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Applied and Environmental Microbiology, March 2007, p. 1851-1857, Vol. 73, No. 6
0099-2240/07/$08.00+0     doi:10.1128/AEM.01722-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Development of a New, Combined Rapid Method Using Phage and PCR for Detection and Identification of Viable Mycobacterium paratuberculosis Bacteria within 48 Hours

Emma C. Stanley,¹ Richard J. Mole,² Rebecca J. Smith,³ Sarah M. Glenn,³ Michael R. Barer,³ Michael McGowan,⁴ and Catherine E. D. Rees^1*

Division of Food Sciences, School of Biosciences, Sutton Bonington Campus, University of Nottingham, Sutton Bonington LE12 5RD, United Kingdom,¹ Biotec Laboratories, 32 Anson Road, Marltesham Heath, Ipswich IP5 3RG, United Kingdom,² Department of Infection, Immunity and Inflammation, University of Leicester, Medical Sciences Building, PO Box 138, University Rd., Leicester LE1 9HN, United Kingdom,³ The University of Queensland, School of Veterinary Science, Brisbane, Queensland, Australia⁴

Received 21 July 2006/ Accepted 28 December 2006

Top Abstract Introduction Materials and Methods Results Discussion References

 
The FASTPlaqueTB assay is an established diagnostic aid for the rapid detection of Mycobacterium tuberculosis from human sputum samples. Using the FASTPlaqueTB assay reagents, viable Mycobacterium avium subsp. paratuberculosis cells were detected as phage plaques in just 24 h. The bacteriophage used does not infect M. avium subsp. paratuberculosis alone, so to add specificity to this assay, a PCR-based identification method was introduced to amplify M. avium subsp. paratuberculosis-specific sequences from the DNA of the mycobacterial cell detected by the phage. To give further diagnostic information, a multiplex PCR method was developed to allow simultaneous amplification of either M. avium subsp. paratuberculosis or M. tuberculosis complex-specific sequences from plaque samples. Combining the plaque PCR technique with the phage-based detection assay allowed the rapid and specific detection of viable M. avium subsp. paratuberculosis in milk samples in just 48 h.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Mycobacterium avium subsp. paratuberculosis is the causative agent of Johne's disease, a degenerative wasting disease that primarily infects ruminants, including domestic livestock, such as cattle, sheep, and goats. In the dairy industry, Johne's disease leads to considerable economic losses due to a decrease in milk production and reduced fertility in infected cattle (16, 17). Due to the possible link between M. avium subsp. paratuberculosis and the development of Crohn's disease in humans (4, 15), M. avium subsp. paratuberculosis has also become of interest as a public health issue despite the fact that a causal link between the organism and human disease has not been unequivocally established. A recent survey in the United Kingdom has shown that viable M. avium subsp. paratuberculosis cells can be cultured from pasteurized retail milk (10), indicating that either M. avium subsp. paratuberculosis may occasionally survive high-temperature, short-time pasteurization treatments or postprocess contamination is occurring.

Both efforts to manage Johne's disease and studies of heat resistance of M. avium subsp. paratuberculosis have been hampered by the lack of rapid, specific detection tests for viable M. avium subsp. paratuberculosis cells. An ideal M. avium subsp. paratuberculosis detection test would be low cost, rapid, and specific and provide live/dead differentiation. Culture is currently regarded as the definitive method for the detection of viable M. avium subsp. paratuberculosis bacteria; however, M. avium subsp. paratuberculosis is an extremely fastidious organism and requires the longest incubation periods of all the mycobacteria cultured to date (6 to 16 weeks). Additionally, harsh chemical decontamination of samples is required to suppress growth of competitive microorganisms, which can reduce the sensitivity of culture to detect M. avium subsp. paratuberculosis (6). PCR assays have been successfully used to detect the presence of M. avium subsp. paratuberculosis based on the amplification of the sequences IS900 (9), F57 (5, 18), and more recently the newly identified multicopy ISMAP02 sequence (22). However, the PCR assays alone cannot provide live/dead differentiation. Reverse transcription (RT)-PCR assays can overcome this limitation to some extent by amplifying IS900 mRNA from M. avium subsp. paratuberculosis cells, demonstrating that gene transcription is taking place (14), but RT-PCR methodology is difficult to apply to the routine testing of bacteria in food samples or other complex sample matrices due to difficulty in extracting cells from the matrix and the carry-through of contaminants that inhibit DNA amplification (11). A variety of immunological tests have been developed that indirectly detect M. avium subsp. paratuberculosis infection by assaying species-specific cell-mediated or humoral immune responses in the host. Immune response assays are generally rapid and easy to perform and are a useful tool for determining M. avium subsp. paratuberculosis infection at the herd level. However, these assays are often limited by issues of specificity and the fact that a variable immunological response is seen during different stages of infection (see reference 25 for a review).

