The Genus Moraxella
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The Genus Moraxella JOHN P. HAYS
The Moraxella genus currently contains at least 14 different species, including M. catarrhalis, M. bovis, M. lacunata, M. osloensis, M. nonliquefaciens, M. atlantae, M. lincolnii, M. ovis, M. caviae, M. canis, M. equi, M. cuniculi, M. caprae and M. boevreii, which colonize both humans and animals. The genus is under constant revision, with recent taxonomic restructuring placing the bacterial species formerly known as Moraxella phenylpyruvica in the genus Psychrobacter as Psychrobacter phenylpyruvica and Moraxella urethralis in the Oligella genus as Oligella urethralis (Fig. 1). Common characteristics of the Moraxella genus include a lack of colony pigmentation; Gram-negative staining coccobacillus and bacillus morphology (except M. catarrhalis, which exhibits a coccoid morphology; Fig. 2), with a tendency to resist decolorization; positive with oxidase reagent and tetra- and dimethyl-pphenylenediamine; and a G+C content of 40– 47.5 mol%. Optimum growth conditions are achieved on blood agar plates under aerobic conditions at a temperature of approximately 33–37°C. Moraxella lacunata colonies form dark haloes on chocolate agar, most often with pitting of the agar (a phenomenon only occasionally observed for other Moraxella spp.). The addition of bile salts and sodium desoxycholate to the growth medium tends to stimulate M. atlantae but inhibits the growth of M. lacunata and M. nonliquefaciens. Combined genetic transformation and nutritional assays for the identification of Moraxella nonliquefaciens have been reported (Juni et al., 1984). In general, however, distinguishing between the different Moraxella species tends to be difficult (Table 1), not least because of the asaccharolytic nature of the genus, though some publications have indicated that 16S rRNA sequence polymorphisms may be a useful adjunct to biochemical testing (Enright et al., 1994a; Pettersson et al., 1998a; Harmsen et al., 2001). When combined, biochemical and 16S rRNA sequence data do not tend to warrant a distinction of the Moraxella genus into two distinct subgenera as suggested by Bovre (1984).
Moraxella lacunata, M. osloensis, M. nonliquefaciens, M. atlantae and M. lincolnii are commensals of human epithelia that are infrequently associated with human disease. Though only infrequently associated with human disease (M. catarrhalis excluded), the range of diseases caused by Moraxella spp. in humans is somewhat broad and includes: 1) ocular infections, e.g., conjunctivitis and endophthalmitis; 2) respiratory infections, e.g., “pneumococcal pneumoniae” and chronic bronchitis; and 3) systemic infections, e.g., bacteremia, septic arthritis, acute thyroiditis, neonatal meningitidis, central venous catheter infection and bone and joint infection (panaritium ossale et articulare). Several Moraxella species are associated with animals, including M. canis (cats and dogs), M. ovis (sheep), M. equi (horses), M. caprae and M. boevreii (goats), M. caviae (guinea pigs) and M. cuniculi (ra
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