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Journal of Clinical Microbiology, May 2005, p. 2201-2206, Vol. 43, No. 5
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.5.2201-2206.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Division of Microbiology & Infectious Diseases, Western Australian Centre for Pathology and Medical Research (PathCentre), Nedlands, Western Australia 6009, Australia
Received 24 November 2004/ Returned for modification 30 December 2004/ Accepted 4 January 2005
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These problems and the rarity of melioidosis outside of northern Australia and Southeast Asia highlight the need for a more standardized culture-based diagnostic pathway. In the present study we evaluated phenotypic identification methods used in Australia to develop a laboratory case definition of melioidosis. This approach aims to integrate previously recommended screening tests with one or more confirmatory tests to complete the identification of suspect isolates. Some centers have reported the development of an in-house B. pseudomallei agglutination test (10, 11, 12). An alternative approach uses bacterial fatty acid methyl ester (FAME) profile analysis by gas-liquid chromatography (GLC) to detect a cellular fatty acid profile that distinguished B. pseudomallei from B. thailandensis (5). In the current study we compared the performance of a proprietary B. pseudomallei monoclonal antibody agglutination test, FAME profile analysis, and a widely used substrate utilization panel (API 20NE) with the PCR-based identification technique used in this center since 1998 to confirm presumptive B. pseudomallei identification.
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TABLE 1. Bacteria used in this study
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Resuscitation. Bacteria were resuscitated by subculture onto 5% horse blood agar and incubation for 24 h at 37°C. A single blood agar plate was used for each isolate, and stock cultures were spread to produce single-colony growth in the third or fourth quadrant. A single colony was then subcultured onto 5% horse blood agar and incubated at 37°C for 24 h to produce pure growth for all subsequent identification tests. Plates were labeled with the relevant culture collection reference code and no other identifying mark.
Identification procedures. Each of the identification procedures used was performed by a single operator for consistency, without reference to the culture collection accession list. Results were recorded in laboratory diaries or worksheets, and the completed work was entered into a master list.
Initial screening investigations. Preliminary screening investigations were used to screen bacterial isolates as previously recommended (3). Gram stains were performed on a thin-smear preparation of fresh culture emulsified in sterile 0.89% NaCl solution (ELP, Bentley, WA). Crystal violet, Lugol's iodine, acetone, and dilute carbol fuchsin were used. Gram reaction, bacterial shape, and the presence or absence of intracellular granules were noted. Oxidase tests were performed by spreading a linear smear from the single colony on filter paper impregnated with oxidase reagent. To be considered positive, a strong purple reaction was required in the paper before 10 s had elapsed. Pseudomonas aeruginosa was used as a positive control. Gentamicin and colistin susceptibilities were determined by demonstration of a zone of inhibition around a 10-µg disk of each agent (Oxoid, Heidelberg, Australia) according to National Accrediting Agency for Clinical Laboratory Sciences Kirby-Bauer disk susceptibility test criteria (9).
Confirmatory identification tests:. A B. pseudomallei-specific agglutination test was performed on each isolate using a B. pseudomallei monoclonal antibody preparation (All Eights Sdn. Bhd., Kuala Lumpur, Malaysia) according to the manufacturer's instructions except that the volumes of emulsified culture and antibody suspension were 4 µl and were dispensed by automatic displacement pipette with sterile disposable tips. New, acid-washed glass microscope slides were used for agglutination reactions, and a new slide was started after each positive test result. B. pseudomallei NCTC 13177 was used for the positive control. Positive-control isolates and all negative isolates were retested after a further 24 h incubation. All agglutination reactions were conducted in a class II biological safety cabinet by gowned and gloved staff.
API 20NE. Substrate utilization tests were performed using the API 20NE panel (Biomerieux, Marcy L'Etoile, France) according to the manufacturer's instructions. The suspension of each isolate was subcultured onto a 5% horse blood agar plate on completion of API inoculation, and gentamicin and colistin disks were added to ensure that there had been no contamination. API test panels were incubated in air in a temperature-controlled incubator at 30°C ± 1°C. The first three tests in each panel were read after a timed 24-h incubation period. The assimilation tests were read after a timed 48 h. All tests were scored against the interpretive chart in the instruction sheet using the manufacturer's interpretive color chart. Results were entered into the current version of the API interpretive software to obtain the final profile. No panel test results were altered to obtain a more meaningful or acceptable identification. Substrate utilization tests were repeated when purity plates showed more than one colony type, suggesting contamination or mixed growth.
Cellular fatty acid analysis. FAME profile analysis was performed on the fatty acid methyl ester derivative of bacterial suspensions using a fine capillary column gas chromatograph (MIDI Systems Inc., Wilmington, DE), according to the manufacturer's instructions, and a previously reported protocol (5). Repeat determinations were performed on all negative results with twice the recommended quantity of bacteria. The method used tryptic soy broth agar (ELP, Bentley, WA). The manufacturer's interpretive software was used to recognize retention time peaks consistent with 2-hydroxymyristic acid (2HMA).