The FASTPlaqueTB assay (FPTB) is an established tool for the rapid detection of Mycobacterium tuberculosis and is used in developing countries as an inexpensive, rapid diagnostic aid for the detection of viable M. tuberculosis cells in sputum (for a review, see reference 19). This type of assay (termed the phage amplification or PhaB assay) is based on the successful replication of phage to indicate the presence of viable host cells, with the end point of the assay being the formation of plaques. In outline, samples containing the M. tuberculosis cells are mixed with phage and are incubated to allow infection. At this point a virucide is added to destroy any phage that have not infected cells, and so only those phage protected by the host cell survive. To detect these protected phage, the sample is mixed with Mycobacterium smegmatis and mixed with agar to form a lawn, and plates are incubated overnight. Lysis of the infected target cell releases phage, which then form plaques by infection of the M. smegmatis cells. Each plaque represents the presence of a mycobacterial cell capable of being infected by the phage in the original sample, and in human sputum this is most likely to be M. tuberculosis. A key part of this assay is the fact that the mycobacteriophage used (D29) has a host range broader than just M. tuberculosis and also infects other types of mycobacterial cells, and therefore, we wished to investigate whether the FPTB assay reagents could also be used for the detection of M. avium subsp. paratuberculosis. To increase the specificity, the phage amplification assay was combined with a PCR-based identification method for molecular confirmation of the cell detected. Here we describe the successful development of this combined phage-PCR method, which gives both live/dead differentiation and molecular characterization of the cell detected, and show that it can be extended for the detection and identification of other mycobacteria from samples containing a mixed flora without the need to remove competitive microflora before detection.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Bacterial strains and culture media.
The mycobacteria strains used in this study are listed in Table 1. M. avium subsp. paratuberculosis cells were cultured on Herrold's egg yolk medium with mycobactin J (37°C for 12 to 16 weeks). M. tuberculosis was cultured on Middlebrook 7H10 oleic acid-albumin-dextrose-catalase agar plates (37°C for 3 to 4 weeks); Mycobacterium bovis bacillus Calmette-Guérin (BCG) cells were cultured (37°C for 3 to 4 weeks) on Löwenstein-Jensen slopes with glycerol (MAST Diagnostics). M. smegmatis cells were grown in Middlebrook 7H9 oleic acid-albumin-dextrose-catalase broth (50 ml; Difco Laboratories) with shaking (37°C for 48 h at 200 rpm).

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TABLE 1. Bacteria and insertion sequences

Phage detection assays.
The FPTB assay reagents were prepared and used according to the manufacturer's instructions (Biotec Laboratories Ltd., Ipswich, United Kingdom). Bacterial samples were prepared by recovering mycobacterial cells from one slope of solid culture medium and directly resuspending these in 1 ml FPTB Media Plus. Actiphage (100 µl containing 10⁸ PFU mycobacteriophage) was added to each test sample and incubated (37°C, 1 h) to allow infection. The sample was then thoroughly mixed with Virusol (100 µl; virucidal solution) and incubated (room temperature, 5 min) to inactivate any remaining extracellular phage. Virusol activity was neutralized by addition of FPTB Media Plus (5 ml) and sensor cells (1 ml; 10⁸ CFU M. smegmatis) and of molten FPTB agar (5 ml, 55°C). Samples were poured into a petri dish and incubated (37°C, 18 h). The presence of mycobacteria in the test sample was indicated by the presence of plaques. Positive and negative assay controls supplied with the kit were included with each analysis, according to the manufacturer's instructions.

