Characteristics of nocardiosis patients with different immune status from a Chinese tertiary general hospital during 8-year period: A STROBE-compliment observational study

Nocardia is an opportunistic pathogen from environment, which is generally thought to infect immunosuppressed patients (ISPs), but recent studies showed it could also cause infections in immunocompetent patients (ICPs).The aim of this study was to compare the clinical characteristics, patients' outcome, Nocardia species' identification, and antibiotic susceptibility profiles of nocardiosis between ICPs and ISPs.The detailed clinical data were collected from all the nonrepetitive nocardiosis patients during 2011 and 2018, from a tertiary general hospital in Beijing, China. Then each Nocardia isolate was identified to species level by DNA sequencing. The antibiotic susceptibility testing was performed by E test method, and interpreted following CLSI M24 document. The clinical and microbiological characteristics between ICPs and ISPs were compared statistically.A total of 23 nonrepetitive nocardiosis patients with detailed clinical data were enrolled in this study. Among them, 9 were ICPs and 14 were ISPs. All the skin and soft tissue infections occurred in ICPs (33.3% vs 0%, P < .05). Bronchiectasis occurred more frequently in ICPs (44.4% vs 21.4%), whereas chronic kidney diseases and coinfection with aspergillosis occurred more frequently in ISPs (35.7% vs 0%, 35.7% vs 0%, respectively), although they did not reach the statistical significance. There were no significant differences in other clinical characteristics, Nocardia species' identification, and antibiotic susceptibility between ISPs and ICPs (P > .05).Nocardiosis could occur in both ISPs and ICPs. Skin and soft tissue infection and bronchiectasis occurred more frequently in ICPs. Chronic kidney diseases and co-infection with aspergillosis occurred more frequently in ISPs. These characteristics should be noticed by physicians in diagnosis of nocardiosis.

Cutaneous nocardiosis in an immunocompromised patient

We describe a case of a 79-year-old man with chronic lymphocytic leukemia (CLL) who presented with ataxia; falls; vision loss; and numerous mobile erythematous nodules on the chin, neck, scalp, and trunk. Computed tomography of the head and chest revealed cavitary lesions in the brain and lungs. Clinically, the skin nodules were believed to represent an infectious process. Two punch biopsies were obtained, which revealed an unremarkable epidermis with a mixed inflammatory infiltrate with abscess formation in the dermis. Gram stain highlighted gram-positive branching bacterial organisms. Similar organisms were identified in a bronchoalveolar lavage specimen. Cultures from skin and blood were positive for Nocardia. Our case serves as a reminder to clinicians and pathologists to keep a broad differential diagnosis when dealing with infectious diseases in immunocompromised patients.

Pulmonary Nocardiosis Caused by Nocardia exalbida in a Patient with Lung Cancer and Radiation Pneumonitis: A Case Report and Literature Review

We report a case of Nocardia exalbida (N.exalbida)-induced pneumonia in a 70-year old Japanese man with lung cancer and radiation pneumonitis. He initially received doripenem (1.5 g/day) for pneumonia treatment, and N.exalbida was identified by a clone library analysis of bronchoalveolar lavage fluid obtained from the pneumonia lesion. The doripenem dosage was therefore increased to 3.0 g/day with adjunctive trimethoprim/sulfamethoxazole, and his pneumonia improved. N. exalbida is susceptible to antibiotics; thus, in nocardiosis, N. exalbida infection might be associated with a good response to treatment, although its clinical findings are non-specific and similar to those of other Nocardia infections.

ENDOGENOUS NOCARDIA ENDOPHTHALMITIS PRESENTING AS A MASS LESION IN A PATIENT WITH METATSTATIC NONSMALL CELL CARCINOMA OF THE LUNG

PURPOSE

To report a case of Nocardia cyriacigeorgica endophthalmitis.

METHODS

Case report.

