INTRODUCTION

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Since the mid-1980s, mycobacterial infections have become increasingly widespread for a number of biological and social reasons, in particular, the human immunodeficiency virus epidemic. Together with the increasing incidence of tuberculosis, the incidences of Mycobacterium avium diseases and other nontuberculous mycobacterial infections have also increased (12, 20, 21). Rapid discrimination between M. tuberculosis and M. avium is of primary importance for the initiation of a correct chemotherapeutic regimen, because the two infections require different types of therapy and management (12, 13, 20).

The advent of PCR has been a breakthrough in the diagnosis of mycobacterial infections. A number of M. tuberculosis-specific sequences can now be amplified (3, 5, 10, 11, 18), and different PCR, restriction enzyme analysis, and hybridization assays have been developed for the diagnosis of M. avium complex infection (2, 4, 6-8, 16, 17, 22, 24-27, 30, 31). However, PCR methods are complicated, limited to few mycobacterial species, and restricted to research laboratories; in addition, the high cost of the commercial methods somewhat hampers the practical use of amplification diagnosis in routine analysis, mainly for different clinical samples.

The aim of this study was to develop a PCR-hybridization technique based on the amplification of a 16S rRNA gene sequence using pan-Mycobacterium primers followed by hybridization of the amplification products to M. tuberculosis- and M. avium-specific probes; this technique may be an alternative in-house method for species differentiation. Moreover, we evaluated the possible use of this method in routine mycobacteriosis diagnosis of Ziehl-Neelsen-positive smears obtained from clinical specimens.

	MATERIALS AND METHODS

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Strains and clinical specimens. The mycobacterial strains (11 American Type Culture Collection [ATCC] strains and 44 clinical isolates) and the nonmycobacterial strains (4 ATCC strains and 9 clinical isolates) listed in Table 1 were used to determine the coverage of the pan-Mycobacterium amplification and the specificity of the M. tuberculosis- and M. avium-specific primers. Hence, 38 acid-fast-bacillus (AFB)-positive smears from different clinical specimens (18 sputum, 9 lymph node biopsy, 6 stool, 2 pus, 2 urine, and 1 pericardial fluid specimens) with culture-confirmed infection by M. tuberculosis or M. avium were subsequently investigated by PCR-hybridization. Thirty AFB-negative smears (13 sputum, 10 lymph node biopsy, 4 pus, and 3 stool specimens) and 21 AFB-positive smears (17 sputum, 2 lymph node biopsy, and 2 pus specimens) obtained from patients affected by mycobacterioses other than those caused by M. tuberculosis and M. avium were also tested as negative controls (Table 2). Sample decontamination, Ziehl-Neelsen staining, cultural isolation, and identification were performed by standard methods (14, 15, 29).

Extraction of DNA from cultures and Ziehl-Neelsen-positive smears. Mycobacterial DNA was extracted from cultures in accordance with the method described by van Embden et al. (28). Stained microscopic preparations were washed in xylol and absolute ethanol, scraped with a sterile blade, and collected in a microcentrifuge tube containing phosphate buffer solution. The samples were centrifuged for 10 min at 13,000 rpm, and pellets were resuspended in 100 µl of lysis buffer (Tris-HCl, 10 mM; KCl, 50 mM; MgCl₂, 2.5 mM; Tween 20, 0.45%; Nonidet P-40, 0.45%; proteinase K, 10 mg/ml) and incubated for 3 h at 56°C or overnight at 37°C. The samples were then incubated for 15 min at 95°C and centrifuged for 15 min at 15,000 × g, and the supernatants were transferred to a new microcentrifuge tube and directly used for PCR.

