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The non–spore-forming gram-positive bacilli are a diverse group of aerobic and anaerobic bacteria. This chapter focuses on the aerobic members of this group. The anaerobic, non–spore-forming gram-positive bacilli such as Propionibacterium species and Actinomyces species are discussed in Chapter 21 on anaerobic infections. Specific genera of both groups, namely Corynebacterium species and Propionibacterium species, are members of the normal flora of skin and mucous membranes of humans and, as such, are frequently contaminants of clinical specimens submitted for diagnostic evaluation. However, among the aerobic Actinomycetes are significant pathogens such as Corynebacterium diphtheriae, an organism that produces a powerful exotoxin that causes diphtheria in humans, and Mycobacterium tuberculosis, the causative agent of tuberculosis. Listeria monocytogenes and Erysipelothrix rhusiopathiae are primarily found in animals and occasionally cause severe disease in humans. Nocardia species and Gordonia and Tsukamurella are emerging pathogens among immunocompromised patients.

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Corynebacterium species and related bacteria tend to be clubbed or irregularly shaped; although not all isolates have the irregular shapes, the terms coryneform or diphtheroid bacteria are convenient ones for denoting this broad group. These bacteria have a high guanosine plus cytosine content and include the genera Corynebacterium, Arcanobacterium, Brevibacterium, Mycobacterium, and others (Table 12-1). Actinomyces and Propionibacterium are classified as anaerobes, but some isolates grow well aerobically (aerotolerant) and must be differentiated from the aerobic coryneform bacteria. Other non–spore-forming gram-positive bacilli have more regular shapes and a lower guanosine plus cytosine content. The genera include Listeria and Erysipelothrix; these bacteria are more closely related to the anaerobic Lactobacillus species, which sometimes grow well in air, to the spore-forming Bacillus and Clostridium species—and to the gram-positive cocci of the Staphylococcus and Streptococcus species—than they are to the coryneform bacteria. The medically important genera of gram-positive bacilli are listed in Table 12-1 and include some spore-forming and anaerobic genera. Anaerobic bacteria are discussed in Chapter 21.

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Table 12-1 Some of the More Common Gram-Positive Bacilli of Medical Importance

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