RESULTS

A 71-year-old man on long-term steroid therapy for poor respiratory function from lung carcinoma and presumed recent liver metastases presented with a four week history of blurry vision in the left eye. Ophthalmic examination revealed a white elevated mass in the macula with hemorrhage, concerning for metastasis. The patient was treated for presumed ocular metastases but had poor response to radiotherapy and was diagnosed four weeks later with N. cyriacigeorgica by retinal biopsy. Despite intravitreal and systemic antibiotics, the patient had progression of disease and died of respiratory involvement.

CONCLUSION

Nocardia endogenous endophthalmitis can present as a mass retinal lesion in immunosuppressed patients with metastatic disease. Early vitreous and retinal biopsy may be required for definitive diagnosis and treatment. A pulmonary source of infection should be suspected and monitored closely.

Imaging Findings of Pulmonary Nocardiosis Mimicking Bronchiectasis

Nocardia species usually cause opportunistic infections, and the frequency of these infections is increasing owing to the growing population of immunocompromised hosts. However, Nocardia may sometimes causes an infectious disease in immunocompetent hosts. Herein, we report two cases of pulmonary nocardiasis in immunocompetent individuals, whose chest computed tomography (CT) findings mimicked bronchiectasis. Samples of bronchalveolar lavage (BAL) fluid obtained by bronchoscopy showed filamentous, branching, gram-positive rods, acid-fast filamentous branching rods, and a colony of suspected Nocardia was cultured. Based on 16sRNA and hsp65 gene sequence analysis, case 1 was identified as N. cyriacigeorgica, but case 2 was not matched. The patients responded well to treatment with the combination of sulfamethoxazole and linezolid.

Primary Cutaneous Nocardia brasiliensis in a Spanish Child

BACKGROUND

We report a case of a primary cutaneous nocardiosis by autochthonous Nocardia brasiliensis in a Spanish immunocompetent 9-year-old boy.

METHODS

N. brasiliensis caused cellulitis showing the patient recovery after drainage and treatment with trimethoprim-sulfamethoxazole. Nocardia grew in pure culture and its identification was confirmed by sequencing (16S rRNA) and by MALDI-TOF MS (Bruker, Daltonics, Germany).

CONCLUSIONS

In Spain although N. brasiliensis cutaneous infections in children are very infrequent should not be ruled out when an insect bite, stuck with a pine needle or an animal scratch has existed and the wound evolution is torpid.

The plant-growth-promoting actinobacteria of the genus Nocardia induces root nodule formation in Casuarina glauca

Actinorhizal plants form a symbiotic association with the nitrogen-fixing actinobacteria Frankia. These plants have important economic and ecological benefits including land reclamation, soil stabilization, and reforestation. Recently, many non-Frankia actinobacteria have been isolated from actinorhizal root nodules suggesting that they might contribute to nodulation. Two Nocardia strains, BMG51109 and BMG111209, were isolated from Casuarina glauca nodules, and they induced root nodule-like structures in original host plant promoting seedling growth. The formed root nodule-like structures lacked a nodular root at the apex, were not capable of reducing nitrogen and had their cortical cells occupied with rod-shaped Nocardiae cells. Both Nocardia strains induced root hair deformation on the host plant. BMG111209 strain induced the expression of the ProCgNin:Gus gene, a plant gene involved in the early steps of the infection process and nodulation development. Nocardia strain BMG51109 produced three types of auxins (Indole-3-acetic acid [IAA], Indole-3-Byturic Acid [IBA] and Phenyl Acetic Acid [PAA]), while Nocardia BMG111209 only produced IAA. Analysis of the Nocardia genomes identified several important predicted biosynthetic gene clusters for plant phytohormones, secondary metabolites, and novel natural products. Co-infection studies showed that Nocardia strain BMG51109 plays a role as a "helper bacteria" promoting an earlier onset of nodulation. This study raises many questions on the ecological significance and functionality of Nocardia bacteria in actinorhizal symbioses.

