Clinical Significance and Taxonomy ofActinobacillus hominis

Abstract

Clinical findings in 36 immunosuppressed patients with lower respiratory tract infection or bacteremia withActinobacillus hominis are described. Animal contact was only recorded for three patients; nine patients died despite appropriate antimicrobial treatment. Although infections with this microorganism seem to be rare, the fact that 37 of 46 strains characterized in this study have been found in Copenhagen indicates that underreporting may occur. A. hominis is phenotypically relatively homogeneous but can be difficult to differentiate from other Actinobacillus species unless extensive biochemical testing is performed. Mannose-positive strains of A. hominis are especially difficult to differentiate from A. equuli. Attempts to identify A. hominis by automatic identification systems may lead to misidentifications. Ribotyping and DNA-DNA hybridization data show that A. hominis is a homogeneous species clearly separated from other species within the genus Actinobacillus.The genus Actinobacillus is comprised of species that are animal pathogens, the only exceptions being Actinobacillus ureae and Actinobacillus hominis, which appear to be highly adapted to humans. A. ureae was first described as a human respiratory tract pathogen in 1960 (8). In 1981, Friis-Møller reported on 17 cases of respiratory tract infections with an "A. ureae-like" bacterium, which was classified as a distinct species, A. hominis (5). Since then, only a few reports of diseases caused by this microorganism have been reported in the literature; these have, however, involved invasive disease (6, 12).Since we now have a collection of 46 A. hominis strains isolated from various European countries, including blood culture isolates, we report here on clinical findings from patients infected with this microorganism. Furthermore, we investigate strain variability within A. hominis and describe its relationship to closely related species of the family Pasteurellaceae as determined by conventional methods, ribotyping, and DNA-DNA hybridization, and we examine the accuracy of commercial identification systems in this area. MATERIALS AND METHODSStrains. The 46 A. hominis isolates examined in the present study are listed in Table 1. Thirty-eight isolates from 37 patients (one patient had A. hominis isolated twice from the sputum with an interval of 15 months) were found in Copenhagen at five different departments of clinical microbiology from 1977 to 1999. There were two isolates each from Greenland, Germany, and France and one each from the Czech Republic and Sweden. The majority of isolates were found in the respiratory tract (sputa and tracheal and bronchial secretions), one was from the pleural fluid, and four were from blood. Selected strains were studied for DNA-DNA hybridization levels and were characterized by automatic identification systems (vide infra). Twenty type and reference strains from taxa related to A. hominis and used for comparison in the bacteriological study are listed in Table 2. Of these, only the Haemophilus species and A. ureae a...

Proposed minimal standards for the description of genera, species and subspecies of the Pasteurellaceae

“…The spectrophotometric, S1 nuclease, hydroxyapatite and microwell methods have all been used with members of Pasteurellaceae (Mutters et al, 1985;Møller et al, 1996;Friis-Møller et al, 2001;Christensen et al, 2002). The accuracy of DNA-DNA hybridization is considered equivalent to whole-genomic sequence comparison and MLSA (Gevers et al, 2005).…”

“…The present work was initiated by the ICSP Subcommittee on the taxonomy of the Pasteurellaceae in 2005 (Kuhnert & Christensen, 2006) based on a draft prepared by Wilhelm Frederiksen. Some of the principles have been discussed in a taxonomic note (Christensen et al, 2001). The main issue of the note was to ensure a wide diversity of the bacterial strain collection used for naming novel taxa, and it was proposed to incorporate such recommendations in the Bacteriological Code.…”

“…As a first step toward reclassification, species found to be unrelated to the three genera Pasteurella, Actinobacillus and Haemophilus were informally excluded by writing the genus name of these species in brackets, leaving only sensu stricto taxa within these three genera (Mutters et al, 1989). Minimal standards for bacteria included in the Pasteurellaceae have been in the pipeline for a long time (Biberstein & Zinneman, 1982;Wauters & Mair, 1982 (Christensen et al, 2001). The main issue of the note was to ensure a wide diversity of the bacterial strain collection used for naming novel taxa, and it was proposed to incorporate such recommendations in the Bacteriological Code.…”

“…In addition, comparative studies have to be performed, including reference strains that represent neighbouring species, in order to give descriptions that are sufficiently detailed to allow differentiation from these neighbours (Christensen et al, 2001). The characters that differentiate the novel species from its neighbours must be given clearly.…”

“…A certain minimum number of strains has also been mentioned in other minimal standards (Graham et al, 1991;Freney et al, 1999;Bernardet et al, 2002). For members of the Pasteurellaceae, descriptions have to be based on as many strains as possible (inclusion of at least five strains is highly desirable), representing different sources with respect to geography and ecology to allow proper characterization both phenotypically and genotypically, to establish the extent of diversity of the cluster to be named (Christensen et al, 2001). Naming of novel taxa associated with serious disease and/or lesions should be considered irrespective of whether they meet these requirements as to geographical and ecological diversity or not, in order to account for and to investigate their pathogenicity.…”