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تسجيل

Pasteurella and Similar Organisms

المؤلف:  Patricia M. Tille, PhD, MLS(ASCP)

المصدر:  Bailey & Scotts Diagnostic Microbiology

الجزء والصفحة:  13th Edition , p391-394

2026-05-21

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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; Table1). Of the organisms listed in Table 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).

Table1. Epidemiology of Selected Pasteurella spp. and  Similar Organisms

Table2. Pathogenesis and Spectrum of Disease of Selected  Pasteurella spp. and Similar Organisms

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 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.

Table3. Antimicrobial Therapy and Susceptibility Testing for Pasteurella spp. and Similar Organisms

Clinical and Laboratory Standards Institute (CLSI) document M45-A2, published in 2010, provides guide lines 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.

Laboratory Diagnosis

SPECIMEN COLLECTION AND TRANSPORT

 No special considerations are required for specimen col lection and transport of the organisms discussed in this chapter.

SPECIMEN PROCESSING

 No special considerations are required for processing of the organisms discussed in this chapter.

DIRECT DETECTION METHODS

 Other than Gram staining, there are no commonly employed procedures for the direct detection of these organisms from primary clinical material. Pasteurella spp. are typically short, straight bacilli, although P. aerogenes may also present as coccobacilli. Bipolar staining is frequent. The bacillus of P. bettyae is usually thinner than those of the other species. M. haemolytica is a small bacillus or coccobacillus. S. indologenes is a broad bacillus of variable length.

Cultivation

Media of Choice

 The bacteria described in this chapter grow well on routine laboratory media such as tryptic soy agar supplemented with 5% sheep blood (blood agar) and chocolate agar. With the exception of P. aerogenes and some strains of P. bettyae and P. pneumotropica, most species do not grow on MacConkey agar. M. haemolytica, Pasteurella spp., and S. indologenes also grow well in broth blood culture systems and common nutrient broths such as thioglycollate and brain-heart infusion. Pasteurella spp. may be differentiated from Haemophilus spp. via CO2 independence and growth on media containing sheep blood.

Incubation Conditions and Duration

Inoculated blood and chocolate agar are incubated at 35° C in ambient air or an environment enriched with 5% CO2 for a minimum of 24 hours. S. indologenes may grow especially slowly on primary media.

Colonial Appearance

 Table 4 describes the colonial appearance and other distinguishing characteristics (e.g., hemolysis and odor) of these genera on blood agar.

Table4. Colonial Appearance and Characteristics of  Selected Pasteurella spp. and Similar Organisms on Sheep  Blood Agar

Approach to Identification

The accuracy of commercial biochemical identification systems has been called into question for the definitive identification of Pasteurella spp. and similar organisms. Table 5 summarizes conventional biochemical tests that can assist in the presumptive differentiation or species confirmation of organisms discussed in this chapter. A more complete conventional biochemical battery, offered as part of a reference laboratory workup, may be required for definitive identification of the isolates. Alternatively, past attempts to definitively identify Pasteurella spp. on the basis of cellular fatty acid analysis have been replaced by 16S rDNA gene sequencing and sodA gene sequencing. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry may provide future utility.

Table5. Key Biochemical Characteristics of Selected Pasteurella spp. and Similar Organisms

Comments Regarding Specific Organisms

 Pasteurella spp. typically yield a positive tetramethyl-p phenylenediamine dihydrochloride-based oxidase result. With the exception of P. bettyae and P. caballi, these organ isms are catalase positive; all Pasteurella spp. reduce nitrates to nitrites. P. aerogenes and some strains of P. dagmatis ferment glucose with the production of gas. P. multocida can be differentiated from other Pasteurella spp. on the basis of positive reactions for ornithine decarboxylase and indole, with a negative reaction for urease. Within P. multocida, subsp. multocida ferments sorbitol and fails to ferment dulcitol, subsp. gallicida ferments dulcitol but not sorbitol, and subsp. septica ferments neither carbohydrate.

M. haemolytica may be differentiated from members of the Pasteurella genus by its inability to produce indole or ferment mannose. S. indologenes can be separated from Pasteurella spp. with a negative nitrate test and is further delineated from Kingella spp. by indole production and sucrose fermentation.

Serodiagnosis

 Serodiagnostic techniques are of little utility for the laboratory diagnosis of infections caused by the organisms discussed in this chapter.

Prevention

 Because these organisms do not generally pose a threat to human health, there are no recommended vaccination or prophylaxis protocols.