Moraxella and Related Organisms

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Moraxella and Related Organisms

Genera and Species to Be Considered

Current Name Previous Name
Moraxella atlantae  
Moraxella canis  
Moraxella lacunata  
Moraxella lincolnii  
Moraxella nonliquefaciens  
Moraxella osloensis  
Neisseria elongata subspecies elongata CDC group M6
Neisseria elongata subspecies glycolytica  
Neisseria elongata subspecies nitroreducens  
Neisseria weaverii CDC group M5

General Characteristics

The organisms discussed in this chapter are either coccobacilli or short to medium-sized, gram-negative rods. This group of bacteria consists of several species within the genera Moraxella and Neisseria, other than the three frequently isolated pathogens, Moraxella catarrhalis, Neisseria. gonorrhoeae, and Neisseria meningitidis. Most of these organisms rarely cause infection and should be considered as potential contaminants. Many Moraxella spp. are considered to be normal mucosal flora with low virulence. Two of these species, N. weaverii and M. canis, are oropharyngeal flora in dogs and cats and are sometimes seen in humans as a result of a bite wound. Subinhibitory concentrations of penicillin, such as occurs in the presence of a 10-unit penicillin disk, cause the coccoid forms of these bacteria to elongate to bacilli morphology. In contrast, true cocci, such as most Neisseria spp. and Moraxella (Branhamella) catarrhalis, with which these organisms may be confused, maintain their original cocci shape in the presence of penicillin. In addition, the organisms discussed in this chapter do not use glucose and most do not grow on MacConkey agar but will grow well on blood and chocolate agar, as well as in commercial blood culture systems. Specific morphologic and physiologic features are presented later in this chapter in the discussion of laboratory diagnosis.

Epidemiology, Spectrum of Disease, and Antimicrobial Therapy

Infections caused by Moraxella spp. and Neisseria elongata most likely result when a breakdown of the patient’s mucosal or epidermal defensive barriers allows subsequent invasion of sterile sites by an organism that is part of the patient’s normal flora (i.e., an endogenous strain; Table 28-1). The fact that these organisms rarely cause infection indicates that they have low virulence. Whenever these organisms are encountered in clinical specimens, the possibility that they are contaminants should be seriously considered. This is especially the case when the specimen source may have come in contact with a mucosal surface.

TABLE 28-1

Epidemiology, Pathogenesis, and Spectrum of Disease

Organism Habitat (Reservoir) Mode of Transmission Virulence Factors Spectrum of Disease and Infections
Moraxella nonliquefaciens, Moraxella lacunata, Moraxella osloensis, Moraxella lincolnii, Moraxella canis, and Moraxella atlantae Normal human flora that inhabit mucous membranes covering the nose, throat, other parts of the upper respiratory tract, conjunctiva, and, for some species (i.e., M. osloensis), the urogenital tract; may also colonize the skin Infections are rare; when they occur, they are probably caused by the patient’s endogenous strains; person-to-person transmission may be possible, but this has not been documented Unknown; because they are rarely associated with infections, they are considered opportunistic organisms of low virulence M. lacunata has historically been associated with eye infections, but these infections also may be caused by other Moraxella spp.; other infections include bacteremia, endocarditis, septic arthritis, and, possibly, respiratory infections
Neisseria elongate Normal flora of upper respiratory tract When infections occur, they are probably caused by the patient’s endogenous strains Unknown; an opportunistic organism of low virulence Rarely implicated in infections; has been documented as a cause of bacteremia, endocarditis, and osteomyelitis
Neisseria weaverii Oral flora of dogs Dog bite Unknown Infections of dog bite wounds


Moraxella catarrhalis is the species most commonly associated with human infections, primarily of the respiratory tract. However, because the cellular morphology of this species is more similar to that of Neisseria spp. than that of the other Moraxella spp., details of this organism’s characteristics are discussed in Chapter 40.

Data collected from the Centers for Disease Control and Prevention (CDC) show that these rare isolates may also be a cause of infection. In a study of the bacteria, Neisseria elongate subsp. nitroreducens, one fourth of the isolates received at the CDC for analysis were from cases of bacterial endocarditis. Data collected during a 16-year period found that most of these isolates were from blood, but they were also recovered from wounds, respiratory secretions, and peritoneal fluid. Individuals at risk had preexisting heart damage or had undergone dental manipulations.

The rarity with which these organisms are encountered as the cause of infection and the lack of validated in vitro susceptibility testing methods does not allow definitive treatment guidelines to be given (Table 28-2). Although many of these organisms may grow on the media and under the conditions recommended for testing other bacteria, this does not necessarily mean that interpretable and reliable results will be produced. Chapter 12 should be reviewed for preferable strategies that can be used to provide susceptibility information when validated testing methods do not exist for a clinically important bacterial isolate.

TABLE 28-2

Antimicrobial Therapy and Susceptibility Testing

Organism Therapeutic Options Potential Resistance to Therapeutic Options Validated Testing Methods*
Moraxella spp. No definitive guidelines; generally susceptible to penicillins and cephalosporins β-lactamase–mediated resistance to penicillins common Not available
Exception: See CLSI document M45 for testing guidelines for M. catarrhalis (see Chapter 40)
Neisseria elongata and Neisseria weaverii No definitive guidelines; generally susceptible to penicillins and cephalosporins None known Not available


*Validated testing methods include those standard methods recommended by the Clinical and Laboratory Standards Institute (CLSI) and those commercial methods approved by the Food and Drug Administration (FDA).

In general, β-lactam antibiotics are thought to be effective against these species. However, some evidence suggests that β-lactamase–mediated resistance may be capable of spreading among Moraxella spp.