Pulmonary nocardiosis masquerading renascence of tuberculosis in an immunocompetent host: a case report from Nepal

BACKGROUND

Pulmonary nocardiosis is an opportunistic infection in an immunocompromised patient; however, often neglected in the immunocompetent patient from the diagnosis considerations.

CASE PRESENTATIONS

We describe a case of pulmonary nocardiosis masquerading renascence of tuberculosis, in a 51-years-Nepali farmer. After a 6 month of presumed successful antitubercular therapy; the patient develops the clinical presentations and radiological features showing similarities with that of tuberculosis and malignancy. MTB complex was not detected with Xpert MTB/RIF assay and cytological examinations were negative for the malignant cells, however. The Ziehl-Neelsen staining of the broncho-alveolar-lavage revealed acid-fast, thin branching filamentous organisms suggestive Nocardia spp. Further, identifications and susceptibility pattern against recommended antibiotics were assessed as per the CLSI guidelines. The case was then, subsequently, diagnosed as pulmonary nocardiosis. Trimethoprim-sulfamethoxazole was prescribed for 12 months. The patient underwent progressive changes and no relapse was noted in a periodic follow-up.

CONCLUSIONS

This case underscores that pulmonary nocardiosis requires diagnostic considerations, regardless of a patient's immunologic status and other mimicking infections.

Incubation time and culture media impact success of identification of Nocardia spp. by MALDI-ToF mass spectrometry

Identification of Nocardia can be challenging, even by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-ToF MS). We examined the impact of incubation duration, culture media and bead-beading on identification success. When isolates were grown for 3 days on SAB (Saboround dextrose agar), then extracted using a bead-beating, ethanol/formic acid (EtOH/FA) procedure, MALDI-ToF MS correctly identified 36.4% to species level. By contrast, 80.9% of isolates were correctly identified to species level when processed with the standard EtOH/FA extraction from Columbia blood agar (CBA) cultures incubated until colonies first appeared. Excluding rare species, the latter approach correctly identified 93.0% of isolates of commonly-encountered Nocardia species when additional database entries were included to expand representation of select species. We demonstrate that while bead-beating has little impact on identification success by MALDI-ToF MS, analysis of early growth is crucial. Additionally, culture media that promotes quick growth of Nocardia (e.g. CBA) is recommended.

Identification of Nocardia species from clinical isolates using MALDI-TOF mass spectrometry

The identification of Nocardia isolates still represents a challenge for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) despite its acceptance for most bacterial and fungal isolates. In this study we evaluate the identification of Nocardia isolates using direct spotting and an updated database. Overall, 82 Nocardia isolates belonging to 13 species were identified by DNA sequence analysis of the 16S rRNA and secA1 genes. Nine of these well-characterized isolates from 6 Nocardia species were used to create an in-house library. The remaining 73 isolates were directly spotted on the target plate and on-plate protein extraction was performed. The protein spectra obtained were analyzed by MALDI-TOF MS using the BDAL database (Bruker Daltonics) updated with 6,903 MSPs or the combination of this commercial database and our in-house library. As a result, the use of the commercial database alone and in combination with the in-house library yielded 94.5% and 95.9% of correct species-level identifications, respectively, No isolate was misidentified at the genus level with either database. Besides, the use of the in-house library allowed the species-level identification of a N. otitidiscaviarum isolate that could only be identified at the genus-level with a score value <1.6 using the commercial database. In conclusion, the implementation of the direct spotting method and the in-house database provided a high rate of correct species assignment of Nocardia isolates despite the low number of isolates added. Further addition of well-characterized Nocardia isolates may ensure the rapid, accurate and inexpensive identification of most isolates encountered in the routine of the microbiology laboratory.

