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Actinomyces neuii rarely causes disease in humans. First described in 1985 in two patients with postcataract endophthalmitis (4), A. neuii represents 17% of all clinical Actinomyces isolates (8), with some 132 cases of infection caused by this microorganism reported to date. The sensitivity of direct Gram staining of skin and soft tissue samples is low (17 to 21%). It grows as round, smooth, convex, opaque, white colonies with complete 0.5- to 1.5-mm-diameter margins on sheep blood agar (SBA) after 48 h of incubation under a 5% CO2 atmosphere or anaerobic conditions. Like most species of the genus, A. neuii is aerotolerant and facultatively anaerobic. Although the different species were traditionally described as branching rods, A. neuii is nonbranching and may appear as short rods or even coccoids (6, 7). A. neuii does not cause typical actinomycosis. Its two subspecies, A. neuii subsp. neuii and A. neuii subsp. anitratus, can be readily identified by the API Coryne test, version 3.0 (bioMérieux). Though most microbiology laboratories use phenotypical methods of identification, the different kits or tests differ in their substrate specificities, so that reactions can vary depending on the system used (2, 17). Identification based on 16S rRNA gene sequence analysis is far more precise (2, 19). The two cases below illustrate its potential pathogenicity.
In case 1, a 28-year-old otherwise healthy man presented with a 3-day fever and an intergluteal indurated cyst. Physical examination revealed a painful, fluctuating, and tender mass diagnosed as a pilonidal sinus. A purulent fluid obtained on aspiration showed many leukocytes and Gram-positive coryneform rods after direct Gram staining.
In case 2, a 48-year-old woman with an uneventful medical history developed a painful lump in the left breast and fever 4 days before admission. On clinical examination, an excruciatingly painful, fluctuating, breast mass with skin erythema was detected. In a sample obtained during surgical debridement, numerous leukocytes but no microorganisms were revealed by Gram staining.
In both cases, Ziehl-Neelsen and modified Ziehl-Neelsen stains were negative. After 48 h of incubation in a 5% CO2 atmosphere and under anaerobic conditions, pure growth was achieved in both cultures on standard microbiological media. In both instances, the microorganism was identified by its staining characteristics, colony morphology, and biochemical reactions as Actinomyces neuii subsp. neuii. Both strains were catalase positive, did not hydrolyze urea or esculin, and fermented many carbohydrates. They showed a characteristic strong CAMP reaction. Phenotypic identification using API Coryne, version 3.0, gave the numerical profile 3410735 in both cases. Species identification of the two strains was confirmed using a BIOLOG GN panel with 95 carbon sources (BIOLOG, Hayward, CA) with a similarity of 96% (T, 0.515) and 100% (T, 0.600), respectively, with Actinomyces neuii subsp. neuii, and by 16S rRNA sequencing (1,157- and 1,390-bp fragments each) using a previously reported method (5). The 16S sequences showed a homology of 99.1% and 99.7% with Actinomyces neuii subsp. neuii (GenBank accession no. AM084228). Both cases were resolved by a course of penicillin V.
In around 50% of cases, the bacterium is detected in patients with abscesses (6, 8, 11). It also appears in patients with diabetic foot osteomyelitis (14), genitourinary infections (8, 13), bloodstream infections and native valve endocarditis (3, 8), pericarditis (12), and biomaterial infections (1, 9, 15, 16, 18). Treatment consists of surgical debridement. Antibiotics are reserved for patients with fever, cellulitis, or immunosuppression. The microorganism is susceptible to most antibiotics but shows diminished susceptibility to aminoglycosides and fluoroquinolones (10). Its frequent interpretation as a skin contaminant hinders its identification.
Published ahead of print on 24 February 2010.