Erysipelothrix rhusiopathiae is the only species in the genus. It is a gram-positive, nonsporulating, pleomorphic rod that has a tendency to form long filaments. It is found worldwide and is a commensal or a pathogen in a very wide variety of vertebrates and invertebrates.
Domestic swine are the major reservoir. Human cases are relatively rare, with infections resulting from occupational exposure. Those individuals whose work involves handling fish and animal products are most at risk. The usual route of infection is through cuts or scratches on the skin. The organism is resistant to salting, pickling, and smoking and survives well in environmental sources such as water, soil, and plant material.
E. rhusiopathiae produces three types of disease in humans: septicemia, endocarditis, and erysipeloid. The incidence of E. rhusiopathiae infections is low. Systemic infection is very uncommon and rarely develops from localized infection. Endocarditis has been seen in patients who have had valve replacements but also in individuals with apparently normal heart valves.
Erysipeloid is a localized skin infection that resembles streptococcal erysipelas. The lesions usually are seen on the hands or fingers because the organisms usually are inoculated through work activities. The incubation period is 1 to 4 days. The infected area is painful and swollen and gives rise to a characteristic lesion-a sharply defined, slightly elevated, purplishred zone that spreads peripherally as discoloration of the central area fades. Low-grade fever, arthralgia, Iymphangiitis, and lymphadenopathy may occur. A mild form of the disease lasts 2 to 4 weeks, but it may
continue for months. Erysipeloid is a self-limiting infection that normally heals within 3 to 4 weeks. A problem is that second attacks can occur and relapses are common. Antimicrobial therapy is effective, with penicillins, cephalosporins, erythromycin, and clindamycin being useful.
Laboratory Diagnosis Microscopy
E. rhusiopathiae is a thin, rod-shaped, gram-positive organism that may form long filaments (Figure 16-8). It is arranged singly, in short chains, or in a Vshape. The latter arrangement is similar to that seen with the Corynebacteria.
Typical specimens received for the isolation of E.rhusiopathiae include tissue biopsy or aspirates from skin lesions. These should be inoculated to a nutrient broth with 1% glucose and incubated in 5% CO2 at 35° C. Subcultures should be made daily to SBA plates.
On SBA, the colonies are nonhemolytic or a-hemolytic and are pinpoint after 24 hours of incubation. After 48 hours of incubation, two distinct colony types are seen. A smaller, smooth form is transparent, glistening, and convex with entire edges. The larger, rough colonies are flatter with a matte surface, curled structure, and irregular edges.
A comparison of Erysipelothrix with Listeria is shown in Figure 16-9. Table 16-3 lists the characteristics of Erysipelothrix and other related gram-positive bacilli. Identification is based on the Gram stain, hydrogen
sulfide production, lack of motility, indole and catalase positive, and a negative VP reaction. Growth of Erysipelothrix in a gelatin stab culture yields a highly characteristic “test tube brush-like” pattern at 22° C. An additional test that can be used to differentiate Erysipelothrix from L. rnonocytogenes is susceptibility to neomycin; Erysipelothrix is resistant and L. rnonocytogenes is susceptible