Version May 2024
INTRODUCTION — Nocardiosis is an uncommon gram-positive bacterial infection caused by aerobic actinomycetes in the genus Nocardia. Nocardia spp have the ability to cause localized or systemic suppurative disease in humans and animals [1-5]. Nocardiosis is typically regarded as an opportunistic infection, but infection can also occur in immunocompetent individuals [4].
Two characteristics of nocardiosis are its ability to disseminate to virtually any organ, particularly the central nervous system, and its tendency to relapse or progress despite appropriate therapy.
The microbiology, taxonomy, and pathogenesis of nocardiosis will be reviewed here. The epidemiology, clinical manifestations, diagnosis, and treatment are discussed separately. (See "Nocardia infections: Epidemiology, clinical manifestations, and diagnosis" and "Treatment of nocardiosis".)
MICROBIOLOGY — Actinomycetes are a group of aerobic and anaerobic bacteria in the order Actinomycetales. These organisms are phylogenetically diverse but morphologically similar, exhibiting characteristic filamentous branching with fragmentation into bacillary or coccoid forms [6]. Aerobic actinomyces that cause human and veterinary disease include Nocardia, Gordona, Tsukamurella, Streptomyces, Rhodococcus, Mycobacteria, and Corynebacteria. Anaerobic genera of medical importance include Actinomyces, Arachnia, and Bifidobacterium.
Nocardia typically appear as delicate filamentous gram-positive branching rods (picture 1) that appear similar to Actinomyces species. Nocardia can usually be differentiated from Actinomyces by acid-fast staining, as Nocardia typically exhibit varying degrees of acid fastness due to the mycolic acid content of the cell wall (picture 2). Another useful clue is that Nocardia grow under aerobic conditions, whereas Actinomyces grow under anaerobic conditions. Further discussion of specific stains for Nocardia is found below. (See 'Stains' below.)
In the laboratory, Nocardia can display both aerial branching and substrate branching into the media or along its surface. These organisms were once considered fungi because of their hyphal-like appearance, but molecular analysis of their cell wall has confirmed their classification as bacteria [5,6]. Culture techniques are described below. (See 'Culture' below.)
Formal species identification is best done using molecular techniques. (See 'Taxonomy' below and 'Molecular tests (eg, PCR)' below.)
TAXONOMY — The genus Nocardia includes more than 90 species, at least 54 of which cause disease in humans [1,4,7,8]. Nocardia species were originally classified based upon biochemical properties. However, molecular techniques are now the preferred methods for speciation. (See 'Molecular tests (eg, PCR)' below.)
Molecular methods including gene sequencing [9] and multilocus sequence analysis (MLSA) [10,11] have led to reclassification and renaming of many Nocardia isolates. For example, isolates that in the past had been identified as N. asteroides complex have now been renamed to other or new species. Because of this, N. asteroides, once a common species, is now rarely seen [12]. Also, molecular studies indicate that N. brasiliensis, N. otitidiscaviarum, and N. transvalensis, once thought to be fairly homogeneous genera, also exhibit diverse characteristics, and it is anticipated that new species will continue to emerge [13-15].
SPECIES PREVALENCE AND DISTRIBUTION — Up to 54 Nocardia species have been shown to cause disease in humans [1,4,6,8] with variation in the frequency of species with geographical region [12,16]. This has not been well characterized due to changes in taxonomy, difficulty in routine identification of Nocardia strains at the species level, and, perhaps, referral and reporting biases.
Among 765 isolates submitted to the United States Centers for Disease Control and Prevention (CDC) between 1995 and 2004, the following species were identified most commonly [17]:
●N. nova complex (28 percent)
●N. brasiliensis (14 percent)
●N. farcinica (14 percent)
●N. cyriacigeorgica (13 percent)
●N. brevicatena (7 percent)
●N. abscessus (6 percent)
A somewhat different distribution of Nocardia spp was noted in a review of 1119 isolates from Spain collected between 2005 and 2014 [12]:
●N. cyriacigeorgica (25 percent)
●N. nova (15 percent)
●N. abscessus (13 percent)
●N. farcinica (11 percent)
●N. carnea (4 percent)
N. farcinica was the most common species in studies from Belgium [18], France [19], and China [10], whereas N. brasiliensis was the most common species in a study from Taiwan [20]. N. nova was the most common species in two series from Australia [9,21].
PATHOGENESIS — Nocardia spp possess multiple mechanisms to overcome the immune response of the host. The ability to combat host resistance to infection appears to vary with the strain and growth phase of the bacteria [22]. Bacteria that are in log-phase exhibit filamentous growth and are resistant to phagocytosis. When phagocytosis does occur, inhibition of phagosome-lysosome fusion has been observed with some nocardial strains, thereby avoiding hydrolysis by the host cell. The production of a bacterial cell surface-associated superoxide dismutase and possibly increased production of catalase may be involved in resistance to human neutrophils [23].
