Relationships among isolates of oral haemophili as determined by DNA-DNA hybridization

Aggregatibacter Actinomycetemcomitans: Clinical Significance of a Pathobiont Subjected to Ample Changes in Classification and Nomenclature

Aggregatibacter actinomycetemcomitans is a Gram-negative bacterium that is part of the oral microbiota. The aggregative nature of this pathogen or pathobiont is crucial to its involvement in human disease. It has been cultured from non-oral infections for more than a century, while its portrayal as an aetiological agent in periodontitis has emerged more recently. A. actinomycetemcomitans is one species among a plethora of microorganisms that constitute the oral microbiota. Although A. actinomycetemcomitans encodes several putative toxins, the complex interplay with other partners of the oral microbiota and the suppression of host response may be central for inflammation and infection in the oral cavity. The aim of this review is to provide a comprehensive update on the clinical significance, classification, and characterisation of A. actinomycetemcomitans, which has exclusive or predominant host specificity for humans.

Classification, identification, and clinical significance of Haemophilus and Aggregatibacter species with host specificity for humans

The aim of this review is to provide a comprehensive update on the current classification and identification of Haemophilus and Aggregatibacter species with exclusive or predominant host specificity for humans. Haemophilus influenzae and some of the other Haemophilus species are commonly encountered in the clinical microbiology laboratory and demonstrate a wide range of pathogenicity, from life-threatening invasive disease to respiratory infections to a nonpathogenic, commensal lifestyle. New species of Haemophilus have been described (Haemophilus pittmaniae and Haemophilus sputorum), and the new genus Aggregatibacter was created to accommodate some former Haemophilus and Actinobacillus species (Aggregatibacter aphrophilus, Aggregatibacter segnis, and Aggregatibacter actinomycetemcomitans). Aggregatibacter species are now a dominant etiology of infective endocarditis caused by fastidious organisms (HACEK endocarditis), and A. aphrophilus has emerged as an important cause of brain abscesses. Correct identification of Haemophilus and Aggregatibacter species based on phenotypic characterization can be challenging. It has become clear that 15 to 20% of presumptive H. influenzae isolates from the respiratory tracts of healthy individuals do not belong to this species but represent nonhemolytic variants of Haemophilus haemolyticus. Due to the limited pathogenicity of H. haemolyticus, the proportion of misidentified strains may be lower in clinical samples, but even among invasive strains, a misidentification rate of 0.5 to 2% can be found. Several methods have been investigated for differentiation of H. influenzae from its less pathogenic relatives, but a simple method for reliable discrimination is not available. With the implementation of identification by matrix-assisted laser desorption ionization-time of flight mass spectrometry, the more rarely encountered species of Haemophilus and Aggregatibacter will increasingly be identified in clinical microbiology practice. However, identification of some strains will still be problematic, necessitating DNA sequencing of multiple housekeeping gene fragments or full-length 16S rRNA genes.

Reclassification of Actinobacillus actinomycetemcomitans, Haemophilus aphrophilus, Haemophilus paraphrophilus and Haemophilus segnis as Aggregatibacter actinomycetemcomitans gen. nov., comb. nov., Aggregatibacter aphrophilus comb. nov. and Aggregatibacter segnis comb. nov., and emended description of Aggregatibacter aphrophilus to include V factor-dependent and V factor-independent isolates

The aim of this study was to reinvestigate the relationships and the generic affiliations of the species Actinobacillus actinomycetemcomitans, Haemophilus aphrophilus, Haemophilus paraphrophilus and Haemophilus segnis. The nicotinamide phosphoribosyltransferase gene (nadV) conferring V factor-independent growth was identified in Haemophilus aphrophilus. The gene encodes a polypeptide of 462 amino acids that shows 74.5 % amino acid sequence identity to the corresponding enzyme from Actinobacillus actinomycetemcomitans. Ten isolates of Haemophilus paraphrophilus all carried a nadV pseudogene. DNA from Haemophilus aphrophilus was able to transform Haemophilus paraphrophilus into the NAD-independent phenotype. The transformants carried a full-length nadV inserted in the former locus of the pseudogene. The DNA–DNA relatedness between the type strains of Haemophilus aphrophilus and Haemophilus paraphrophilus was 77 %. We conclude that the division into two species Haemophilus aphrophilus and Haemophilus paraphrophilus is not justified and that Haemophilus paraphrophilus should be considered a later heterotypic synonym of Haemophilus aphrophilus. Forty strains of Actinobacillus actinomycetemcomitans, Haemophilus aphrophilus and Haemophilus segnis were investigated by multilocus sequence analysis. The 40 strains form a monophyletic group clearly separate from other evolutionary lineages of the family Pasteurellaceae. We propose the transfer of Actinobacillus actinomycetemcomitans, Haemophilus aphrophilus and Haemophilus segnis to a new genus Aggregatibacter gen. nov. as Aggregatibacter actinomycetemcomitans comb. nov. (the type species; type strain ATCC 33384T=CCUG 13227T=CIP 52.106T=DSM 8324T=NCTC 9710T), Aggregatibacter aphrophilus comb. nov. (type strain ATCC 33389T=CCUG 3715T=CIP 70.73T=NCTC 5906T) and Aggregatibacter segnis comb. nov. (type strain HK316T=ATCC 33393T=CCUG 10787T=CCUG 12838T=CIP 103292T=NCTC 10977T). The species of the genus Aggregatibacter are independent of X factor and variably dependent on V factor for growth in vitro.

