Version July 2025
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 [1-6]. Nocardiosis is typically regarded as an opportunistic infection, but infection can also occur in seemingly immunocompetent individuals [4].
Two characteristics of nocardiosis are its ability to disseminate to virtually any organ, particularly the central nervous system, and its potential 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 [7]. Aerobic actinomyces that cause human and veterinary disease include Nocardia, Gordonia, Tsukamurella, Streptomyces, Rhodococcus, Mycobacteria, and Corynebacteria. Anaerobic genera of medical importance include Actinomyces, 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 [5-7]. Culture techniques are described below. (See 'Culture' below.)
Formal species identification is best done using molecular and proteomic techniques. (See 'Taxonomy' below and 'Molecular and proteomic testing' below.)
TAXONOMY —
The genus Nocardia is complex and new species continue to emerge. It now includes over 100 species with approximately half associated with disease in humans [1,4,6-9]. Nocardia species were originally classified based upon biochemical properties. However, molecular and proteomic techniques are now the preferred methods for speciation. (See 'Molecular and proteomic testing' below.)
Species-level identification utilizing molecular methods including gene sequencing, multilocus sequence typing (MLST), and whole genome analysis have led to reclassification and renaming of many Nocardia isolates [6,10-13]. This is perhaps best illustrated by N. asteroides complex. Isolates previously identified as N. asteroides complex have now been renamed to other or new species and this once common species is now rarely seen [1,14,15].
SPECIES PREVALENCE AND DISTRIBUTION —
Roughly half of the Nocardia species identified to date have been shown to cause disease in humans [1,4,6-9] with variation in the frequency of species based on geographical region [14,16-27]. Historically, 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)
An analysis of over 260 Nocardia infections in three regions of the US from 2011 to 2018 demonstrated further distinction in species distribution with N. nova complex, N. cyriacigeorgica and N. brasiliensis infections predominating within midwestern, southwestern and southeastern regions, respectively [21].
The diverse geographic distribution continues to be detailed throughout the world. In the largest series from Western Australia, inclusive of 960 Nocardia isolates from 2000 to 2021, N. cyriacigeorgica (27 percent) predominated, followed by N. nova complex (20 percent), N. brasiliensis (9 percent), N. farcinica (9 percent), and N. transvalensis complex (3 percent) [22]. However, in neighboring Queensland, Australia, an analysis of 484 isolates between 2004 and 2018 revealed N. nova complex was most common (19 percent) followed by N. farcinica (16 percent) [23].
Similarly, N. cyriacigeorgica (25 percent) and N. nova complex (15 percent) were the most common species in a review of 1119 isolates from Spain from 2005 to 2014 [14]. N. brasiliensis and N. cyriacigeorgica predominated in studies from Taiwan [20,24,25] whereas N. farcinica was the most common species in studies from Belgium [18], France [19], Japan [26], and China [27].
PATHOGENESIS —
The development of infection is mitigated by a variety of factors including the nature of exposure, host defenses, and the infecting Nocardia spp.
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 nocardiosis.
The initial host response to Nocardia spp. involves neutrophils and local macrophages, which inhibit but do not kill the bacteria [28,29]. 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 [5,30].
Gamma delta T lymphocytes may play a crucial role in the host defenses against Nocardia spp. In a murine model, gamma delta T lymphocyte-deficient mice died within 14 days after inoculation with N. asteroides at a dose that was not lethal to control mice [31]. 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.
Impaired phagocytosis and resultant increased susceptibility to nocardiosis is seen in patients with chronic granulomatous disease [32] and anti-cytokine autoantibodies (eg anti-GM-CSF autoantibodies) [33]. In fact, Nocardia infection may disclose previously unrecognized immunodeficiencies in hosts [34].
The role of humoral immunity in nocardiosis is unknown. Murine models indicate that humoral immunity is not as critical as cell-mediated [35,36], but antibody has been demonstrated to enhance phagocytosis and the microbicidal activity of activated macrophages in a rabbit model [37]. There is no evidence that B cells directly influence host defenses in nocardial infections [38].
There is some in vitro evidence suggesting estrogen may be protective against Nocardia infections, a finding that could partially explain the male predominance noted in the literature. For example, in a murine model of N. brasiliensis–induced actinomycetoma, estradiol limited mycetoma lesions [39]. In contrast, a murine model of N. farcinica lung infection demonstrated higher bacterial burden, inflammatory response, and mortality in association with estradiol [40]. The impact of estradiol and other sex steroids hormones, such as progesterone and testosterone, on disease pathogenesis necessitates further evaluation [41].
Immune evasion and virulence — Nocardia spp possess multiple mechanisms to overcome the host immune response. The ability to combat host resistance to infection appears to vary with the strain and growth phase of the bacteria [42]. 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 [43].Host apoptotic cell death via caspase activation and proteasome inhibition by Nocardia spp may further contribute to disease progression [44].
