemolytic activity but is significantly less cytotoxic to porcine alveolar macrophages. However, no differences were found in gene expression and secretion of the haemolytic and cytotoxic toxins ApxI and ApxII, both secreted by A. pleuropneumoniae serotype 1. This suggests that the outer core truncation mediated by the galU mutation affects the toxins in their cytotoxic activities. Using both ELISA and surface plasmon resonance binding assays, we demonstrate a novel interaction between LPS and the ApxI and ApxII toxins via the core oligosaccharide. Our results indicate that the GalNAc-Gal II-Gal I trisaccharide of the outer core is fundamental to mediating LPS/Apx interactions. The present study suggests that a lack of binding between LPS and ApxI/II affects the cytotoxicity and virulence of A. pleuropneumoniae.     

Truncation of the lipopolysaccharide outer core affects susceptibility to antimicrobial peptides and virulence of Actinobacillus pleuropneumoniae serotype 1

We reported previously that the core oligosaccharide region of the lipopolysaccharide (LPS) is essential for optimal adhesion of Actinobacillus pleuropneumoniae, an important swine pathogen, to respiratory tract cells. Rough LPS and core LPS mutants of A. pleuropneumoniae serotype 1 were generated by using a mini-Tn10 transposon mutagenesis system. Here we performed a structural analysis of the oligosaccharide region of three core LPS mutants that still produce the same O-antigen by using methylation analyses and mass spectrometry. We also performed a kinetic study of proinflammatory cytokines production such as interleukin (IL)-6, tumor necrosis factor-alpha, IL1-beta, MCP-1, and IL8 by LPS-stimulated porcine alveolar macrophages, which showed that purified LPS of the parent strain, the rough LPS and core LPS mutants, had the same ability to stimulate the production of cytokines. Most interestingly, an in vitro susceptibility test of these LPS mutants to antimicrobial peptides showed that the three core LPS mutants were more susceptible to cationic peptides than both the rough LPS mutant and the wild type parent strain. Furthermore, experimental pig infections with these mutants revealed that the galactose (Gal I) and d,d-heptose (Hep IV) residues present in the outer core of A. pleuropneumoniae serotype 1 LPS are important for adhesion and overall virulence in the natural host, whereas deletion of the terminal GalNAc-Gal II disaccharide had no effect. Our data suggest that an intact core-lipid A region is required for optimal protection of A. pleuropneumoniae against cationic peptides and that deletion of specific residues in the outer LPS core results in the attenuation of the virulence of A. pleuropneumoniae serotype 1.     

Electron microscopic examination of capsular material from various serotypes of Actinobacillus pleuropneumoniae.

The capsular material on PPLO broth-grown cells of Actinobacillus pleuropneumoniae representing serotypes 1 to 10 was visualized by transmission electron microscopy after polycationic ferritin labeling and also after stabilization with specific antibodies. All the isolates examined were covered with a layer of capsular material whose thickness varied between 80 to 90 nm and 210 to 230 nm when examined by immunostabilization. We were also able to visualize A. pleuropneumoniae in lungs of infected pigs and to estimate the amount of capsular material covering the cells. Our results indicate that differences in capsular structure exist among the different A. pleuropneumoniae serotypes, and this result may explain in part why the serotypes are not equally virulent.     

Inhibition of Adherence of Actinobacillus pleuropneumoniae to Porcine Respiratory Tract Cells by Monoclonal Antibodies Directed Against LPS and Partial Characterization of the LPS Receptors

Actinobacillus pleuropneumoniae is the causative agent of porcine fibrinohemorrhagic necrotizing pleuropneumonia. We have previously identified the lipopolysaccharides (LPS) as the major adhesin of A. pleuropneumoniae involved in adherence to porcine respiratory tract cells. In the present study, adherence of A. pleuropneumoniae to porcine tracheal frozen sections was inhibited by homologous monovalent Fab fragments produced from monoclonal antibodies 5.1 G8F10 and 102-G02 directed, respectively, against the A. pleuropneumoniae serotype 1 or serotype 2 O-antigens. These results confirm the important role played by LPS in adherence of A. pleuropneumoniae and suggest that these adhesins might represent good vaccine candidates. We also investigated the presence of A. pleuropneumoniae receptors in tracheal cell preparations from piglets of four different breeds. Using Far-Western binding assays, we identified proteins recognized by whole cells of A. pleuropneumoniae reference strains for serotype 1 and 2, and local isolates belonging to the same serotypes, and also recognized by extracted LPS from both reference strains. We confirmed the proteinaceous nature of these LPS-binding molecules by their staining with Coomassie brilliant blue, sensitivity to proteinase K digestion, resistance to sodium m-periodate oxidation, and their inability to stain with glycoprotein-specific reagents. Four low-molecular-mass bands (14–17 kDa) seemed to correspond to histones. We also identified proteins at Mr 38,500 that could represent putative receptors for A. pleuropneumoniae LPS in swine respiratory tract cells. Received: 16 April 1999 / Accepted: 1 July 1999     