Multilocus sequence phylogenetic study of the genus Haemophilus with description of Haemophilus pittmaniae sp. nov

The phylogeny of human isolates of Haemophilus species was estimated based on partial sequences of four separate housekeeping genes. The clustering of each set of sequences was in accordance with speciation of the strains with few exceptions: of 108 gene fragments examined, only three appeared to have been subject to recombination events across the species barrier. Housekeeping gene similarity supported previous DNA-DNA hybridization data for the genus rather than the phylogeny inferred from 16S rRNA gene sequence comparison. The similarity of sequences of Haemophilus parainfluenzae with those of Haemophilus influenzae suggested preservation of the former species in the genus Haemophilus. Three strains representing a novel taxon were unique with respect to the four investigated gene loci. 16S rRNA gene sequence analysis suggested that this taxon belonged to the Parainfluenzae cluster. DNA-DNA hybridization data supported this generic placement. Nine strains of the novel taxon were available for analysis. They were distinct from representatives of all current species of the genus Haemophilus by conventional phenotypic characterization. Genotypic and phenotypic data show that the strains merit recognition as a novel species of Haemophilus. The name Haemophilus pittmaniae sp. nov. is proposed, with HK 85T (=CCUG 48703T=NCTC 13334T) as the type strain.

Population structure and genetic diversity of Actinobacillus actinomycetemcomitans strains isolated from localized juvenile periodontitis patients

The phylogeny of 20 Actinobacillus actinomycetemcomitans strains isolated from patients with localized juvenile periodontitis (LJP) was investigated by using partial sequence analysis of 16S rRNA genes, arbitrarily primed PCR (AP-PCR), and four additional PCR assays that amplified polymorphic regions in the leukotoxin (lkt), cytolethal distending toxin (cdt), major fimbrial subunit (flp-1), and serotype-specific O polysaccharide gene clusters. Our analysis also included four strains isolated from healthy subjects and nine reference strains. We found that A. actinomycetemcomitans strains comprised three major phylogenetic lineages. One lineage consisted of serotype b strains, a second lineage consisted of serotype c strains, and a third lineage consisted of serotype a, d, e, and f strains. 16S rRNA sequences within each lineage were highly conserved (<1% base substitutions), whereas sequences between lineages were exceptionally divergent (1.9 to 5.0% substitutions). Two strains exhibited 16S rRNA sequences that were even more distantly related to those of the three major lineages (2.7 to 6.7% substitutions), indicating that additional minor lineages or variants exist. The distribution of 16S rRNA sequences and lkt, cdt, flp-1, and AP-PCR genotypes was consistent with a clonal population structure, with little evidence of assortative recombination between strains of different serotypes. Strains from all three major lineages were recovered from LJP patients, suggesting that phylogenetically diverse strains of A. actinomycetemcomitans carry pathogenic potential.

Phylogeny of the genus Haemophilus as determined by comparison of partial infB sequences

A 453 bp fragment of infB, the gene encoding translation initiation factor 2, was sequenced and compared from 66 clinical isolates and type strains of Haemophilus species and related bacteria. Analysis of the partial infB sequences obtained suggested that the human isolates dependent on X and V factor, H. influenzae, H. haemolyticus, H. aegyptius and some cryptic genospecies of H. influenzae, were closely related to each other. H. parainfluenzae constituted a heterogeneous group within the boundaries of the genus, whereas H. aphrophilus/paraphrophilus and Actinobacillus actinomycetemcomitans were only remotely related to the type species of the genus Haemophilus H. parahaemolyticus and H. paraphrohaemolyticus took up an intermediary position and may not belong in the genus Haemophilus sensu stricto. Ambiguous results were obtained with seven isolates tentatively identified as H. segnis, which fell into two discrete clusters. The delineation of 'Haemophilus sensu stricto' as suggested by infB analysis supports previous results obtained by DNA hybridization, in contrast to the delineation inferred from 16S rRNA sequence comparison.

