General Characteristics

The organisms discussed in this chapter are dysgonic—that is, they grow slowly (48 hours at 35° to 37° C) or poorly. Although they all ferment glucose, their fastidious nature requires that serum be added to the basal fermentation medium to enhance growth and detect fermentation byproducts. These bacteria are capnophiles—that is, they require additional carbon dioxide (5% to 10% CO₂) for growth, and most species will not grow on MacConkey agar. Actinobacillus actinomycetemcomitans has been reclassified to be included in the Aggregatibacter genus based on 16sRRNA sequencing. Haemophilus aphrophilus and Haemophilus paraphrophilus have been reclassified as a single species based on multilocus sequence analysis. Aggregatibacter aphrophilus now includes both the hemin-dependent and hemin-independent isolates. Haemophilus segnis has been reclassified as Aggregatibacter segnis. A. segnis requires V-factor, but does not require X-factor.

Epidemiology, Pathogenesis, and Spectrum of Disease, and Antimicrobial Therapy

The organisms listed in Table 31-1 are part of the normal flora of the nasopharynx or oral cavity of humans and other animals and are parasitic. Species associated with animals are specifically indicated in the table at the beginning of the chapter. As such, they generally are of low virulence and, except for those species associated with periodontal infections, usually only cause infections in humans after introduction into sterile sites following trauma such as bites, droplet transmission from human to human, sharing paraphernalia, or manipulations in the oral cavity. Cardiobacterium spp. are not only associated with the human oropharynx and oral cavity, but they may also be identified in the gastrointestinal and urogenital tract. The natural habitat for Dysgonomonas is unknown. Rare isolates have been identified in the feces of immunocompromised patients.

The types of infections caused by these bacteria vary from periodontitis to endocarditis (Table 31-2). Actinobacillus spp. cause granulomatous disease in animals and have been associated with soft tissue infection in humans following animal bites. Additionally, A. equuli and A. suis have been isolated from the human respiratory tract. Additional species have been isolated from patients that have developed meningitis following trauma or surgery. Actinobacillus spp. may harbor a pore-forming protein toxin known as an RTX toxin that is cytotoxic and hemolytic. A. actinomycetemcomitans is often associated with periodontitis. Virulence factors include the RTX leukotoxin, cytotoxic distending toxin, and the EmaA adhesin. Three of these organisms, Aggregatibacter actinomycetemcomitans, Cardiobacterium hominis, and Kingella spp., are the A, C, and K, respectively, of the HACEK group of organisms that cause slowly progressive (i.e., subacute) bacterial endocarditis, soft tissue infections, and other infections. Capnocytophaga are associated with septicemia and endogenous infections in immunocompromised patients. Infections with C. canimorsus and C. cynodegmi following a dog or cat bat can result in serious illness including disseminated intravascular coagulation, renal failure, shock, and hemolytic-uremic syndrome. Kingella spp. can also be involved in other serious infections involving children, especially osteoarthritic infections. The pathogenic mechanisms are unknown, and disease associated with Dysgonomonas spp. is quite variable and includes diarrhea, bacteremia, blood, and wound infections.

Infections are frequently treated using β-lactam antibiotics, occasionally in combination with an aminoglycoside (Table 31-3). β-lactamase production has been described in Kingella spp., but the impact of this resistance mechanism on the clinical efficacy of beta-lactams is uncertain. When in vitro susceptibility testing is required, Clinical and Laboratory Standards Institute (CLSI) document M45 does provide guidelines for testing A. actinomycetemcomitans, Cardiobacterium spp., and Kingella spp.