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.     

Serological Characterization of Actinobacillus pleuropneumoniae Biotype 1 Strains Antigenically Related to both Serotypes 2 and 7

Nine Danish Actinobacillus pleuropneumoniae biotype 1 isolates were shown by latex agglutination and indirect haemagglutination to possess capsular polysaccharide epitopes identical to those of serotype 2 strain 1536 (reference strain of serotype 2) and strain 4226 (Danish serotype 2 strain). Immunodiffusion confirmed the antigenic relationship with serotype 2 and further demonstrated an antigenic relationship with strain WF83 (reference strain of serotype 7). SDS-PAGE with LPS from strains 1536, 4226, WF83 and strain 7317 (representative of the 9 isolates examined) showed that strains WF83 and 7317 had an identical smooth ladder pattern whereas LPS from strains 1536 and 4226 showed a distinctly different pattern. The antigenic similarities of the LPS of strains WF83 and 7317 were confirmed by immunoblots using rabbit or pig antisera prepared against the 3 strains. No antigenic similarities in the LPS of strains 1536 and 7317 were revealed. Since an antigenic determinant specific for the 9 isolates could not be demonstrated with the methods used, the strains are proposed to be designated K2:O7.     

Immobilization of Pseudorabies Virus in Porcine Tracheal Respiratory Mucus Revealed by Single Particle Tracking

Pseudorabies virus (PRV) initially replicates in the porcine upper respiratory tract. It easily invades the mucosae and submucosae for subsequent spread throughout the body via blood vessels and nervous system. In this context, PRV developed ingenious processes to overcome different barriers such as epithelial cells and the basement membrane. Another important but often overlooked barrier is the substantial mucus layer which coats the mucosae. However, little is known about how PRV particles interact with porcine respiratory mucus. We therefore measured the barrier properties of porcine tracheal respiratory mucus, and investigated the mobility of nanoparticles including PRV in this mucus. We developed an in vitro model utilizing single particle tracking microscopy. Firstly, the mucus pore size was evaluated with polyethylene glycol coupled (PEGylated) nanoparticles and atomic force microscope. Secondly, the mobility of PRV in porcine tracheal respiratory mucus was examined and compared with that of negative, positive and PEGylated nanoparticles. The pore size of porcine tracheal respiratory mucus ranged from 80 to 1500 nm, with an average diameter of 455±240 nm. PRV (zeta potential: −31.8±1.5 mV) experienced a severe obstruction in porcine tracheal respiratory mucus, diffusing 59-fold more slowly than in water. Similarly, the highly negatively (−49.8±0.6 mV) and positively (36.7±1.1 mV) charged nanoparticles were significantly trapped. In contrast, the nearly neutral, hydrophilic PEGylated nanoparticles (−9.6±0.8 mV) diffused rapidly, with the majority of particles moving 50-fold faster than PRV. The mobility of the particles measured was found to be related but not correlated to their surface charge. Furthermore, PEGylated PRV (-13.8±0.9 mV) was observed to diffuse 13-fold faster than native PRV. These findings clearly show that the mobility of PRV was significantly hindered in porcine tracheal respiratory mucus, and that the obstruction of PRV was due to complex mucoadhesive interactions including charge interactions rather than size exclusion.     

