Nocardia

Nocardia

General Characteristics

The organisms in this genus are aerobic, gram-positive bacilli that often form branched hyphae. The hyphae are easily disrupted into rods and cocci. Even though Nocardia spp. are, strictly speaking, gram-positive, organisms will often stain gram-variable and may be weakly acid fast (using weak acid as decolorizer in acidfast staining). The acid-fast stain is used to visualize the mycobacterium and is discussed in Chapter 26.

The colonial and microscopic morphology, and the types of infections caused, resemble those of the fungi, but these organisms are true bacteria. Nocardia spp.

grow well on standard nonselective media. Growth may take a week or more. The organisms in this genus are commonly found in soil. Generally, infections caused by Nocardia organisms are seen in immunocompromised patients. Reports of infection in patients with no apparent illness or immunosuppressive therapy are increasing,however. Sixteen species have been implicated inhuman infections with wide diversity of geographicprevalence throughout the world. The most commonly encountered species are N. asteroides, N.brasiliensis,

N. {arcinica, and N. nova. Less commonly encounter species include N. otitidiscaviarum, N. pseudobrasiliensis, N. abscessus, N. africana, and N. transvalensis. The majority of isolates received for identification were

from sputum and wounds. Virulence Factors The role of such factors as toxins and extracellular proteins in nocardiosis is unclear. No virulence factors have been identified, although virulence has beencorrelated with alterations in the components in thecell wall. The precise role of the various cell wall molecules

in virulence is unknown. Nocardia organisms produce a superoxide dismutase and catalase that may provide resistance to oxidative killing by phagocytes.

It also produces an iron-chelating compound called nocobactin. A correlation has been reported between the amount of nocobactin produced by the organism and its virulence. Clinical Infections Nocardia spp. are found worldwide in soil and on plant material. Infection occurs by two routes: pulmonary and cutaneous. Pulmonary infection by Nocardia spp.

occurs from the inhalation of the organism present indust or soil. The disease appears to be associated with impaired host defenses, because most persons seen with nocardiosis have an underlying disease or compromised immune defenses. Even so, approximately 109(,of Nocardia infections occur in seemingly normal patients with no obvious immune impairment. Infection

with Nocardia organisms can be serious. Approximately40% of the diagnoses are made at autopsy. Themortality rate is high, and those who survive often

suffer significant tissue damage. Pulmonary Infections The majority of pulmonary infections are caused by N. asteroides complex. The most common manifestation of infection is a confluent bronchopneumonia that is usually chronic but may be acute or relapsing. The disease generally progresses more rapidly than tuberculosis but is measured in months rather than years. In the acute form, which is often seen in patients with underlying immune defects, the time course is a matter of weeks.The initial lesion in the lung is a focus of pneumonitisthat advances to necrosis. The abscesses that form may extend into the tissue and coalesce with eachother. Extensive tissue involvement and damage result. Unlike some pneumonias, little inflammatory responseor scarring, no encapsulation of the abscesses, andno granuloma formation occur. Dissemination to otherorgans, especially the brain, may occur, with reports

in the literature of involvement of virtually every organ.

The sputum is thick and purulent. Unlike infection bythe anaerobicactinomycetes, no sulfur granules or

sinus tract formation occurs

Cutaneous Infections

Cutaneous infection occurs following inoculation of the organism into the skin or subcutaneous tissues.N. brasiliensis is the most frequent cause of this form

of nocardiosis, which is usually seen in the hands andfeet as a result of outdoor activity. The trauma mostlikely is minor, such as from a thorn or wood sliver.

The infection begins as a localized subcutaneousabscess that is invasive and quite destructive of thetissues and underlying bone. These lesions are termed

mycetomas (certain species of fungi also cause mycetomas).

Mycetomas are characterized by swelling,draining sinuses, and granules. About half of the mycetomas seen clinically are caused by the actinomycetes,and the remaining half are caused by fungi. As theinfection progresses, burrowing sinuses open to theskin surface and drain pus. The pus may be pigmented

and contain sulfur granules (Figure 17-5), which are masses of filamentous organisms bound together by calcium phosphate. They often appear yellow or orangeand have a distinct granular appearance.