PCR assay. The primers and probes were designed on the basis of the published 16S rRNA fragment sequence (2, 22). The first step of the PCR-hybridization assay was DNA amplification using a pair of pan-Mycobacterium primers able to amplify all mycobacterial species: M-OU-1 (5'-ATAAGCCTGGGAAACTGGGT-3' [positions 142 to 162]) and M-OL-2 (5'-CACGCTCACAGTTAAGCCGT-3' [positions 614 to 633]) (Genset, Paris, France). The amplification was performed in a total volume of 50 µl. The reaction mixture consisted of 5 pmol of primers, 200 µM deoxynucleosides triphosphates (dATP, dCTP, and dGTP), 190 µM dTTP, 2.5 U of Taq polymerase Gold (Perkin-Elmer, Norwalk, Conn.), 5 µl of 10× Taq polymerase buffer, 1.5 mM MgCl₂, 1 µg of target DNA (from either cultures or smears), and 10 µM digoxigenin (DIG)-11-dUTP (Boehringer GmbH, Mannheim, Germany). The cycling parameters were as follows: 10 cycles of annealing at 58°C for 1 min, elongation at 72°C for 90 s, and denaturation at 94°C for 1 min; 40 cycles of annealing at 58°C for 30 s, elongation at 72°C for 40 s, and denaturation at 94°C for 20 s; and a final annealing (58°C for 1 min) and elongation (72°C for 5 min) step.

Hybridization assay. After amplification, the DIG-labeled amplified product was hybridized with two biotinylated probes designed to differentiate between M. tuberculosis (5'-ACCACAAGACATGCATCCCG-3' [positions 182 to 201]) and M. avium (5'-ACCAGAAGACATGCGTCTTG-3' [positions 182 to 201]).

	RESULTS

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The PCR-hybridization assay was performed on 55 strains (11 ATCC strains and 44 clinical isolates) of different cultured mycobacteria (21 M. tuberculosis, 21 M. avium, and 13 other mycobacterial strains).

Amplification of the 484-bp fragment by pan-Mycobacterium primers was successful in all 55 mycobacterial isolates. No amplification of human DNA or DNA from 13 bacterial species other than mycobacteria was ever observed under these conditions, indicating the mycobacterial specificity of the primers. The M-OU-1 and M-OL-2 primers efficiently amplified DNA from all mycobacterial isolates, confirming their ability in the identification of all the Mycobacterium species (Table 1). The limit of detection of this method was approximately 100 fg of mycobacterial DNA (Fig. 1). Biotinylated M-TB and M-AV oligonucleotides, specific for M. tuberculosis and M. avium, respectively, were used as probes with 10 µl of mycobacterial DNA amplified with the M-OU-1 and M-OL-2 primers. Twenty-one M. tuberculosis strains were found to be positive when hybridized with the M. tuberculosis-specific probe (M-TB) and negative when hybridized with the M. avium-specific probe (M-AV). All 21 M. avium strains generated a positive signal with the M-AV probe and no signal with the M-TB probe. There was no cross-hybridization with any of the other mycobacterial strains tested, except for M. bovis. No cross-hybridization with 13 other bacterial species or human chromosomal DNA was observed (Table 3).

View larger version (39K): [in this window] [in a new window]   FIG. 1.   Detection limit for the PCR-hybridization assay, as estimated from serial dilutions of standard amounts of human DNA and mycobacterial DNA (M. tuberculosis H37Ra and M. avium ATCC 15769). O.D., optical density.

The detection limit of the hybridization assay was 10 fg of mycobacterial DNA (Fig. 1).

To test the possible use in clinical practice of the PCR-hybridization assay, 59 Ziehl-Neelsen-positive and 30 Ziehl-Neelsen-negative smears obtained from clinical samples were tested in parallel with conventional culturing. Amplification of the -globin gene segment was achieved in all 89 samples. PCR amplification with the M-OU-1 and M-OL-2 primers was successful in all of the 59 Ziehl-Neelsen-positive smears. The amplified products were hybridized with the DIG-labeled M-TB and M-AV oligonucleotides. Consistent with the results obtained from culture samples, the hybridization method applied to clinical specimens was positive exclusively with the corresponding species (specificity, 100%). The same was true for both microscopically strongly positive and paucibacillary specimens (sensitivity, 100%). The probes did not hybridize to the amplification products of any other mycobacterial species or mycobacterial culture-negative specimens. The results are summarized in Table 2.