Haemagglutination of Bacteroides fragilis

Abstract The relationship between the ability to cause haemagglutination (HA) and the presence of capsules and/or pili was examined for 50 clinical isolates of Bacteroides fragilis . HA was tested using a slide technique, and bovine, porcine, guinea pig, rat, rabbit, horse, human, chicken and pigeon erythrocytes. Chicken and pigeon erythrocytes were the best indicators for HA with 43 (86%) of the strains tested causing HA and 39 (78%) with strong reactions. Capsule staining showed that the same 43 strains causing HA also produced a demonstrable capsule. No pili were found on either encapsulated or non-encapsulated strains using transmission electron microscopy. These results suggest that adherence of B. fragilis is related to the presence of capsular material, not pili.     

Identification,cloning and characterization of rfaE of Actinobacillus pleuropneumoniae serotype 1, a gene involved in lipopolysaccharide inner-core biosynthesis

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia and its lipopolysaccharides (LPS) have been identified as important adhesins involved in adherence to host cells. To better understand the role of LPS core in the virulence of this organism, the aim of the present study was to identify and clone genes involved in LPS core biosynthesis by complementation with Salmonella enterica serovar Typhimurium mutants (rfaC, rfaD, rfaE and rfaF). Complementation with an A. pleuropneumoniae 4074 genomic library was successful with Salmonella mutant SL1102. This Salmonella deep-rough LPS mutant is defective for the rfaE gene, which is an ADP-heptose synthase. Novobiocin was used to select transformants that had the smooth-LPS type, since Salmonella strains with wild-type smooth-LPS are less permeable, therefore more resistant to hydrophobic antibiotics like novobiocin. We obtained a clone that was able to restore the wild-type smooth-LPS Salmonella phenotype after complementation. The wild-type phenotype was confirmed using phage (Felix-O, P22c.2 and Ffm) susceptibility and SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis). One of the open reading frames contained in the 3.3-kb insert in the plasmid encoded a 475-amino-acid protein with 71% identity and 85% similarity to the RfaE protein of S. enterica. We then attempted to generate an A. pleuropneumoniae rfaE mutant by gene replacement. The rfaE gene seems essential in A. pleuropneumoniae viability as we were unable to isolate a heptose-less knockout mutant.     

Evaluation of the hemoglobin-binding activity of Actinobacillus pleuropneumoniae using fluorescein-labeled pig hemoglobin and flow cytometry

In the present study, the hemoglobin (Hb)-binding activity of Actinobacillus pleuropneumoniae was examined using fluorescein-labeled pig Hb and flow cytometry. Comparison of the Hb-binding activity of A. pleuropneumoniae serotype 1 strain 4074 grown under iron-restricted conditions with cells grown under iron-sufficient conditions indicated that iron-restriction in A. pleuropneumoniae promotes the expression of Hb receptors, and that Hb-binding activity is, at least in part, iron-repressible. Hb-binding activity was also observed in representative strains of A. pleuropneumoniae belonging to serotypes 1 and 2. In addition, A. pleuropneumoniae serotype 1 LPS or capsule isogenic mutants were tested in flow cytometry in order to understand the influence of surface polysaccharides on Hb-binding activity. Experiments with an acapsulated mutant indicated that surface molecules with Hb-binding activity are more exposed at the cell surface in the absence of capsular polysaccharides. However, the Hb-binding activity of LPS mutants analyzed in this study was unchanged compared to the parent strain. The outer membrane proteins profile of A. pleuropneumoniae serotype 1 grown under iron-restricted or iron-sufficient conditions was also evaluated by polyacrylamide gel electrophoresis. Iron-regulated outer membrane proteins were observed under iron-restricted growth conditions which suggests that one or more of these outer membrane proteins may play a role in the Hb-binding activity detected by flow cytometry.     

Detection of Actinobacillus pleuropneumoniae by PCR on field strains from healthy and diseased pigs

We investigated whether primers able to specifically amplify a 0.7-kb DNA fragment from the conserved cpx genes could be applied to analyze A. pleuropneumoniae field isolates. The specific cpx primers were tested on 120 strains of A. pleuropneumoniae and other NAD-dependent field isolates from healthy and diseased animals to analyze A. pleuropneumoniae isolates from pigs in Brazil. We found that PCR and hybridization were able to discriminate between isolates of A. pleuropneumoniae and other bacteria. The 0.7-kb cpx DNA fragments were amplified from all 63 A. pleuropneumoniae isolates from herds with clinical symptoms and were isolated from lesions of acute cases of swine pleuropneumonia, both serotypable and nonserotypable. The PCR was also applied to 57 field isolates obtained from animals of apparently healthy herds, and the amplified cpx product was present in four serotypable and only two out of eleven A. pleuropneumoniae nonserotypable isolates. All nonserotypable A. pleuropneumoniae isolates revealed the apxA amplification pattern compatible with previously known serotypes. Some nonserotypable isolates might represent a population of isolates that originally were serotypable but lost the ability to react with serotype-specific antisera or might belong to novel serotypes. The PCR method applied is highly sensitive for serotypable A. pleuropneumoniae strains and for nonserotypable strains isolated from acute cases of swine pleuropneumoniae in Brazil.     

