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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 microbiota of skin and mucous membranes of humans and, as such, are frequently contaminants of clinical specimens submitted for diagnostic evaluation. However, among the aerobic gram-positive bacilli are significant pathogens such as Corynebacterium diphtheriae, an organism that produces a powerful exotoxin that causes diphtheria in humans, and Mycobacterium tuberculosis (see Chapter 23), the causative agent of tuberculosis. Listeria monocytogenes and Erysipelothrix rhusiopathiae are primarily found in animals and occasionally cause severe disease in humans. Nocardia and Rhodococcus species are found in the soil and are significant pathogens among immunocompromised patients.

Corynebacterium species and related bacteria tend to be clubbed or irregularly shaped; although not all isolates have the irregular shapes, the terms “coryneforms” 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, 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 genera—than they are to the coryneform bacteria. The medically important genera of aerobic gram-positive bacilli are listed in Table 12-1. Anaerobic bacteria are discussed in Chapter 21.

Table 12-1Common Aerobic Gram-Positive Bacilli and Their Disease Associations

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