Preparation of genomic and plaque DNA.
For M. avium subsp. paratuberculosis, M. smegmatis, and M. bovis BCG genomic DNA, a loopful (10 µl) of bacteria was resuspended in high-performance-liquid-chromatography-grade sterile water (1 ml), heated (100°C, 20 min), and then centrifuged (13,000 x g, 15 min) and the supernatant (5 µl) used for PCR amplification. M. tuberculosis H37Rv genomic DNA was a gift of the Department of Microbiology, Immunology and Pathology, Colorado State University. Plaque DNA samples were prepared by excising an agar plug (approximately 10 µl) from the center of a plaque using a sterile polypropylene 1,000-µl micropipette tip. The agar plug was transferred to high-performance-liquid-chromatography-grade sterile water (10 µl), heated (95°C, 5 min), and immediately frozen (–20°C). After a minimum of 15 min at this temperature, samples were thawed and 10 µl of supernatant was used per PCR.

PCR conditions.
Primers used for the amplification of signature sequences were P90 and P91 (IS900 [26]), JB21 and JB22 (500-bp M. bovis gene fragment [21]), TB850 and TB284 (IS6110 [27]), and BW6 and BW7 (IS1081 [24]). PCRs were carried out in a Techne PCR block (Progene) in a reaction volume of 50 µl using ABgene 10x buffer, 250 µM of each deoxyribonucleotide triphosphate (Promega), 2.5 mM MgCl₂, 1 U of Taq polymerase (ABgene), and 250 ng primer. For duplex or multiplex PCR, 3 mM MgCl₂ was used and each primer was used at 150 ng. Amplification conditions used were initial denaturation at 94°C for 5 min, followed by 37 cycles of 94°C for 30 s, 62°C for 30 s, and 72°C for 1 min, with a final extension step of 72°C for 4 min. PCR products were visualized after agarose gel (2% [wt/vol]) electrophoresis by staining with ethidium bromide (0.33 µg ml^–1) and UV illumination. PCR products were sequenced by the University of Nottingham, Sutton Bonington Campus sequencing unit.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Detection of M. avium subsp. paratuberculosis using the FPTB assay.
To determine if the FPTB assay could be used to detect M. avium subsp. paratuberculosis, a cell suspension of M. avium subsp. paratuberculosis ATCC 19851 was 10-fold serially diluted in FPTB Media Plus and samples tested with the FPTB assay. From the optical density of the suspension, it was predicted that approximately 1 x 10⁷ CFU ml^–1 were present and that numbers in the countable range would be reached at the 10^–5 dilution. Plaques were detected only at the end point of the assay when M. avium subsp. paratuberculosis cells were added, indicating that the phage was able to infect M. avium subsp. paratuberculosis cells, and the number of plaques detected (57 at the 10^–5 dilution [Table 2]) correlated with the expected number of M. avium subsp. paratuberculosis cells in the test sample. When samples containing heat-killed M. avium subsp. paratuberculosis cells (1 x 10⁷ CFU ml^–1 M. avium subsp. paratuberculosis ATCC 19851 heat treated at 100°C for 20 min) were tested, no plaques were formed, indicating that the M. avium subsp. paratuberculosis cells must be viable for detection by the FPTB assay. This finding indicated that the current FPTB assay format and components were suitable for the detection of viable M. avium subsp. paratuberculosis cells.

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TABLE 2. Comparison of the M. avium subsp. paratuberculosis cell numbers and plaques recorded

PCR from plaques generated by the FPTB M. avium subsp. paratuberculosis assay.
To add specificity to this assay, we investigated whether the mycobacterial cell giving rise to an individual plaque could be identified by PCR. Primers P90 and P91 (26) were chosen to amplify a 400-bp fragment of the multicopy IS900 gene found in all M. avium subsp. paratuberculosis strains (Tables 1 and 2). These primers were chosen because the IS900 method is a well-established identification method for M. avium subsp. paratuberculosis and because it was more likely that a multicopy gene could be detected in the DNA originating from a single cell. After M. avium subsp. paratuberculosis cells had been processed through the FPTB assay, DNA was extracted by excising agar from the center of a plaque and heating at 95°C. Samples were then frozen at –20°C for a minimum of 15 min, since this was found to give the most efficient separation of the DNA from the agar. Samples were thawed at room temperature, and the supernatant was used as the template for the PCR.