B. pseudomallei PCR. A single colony of B. pseudomallei grown on blood agar was resuspended in deionized water, treated with diethylpyrocarbonate to remove nucleases. The suspension was heated at 100°C for 15 min and centrifuged at 9,000 x g to pellet the cell debris. The supernatant was used as the template for all subsequent seminested PCR assays. The PCR primers used for identification were as described previously (7). Briefly, the first-round primers were bp1 (5'-CGATGATCGTTGGCGCTT) and bp4 (5'-CGTTGTGCCGTATTCCAAT), and the seminested second-round primers were bp1 and bp3 (5'-ATTAGAGTCGAACAAT). The first-round mix consisted of 0.5 units of Taq polymerase (Applied Biosystems, Foster City, CA), 2 µl of buffer, 0.2 mM of pooled deoxynucleoside triphosphates, 1.5 mM of MgCl2, and 0.2 µM each of primers bp1 and bp4 (product = 302 bp). To this mix 8 µl of template DNA was added, giving a total volume of 20 µl. The second-round mix was identical to the first round but for use of primers bp1 and bp3 (product = 285 bp). To this mix 0.4 µl of first-round product was added, giving a total volume of 20.4 µl. The first-round cycling program consisted of a pre-PCR of 5 min at 94°C to fully denature the template DNA followed by 45 cycles of 30 s at 94°C for denaturation, 30 s at 55°C annealing, and 45 s at 72°C extension. The samples were maintained at 72°C for a further 7 min following the final cycle. After inoculation of first-round product into the second-round tubes, cycling was carried out under the same conditions as for the first round but with a 50°C annealing temperature. Second-round PCR products were demonstrated by ethidium bromide gel electrophoresis on 2.5% agar gels. Digital gel images were captured and optimized for brightness and intensity using a UVIdoc capture system (Cambridge, UK).
Sequencing. Sequencing was carried out using the previously mentioned first-round product and primers bp1 and bp4 for B. pseudomallei NCTC 13177 (second-round PCR product positive control) and B. thailandensis BCC 89 (second-round PCR product negative control). The PCR products were treated with presequencing clean-up enzyme (ExoSap-It USB Corp., Cleveland, Ohio) and then used as the template in a sequencing mix (Applied Biosystems, BigDye terminator v3.1). The now labeled products of the sequencing reaction were then filter purified using Microcon PCR filters (Amicon Millipore, North Ryde, Australia) and sequenced in an Applied Biosystems 3100 Avant genetic analyzer.
Statistical methods. The statistical method and graphing functions used were the Fisher's exact test and the cumulative trend curves (Prism v2.01; GraphPad, San Diego, CA).
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Monoclonal antibody agglutination test. The strain used as a positive control (B. pseudomallei NCTC 13177) gave clear positive results visible behind safety cabinet glass with 4-µl volumes of reconstituted monoclonal antibody and bacterial suspension. The recommended volumes of 10 µl each produced too large a drop to safely handle on the glass microscope slide. A smaller volume of 2 µl each made the positive result difficult to read. A total of 86% of the B. pseudomallei isolates were positive on first agglutination test, and a further 8% were positive after another 24 h of incubation. Overall, 67 (94%) B. pseudomallei isolates were positive by agglutination test, and four other Burkholderia spp. were agglutination positive (Table 2). There were four persistently negative B. pseudomallei isolates: one was a mucoid environmental strain, two were clinical strains from sputa, and one was from a blood culture in three epidemiologically unrelated patients. One B. cepacia isolate found to be consistently agglutination positive had other features strongly suggesting incorrect identification by PCR (wrinkled colony surface, 2HMA positive). Repetition of B. pseudomallei-specific PCR produced a consistently negative result. The minimum time to completion of an agglutination test result was 5 min, including preparatory stages.
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TABLE 2. Confirmatory identification test results
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TABLE 3. Comparison of test resultsa
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Identification tests in combination. The cumulative performance of the combination of B. pseudomallei identification tests was evaluated in this study. The best and most timely approach was the combination of agglutination performed on two consecutive days and FAME-GLC (98%) (Table 4). The addition of the API 20NE result added nothing to the combination of agglutination test and GLC. When performed as the mainstay of identification the API 20NE both delayed the result and required the addition of an agglutination test to compare favorably with the alternative. The first combination of these methods approached the sensitivity of the PCR-based method, and it is notable that both the agglutination test and the FAME profile analysis identified one PCR-negative isolate as a possible B. pseudomallei isolate.