Humoral immunity to commensal oral bacteria: quantitation, specificity and avidity of serum IgG and IgM antibodies reactive with Actinobacillus actinomycetemcomitans in children

The levels, specificity and avidities of serum IgM and IgG antibodies reactive with Actinobacillus actinomycetemcomitans (Aa) serotypes a, b and c were determined in periodontally healthy (PH) children and compared with subjects with localized juvenile periodontitis (LJP). All PH children exhibited IgM and IgG Aa-reactive antibodies whether or not Aa was detected subgingivally but the antibodies were not specific for Aa. In contrast, LJP sera contained high concentrations of IgM and IgG antibodies reactive with Aa that were largely specific for this bacterium. IgM and IgG antibodies in both PH and LJP subjects were of low avidity. With one exception, the avidities of IgG anti-Aa antibodies were significantly greater than those of IgM antibodies in both PH and LJP subjects. However, although the LJP subjects had as much as 115-fold more Aa-reactive IgG antibody than did the PH subjects the avidities of their IgG antibodies were no greater than those of the PH group. The induction by the host of low-avidity antibodies, that are ineffective in immune elimination, may be a reason why commensal bacteria persist at mucosal surfaces and why persons with LJP fail to eliminate Aa from their periodontal pockets.

Identification and analysis of the gap region in the 23S ribosomal RNA from Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans is a Gram-negative coccobacillus which can cause certain severe extra-oral infections as well as forms of human periodontal disease such as localized juvenile periodontitis. In contrast to many prokaryotic and eukaryotic species which exhibit an intact 23S ribosomal RNA (rRNA) molecule, examination of six A. actinomycetemcomitans strains--including three serogroup representative strains and two strains from non-human primates--revealed that this micro-organism does not produce an intact 23S ribosomal RNA (rRNA) molecule but, rather, two smaller forms of 1.8 kb and 1.2 kb designated as 23S alpha and 23S beta fragments. On the other hand, 14 other strains of Actinobacillus, Haemophilus, and Pasteurella species demonstrated intact 23S rRNA. The sequence of the region of the 23S rRNA gene in A. actinomycetemcomitans strain ATCC 43718 containing the cleavage site was determined by dideoxynucleotide sequencing, while the location of the 3' and 5' termini of the 23S alpha and 23S beta fragments was resolved by S1 nuclease mapping and cDNA primer-extension. A deletion of 112 bases was noted in comparisons of base sequences from A. actinomycetemcomitans rRNA and rDNA. The DNA intervening sequence was localized to nucleotide 1180 of the Escherichia coli 23S rRNA map. While the primary structure of the gap region showed little homology with the gap regions described in other organisms, the secondary structure was similar to that previously described in the parasitic helminth Schistosoma japonicum. Restriction enzyme and nucleotide sequence analysis of the gap region in eight other A. actinomycetemcomitans strains showed it to be highly conserved.

Identification of Haemophilus aphrophilus and Actinobacillus actinomycetemcomitans by DNA-DNA hybridization and genetic transformation

DNA-DNA hybridization was used to identify clinical isolates as Haemophilus aphrophilus or Actinobacillus actinomycetemcomitans. Some of the isolates were naturally competent for genetic transformation and were also used as DNA recipients for identification of other isolates. The results obtained by hybridization were supported by interstrain-to-intrastrain transformation ratios. Distinction between the closely related species H. aphrophilus and A. actinomycetemcomitans was generally clear-cut by both methods. Distinction of H. aphrophilus and A. actinomycetemcomitans from type and reference strains of a diversity of species in the family Neisseriaceae and other gram-negative species was also demonstrated by both methods. This is the first description of the identification of clinical isolates of H. aphrophilus or A. actinomycetemcomitans by using them as recipients in genetic transformation. The results suggest that this is a reliable system for identification of new clinical isolates belonging to these taxonomic entities.

Nucleic acid probes as potential tools in oral microbial epidemiology

The present review deals with genetic tools in the microbial diagnostic laboratory, types of nucleic acid probes, their construction and cloning, hybridization reactions, their sensitivity and specificity, and their advantages and limitations. The nucleic acid probes seem to possess a number of advantages over traditional diagnostic methods. DNA, RNA, and oligonucleotide probes are expected to have increased impact on our understanding of the pathogenesis and the treatment of infectious inflammatory periodontal diseases. In addition, these probes have potential application in epidemiological studies of oral microorganisms. However, the limitations of the probes must be kept in mind, and newly developed probes must be controlled with respect to the critical issues of sensitivity and specificity.