Four different strains of M. avium subsp. paratuberculosis were processed through the assay to generate plaques containing M. avium subsp. paratuberculosis DNA (Fig. 1). A PCR product of 400 bp for the target IS900 gene was amplified from plaque DNA extracted from all four M. avium subsp. paratuberculosis strains tested (Fig. 1, lanes 2 to 13). Sequence analysis of the PCR products was used to confirm that the PCR products arose from the specific amplification of the IS900 gene. The amount of PCR product achieved for the different M. avium subsp. paratuberculosis strains tested varied and may reflect either variation in the copy number of the IS900 sequence (generally strains contain between 14 and 18 copies of IS900) or the propensity of the M. avium subsp. paratuberculosis cells to clump (hence plaques may arise from a group of cells rather than a single cell). As expected, the amount of product amplified from DNA extracted from the plaques was generally smaller than that of the control reaction carried out using genomic DNA as the template (Fig. 1, lane 15). As a negative control, M. smegmatis cells were processed through the FPTB assay and plaque DNA tested by PCR for the presence of IS900 sequences. In this case no PCR product was generated (Fig. 1, lane 14).

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FIG. 1. Amplification of IS900 sequences from plaque DNA. M. avium subsp. paratuberculosis strains ATCC 19851, 377 pw, B4, DVL 943, and M. smegmatis were processed through the FPTB assay. Plaque DNA was extracted from four plaques for each M. avium subsp. paratuberculosis strain and for M. smegmatis. Samples were then randomized, and PCR amplification of IS900 was performed using primers P90 and P91. PCR products (30 µl) were analyzed by agarose gel electrophoresis (2% [wt/vol]). After PCR amplification, samples were decoded and grouped by sample type before gel analysis. Lanes: 1 and 17, 100-bp DNA size markers; 2 to 4, M. avium subsp. paratuberculosis ATCC 19851; 5 to 7, M. avium subsp. paratuberculosis B4; 8 to 10, M. avium subsp. paratuberculosis DVL 943; 11 to 13, M. avium subsp. paratuberculosis pw 377; 14, M. smegmatis; 15, M. avium subsp. paratuberculosis ATCC 19851 genomic DNA (positive control); 16, sterile distilled water (negative control). Expected band size for the P90 and P91 primers is 400 bp.

The finding that the DNA of the original M. avium subsp. paratuberculosis cell was preserved in the plaque after overnight incubation was somewhat surprising. To rule out the possibility that we were simply detecting uninfected M. avium subsp. paratuberculosis cells carried through the assay into the agar, M. avium subsp. paratuberculosis cells were processed through the assay and agar plugs extracted both from plaques and also from the lawn of the same plates in areas where no plaques were present (n = 18 for each sample type). DNA was extracted from both types of sample and used as a template for PCR amplification of the IS900 sequence. The 400 bp-band was amplified from all the plaque DNA samples as expected, and no PCR products were generated from the PCR using agar plugs extracted from the lawn area (data not shown). This finding confirmed that the amplification of the 400-bp region of the IS900 gene was from the M. avium subsp. paratuberculosis progenitor cell within the plaque and was not due to any uninfected M. avium subsp. paratuberculosis cells present in the surrounding agar.

Specificity of the plaque PCR.
A blind trial was performed to test the reproducibility and reliability of the IS900 plaque PCR assay in that the identities of the samples being tested were not known until after the PCR results were available. The sample group consisted of 16 M. avium subsp. paratuberculosis plaques (4 plaque samples from 4 different M. avium subsp. paratuberculosis strains) and 4 plaques originating from M. smegmatis cells. The operator was blinded to the origins of the samples until after the results interpretation. A total of 20 plaques were tested by PCR for the presence of the IS900 gene, and only the 16 plaque samples originating from M. avium subsp. paratuberculosis cells gave a positive PCR result. Besides this blind trial, when using the combine phage-PCR assay, we achieved figures of 100% reproducibility and 100% specificity overall (n = 73) (i.e., samples containing M. avium subsp. paratuberculosis cells were always plaque positive, and IS900 PCR products were achieved only when M. avium subsp. paratuberculosis cells were present).

