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Journal of Clinical Microbiology, October 2003, p. 4497-4501, Vol. 41, No. 10
0095-1137/03/$08.00+0 DOI: 10.1128/JCM.41.10.4497-4501.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Nocardiosis
Review of Clinical and Laboratory Experience
Michael A. Saubolle1,2* and Den Sussland1
Laboratory Sciences of Arizona/Sonora Quest Laboratories, Phoenix,1
Department of Medicine, University of Arizona College of MedicineTucson Arizona2

INTRODUCTION
Members of the genus
Nocardia are associated with the group
of microorganisms known as the aerobic actinomycetes and belong
specifically to the family
Mycobacteriaceae. The nocardiae contain
tuberculostearic acids but differ from the mycobacteria by possession
of shorter-chained (40- to 60-carbon) mycolic acids. They have
a type IV cell wall, characterized by a peptidoglycan made up
of
meso-diaminopimelic acid, arabinose, and galactose (
15).
The systematics of this group of organisms originated from intuitive
principles based on microscopic morphology and phenotypic characterization.
The nocardiae are gram-positive, bacillary, branching bacteria
whose hyphae often fragment to coccobacillary forms. Recent
application of modern taxonomic procedures, inclusive of more
extensive phenotypic evaluation, molecular characterization,
and numerical taxonomic methods, has expanded our knowledge
of their phylogenetic relatedness and taxonomic status (
9,
13,
15). The taxonomy within the genus
Nocardia is changing rapidly
as the recognition and description of new species continue.
As expected, there are differences of opinion as to the number
of validly described species within the genus at this time,
with recent publications citing from 22 to 30 such valid species
(
3,
14). Although a large number of species have been characterized
both phenotypically and genotypically within the genus, the
genotype remains greatly heterogeneous and will continue to
evolve (
3,
14). Sixteen species have been implicated in human
infections (Table
1); but the geographic prevalence of each
may change dramatically throughout the world, and some are uncommon.
The species found most frequently in Arizona (Table
2) may vary
substantially from those isolated in other parts of the United
States.
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TABLE 2. Isolates of Nocardia spp. recovered from individual patients in the Phoenix cosmopolitan area between 1998 and 2002
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NATURAL HABITAT AND EPIDEMIOLOGY
Nocardiae are found extensively worldwide and are saprophytic,
making up an important component of the normal soil microflora
and often being associated with water. They may also be associated
with decomposing plant material, dust, and air (
3,
15). As a
species,
Nocardia asteroides sensu stricto type VI is distributed
evenly throughout the United States.
N. farcinica is also found
evenly throughout the United States, although it is less prevalent
then
N. asteroides. The distribution of other species varies
regionally.
N. nova is less commonly isolated in the Southwest
(authors' personal observations). Frequently, the term "
N. asteroides complex" is used to include
N. asteroides sensu stricto type
VI,
N. farcinica,
N. nova, and more recently,
N.
abscessus because
earlier reports failed to differentiate between the four species
(
3,
15).
N. brasiliensis is associated with tropical environments,
although in the United States it is the second most common isolate
and has a higher prevalence in the southwestern and southeastern
regions (
3,
11,
15).
N. otitidiscaviarum has infrequently been
recovered from soil throughout the world. As with many other
rarely reported nocardial species, the specific natural habitat
of
N. transvalensis has not yet been identified (
3,
11,
15).
The majority of nocardial infections in the United States are acquired through inhalation (3, 15). It is the authors' personal observation that in the United States the overall number of nocardial infections seems to be greatest in association with the dry warm climates of the Southwest (Table 2). It may be that the dry, dusty, and often windy conditions in that region facilitate the aerosolization and dispersal of fragmented nocardial cells and enhance their acquisition via the respiratory route. A smaller number of infections are caused by traumatic introduction of organisms percutaneously. Normally, primary infections with N. brasiliensis and N. otitidiscaviarum in an immunocompetent host are associated with implantation via a foreign object. It is now known that many of the invasive infections thought to have been caused by N. brasiliensis were actually caused by a more recently recognized species, N. pseudobrasiliensis (17). Newly recognized species such as N. africana, N. paucivorans, and N. veterana have also been reported to cause disease in humans, although little is known of their epidemiology (6, 8, 13).
Nocardial infections are not thought to be transmitted from person to person and are not usually acquired nosocomially (15). However, there have been rare reports of interesting clusters of patients infected with identical strains of nocardia while occupying beds in close proximity on hospital wards. In such cases nosocomial acquisition was probable. It was difficult to ascertain, however, whether the infections were primary acquisitions from a common environmental source (environmental transmission) or secondary acquisitions from an initially infected patient (person-to-person transmission).