Is the CvaA* protein, encoded within the colicin V export gene cvaA, required for colicin V transport?

Abstract We investigated the expression of important Actinobacillus pleuropneumoniae surface polysaccharides, namely, capsular polysaccharides (CPS) and lipopolysaccharides (LPS), after growth under iron-restricted conditions. Iron restriction did not seem to affect the production of CPS, as determined by labelling with a monoclonal antibody (mAb) against the serotype l K-antigen and flow cytometry analysis, and also as determined by electron microscopy. SDS-PAGE revealed that the LPS profiles of these cells were also unaffected by iron restriction. Using flow cytometry analysis, however, we observed that binding of mAb against serotype 1 O-antigen was altered in cells of A. pleuropneumoniae serotype l reference strain (4074) grown under iron-restricted conditions. This strain exhibited two subpopulations with distinct patterns of reactivity with the mAb against the O-antigen. When strain 4074 was grown under iron-restricted conditions, a shift from one cell subpopulation (moderately fluorescent) to another cell subpopulation (highly fluorescent, thus binding more antibodies) was observed. Our results indicate that growth of A. pleuropneumoniae serotype l under iron-restricted conditions did not seem to affect CPS production, but might alter, at least for the reference strain, the expression of LPS.     

Proteolytic activity of clinical Candida albicans isolates in relation to genotype and strain source

Proteolytic activity is regarded as one of the most important virulence factors of Candida albicans. Several authors recently demonstrated that some karyotypes and genotypes harbouring a group I self-splicing intron (CaLSU) located in the gene encoding the large rRNA subunit showed a high level of proteinase production. The aim of this study was to investigate the correlation between the level of proteinase production and the presence of the CaLSU intron in C. albicans isolates originating from the blood and respiratory tracts (sputum/pharyngeal swabs) of patients with and without oropharyngeal candidosis. The results revealed statistically significant differences in genotype distribution and the level of proteinase production between the C. albicans isolates obtained from blood and from the respiratory tract. Genotype A, without the intron, was prevalent in all groups of strains and its prevalence was higher among isolates from blood (75%) and from patients with candidosis (80%) compared with strains from colonisation (as opposed to infection) (57.8%). Isolates from blood produced significantly less proteinase than isolates from the respiratory tract (p<0.02), and this difference should be attributed to lower proteinase production of genotypes B and C from blood compared with genotypes B and C from the respiratory tract (p<0.01). The higher proteinase production of genotype B than of genotype A was found among respiratory tract isolates only. The presented data indicate that the association between proteinase production and the CaLSU intron depends on the strains' population. Further study is needed on well-defined groups of clinical isolates to elucidate whether the observed diversity in proteinase production plays a role in the selection of strains inducing bloodstream infections.     

Actinobacillus Pleuropneumoniae Serotype 5, Subtypes a and b: Cross Protection Experiments

Vaccination of pigs with a killed culture of A. pleuropneumoniae serotype 5, strain K17 (subtype a) afforded a high degree of protection against challenge with strains L20 and T928 (subtype b). The reverse experiment showed that strain L20 gave good protection against challenge with strain K17 whereas strain T928 did not afford an acceptable protection against challenge with this strain. The considerable cross immunity shown to exist between strains K17 and L20 indicates a high degree of homogeneity of the antigenic determinants of the two strains involved in induction of protective immunity and suggest that antibodies to capsular subtype specific determinants may not play a significant role in the specific defence against A. pleuropneumoniae strains belonging to serotype 5. The finding that a vaccine prepared from strain T928 did not afford an acceptable protection against challenge with strain K17 indicates a variable expression among serotype 5 strains of the antigenic determinants which induce protective immunity against A. pleuropneumoniae infection.     