Nocardia zhihengii sp. nov., an actinobacterium isolated from rhizosphere soil of Psammosilene tunicoides

A Nocardia-like actinobacterial strain, designated YIM TG2190^T, was isolated from rhizosphere soil of Psammosilene tunicoides collected from Gejiu, Yunnan province, China. The cells of strain YIM TG2190^T were observed to be Gram-stain positive and non-motile. The strain forms extensively branched substrate mycelia that fragments into rod-shaped elements. The 16S rRNA gene sequence analysis showed that strain YIM TG2190^T is closely related to Nocardia nova (97.5%), Nocardia jiangxiensis (97.1%) and Nocardia miyunensis (96.8%). Growth occurs at 4-30 °C (optimum 28 °C), pH 6.0-8.0 (optimum pH 7.0) and the strain can tolerate NaCl (w/v) up to 3% (optimum 0-1%). The cell walls were found to contain meso-diaminopimelic acid. The whole-cell sugars were identified as glucose, mannose, ribose, galactose, arabinose and fucose. The polar lipids were identified as diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol mannosides, phosphatidylglycerol and an unidentified phospholipid. The menaquinones detected were MK-9 (H₂) and MK-8 (H₄). The major fatty acids (> 5%) were found to be C_16:0 (33.9%), summed feature 3 (21.7%), C_18:0 10-methyl TBSA (13.7%) and C_18:1ω9c (7.0%). The DNA G+C content was determined to be 61.1 mol%. DNA-DNA relatedness between the strain YIM TG2190^T and N. nova CGMCC 4.1705^T, N. jiangxiensis CGMCC 4.1905^T and N. miyunensis CGMCC 4.1904^T were 46.9 ± 2.6, 36.8 ± 1.3, and 35.7 ± 2.6%, respectively, values which are less than the threshold value (70%) for the delineation of prokaryotic genomic species. The phenotypic, chemotaxonomic and phylogenetic data indicates that strain YIM TG2190^T represents a novel species of the genus Nocardia, for which the name Nocardia zhihengii sp. nov. is proposed. The type strain is YIM TG2190^T (=KCTC 39596^T = DSM 100515^T).

Role of FNA and Special Stains in Rapid Cytopathological Diagnosis of Pulmonary Nocardiosis

BACKGROUND

Nocardia, a gram-positive aerobic bacillus of the Actinomycetales family, is a significant opportunistic pathogen in immunocompromised individuals. Clinical and radiological features of pulmonary nocardiosis are nonspecific and can be misdiagnosed as tuberculosis, pneumocystis, staphylococcal or fungal infections, or as malignancy. Aspiration cytology with special stains is a quick and effective approach for accurate diagnosis.

MATERIALS AND METHODS

We present 7 cases of pulmonary nocardiosis, admitted to the pathology department in a tertiary-care hospital in Punjab. Clinical findings, immune status, laboratory tests, chest radiographs, and computed tomography scans were reviewed. Cytologically, special stains like 1% Ziehl-Neelsen (ZN), 20% ZN, periodic acid-Schiff (PAS), Grocott methenamine silver (GMS), and reticulin stains were studied along with May-Grünwald Giemsa, Papanicolaou, and hematoxylin and eosin.

RESULTS

All the patients were immunocompromised. The radiological changes were nonspecific. Cytomorphology showed acute and chronic inflammatory infiltrates with necrosis. None of the cases showed well-defined granulomas. GMS, modified 1% ZN and, Gordon and Sweet reticulin stains highlighted the delicate filamentous bacteria in all cases. PAS and 20% ZN stain for tuberculous bacilli were uniformly negative.

CONCLUSION

FNAC can provide a quick and accurate diagnosis of nocardiosis and thereby facilitate timely medical management.

Nocardia infection over 5 years (2011-2015) in an Italian tertiary care hospital

This study was conducted reviewing clinical records of 14 patients affected by nocardiosis over 5 years in a tertiary care hospital. Nocardia abscessus was responsible for one third of infections, deviating significantly from the results reported by other epidemiological investigations and highlighting the key role of molecular identification tests. Indeed, a precise identification of species is crucial for the determination of antibiotic sensitivity patterns and, consequently, for the choice of antibiotic treatment. Noteworthy, 40% of isolates of N. abscessus (formerly N. asteroides complex) showed resistance to carbapenems, which are usually recommended for empirical therapy.