SPECTRUM OF DISEASE
Nocardiosis is usually an opportunistic infection and most commonly
presents as pulmonary disease. The majority of patients with
clinically recognized disease have underlying debilitating factors
(
3,
11,
15). Arguably, the most common condition predisposing
the patient to nocardiosis is underlying chronic lung disease,
often in association with long-term corticosteroid therapy (authors'
unpublished observations). In a review of 16 patients with nocardiosis
admitted to Banner Good Samaritan Medical Center, Phoenix, Ariz.,
over a 1-year period, nearly 75% had an underlying chronic pulmonary
condition. Other predisposing conditions include diabetes mellitus,
hematologic and other malignancies, transplantation, and AIDS.
The authors estimate that less than 10% of patients with nocardiosis
have no definable underlying predisposing factor. Healthy hosts
with nocardial infections often have undergone percutaneous
trauma and soft tissue inoculation (
3,
4,
15).
The majority of primary cases present as pulmonary disease, although traumatically induced local abscesses occur as well. Dissemination from the lungs may be manifested as bacteremia, empyema, brain abscess, pericarditis, synovitis, and soft tissue infection (Table 3). Peritonitis and corneal ulcers have been described. Typically, nocardiosis is characterized by an acute inflammatory response terminating in necrosis and abscess formation; granulomas are not normally formed (3, 4, 15).
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TABLE 3. Individual source or specimen type of 470 Nocardia isolates recovered within the Phoenix, Ariz., area between 1998 and 2002
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In the United States most human infections are caused by
N. asteroides sensu stricto type VI,
N.
farcinica,
N. nova,
N.
brasiliensis,
N.
otitisdiscaviarum, and possibly,
N.
pseudobrasiliensis.
N. transvalensis,
N. africana,
N. brevicatena, and the newly
described species are rarely encountered at this time. Members
of the
N. asteroides complex primarily cause pulmonary disease
and, except for
N.
nova, are prone to extrapulmonary dissemination.
Dissemination is especially prevalent with
N.
farcinica (
3,
4,
15,
16) (Table
3).
N. brasiliensis and
N. transvalensis typically
produce localized infection induced by an abrasion, although
the latter species is uncommon in the United States.
N. brasiliensis can be associated with a lymphocutaneous sporotrichoid presentation
(Fig.
1).

DIAGNOSTIC METHODS
The clinical diagnosis of nocardiosis is difficult. Signs, symptoms,
and radiologic studies may suggest the diagnosis but are not
pathognomonic. Serologic diagnosis is unreliable, and serologic
tests are not available commercially. The evaluation of appropriate
specimens by smear and culture remains the principal method
of diagnosis. Detection in smears and isolation of nocardiae
on primary and/or selective media are not usually difficult.
This was demonstrated by an earlier study of 80 patients in
whom nocardiosis was diagnosed by microbiologic or histopathologic
techniques at Banner Good Samaritan Medical Center (M. A. Saubolle,
unpublished observations, 1978 to 1989). Seventy-six of 80 (95%)
patients studied had positive cultures, and in 50 of the 80
(64%) patients, infection was detected by Gram staining of infected
material (Fig.
2). Of the 50 patients with specimens positive
by Gram staining, only 26 (51%) were also positive by modified
acid-fast staining (Fig.
3). Gram staining did not miss any
acid-fast-positive specimens. Of the 76 patients from whom nocardiae
were recovered, the isolates were recovered from 63 (83%) by
culturing specimens on routine microbiologic media, and of these,
44 (70%) had Gram stain-positive primary specimens. Sixty-four
of 76 (84%) patients had pulmonary involvement. Of the 51 of
64 patients from whom expectorated sputum was submitted for
routine culture, nocardiae were detected in 40 (78%) by Gram
staining and nocardiae were recovered from 45 (88%). Thus, Gram
staining is the most sensitive method by which to visualize
and recognize nocardiae in clinical specimens. The modified
acid-fast stain is not reliable and should be used only to confirm
the acid fastness of organisms detected by Gram staining.
Nocardiae normally appear within 2 to 7 days on most routine
bacteriologic media such as 5% sheep blood agar, chocolate agar,
and BACTEC blood culture broth media. In cases in which suspicion
of nocardia is high or in which the organism is visualized in
specimens that are heavily contaminated, recovery may be optimized
by the addition of selective media such as colistin-nalidixic
acid agar, modified Thayer-Martin agar, and buffered charcoal-yeast
extract (BCYE) and selective BCYE agars (
3,
7,
15). The nocardiae
seem to grow well on fungal media as well, including cycloheximide-containing
agar, such as Mycosel. Some strains may, however, be inhibited
by the gentamicin present in inhibitory mold agar. Media should
be examined for up to 2 or 3 weeks for possible slowly growing
nocardial strains, although the majority will be detected within
the first several days of incubation. Recognition of the nocardiae
can be optimized by seeing filamentous, white to yellow to orange
colonies with aerial mycelia and delicate, dichotomously branched
substrate mycelia with a dissecting microscope (
15).
Although nocardiae are frequently isolated during culture for mycobacteria, procedures used for decontamination of contaminated specimens submitted for mycobacterial culture may be deleterious to the nocardia (12). The procedures used for specimens submitted for such culture should be supplemented with less selective procedures and culture media when nocardia is high on the differential diagnosis list.