Capsule phase variation in Neisseria meningitidis serogroup B by slipped-strand mispairing in the polysialyltransferase gene (siaD): correlation with bacterial invasion and the outbreak of meningococcal disease

A mechanism of capsular polysaccharide phase variation in Neisseria meningitidis is described. Meningococcal cells of an encapsulated serogroup B strain were used in invasion assays. Only unencapsulated variants were found to enter epithelial cells. Analysis of one group of capsule-deficient variants indicated that the capsular polysaccharide was re-expressed at a frequency of 10^?3. Measurement of enzymatic activities involved in the biosynthesis of the α-2,8 polysialic acid capsule showed that polysialyltransferase (PST) activity was absent in these capsule-negative variants. Nucleotide sequence analysis of siaD revealed an insertion or a deletion of one cytidine residue within a run of (dC)₇ residues at position 89, resulting in a frameshift and premature termination of translation. We analysed unencapsulated isolates from carriers and encapsulated case isolates collected during an outbreak of meningococcal disease. Further paired blood-culture isolates and unencapsulated nasopharyngeal isolates from patients with meningococcal meningitis were examined. In all unencapsulated strains analysed we found an insertion or deletion within the oligo-(dC) stretch within siaD, resulting in a frameshift and loss of capsule formation. All encapsulated isolates, however, had seven dC residues at this position, indicating a correlation between capsule phase variation and bacterial invasion and the out-break of meningococcal disease.     

表达ApxIA的血清7型胸膜肺炎放线杆菌弱毒菌株的构建及特性分析

猪传染性胸膜肺炎是由胸膜肺炎放线杆菌引起的一种高度接触传染疾病,严重阻碍着全球养猪业的发展,疫苗接种是控制该病的有效措施。为提高胸膜肺炎放线杆菌弱毒疫苗的免疫效力,以及探索胸膜肺炎放线杆菌弱毒疫苗作为呼吸系统病原疫苗载体的可行性,通过穿梭质粒pJFF224-XN将完整的apxIA基因导入apxIIC基因缺失突变株HB04C-中,构建了含有apxIA和apxIIA基因的弱毒疫苗菌株HB04C2(apxIIC-/apxIIA+/apxIA+)。通过对HB04C2的生物学特性分析发现,穿梭质粒可稳定传代,并表达ApxIA,其生长特性未受穿梭质粒的影响。将HB04C2以气管接种方式免疫仔猪,可产生针对ApxIA和ApxIIA的抗体。二免后2周以高致病性的血清1型胸膜肺炎放线杆菌攻毒,该弱毒疫苗可提供良好的免疫保护效果。     

Prevalence of <Emphasis Type="Italic">H. influenzae</Emphasis> type b &; f among unvaccinated patients with respiratory tract infection and meningitis

Haemophilus influenzae type b meningitis has been reported predominantly among infants who have not completed the primary immunization series, whereas infection by capsular f has been reported mostly in patients with significant underlying diseases. The present study analyses H. influenzae type b & f infection among 25 unvaccinated patients with respiratory tract infections and symptoms of meningitis by PCR. 5/7 isolates from CSF sample were capsular type b; whereas capsular type f was detected in 2/10 and 5/8 isolates from throat swab and sputum samples, respectively. Rest 13 strains were negative for both the capsular types. To conclude, in India where H. influenzae type b conjugate vaccine is not included in national immunization program, individuals with respiratory tract infections and meningitis are at a greater risk of developing infection with H. influenzae type b strains along with other capsular types and non-capsular types.     

The role of encapsulation in the pathogenesis of anaerobic gram-positive cocci

The pathogenicity of 22 anaerobic and facultative Gram-positive cocci (AFGPC) was investigated by inoculating them into mice and determining their ability to cause subcutaneous abscesses. Only 11 heavily encapsulated isolates (greater than 50% of the cells were encapsulated) induced abscesses. However, when the other 11 isolates were injected with Bacteroides sp. or facultative and aerobic bacteria, abscesses were formed in 8 of the 11 combinations. The AFGPC recovered from the mixed infections contained many encapsulated cells. Encapsulation also occurred in cocci injected with capsular material or with Formalin-killed cells of Klebsiella pneumoniae or capsule-positive Bacteroides sp. After acquisition of capsules, these strains could induce abscesses on reinoculation in mice.