Relationship between the duration of trimethoprim/sulfamethoxazole treatment and the clinical outcome of pulmonary nocardiosis

BACKGROUND

Despite treatment, pulmonary nocardiosis, which is a rare opportunistic disease caused by Nocardia species, has poor clinical outcomes including recurrence and death. Currently, the treatment regimen and duration for pulmonary nocardiosis are not fully understood. The present study aimed to clarify the factors related to the clinical outcome of pulmonary nocardiosis.

METHODS

The medical records of 24 patients with pulmonary nocardiosis were retrospectively reviewed. The patients were divided into two groups based on the outcomes within 2 years: patients with controlled disease (n = 14) and patients who developed recurrence or died (n = 10).

RESULTS

Nocardia was identified by 16S ribosomal RNA sequencing in 17 patients (70.8%) and by conventional biochemical test in five patients (20.8%). The patients' characteristics, clinical findings, radiological features, and treatment history were not different between the two groups. Compared with patients who developed recurrence or died, those with controlled disease had significantly longer total duration of treatment with antibiotics, especially trimethoprim/sulfamethoxazole (67.5 ± 111.6 days vs. 9.0 ± 6.5 days; p = 0.01). Pancytopenia was the most frequent adverse effect of trimethoprim/sulfamethoxazole.

CONCLUSIONS

Longer duration of trimethoprim/sulfamethoxazole treatment was significantly associated with better outcomes of pulmonary nocardiosis. In such cases, antibiotics, especially trimethoprim/sulfamethoxazole, should be administered for more than 3 months.

Epidemiology and susceptibility to antimicrobial agents of the main Nocardia species in Spain

Objectives

The aims of this study were to explore the clinical distribution, by species, of the genus Nocardia and to assess the antimicrobial susceptibilities of the 10 most prevalent species identified in Spain.

Methods

Over a 10 year period (2005-14), 1119 Nocardia strains were molecularly identified and subjected to the Etest. The distribution and resistance trends over the sub-periods 2005-09 and 2010-14 were also examined.

Results

Of the strains examined, 82.9% belonged to the following species: Nocardia cyriacigeorgica (25.3%), Nocardia nova (15.0%), Nocardia abscessus (12.7%), Nocardia farcinica (11.4%), Nocardia carnea (4.3%), Nocardia brasiliensis (3.5%), Nocardia otitidiscaviarum (3.1%), Nocardia flavorosea (2.6%), Nocardia rhamnosiphila (2.6%) and Nocardia transvalensis (2.4%). Their prevalence values were similar during 2005-09 and 2010-14, except for those of N. abscessus , N. farcinica and N. transvalensis , which fell significantly in the second sub-period ( P ≤  0.05). The major location of isolation was the respiratory tract (∼86%). Half (13/27) of all strains from the CNS were N. farcinica . Significant differences in MIC results were recorded for some species between the two sub-periods. According to the CLSI's breakpoints, low resistance rates (≤15%) were recorded for seven species with respect to cefotaxime, imipenem and tobramycin; five species showed similar rates with respect to trimethoprim/sulfamethoxazole. Linezolid and amikacin were the most frequently active agents.

Conclusion

The accurate identification of the infecting species and the determination of its susceptibility to antimicrobial agents, given the large number of strains with atypical patterns, are crucial if patients with nocardiosis are to be successfully treated.

Granulomatous Thyroiditis: A Case Report and Literature Review

BACKGROUND

Granulomatous disease in the thyroid gland has been linked to viral, bacterial and autoimmune etiologies. The most common granulomatous disease of the thyroid is subacute granulomatous thyroiditis, which is presumed to have a viral or post-viral inflammatory cause. Bacterial etiologies include tuberculosis, actinomycosis, and nocardiosis, but are extremely rare. Disseminated actinomycosis and nocardiosis more commonly affect organ-transplant patients with the highest susceptibility within the first year after transplant surgery.