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TABLE 4. Identification test performance
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The preliminary identification methods used in this study were chosen to be inclusive of other Burkholderia species and closely related bacteria (3) and would have excluded only one isolate housed in our culture collection. Inclusion of polymyxin susceptibility results in the screening process, though not evaluated in this study, may help alert laboratory staff to the need to consider confirmatory identification procedures.
Given the greater gravity of a missed diagnosis of melioidosis, we placed more emphasis on the sensitivity of B. pseudomallei identification methods than on their specificity. We proceeded to confirm a presumptive identification of possible B. pseudomallei by a more definitive method. The conventional approach has been to use a substrate utilization panel such as the API 20NE, despite mixed reports on its performance (3, 6, 8). After an earlier report questioning the reliability of such test systems (6), the identification data bank appears to have expanded to include API panel results from additional B. pseudomallei strains. Some centers continued to use the API 20NE on the understanding that it had been improved or used the API 20NE alongside the API 20E as a reference point for other biochemical identification systems (8). However, the results of this study show that the API 20NE fails to reliably identify B. pseudomallei when used precisely according to manufacturer's instructions to identify an unknown Burkholderia sp. isolate from a diverse range of sources. In our hands, the probability of B. pseudomallei being given an incorrect bacterial identity was unacceptably high. We no longer consider the API 20NE to have a major part in preliminary B. pseudomallei identification. One study claiming a much higher sensitivity and specificity for the API 20NE used 48-h-old B. pseudomallei cultures (8), further extending the time taken to produce a definitive result. The higher specificity of the API 20NE substrate utilization panel suggests that it may be more suited to a supportive role in B. pseudomallei identification, once the possibility of the species has already been seriously considered. Modifications could then be made to the manufacturer's recommended procedure to improve the prospect of an accurate confirmatory result, supported by complementary confirmatory procedures.
By contrast, the rapid agglutination test correctly identified the majority of B. pseudomallei isolates. This test was quick to perform, producing a useful positive result on the same day as the presumptive screening tests, and had a high sensitivity. We used a commercially available reagent developed in Malaysia. Previous reports have been based on in-house reagents that have not yet been independently validated to the standard expected of commercially available diagnostic test kits (10, 11, 12). The lack of validated diagnostic reagents for either immunological or molecular diagnostic tests for B. pseudomallei has been recognized as a significant hindrance to reliable laboratory diagnosis of melioidosis (13). Our agglutination test results provide a reference point for future comparisons with in-house methods. However, the agglutination reaction was sometimes difficult to read behind the glass of a microbiological safety cabinet, possibly explaining why a small proportion of isolate results were positive after a further 24 h of incubation. It is notable that the test was reliably positive at just less than half the recommended volume of 10 µl. However, a lower sensitivity than the PCR identification method highlights the need for another confirmatory identification method. This observation places the B. pseudomallei agglutination test result somewhere between a presumptive and definitive identification. Of the tests considered in this study, only the B. pseudomallei agglutination test offered any prospect of shortening the time to culture-based diagnosis.
FAME profile analysis was more sensitive than the agglutination test but significantly better than the API 20NE. The longer processing time makes GLC analysis less suited to a routine confirmatory role. In combination with the agglutination test, GLC analysis was more effective as a supplementary confirmatory tool. Its additive contribution to B. pseudomallei identification suggests that GLC might have a supporting role to the agglutination test in centers that are unable or unwilling to use PCR methods for bacterial identification, particularly when the API 20NE has produced an unclear or equivocal result. We recognize that most centers with a FAME-GLC capability will also have bacterial identification PCR capability and may prefer to validate their in-house PCR protocol.
At present we use a PCR-based method to confirm one or another of the three phenotypic methods compared here, as indicated in the flow diagram (Fig. 1). In future it is possible that the greater reliability, shorter time to a result, simplicity, and lower cost of PCR may make it the preferred confirmatory method for B. pseudomallei identification. One method combination came close to the sensitivity but not the specificity of PCR. The observation of one PCR-negative, agglutination-positive, 2HMA-positive isolate indicates a need to approach PCR-based identification of B. pseudomallei with caution. Excessive reliance on specific PCR protocols for detection of human pathogens will leave a diagnostic laboratory vulnerable to unpredictable external factors such as genetic variation at the primer binding sites. Further work is needed on the use of PCR-based protocols to identify presumptive B. pseudomallei from clinical diagnostic media. As an interim measure, we propose that the B. pseudomallei discovery pathway depicted in Fig. 1 be used for identification of referred bacterial isolates that meet the recommended preliminary screening criteria of oxidase-positive, gentamicin- and polymyxin-resistant, Gram-negative bacilli.
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FIG. 1. Proposed discovery pathway for clinical laboratory identification of possible B. pseudomallei from primary or referred cultures.
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