General Characteristics

The organisms discussed in this chapter are considered together because, except for CDC group NO-1, they are all oxidase negative and grow on MacConkey agar, as do the Enterobacteriaceae. However, unlike the Enterobacteriaceae, which ferment glucose, these organisms either oxidize glucose (i.e., they are saccharolytic), or they do not utilize glucose (i.e., they are nonoxidizers, or asaccharolytic). Although CDC group NO-1 is oxidase negative and does not usually grow on MacConkey agar, it is included here because it must be distinguished from the asaccharolytic Acinetobacter spp. Based on molecular studies, approximately 21 species and/or strains of Acinetobacter spp. have been identified. The specific morphologic and physiologic features of the organisms are considered later in this chapter in the discussion of laboratory diagnosis. Of note, only Acinetobacter and Stenotrophomonas spp. are routinely found in clinical specimens. Bordetella parapertussis is included in Table 21-4 in this chapter but is discussed in Chapter 37.

Epidemiology

The organisms discussed in this chapter inhabit environmental niches. Acinetobacter spp. and Stenotrophomonas maltophilia are widely distributed in moist natural and hospital environments (Table 21-1). Acinetobacter spp. can be found on fomites and in soil, water, and animal food products. These organisms are capable of survival on inanimate objects for extended periods. Acinetobacter spp. is a human skin colonizer in 0.5% to 3% of the general population and has been identified from a number of human sources, including sputum, urine, feces, and vaginal secretions. S. maltophilia may be found in tap water and salads. Although none of these organisms are considered normal human flora, the relatively high prevalence of Acinetobacter spp. and S. maltophilia in hospitals frequently results in colonization of the skin and respiratory tract of patients. The prevalence of these organisms is evidenced by the fact that, excluding the Enterobacteriaceae, Acinetobacter spp. and S. maltophilia are the second and third most common gram-negative bacilli, respectively, encountered in clinical specimens. In contrast, Pseudomonas luteola, Pseudomonas oryzihabitans, and CDC group NO-1 are not commonly found in clinical specimens but have been isolated from wounds, blood cultures, and dialysis fluids.

Pathogenesis and Spectrum of Disease

All of the organisms listed in Table 21-2 are opportunistic pathogens for which no definitive virulence factors are known. Because Acinetobacter spp. and S. maltophilia are relatively common colonizers of hospitalized patients, their clinical significance when found in patient specimens can be difficult to establish. In fact, these organisms are more frequently isolated as colonizers than as infecting agents. When infection does occur, it usually is seen in debilitated patients, such as those in burn or intensive care units and those who have undergone medical instrumentation and/or have received multiple antimicrobial agents. Acinetobacter baumannii is typically the species identified in hospital-acquired infections. Infections caused by Acinetobacter spp. and S. maltophilia usually involve the respiratory or genitourinary tract, bacteremia and, occasionally, wound infections, although infections involving several other body sites have been described. Community-acquired infections with these organisms can occur, but the vast majority of infections are nosocomial.

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.

Cultivation

Media of Choice

In addition to their ability to grow on MacConkey agar, all of the genera described in this chapter, except CDC group NO-1, grow well on 5% sheep blood and chocolate agars. These organisms also grow well in the broth of blood culture systems and in common nutrient broths, such as thioglycollate and brain-heart infusion.

Incubation Conditions and Duration

These organisms generally produce detectable growth on 5% sheep blood and chocolate agars when incubated at 35°C in carbon dioxide or ambient air for a minimum of 24 hours. MacConkey agar should be incubated only in ambient air.

Colonial Appearance

Table 21-3 describes the colonial appearance and other distinguishing characteristics (e.g., hemolysis and odor) of each genus when grown on 5% sheep blood and MacConkey agars.

TABLE 21-3

Colonial Appearance and Characteristics

Organism	Medium	Appearance
Stenotrophomonas maltophilia	BA	Large, smooth, glistening colonies with uneven edges and lavender-green to light purple pigment; greenish discoloration underneath growth; ammonia smell
	Mac	NLF
Acinetobacter spp.	BA	Smooth, opaque, raised, creamy, and smaller than Enterobacteriaceae; some genospecies are beta-hemolytic
	Mac	NLF, but colonies exhibit a purplish hue that may cause the organism to be mistaken for LF (Figure 21-1)
Burkholderia gladioli	BA	Yellow
	Mac	NLF
Bordetella parapertussis	BA	Smooth, opaque, beta-hemolytic
	Mac	NLF, delayed growth
Bordetella holmesii	BA	Punctate, semiopaque, convex, round, with greening of blood usually accompanied by lysis
	Mac	NLF, delayed growth
Bordetella trematum	BA	Convex, circular, grayish cream to white
	Mac	NLF
Pseudomonas oryzihabitans	BA	Wrinkled, rough or smooth, transparent, yellow
	Mac	NLF
Pseudomonas luteola	BA	Maybe rough and smooth, opaque, yellow
	Mac	NLF
CDC group NO-1	BA	Small colonies that can be transferred intact with an inoculating needle
	Mac	NLF, but only 20% of strains grow

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