L-forms are cell wall-deficient variants of some bacterial species, including Nocardia. L-forms have been recovered from cerebrospinal fluid in human nocardiosis and cause disease in animal models. Lifelong persistence of L-forms has been demonstrated in murine models, and it has been postulated that L-forms may be related to the tendency of nocardiosis to relapse and to recur years after successful initial antimicrobial therapy [5].
Some species have enhanced virulence. N. farcinica appears to be more virulent than some other species, since infection with this species is more likely to result in disseminated disease and tends to be more resistant to antimicrobials [24-26]. (See "Treatment of nocardiosis", section on 'Antibiotic susceptibility'.)
Host defenses — The interplay between host defenses and nocardial infections has been studied extensively both in vivo and in vitro. Although not all mechanisms are fully understood, it is clear that cell-mediated immunity is crucial in containing Nocardia spp infection.
The initial host response to nocardiosis involves neutrophils and local macrophages, which inhibit but do not kill the bacteria [27,28]. This inhibition helps to limit the spread of infection until a specific cell-mediated response can occur. A population of immune-primed T cells enhances phagocytosis, stimulates cellular response, and may be capable of direct cytotoxicity to the bacteria [5,29].
Gamma delta T lymphocytes may play a crucial role in the host defenses against Nocardia spp. In a murine model, gamma delta T cell-deficient mice died within 14 days after inoculation with N. asteroides at a dose that was not lethal to control mice [30]. Lung tissue from these mice showed severe damage and growth of the organism compared with a neutrophil response and clearance of the bacteria in the control animals.
The role of humoral immunity in nocardiosis is unknown. Murine models indicate that humoral immunity is not as critical as cell-mediated immunity [31,32], but antibody has been demonstrated to enhance phagocytosis and the microbicidal activity of activated macrophages in a rabbit model [33]. There is no evidence that B cells directly influence host defenses in nocardial infections [34].
There is some in vitro evidence that estrogen might be protective against nocardia infections, a finding that could explain a male predominance noted in most case series. For example, in a murine model of N. brasiliensis–induced actinomycetoma, estradiol limited mycetoma lesions [35].
SPECIFIC MICROBIOLOGIC TESTS
Stains — In individuals with compatible syndromes, a rapid diagnosis of nocardial infection can be made if gram-positive partially acid-fast filamentous branching bacilli are visualized in clinical specimens (picture 1 and picture 3 and picture 4). The value of direct microscopy of stained specimens cannot be overemphasized. (See "Nocardia infections: Epidemiology, clinical manifestations, and diagnosis", section on 'Microbiologic tests'.)
Staining can be performed on primary clinical samples (eg, sputum, tissue) or on colonies grown in culture. Two slides should be prepared when staining for Nocardia: one for Gram stain and the other for modified acid-fast staining.
If granules are seen in clinical specimens, they should be carefully washed in sterile saline, crushed between two microscopic slides, and stained for microscopic examination; granules are comprised of countless organisms and are most commonly found in cutaneous mycetomas, as discussed in further detail elsewhere (picture 5) [1,36-38]. (See "Nocardia infections: Epidemiology, clinical manifestations, and diagnosis", section on 'Skin' and "Eumycetoma", section on 'Diagnosis'.)
Many infections that can mimic nocardial infection will stain positive by stains used to detect Nocardia. In most cases, an experienced laboratory technician can differentiate the organisms based on the morphologic shape and pattern of the organisms along with their staining patterns.
Gram stain — Nocardia spp appear as delicate, filamentous, sometimes beaded, branching gram-positive bacilli on Gram stain (picture 1 and picture 3) [1]. Some strains have alternating gram-positive and gram-negative areas, particularly if beading is present.
The Gram-stain appearance of Nocardia spp is indistinguishable from that of Actinomyces spp and other bacteria that cause mycetoma (eg, Actinomadura, Streptomyces) [39]. Nocardia can be distinguished from these organisms by the modified acid-fast stain, described below.
Modified acid-fast stain — Samples that reveal branching gram-positive bacilli should undergo modified acid-fast staining; among the various branching gram-positive bacilli, only Nocardia spp will stain positive [36,39].
However, the acid-fastness of Nocardia may be variable and modified acid-fast stains (eg, modified Kinyoun, Ziehl-Neelsen, or auramine O stains) require experienced laboratory technicians for proper performance and interpretation (picture 4 and picture 6) [1,38,40,41]. Furthermore, sensitivity is suboptimal, particularly on older culture specimens; in a study of 50 patients with nocardial infection who had positive Gram stains, only 26 (51 percent) were positive by modified acid-fast staining.