Differentiation of M. avium subsp. paratuberculosis from M. tuberculosis complex (TB complex).
Having demonstrated that the target organism in the plaque can be identified by PCR, we next attempted to develop a duplex PCR to differentiate between M. avium subsp. paratuberculosis and M. bovis, since these two mycobacteria may also cause disease in cattle. For detection of M. bovis, primers JB21 and JB22, which target a single copy 500-bp gene fragment from M. bovis (21), were chosen. For detection of M. avium subsp. paratuberculosis, the IS900 primers (P90 and P91) were used again.

Reaction conditions for duplex PCR were optimized using genomic DNA, and it was shown that the 400-bp IS900 fragment from M. avium subsp. paratuberculosis ATCC 19851 and the 500-bp fragment from M. bovis BCG NCTC 5692 could be amplified, either individually or in combination (Fig. 2, lanes 2 to 4), showing no interference by either primer set. However, when M. bovis BCG plaque DNA was use as the template for the duplex PCR, no PCR product was generated, while the M. avium subsp. paratuberculosis plaque samples were PCR positive (Fig. 2, lanes 5 to 7). The failure of the JB21 and JB22 primers to amplify a single-copy gene from M. bovis BCG plaque DNA may indicate that single-copy genes are below the limit of the PCR identification step.

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FIG. 2. Duplex PCR amplification of plaque DNA samples. Purified genomic DNA was used as a template to optimize duplex PCR conditions for the P90-P91 and JB21-JB22 primer pairs (lanes 2 to 4). For plaque samples, M. avium subsp. paratuberculosis strain ATCC 19851, M. bovis BCG, and M. smegmatis were processed through the FPTB assay. DNA was extracted from plaques for each strain, and duplex PCR amplification was performed (lanes 5 to 9). For mixed plaque samples, DNAs from two separate plaques were combined in one tube, and then 10-µl samples of the mixture were added to the PCR. Lanes 1 and 10, 100-bp DNA size marker; 2, M. avium subsp. paratuberculosis and M. bovis BCG genomic DNA; 3, M. avium subsp. paratuberculosis genomic DNA; 4, M. bovis BCG genomic DNA; 5, mixed M. avium subsp. paratuberculosis and M. bovis BCG plaque DNA; 6, M. avium subsp. paratuberculosis plaque DNA; 7, M. bovis BCG plaque DNA; 8, M. smegmatis plaque DNA; 9, sterile distilled water (negative control). Expected band sizes are 400 bp for the M. avium subsp. paratuberculosis-specific IS900 sequence and 500 bp for the M. bovis-specific sequence.

As an alternative strategy, a multiplex PCR based on multicopy insertion sequences from the TB complex was designed (Table 1). Using a combination of the three sets of primers (P90 and P91 for M. avium subsp. paratuberculosis [IS900], TB850 and TB284 [IS6110 {27}], and BW6 and BW7 [IS1081 {24}] for TB complex bacteria), multiplex PCRs were performed using either genomic DNA or plaque DNA (Fig. 3). The same PCR conditions were used as those optimized for the IS900 PCR, but MgCl₂ concentrations were increased to 3 mM due to the higher primer concentrations. The multiplex PCR was able to amplify all three insertion sequence fragments from genomic DNA from the respective mycobacterial species with or without the presence of competing DNA from the other cell types (Fig. 3, lanes 2 to 5). No nonspecific PCR products were seen, indicating that nonspecific priming was not occurring when unrelated mycobacterial DNA was present (Fig. 3, lane 2). However, when genomic DNA from M. smegmatis was used as a template, a 200-bp fragment was amplified (lane 6). Each primer pair was tested individually with the M. smegmatis genomic DNA template, and it was found that this band was seen only when using the IS1081 primers (data not shown). BLAST analysis of the IS1081 primers against the partially annotated M. smegmatis MC2 155 genomic sequence (TIGR) showed that primer BW6 could be aligned with a sequence at position 6962740 with a match of 21 out of 23 bases, suggesting that a mispriming is possible; however, no homology with the BW7 sequence primer was found. Sequence analysis of the amplified fragment did not show homology with any of the M. smegmatis sequence currently in the database; however, a complete genome sequence for this organism is not yet available, and the sequence amplified may be specific to the strain supplied in the FPTB kit. While we could not positively determine the origin of the 200-bp band, it fortuitously provides us with an internal positive control for the combined phage-multiplex PCR test, since M. smegmatis DNA is always present in plaque DNA samples.