CASE

A 45-year-old African American male, who received his third kidney transplant for renal failure secondary to Alport Syndrome, presented with numerous subcutaneous nodules and diffuse muscle pain in the neck. Further workup revealed bilateral nodularity of the thyroid. Fine needle aspiration of these nodules demonstrated suppurative granulomatous thyroiditis. Subsequent right thyroid lobectomy showed granulomatous thyroiditis with filamentous micro-organisms, morphologically resembling Nocardia or Actinomyces.

CONCLUSION

Disseminated granulomatous disease presenting in the thyroid is very rare, and typically afflicts immune-compromised patients. The overall clinical, cytologic and histologic picture of this patient strongly points to an infectious etiology, likely Nocardia, in the setting of recent organ transplantation within the last year.

[Disseminated nocardiosis caused by Nocardia nova with brain abscesses and osteomyelitis in an immunocompetent patient]

INTRODUCTION

Nocardia is an opportunist bacteria involved in patients with cellular immunodepression or chronic lung disease. The most frequent portals of entry are the respiratory tract by inhalation or direct inoculation through a cutaneous effraction. Nocardiosis may be localised or disseminated.

CASE REPORT

We report a rare case of disseminated nocardiosis to Nocardia nova with pulmonary, cutaneous, cerebral attacks and femoral osteomyelitis. The diagnosis was confirmed by prolonged cultures of the bronchoalveolar fluid and the pus extracted from a cutaneous lesion. The outcome was favorable under adapted and prolonged antibiotherapy with imipenem and amikacine and then cotrimoxazole.

CONCLUSION

This observation is original because it involves a immunocompetent patient with an association of two exceptional locations for N. nova: brain abscesses and femoral osteomyelitis. Screening for cerebral involvement should be systematic, even in the lack of neurological signs. An adapted and prolonged antibiotherapy must be conducted.

Identification of Nocardia, Streptomyces, and Tsukamurella using MALDI-TOF MS with the Bruker Biotyper

Nocardia species are the most commonly isolated aerobic actinomycetes from human clinical specimens. Our objective was to assess the identification of clinically relevant actinomycetes using the Bruker Biotyper MALDI-TOF system, including comparison of extraction methods, Biotyper library versions, score cutoffs, and media. Banked Streptomyces (n=10), Tsukamurella (n=2), and Nocardia isolates (n=60) were cultured and extracted using three methods: mycobacterial extraction, ethanol formic acid extraction, or direct on-target extraction. Following MALDI-TOF analysis, spectra were analyzed using versions 5 and 6 of the BDAL Biotyper library. Optimal species-level identifications for Nocardia were achieved using BDAL v6 at a score cutoff of ≥1.8 after direct extraction (49/60, 82%). Overall, the Biotyper platform with BDAL v6 accurately identified 12/16 species of Nocardia, demonstrating the utility of MALDI-TOF for identification of clinically relevant actinomycetes without the need for supplementation of the database.

Isolation, biochemical and molecular identification of Nocardia species among TB suspects in northeastern, Tanzania; a forgotten or neglected threat?

BACKGROUND

Pulmonary nocardiosis mimic pulmonary tuberculosis in most clinical and radiological manifestations. In Tanzania, where tuberculosis is one of the major public health threat clinical impact of nocardiosis as the cause of the human disease remains unknown. The objective of the present study was to isolate and identify Nocardia isolates recovered from TB suspects in Northeastern, Tanzania by using biochemical and molecular methods.

METHODS

The study involved 744 sputum samples collected from 372 TB suspects from four periphery diagnostic centers in Northeastern, Tanzania. Twenty patients were diagnosed as having presumptively Nocardia infections based on microscopic, cultural characteristics and biomèrieux ID 32C Yeast Identification system and confirmed using 16S rRNA and hsp65 gene specific primers for Nocardia species and sequencing.

