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So Asthma Mark would sit on the corner
And he would play his Diphtheria Blues
—Frank Zappa
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Overview
Corynebacteria are small, pleomorphic gram-positive rods that include Corynebacterium diphtheriae, the foremost pathogen and cause of diphtheria. Other species are common skin flora, rarely cause disease, and often are found as contaminants in blood cultures. Diphtheria is the disease resulting from the local and systemic effects of diphtheria toxin (DT), a potent inhibitor of protein synthesis. The local disease is a severe pharyngitis typically accompanied by a plaque-like pseudomembrane that adheres to and ultimately occludes the throat and trachea. The life-threatening aspects of diphtheria are from suffocation and the consequences of absorption of the DT across the pharyngeal mucosa and its circulation in the bloodstream. Multiple organs are affected, but the most important is the heart, where the toxin produces an acute myocarditis.
Listeria monocytogenes is the only pathogen in its genus. Importantly, it is catalase positive like corynebacteria, which it resembles morphologically, and unlike catalase-negative Group B streptococci, which it otherwise mimics when isolated on sheep blood agar. Listeria monocytogenes causes listeriosis for which pregnant women are at greatest risk. Although insidious in onset, listeriosis can be devastating for the fetus and may result in stillbirth or multiorgan involvement and fulminant sepsis. Listeria also causes meningitis in newborns and immunocompromised adults. Although susceptible to ampicillin, Listeria are intrinsically resistant to all cephalosporins.
The genus Bacillus has hundreds of species of aerobic spore-forming gram-positive bacilli; only two are human pathogens and of these Bacillus anthracis is paramount both for its bioterrorism potential and the severity of disseminated disease owing to its potent tri-component exotoxin (lethal factor, protective antigen, and edema factor). Anthrax occurs in two clinical forms: cutaneous and systemic. When spores are inoculated with trauma to the skin, a localized eschar with surrounding edema ensues (Figure 26–9). When spores are inhaled, lethal hemorrhagic pneumonia, mediastinitis, and meningitis may result. Bacillus cereus can cause rapidly destructive lesions of the eye following trauma, opportunistic infections, and food poisoning owing to enterotoxin production.
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This chapter includes a variety of highly pathogenic gram-positive rods that are not currently common causes of human disease. Their medical importance lies in the lessons learned when they were more common, and the continued threat their existence poses. Corynebacterium diphtheriae, the cause of diphtheria, is a prototype for toxigenic disease. Listeria monocytogenes is a sporadic cause of meningitis and other infections in the fetus, newborn, and immunocompromised host. Occurrences in 2001 have served as a painful reminder that B anthracis, the cause of anthrax, is still the agent with the most potential for use in bioterrorism. The characteristics of these bacilli are presented in Table 26–1.
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