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FIG. 3. Multiplex PCR amplification of M. avium subsp. paratuberculosis and TB complex sequences. Purified genomic DNA was used as a template to optimize the multiplex PCR conditions for the P90-P91 (IS900), TB850-TB284 (IS6110), and BW6-BW7 (IS1081) primer pairs (lanes 2 to 6). For mixed DNA samples, equal amounts of purified genomic DNA were mixed together and a sample (5 µl) of this used as template. For plaque samples, M. avium subsp. paratuberculosis strain ATCC 19851, M. bovis BCG, M. tuberculosis, and M. smegmatis were processed through the FPTB assay. DNA was extracted from plaques for each strain, and duplex PCR amplification was performed (lanes 6 to 9). For mixed plaque samples, DNAs from two separate plaques were combined in one tube, and then samples (10 µl) of the mixture were added to the PCR. Lanes: 1 and 12, 100-bp DNA size marker; 2, mixed M. avium subsp. paratuberculosis and M. bovis BCG genomic DNA; 3, M. avium subsp. paratuberculosis genomic DNA; 4, M. bovis BCG genomic DNA; 5, M. tuberculosis genomic DNA; 6, M. smegmatis genomic DNA; 7, mixed M. avium subsp. paratuberculosis and M. bovis BCG plaque DNA; 8, M. avium subsp. paratuberculosis plaque DNA; 9, M. bovis BCG plaque DNA; 10, M. tuberculosis plaque DNA; 11, M. smegmatis plaque DNA. Expected band sizes are 400 bp (IS900), 550 bp (IS6110), and 306 bp (IS1081). The 200-bp band is generated by amplification of an uncharacterized sequence from the M. smegmatis genome by the BW6-BW7 primer pair.

The multiplex PCR was then carried out using M. tuberculosis, M. bovis BCG, and M. avium subsp. paratuberculosis plaque DNA either alone or in combination. The appropriately sized PCR products were amplified from M. avium subsp. paratuberculosis (400 bp), M. bovis BCG, and M. tuberculosis plaques (550 bp and 306 bp), respectively (Fig. 3, lanes 7 to 11). In addition, as expected, the 200-bp band was amplified from M. smegmatis DNA present in all DNA plaque samples. Only the 306-bp product from the IS1081 element was amplified from M. bovis BCG plaque DNA (Fig. 3, lane 9). This is in contrast to the result using genomic DNA (Fig. 3, lane 4), where both the IS1081 and IS6110 bands were amplified. However, M. bovis BCG is known to contain only one to two copies of the IS6110 element, and again this points to the fact that the PCR identification step is not able to reliably amplify a single-copy genomic sequence from the progenitor cell in a plaque from the FPTB assay. The 300-bp product was successfully amplified from the M. tuberculosis plaque, consistent with the fact that they are reported to contain 18 to 20 copies of the IS6110 element (Fig. 3, lane 10).