RESULTS

Biochemically, the majority of the isolates were N. asteroides (n = 8/20, 40%), N. brasiliensis (n = 4/20, 20%), N. farcinica (n = 3/20, 15%), N. nova (n = 1/20, 5%). Other aerobic actinomycetales included Streptomyces cyanescens (n = 2/20, 10%), Streptomyces griseus, Actinomadura madurae each (n = 1/20, 5%). Results of 16S rRNA and hsp65 sequencing were concordant in 15/17 (88. 2%) isolates and discordant in 2/17 (11.8%) isolates. Majority of the isolates belonged to N. cyriacigeorgica and N. farcinica, four (23.5%) each.

CONCLUSIONS

Our findings suggest that Nocardia species may be an important cause of pulmonary nocardiosis that is underdiagnosed or ignored. This underscores needs to consider pulmonary nocardiosis as a differential diagnosis when there is a failure of anti-TB therapy and as a possible cause of human infections.

[Brain abscesses caused by Nocardia farcinica in a kidney transplanted man]

Nocardiosis is primarily an opportunistic infection caused by aerobic Gram-positive bacteria of the genus Nocardia. In this case report we describe a male patient who previously received a kidney transplant and was admitted to hospital with chills, headache and pain in the neck and left eye, tremor and coordination problems. A magnetic resonance scan of the brain showed multiple abscesses, and blood culture was positive for Nocardia farcinica. Nocardiosis of the central nervous system is a rare, but serious differential diagnosis in immunosuppressed patients with signs of high intracranial pressure.

Dual color fluorescence in situ hybridization (FISH) assays for detecting Mycobacterium tuberculosis and Mycobacterium avium complexes and related pathogens in cultures

Two rapid dual color fluorescence in situ hybridization (FISH) assays were evaluated for detecting M. tuberculosis and related pathogens in cultures. The MN Genus-MTBC FISH assay uses an orange fluorescent probe specific for the Mycobacterium tuberculosis complex (MTBC) and a green fluorescent probe specific for the Mycobacterium and Nocardia genera (MN Genus) to detect and distinguish MTBC from other Mycobacteria and Nocardia. A complementary MTBC-MAC FISH assay uses green and orange fluorescent probes specific for the MTBC and M. avium complex (MAC) respectively to identify and differentiate the two species complexes. The assays are performed on acid-fast staining bacteria from liquid or solid cultures in less than two hours. Forty-three of 44 reference mycobacterial isolates were correctly identified by the MN Genus-specific probe as Mycobacterium species, with six of these correctly identified as MTBC with the MTBC-specific probe and 14 correctly as MAC by the MAC-specific probe. Of the 25 reference isolates of clinically relevant pathogens of other genera tested, only four isolates representing two species of Corynebacterium gave a positive signal with the MN Genus probe. None of these 25 isolates were detected by the MTBC and MAC specific probes. A total of 248 cultures of clinical mycobacterial isolates originating in India, Peru and the USA were also tested by FISH assays. DNA sequence of a part of the 23S ribosomal RNA gene amplified by PCR was obtained from 243 of the 248 clinical isolates. All 243 were confirmed by DNA sequencing as Mycobacterium species, with 157 and 50 of these identified as belonging to the MTBC and the MAC, respectively. The accuracy of the MN Genus-, MTBC-and MAC -specific probes in identifying these 243 cultures in relation to their DNA sequence-based identification was 100%. All ten isolates of Nocardia, (three reference strains and seven clinical isolates) tested were detected by the MN Genus-specific probe but not the MTBC- or MAC-specific probes. The limit of detection for M. tuberculosis was determined to be 5.1x10⁴ cfu per ml and for M. avium 1.5x10⁴ cfu per ml in liquid cultures with the respective MTBC- and MAC-specific probes in both the MN Genus-MTBC and MTBC-MAC FISH assays. The only specialized equipment needed for the FISH assays is a standard light microscope fitted with a LED light source and appropriate filters. The two FISH assays meet an important diagnostic need in peripheral laboratories of resource-limited tuberculosis-endemic countries.