General Characteristics and Taxonomy

The organisms discussed in this chapter are small, gram-negative, non-motile, oxidase-positive bacilli that ferment glucose. The majority of the organisms discussed in this chapter will not grow on MacConkey agar. Their individual morphologic and physiologic features are presented later in this chapter in the discussion of laboratory diagnosis.

Taxonomy of Pasteurella spp. and similar organisms has significantly changed since the early 2000s and may be subject to additional revision. Genera now classified into the Pasteurellaceae family include Actinobacillus, Aggregatibacter (aggregation of the former Actinobacillus actinomycetemcomitans, Haemophilus aphrophilus, H. paraphrophilus, and H. segnis), Haemophilus, and Pasteurella.

Epidemiology, Spectrum of Disease, and Antimicrobial Therapy

Most of the organisms presented in this chapter constitute portions of both domestic and wild animal flora and are transmitted to humans during close animal contact, including bites. For most of these species, virulence factors are not recognized. As a result, the organisms may be considered opportunistic pathogens that require mechanical disruption of host anatomic barriers (i.e., bite-induced wounds; Table 30-1). Of the organisms listed in Table 30-2, P. multocida subsp. multocida is most commonly encountered in clinical specimens. Reported virulence factors for this subspecies include lipopolysaccharide, cytotoxin, six serotypes of the antiphagocytic capsule, surface adhesins, and iron-acquisition proteins. Other manifestations of infection by P. multocida subsp. multocida can include respiratory disease and systemic disease such as endocarditis and septicemia. Liver cirrhosis is viewed as a risk factor for systemic disease. Other Pasteurella spp. can be agents of systemic infection (P. pneumotropica) and genital tract-associated disease (P. bettyae).

An unusual feature of the organisms considered in this chapter is that most are susceptible to penicillin. Although most other clinically relevant Gram-negative bacilli are intrinsically resistant to penicillin, it is the drug of choice for infections involving P. multocida and several other species listed in Table 30-3. The general therapeutic effectiveness of penicillin and the lack of resistance to this agent among Pasteurella spp. suggest that in vitro susceptibility testing is typically not indicated. This is especially true with isolates emanating from bite wounds. Moreover, bite wounds can be complicated by polymicrobial infection. In this case, the empiric therapy directed toward multiple agents is generally also effective against Pasteurella spp. As a result, antimicrobial susceptibility testing for Pasteurella spp. may have greater utility for isolates recovered from sterile sources (blood, deep tissue) and from respiratory specimens obtained from immunocompromised patients.

Clinical and Laboratory Standards Institute (CLSI) document M45-A2, published in 2010, provides guidelines for broth microdilution (cation-adjusted Mueller Hinton broth medium supplemented with 2.5% to 5% lysed horse blood) and disk diffusion (Mueller Hinton agar medium supplemented with 5% sheep blood) susceptibility testing of Pasteurella spp. Both formats are incubated in 35° C ambient air. Interpretation of disk diffusion and broth microdilution formats occurs at 16 to 18 hours and 18 to 24 hours of incubation, respectively. Antimicrobial agents to consider for testing include penicillin, ampicillin, amoxicillin, amoxicillin-clavulanate, ceftriaxone, moxifloxacin, levofloxacin, tetracycline, doxycycline, erythromycin, azithromycin, chloramphenicol, and trimethoprim-sulfamethoxazole. Of these agents, breakpoints for categorical interpretation of resistance or intermediate susceptibility have only been established for erythromycin.

No special considerations are required for specimen collection and transport of the organisms discussed in this chapter. Refer to Table 5-1 for general information on specimen collection and transport.