	DISCUSSION

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Differentiation between M. tuberculosis and M. avium is essential for the diagnosis and treatment of mycobacterial infections in AIDS patients. Cultural isolation and identification of mycobacterial species usually are performed by time-consuming biochemical tests or by genetic probes, which are quite expensive. Several molecular genetic methods have also been recently reported. These include amplification of species-specific sequences, PCR amplification, restriction enzyme analysis, hybridization with species-specific oligonucleotide probes, and nucleic acid sequence determination. Thus, PCR methods could potentially provide very sensitive, specific, and rapid tests for the detection of mycobacteria. Over the years, several studies have been published proposing different amplification protocols directly feasible for clinical specimens, such as sputum, bronchial washings, and tissue biopsies, mostly for the identification of M. tuberculosis (1, 3, 9). However, no easy method is currently available for the detection and identification of M. tuberculosis and of M. avium. The direct identification of mycobacterial species is hampered by the poor performance of the methods and is often possible only in a few reference centers. Moreover, the wide use of commercial PCR methods still remains quite expensive for routine analysis, rendering impractical direct application to all samples based only on clinical suspicion.

In this study, we propose a simple, in-house nucleic acid amplification protocol that allows direct detection and species identification of M. avium and M. tuberculosis (the most relevant mycobacteria in the clinical setting) with DNA extracted from Ziehl-Neelsen-positive smears.

The good performance of the method that we developed for the extraction of DNA from smears is evidenced by the maintenance of intact template DNA and by the absence of PCR inhibitors, as shown by the results for the -globin internal control.

Our results show that the PCR-hybridization assay using M-TB and M-AV as probes specific for M. tuberculosis and M. avium, respectively, was highly sensitive and specific; correct identification of M. tuberculosis and M. avium organisms was achieved in all samples. In fact, 10 fg of nucleic acids, corresponding to approximately 3 organisms, yielded a clear-cut positive result. In parallel, the results for Ziehl-Neelsen-positive smears obtained from clinical specimens (sputum, lymph node, stool, pus, urine, and pericardial fluid) proved that our PCR-hybridization assay also allows direct identification of M. tuberculosis and M. avium even in paucibacillary specimens. The applicability of the method directly to DNA extracted from smears could become a valuable alternative approach for the routine diagnosis of mycobacterial infections when used with Ziehl-Neelsen-positive smears.

The PCR-hybridization method with Ziehl-Neelsen-positive smears coupled the advantage of the amplification techniques for rapid diagnosis and the concomitant identification of the different mycobacterial species, allowing the early establishment of the appropriate therapeutic regimens and the prompt isolation of infected patients.

Our method is relatively simple, rapid, and widely applicable in experienced clinical laboratories, and the fact that it is "home brew" means that it is relatively inexpensive. Moreover, limiting the use of the PCR-hybridization assay only to AFB-positive smears could reduce both the costs and the probability of false-positive PCR results. Our assay can be easily performed on stored specimens and therefore can be used in retrospective studies. The use of different specific probes allows direct etiological diagnosis rather than diagnosis by exclusion, as with the M. tuberculosis probe. In this work, we designed the two specific probes for the most clinically important mycobacterial species but, by using available software (GenBank), it is also possible to design specific probes within the 16S rRNA of almost all mycobacterial species and to adapt the screening panel to the species prevalence in each geographical area. However, no differentiation between members of the M. tuberculosis complex can be made, because of the 16S rRNA homology within these sequences.

Further studies will help to establish whether the PCR-hybridization technique can also be used to detect and identify M. tuberculosis and M. avium from particular biological samples (formalin-fixed, paraffin-embedded tissues) and especially from blood specimens at an early stage of growth, given the occurrence of bacteremia observed in immunocompromised populations, such as human immunodeficiency virus-infected subjects.