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].
Whole genome sequencing has demonstrated secondary metabolites of Nocardia spp that may also contribute to immune evasion and virulence [45]. For example, siderophores such as nocobactin NA are common secondary metabolites produced by Nocardia spp and have been shown to inhibit notch signaling with resultant evasion from cell-mediated immunity [45,46]. Nocardia spp are also quite heterogenous with reported genome sizes ranging from 6.3 to 9.4 million base pairs for N. farcinica and N. brasiliensis, respectively [47]. Accordingly, while species such as N. farcinica have been associated with disseminated and multidrug-resistant disease [48], advanced molecular technologies are key in delineating clinically relevant genomic features indicative of virulence as well as other factors mitigating pathogenesis.
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.
Specimens should be inspected for granules and when present should be gently washed in sterile saline, crushed between two microscopic slides, and stained for microscopic examination [1,49]. Granules or "grains" associated with Nocardia spp are generally less than 1 mm in diameter in comparison to larger sulfur granules more commonly, but not exclusively, seen with Actinomyces infection (picture 5 and picture 6). Granules are comprised of countless organisms and most commonly found in cutaneous mycetomas, as discussed in further detail elsewhere [1,49-51]. (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) [52]. 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 [49,52].
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) [1,51,53,54]. 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 [51]. 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 [55,56]. 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) [51,57,58]. 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 [3,4,7].
●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 [1,2,49,59].
●Avoiding certain sputum decontamination solutions that are toxic to Nocardia spp, particularly sodium hydroxide, N-acetylcysteine, and benzalkonium chloride [2,49].
The overall sensitivity of culture ranges between 85 and 95 percent in most studies [49,53]. However, the yield from blood cultures is low, even in disseminated disease [60,61].
Antimicrobial susceptibility testing — Due to challenges associated with obtaining accurate susceptibility results, many centers 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 antimicrobial susceptibility patterns. By identifying individual species, susceptibility patterns may be accurately predicted for many commonly isolated species. However, such testing requires advanced molecular testing that may not be widely available outside of reference laboratories [1,6,7,14,62].
The traditional method of nocardial susceptibility testing, as recommended by the United States Clinical and Laboratory Standards Institute (CLSI), is the broth microdilution method with select application of disk diffusion methods for verification of sulfonamide MIC results only [63,64]. However, the broth microdilution method can be difficult to perform and interpret and is not available in many clinical laboratories [1,2,4,6,7,63-65]. Disk diffusion and E-test methods of susceptibility testing are considered by some experts to be appropriate alternative methods for susceptibility testing of Nocardia spp [1,14,19,66].
Antibiotic susceptibility patterns of different species of Nocardia spp and additional susceptibility testing considerations are discussed separately. (See "Treatment of nocardiosis", section on 'Antibiotic susceptibility'.)
Molecular and proteomic testing — Compared with traditional cultures, molecular and proteomic tests have improved sensitivity, specificity, and turn-around time, and they can identify the organism to the species level [1,6,9,16]. Such tests include polymerase chain reaction (PCR) [67,68], gene sequencing [69], MLST [10], whole genome analysis [70], and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF) [71,72].
DNA sequencing of gene targets, such as 16S ribosomal RNA (rRNA), DNA gyrase subunit B and secA1, may be utilized for identification of Nocardia spp. However, 16S rRNA is utilized most frequently and considered the gold standard. MLST and whole genome analysis can be utilized to overcome challenges of single gene analysis such as discriminating between closely related species [1,6,9,10,70].
MALDI-TOF is increasingly applied as a rapid and cost-efficient means of organism identification in clinical labs. Important limitations with Nocardia spp include inadequate reference spectrum databases such that additional molecular methods for identification may be required [6,71,72].
A molecular technique known as metagenomic next-generation sequencing uses a "shot-gun" approach to detect unknown or unidentifiable pathogens and has been used successfully from varied clinical specimens to diagnose Nocardia infection [73-76]. Other examples of rapid, nonculture based identification methods include the application of in situ hybridization with DNA probes for specific gene sequences to formalin-fixed, paraffin-embedded tissue [77] and genus-specific PCR testing of respiratory and other clinical specimens [67]. The latter may be difficult to interpret when applied to respiratory samples given the potential for Nocardia colonization [68].
Molecular and proteomic tests are not available in all clinical laboratories, can have significant expenses, and require specialized equipment and expertise [1,6].
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 100 species, approximately half 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 and proteomic tests are available in some clinical laboratories. Antimicrobial susceptibility testing can be challenging and often requires performance in reference laboratories with significant experience with Nocardia spp. (See 'Specific microbiologic tests' above.)