Detection of M. avium subsp. paratuberculosis in naturally infected milk samples.
To determine whether the assay could be used to detect M. avium subsp. paratuberculosis in milk recovered from naturally infected animals, a small trial was carried out with a herd which was confirmed to be infected with Johne's. Individual milk samples were collected from cows showing signs of weight loss and/or scouring and then chilled on ice before being transported to the laboratory. Bacterial cells were sedimented (2,500 x g, 15 min), the supernatant removed by aspiration, and the pellet resuspended in 1 ml of Media Plus before directly proceeding with the Phage amplification assay without any further decontamination steps. After overnight incubation, some growth of competitive microflora could be seen in the lawn, although this was not sufficient to obscure the development of plaques. Milk from 10 out of the 15 animals tested was plaque positive in the assay (Table 3). Plaques were then picked and DNA extracted for PCR analysis and in each case IS900 was detected in some of the plaques. In general, those samples with the highest numbers of plaques had the highest percentages of IS900-positive samples. The fact that some plaques were not positive for IS900 indicates that mycobacteria other than M. avium subsp. paratuberculosis were also present in the samples and demonstrates the requirement for the combined phage-PCR test to increase the specificity of the phage-based assay.

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TABLE 3. Detection of M. avium subsp. paratuberculosis in naturally infected samples

Top Abstract Introduction Materials and Methods Results Discussion References

 
Studies of the physiology and ecology of M. avium subsp. paratuberculosis are limited by its fastidious nature and slow growth. When isolating the organism from samples containing a complex microflora, harsh decontamination methods are required to prevent overgrowth during the extended periods of incubation. Molecular tests, such as RT-PCR, can be used to detect M. avium subsp. paratuberculosis and offer live/dead differentiation but are also limited by the need to decontaminate and further purify samples to remove inhibitors of the PCR. This again leads to a loss of sensitivity and makes the assays both time consuming and complex. In this work we describe an adaptation of the FPTB assay for the detection of viable M. avium subsp. paratuberculosis cells. The assay is rapid and simple, detecting and identifying M. avium subsp. paratuberculosis in 48 h using standard laboratory reagents and equipment.

Here we have used four bovine M. avium subsp. paratuberculosis strains from different sources, including those from a natural infection, and in each case the FPTB assay was able to detect the presence of viable cells. In addition to the strains used in the development of this test, we have also shown that the sequence strain K10, originally isolated from bovine feces, can also be detected using this assay. It will be interesting to extend the range of M. avium subsp. paratuberculosis isolates tested to include ovine strains; however, given the fact that the broad host range of the phage is an essential feature of the test to amplify the phage using a fast-growing Mycobacterium, it seems likely that they too will be sensitive to the phage infection. Indeed, although the assay was developed for the detection of TB complex bacteria, other studies have previously reported the use of the phage amplification assay to detect other pathogenic mycobacteria (2, 3).

This study is the first report that actiphage (D29) infects M. avium subsp. paratuberculosis. This result is in contrast to a previous report by Foley-Thomas et al. (8) that D29 did not form plaque on a lawn of freshly grown M. avium subsp. paratuberculosis K10 cells. The problem here may have been the slow rate of M. avium subsp. paratuberculosis lawn development, leading to a failure to form plaques rather than an inability of the phage to infect. When using the FPTB assay, only one round of phage replication is dependent on M. avium subsp. paratuberculosis, with subsequent rounds of replication occurring in the fast-growing M. smegmatis used to generate the lawn. Therefore, the FPTB assay could be used in studies of mycobacteriophage to determine the infection range with other slow-growing mycobacteria, circumventing the need to generate lawns of the host cell. This would be of particular relevance for difficult-to-culture mycobacteria, such as Mycobacterium leprae, and this assay could be used to determine if phage-based genetic tools (see reference 20) could be used in the study of such organisms.

Carrying out viable count measurements for M. avium subsp. paratuberculosis is problematic due to clumping of cells and contamination of samples during extended periods of incubation, and therefore, a quantitative comparison of plaque number and CFU was not carried out. However, based on an estimate of cell mass by optical density, the number of plaques recorded was always in the expected range, indicating that the phage infected all these strains with the same efficiency, but further work will be needed to determine whether this is true for all M. avium subsp. paratuberculosis strains. Comparison of the efficacy of the FPTB assay and culture techniques would be necessary to determine any difference in the detection of viable M. avium subsp. paratuberculosis by the two techniques. By using killed cells in the assay, we have demonstrated that only viable cells are detected. This is in accordance with the use of the FPTB assay to determine sensitivity to antibiotics (FPTB multidrug resistance assay), where cells that are killed by antibiotic treatment do not give rise to plaques (1, 7, 13). Taken together, this work suggests that this assay provides a rapid and simple way to quantify viable cell numbers.

