Nocardia cyriacigeorgica, an emerging pathogen in the United States

Abstract

Nocardia cyriacigeorgica is recognized as an emerging pathogen in many parts of the world. We present the first case description of invasive N. cyriacigeorgica pulmonary infection in the United States identified to the species level by 16S rRNA and hsp65 sequence analysis. A subsequent retrospective molecular screening of recent Nocardia clinical isolates at our New York City medical center yielded an additional six N. cyriacigeorgica isolates. Because routine laboratory algorithms for the phenotypic identification of Nocardia species are limited in practice, the true prevalence of N. cyriacigeorgica infections may be greater than currently appreciated. Indeed, we present evidence confirming that N. cyriacigeorgica is coincident with the unofficial species designation Nocardia asteroides complex antimicrobial susceptibility pattern type VI and distinct from the N. asteroides sensu stricto strain ATCC 19247(T). As nocardial species identity can predict antimicrobial susceptibility and guide clinical management, we offer simplified phenotypic and molecular protocols to assist the identification of N. cyriacigeorgica.

FIG. 1.

Radiological and histopathological findings from the index patient. Posterior-anterior chest X ray (A) and chest computerized tomography (B) show nodular opacities in the right lower lobe and dense consolidation in the left lower lobe (arrowheads) in addition to other small irregular opacities in the right upper lobe and elsewhere. The chest computerized tomography was performed 6 days and the chest X ray 2 days before the first positive sputum specimen was obtained. (C) A computerized tomography-guided biopsy from the left lung obtained 4 days later showed gram-positive, branching, beaded filaments and an acute inflammatory infiltrate (original magnification, ×400).

FIG. 2.

Unrooted phylogenetic tree based on the concatenated 16S rRNA and hsp65 gene sequences (1,872 bp in total). Sequences were aligned using Molecular Evolutionary Genetics Analysis software, version 3.1, by ClustalW, and a phylogenetic tree was constructed using the neighbor-joining method and tested for robustness by bootstrapping with 2,000 replicates. N. farcinica strain DSM 43665^T was used as the outgroup. The reference bar marks a 0.2% estimated sequence variance. An asterisk denotes the isolate from the index patient.

FIG. 3.

Unique macroscopic and microscopic features distinguish N. cyriacigeorgica. (A to F) Growth of N. cyriacigeorgica strain 06-51518 at 48 h (day 2) and day 7 on Columbia 5% sheep's BAP BCYE agar, and Middlebrook 7H11 agar, as indicated. (G) Mega-colony of type VI strain ATCC 14759^T after 4 days of incubation on SAB agar. (H) Microscopic view (original magnification, ×100) of the growing mega-colony edge of panel G. Gram stains of ATCC 14759^T after growth for 24 h (I) and 7 days (J) on SAB agar (original magnification, ×400) are shown.

FIG. 4.

Unique macroscopic and microscopic features distinguish the N. asteroides laboratory type strain ATCC 19247^T. Colony morphology varied with medium after 7 days of incubation at 35°C. (A) Individual colonies were shiny, honey-colored, slightly elevated, and growing to a peak with an eroded surface texture on BAP. (B) Colonies were similar but cream colored on BCYE agar. (C) Colonies were white and dome shaped, with a buff velvety surface on 7H11 agar. (D) Mega-colonies after 4 days of incubation on SAB agar were coral colored, flat, and moderately ruffled. (E) Microscopically, aerial hyphae protruding from these large colonies had a bundled, pointed-end appearance. Note that all N. cyriacigeorgica strains differed from the N. asteroides species type strain in each of the above descriptions (compare with Fig. 3) as well as by urease activity and the ability to grow at 45°C (Table 3).

FIG. 5.

Differential carbapenem susceptibility of N. cyriacigeorgica and synergistic activity of amikacin and imipenem. (A) Antimicrobial susceptibility and synergy were evaluated for amikacin (upper right), imipenem (upper left/lower right), and meropenem (lower left) by disk diffusion. (B) MICs of imipenem (left) and meropenem (right) were determined by Etest. Results for type VI strain ATCC 14759^T are shown and are representative of all N. cyriacigeorgica clinical isolates. N. cyriacigeorgica strains also produced a brown pigment when grown on Mueller-Hinton agar that was clearly visible on the plate reverse (not shown). Note that, based upon preliminary testing, disks were spaced at a nonstandard 3.5-cm distance apart in order to best illustrate synergy between amikacin and imipenem.