Other organisms that stain positive by modified acid-fast staining include mycobacteria and Rhodococcus, both of which can appear as gram-positive bacilli but reveal only rudimentary or no branching on Gram stain [38]. However, on older specimens, incorrect diagnoses of mycobacteria or Rhodococcus infection may occur because nocardial filaments can break down into individual bacilli and cocci that retain acid-fastness [42,43]. Acid-fast staining for mycobacteria and Rhodococcus is discussed in detail elsewhere. (See "Microbiology of nontuberculous mycobacteria", section on 'Microscopy' and "Microbiology, epidemiology, and pathogenesis of Rhodococcus equi infections", section on 'Microbiology'.)
Methenamine silver stain — This stain can reliably detect Nocardia species in fixed tissue samples examined via histopathology (picture 7) [38,44-46]. Other organisms that will stain positive by this stain include all fungal pathogens (eg, Aspergillus, Histoplasma, Pneumocystis), in addition to Actinomyces, mycobacteria, Rhodococcus, and other organisms that can mimic nocardial infection.
Culture — Recovery of Nocardia in cultures can be difficult, but it is critical for diagnosis and for determining antibiotic susceptibility.
When Nocardia infection is suspected, the clinical laboratory should be notified so measures can be taken to optimize growth of the organism. Such measures include the following:
●Extending incubation time to 21 days (28 days for blood cultures); Nocardia spp usually require 5 to 21 days for growth [3,5,47].
●Using special media. Although most routine bacterial, fungal, and mycobacterial culture media support Nocardia growth, cultures from nonsterile sites may benefit from the use of selective media (eg, buffered charcoal yeast extract, modified Thayer-Martin agar) to decrease overgrowth of contaminating organisms [1,2,36,48].
●Avoiding certain sputum decontamination solutions that are toxic to Nocardia spp, particularly sodium hydroxide, N-acetylcysteine, and benzalkonium chloride [2,36].
The overall sensitivity of culture ranges between 85 and 95 percent in most studies [36,40]. However, the yield from blood cultures is low, even in disseminated disease [49,50].
Antimicrobial susceptibility testing — Due to challenges associated with obtaining accurate susceptibility results, we send isolates of Nocardia to reference laboratories for species identification and susceptibility testing. Reference laboratories that provide such testing in the United States are listed elsewhere. (See "Nocardia infections: Epidemiology, clinical manifestations, and diagnosis", section on 'Antimicrobial susceptibility testing'.)
Different Nocardia species have different susceptibility patterns. By identifying individual species, susceptibility patterns can be accurately predicted. However, such testing requires advanced molecular testing that is not widely available outside of reference laboratories [1,12,25,47,51-54].
The traditional method of nocardial susceptibility testing, as recommended by the United States Clinical and Laboratory Standards Institute (CLSI), is the microdilution method [55]. However, this method is not available in many clinical laboratories and can be difficult to perform and interpret [1,2,25,47,51-56]. The E-test method of susceptibility testing is considered by some experts to be an appropriate alternative method for susceptibility testing of Nocardia spp [1,12,57,58].
Antibiotic susceptibility patterns of different species of Nocardia spp are discussed separately. (See "Treatment of nocardiosis", section on 'Antibiotic susceptibility'.)
Molecular tests (eg, PCR) — Compared with traditional cultures, molecular tests have improved sensitivity, specificity, and turn-around time, and they can identify the organism to the species level [12,16,59-78]. Such tests include polymerase chain reaction (PCR), gene sequencing, and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF).
A molecular technique known as metagenomic next-generation sequencing uses a "shot-gun" approach to detecting unknown or unidentifiable pathogens and has been used successfully to diagnose Nocardia infection [75,78].
Molecular tests are not available in many clinical laboratories and have significant expense and equipment requirements [60,65,66].
SUMMARY AND RECOMMENDATIONS
●Microbiology – Nocardia typically appear as delicate filamentous gram-positive branching rods that appear similar to Actinomyces species. Nocardia spp can be differentiated from Actinomyces by acid-fast staining and their ability to grow under aerobic conditions, neither of which is a characteristic of Actinomyces. (See 'Taxonomy' above.)
●Taxonomy – The genus Nocardia includes more than 90 species, at least 54 of which cause disease in humans. (See 'Taxonomy' above.)
●Species epidemiology – The most common Nocardia species to cause disease in the United States are members of the N. nova complex, N. brasiliensis, N. farcinica, and N. cyriacigeorgica. The species prevalence has varied in studies from different geographic regions. (See 'Species prevalence and distribution' above.)
●Pathogenesis – Nocardia spp possess multiple mechanisms to overcome the immune response of the host. Cell-mediated immunity is crucial in containing Nocardia spp infection. (See 'Pathogenesis' above.)
●Microbiologic tests – For diagnosis, multiple microbiologic tests are available, including stains (eg, Gram stain, modified acid-fast stain) and culture. Molecular tests (eg, polymerase chain reaction [PCR]) are available in some clinical laboratories. Antimicrobial susceptibility testing can be challenging and often requires testing in reference laboratories with significant experience with Nocardia spp. (See 'Specific microbiologic tests' above.)
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