In the development of this assay, we have used the IS900 multicopy element for the PCR confirmation of M. avium subsp. paratuberculosis cells. While some controversy exists concerning the specificity of this as a signature sequence for M. avium subsp. paratuberculosis, it is still the most widely used molecular diagnostic test. In the future, amplification of the ISMAP02 element may increase the specificity of this assay (22). The fact that these elements are present in multicopies is beneficial for the amplification of DNA from single-cell samples, where losses may occur during the DNA extraction process. Ultimately the development of more-efficient DNA extraction procedures or the incorporation of nested PCR procedures to increase sensitivity may be possible and allow the reproducible amplification of single-copy genes, such as F57 (5, 18). Originally the focus of this work was the identification of M. avium subsp. paratuberculosis alone. However, samples from infected animals, such as milk, could potentially contain more than one Mycobacterium species, most notably M. bovis. By combining the phage detection method with PCR amplification, the assay now gives both live/dead differentiation and genotypic identification of cells. This is particularly relevant to the detection of M. avium subsp. paratuberculosis in unpasteurized milk, where other pathogenic mycobacteria may also be found.

Development of the multiplex PCR allows the identity of the cell giving rise to the plaque to be established using only one PCR test (M. avium subsp. paratuberculosis/TB complex differentiation). The finding that the IS6110 element was not amplified from M. bovis BCG was not surprising, since this strain is known to contain only one copy of the gene (12), and again supports our conclusion that amplification of single-copy genes is currently beyond the limit of detection for plaque DNA samples. Wild-type M. bovis strains are known to contain variable numbers of this element, with the majority of them having six; however, isolates of M. bovis with higher numbers of this element have been characterized (23). Therefore, amplification of both bands may be expected when testing clinical samples, and so the absence of the IS6110 band cannot be used to differentiate between M. bovis and the rest of the TB complex. Of the four members of the TB complex, only M. bovis and M. tuberculosis would be expected to be isolated from bovine unpasteurized milk in the United Kingdom. The remaining members of the TB complex, Mycobacterium microti and Mycobacterium africanum, are isolated predominantly from rodents suffering from tuberculosis and human tuberculosis patients in Africa, respectively. Therefore, even though the level of molecular identification achieved is only within the TB complex, this is unlikely to be of practical significance within Europe or America. However, irrespective of the exact species of bacterium detected, the presence of TB complex bacteria in milk samples is likely to warrant further investigation of the herd. Hence, we have developed a generically useful test that can now be applied for the identification of either M. avium subsp. paratuberculosis or TB complex from milk samples, and further work is being undertaken to develop the test further and to carry out a full evaluation of its use both in the routine testing of milk samples and in clinical applications.

 
This work was funded by the BBSRC and Biotec Laboratories Ltd.

We thank I. Grant, Queen's University Belfast, for the donation of M. avium subsp. paratuberculosis strains and the Department of Microbiology, Immunology and Pathology, Colorado State University, for the donation of the M. tuberculosis genomic DNA.

 
* Corresponding author. Mailing address: Division of Food Sciences, School of Biosciences, Sutton Bonington Campus, University of Nottingham, Sutton Bonington LE12 5RD, United Kingdom. Phone: 44 115 9516167. Fax: 44 115 9516162. E-mail: cath.rees{at}nottingham.ac.uk.

Published ahead of print on 26 January 2007.

Top Abstract Introduction Materials and Methods Results Discussion References

 

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Applied and Environmental Microbiology, March 2007, p. 1851-1857, Vol. 73, No. 6
0099-2240/07/$08.00+0     doi:10.1128/AEM.01722-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

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• Altic, L. C., Rowe, M. T., Grant, I. R. (2007). UV Light Inactivation of Mycobacterium avium subsp. paratuberculosis in Milk as Assessed by FASTPlaqueTB Phage Assay and Culture. Appl. Environ. Microbiol. 73: 3728-3733 [Abstract] [Full Text]  

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