CLINICAL RELEVANCE
The presence of nocardiae in the environment can lead to contamination
and/or colonization of clinical specimens and can cause confusion
over their clinical relevance. Nocardiae are rarely seen as
contaminants in the laboratory, and each isolate must be carefully
evaluated as to its clinical significance (
4,
15). The presence
of nocardia in normally sterile sites or on direct microscopic
examination of potentially contaminated specimens, such as sputum,
greatly increases the likelihood of the organism's role as an
etiologic agent. Corticosteroid use significantly increases
the clinical relevance of a sputum isolate.

IDENTIFICATION
Initial visualization of phenotypic colony coloration and morphology,
together with the presence of aerial hyphae, with a dissecting
microscope often provides initial clues to the genus of the
isolate. Presumptive identification can be achieved if a filamentous,
branched isolate stains with the carbolfuchsin modified acid-fast
stain with a weak (0.5% to 1%) sulfuric acid decolorizing solution
but not with the traditional Kinyoun acid-fast stain (
15). Resistance
to lysozyme differentiates
Nocardia species from
Streptomyces species. On occasion, examination of cell wall components by
high-pressure liquid chromatography or thin-layer chromatography
is needed for identification to the genus level. Identification
to the species level may be more tedious and problematic. Originally,
identification of the nocardial species was based on hydrolysis
of casein, tyrosine, xanthine, and hypoxanthine. However, different
stable susceptibility profiles among
N. asteroides isolates
showed that at least six unique species were identifiable (
3,
9,
15,
17). Molecular as well as further phenotypic studies
of the species confirmed their disparity and uniqueness. At
present, molecular methods used to successfully identify the
nocardiae to the species level include restriction endonuclease
analysis of an amplified portion of the 16S rRNA gene, restriction
fragment length polymorphism analysis of the amplified
hsp gene,
and sequencing methodologies, such as sequencing of the 16S
rRNA or DNA (
5,
10,
14,
18). Unfortunately, such molecular studies
are often limited to research-oriented laboratories and are
rarely preformed in routine clinical laboratories. Although
combinations of phenotypic and genotypic characterization are
most successful in identifying all nocardial species, the majority
of the frequently isolated, clinically relevant species can
be initially identified by the use of a number of phenotypic
characteristics. Use of arylsulfatase, acetamide utilization,
gelatin liquefaction, growth patterns at 35 and 45°C, opacification
of 7H11 agar, and reactions with a number of commercial systems,
together with susceptibility profiles, can be used to identify
most isolates within a reasonable time frame of less than 7
days. Table
4 provides a simple guide for the preliminary identification
of the nocardiae in the clinical laboratory. Additional testing
may be needed for isolates not keying out with the limited number
of characteristics. The reader is referred to previous reports
(
3,
9,
15) for more extensive summaries and flowcharts.

THERAPY AND SUSCEPTIBILITY STUDIES
Sulfa-containing antimicrobials remain the drugs of choice and
may improve survival when used alone or in combination with
other antimicrobials (
3,
4,
15). Primary agents that have been
used successfully are minocycline, amikacin, imipenem, and linezolid.
Combination therapy with a sulfa-containing agent and one of
the primary agents has been recommended for serious, systemic
disease (
4). The use of amikacin in combination with imipenem
has also been suggested for serious infections. Other potentially
efficacious choices include the extended-spectrum cephalosporins,
amoxicillin-clavulanate, newer macrolides, other aminoglycosides,
and the fluoroquinolones (
3,
4,
15). The duration of therapy
is uncertain, but it should be protracted because of the occurrence
of considerable numbers of relapses after shorter courses of
therapy (
4).
Nocardia species can vary in their antimicrobial susceptibility patterns. Therapeutic efficacy in individual patients may depend on species identity and on in vitro susceptibility studies (3, 4, 15). Susceptibility testing should especially be considered in refractory cases. A standard for susceptibility testing by broth microdilution and with cation-supplemented Mueller-Hinton broth has been aproved by the NCCLS (19). Interpretative guidelines are provided in that publication. Disk agar diffusion, agar dilution, gradient strip agar dilution (E test), and BACTEC radiometric methods have all also been used for susceptibility testing (1, 2, 3, 15). Studies have shown rates of inter- and intralaboratory agreement and reproducibility above 90% between these methods (1, 2, 3, 15). Prospective clinical studies attempting to correlate the results of susceptibility testing to patient therapy and outcomes have not been systematically performed.

ACKNOWLEDGMENTS
We thank Peter P. McKellar for reviewing the manuscript and
Phillip Rubin for providing the picture of the case.

FOOTNOTES
* Corresponding author. Mailing address: Department of Clinical Pathology, Banner Good Samaritan Medical Center, 1111 E. McDowell Rd., Phoenix, AZ 85006. Phone: (602) 239-3485. Fax: (602) 239-5605. E-mail:
Mike.Saubolle{at}bannerhealth.com.


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Journal of Clinical Microbiology, October 2003, p. 4497-4501, Vol. 41, No. 10
0095-1137/03/$08.00+0 DOI: 10.1128/JCM.41.10.4497-4501.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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