Laboratory Diagnosis

Microscopy

The gram-positive branching filaments characteristic of Nocardia organisms are often seen in sputum and exudates or aspirates from skin or abscesses. The specimen often contains coccobacillary bodies as well. The Gram reaction may be weak or irregular, causing a “beading” appearance similar to the appearance of chains of gram-positive cocci. This morphology may easily confuse the microscopist. Presumptive identification of Nocardia can be made based on observation of filamentous, branching isolate that is acid fast upon staining with carbolfuchsin and decolorizing with aweak acid (0.5% to I%sulfuric acid), but not with theKinyoun acid-fast stain. Additional testing is needed for speciation and to distinguish Nocardia spp. From Streptomyces spp Wet mounts should also be performed on clinical specimens. Granules may be seen in specimens from cutaneous infection. Tissue and pus from the draining

sinuses are the specimens of choice for direct examination. The granules may be visualized by separating them from the pus with an inoculating needle and then washing in sterile saline. The granules of N. aste ro ides, N. brasiliensis, and N. otitidiscaviarum are soft, whiteto- cream colored, and 0.5 to 1 mm in size. They may be crushed between two glass slides to visualize the branching and cellular morphology comprised of grampositive, interwoven, thin (0.5 to 1.0 mm in diameter)filaments. The granules may also be used to inoculate the appropriategrowth media. The granules of a fungal mycetoma (eumycotic mycetoma) are composed of broad, interwoven, septate hyphae that are wider (2 to 5 mm) than those of the actinomycetes (actinomycotic mycetoma).

Cultural Characteristics

The growth requirements of Nocardia spp. are not as well defined as those of many other medically important bacteria. These organisms show an oxidative-type metabolism, and as a genus, they use a wide variety of sugars. They do not require specific growth factors as do Haernophilus and Francisella organisms. Nocardia spp. grow well on most common nonselective laboratory media incubated at temperatures between 22° and 37° C, although 3 to 6 days or longer may pass before growth is seen. In fact, Nocardia spp. grow on simple media containing a single organic molecule as a source of carbon. Some characteristics of the aerobic actinomycetes are listed in Table 17-3.

Colonies of Nocardia organisms may have a chalky, matte, or velvety appearance and may be pigmented. They have a dry, crumbly appearance that is likened to that of breadcrumbs. Table 17-4 outlines the colonialappearance of the aerobic actinomycetes. Examination of colonies with a dissecting microscope can reveal the presence of aerial hyphae. These macroscopic and microscopic phenotypic colony morphologies provide the first clues to the identity of the organism as belonging to the genus Nocardia.

Identification

Nocardia spp. grow well on standard nonselective laboratory media, including those used for fungal cultures.

Media containing antimicrobial agents used for isolating fungi should not be used, however, because Nocardia spp. are susceptible to many of the agents used in these media. The possibility of isolating Nocardia organisms is increased by the paraffin bait technique, which takes advantage of the fact that

Nocardia organisms use paraffin as an energy source and other aerobic bacteria do not.

An isolate showing branching filaments that are gram-positive and partially acid fast should be suspected of belonging to the genus Nocardia (Figure 17-6). Tentative identification and differentiation of the aerobic actinomycetes is outlined in Table 17-3. Simple, phenotypic tests generally can result in proper identification of the majority of clinically relevant Nocardia spp. Methods employed for identification include (1) substrate hydrolysis (casein, tyrosine, xanthine, and hypoxanthine), (2) other substrate and carbohydrate utilization (arylsulfatase, gelatin liquefaction, and carbohydrate utilization), (3) susceptibility antibiogram profile, (4) fatty acid analysis by high-performance liquid chromatography, and (5) molecular methods based on amplification and sequencing of DNA. Some of these tests are beyond the capabilities of most clinical laboratories. If routine tests used do not result in identification, confirmation of the identity can be confirmed at a reference laboratory with experience in identification of such organisms.

Treatment

Treatment of Nocardia infection often involves drainage and surgery and antimicrobials. The organisms are resistant to penicillin but susceptible to sulfonamides. Antifungal agents, of course, have no activity against Nocardia organisms; this fact underscores the importance of laboratory diagnosis, because many of the clinical manifestations of pulmonary and cutaneous infection are shared with other organisms, including fungi. This organism represents a classic example of a situation in which laboratory results are absolutely essential for proper antimicrobial treatment .

figure 17-5 figure 17-6 table 17-3 table 17-4

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