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Journal of Clinical Microbiology, September 2005, p. 4349-4356, Vol. 43, No. 9
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.9.4349-4356.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Erasmus MC University Medical Center Rotterdam, Department of Medical Microbiology & Infectious Diseases, Dr. Molewaterplein 40, 3015 GD Rotterdam, The Netherlands,1 Keygene N.V., Department of Microbial Genomics, Agro Businesspark 90, 6708 PW Wageningen, The Netherlands,2 PathoFinder BV, Canisius-Wilhelmina Hospital, Weg door Jonkerbos 100, 6532 SZ Nijmegen, The Netherlands,3 Mycetoma Research Group, Institute of Endemic Diseases, and Faculty of Medical Laboratory Sciences, University of Khartoum, Khartoum, Sudan4
Received 11 February 2005/ Returned for modification 31 March 2005/ Accepted 2 June 2005
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Mycetoma can be caused by a great variety of microorganisms, but these are not evenly distributed throughout the "mycetoma belt" (17). In Sudan the most frequently encountered causative organism is Madurella mycetomatis. M. mycetomatis is a slowly growing fungus which forms a dark, sterile mycelium (4). Only two reports on genetic variability or the lack thereof in M. mycetomatis isolates have appeared. In a report published by Lopes et al., it was shown that random amplification of polymorphic DNA (RAPD) and restriction endonuclease assays differentiated M. mycetomatis strains from different countries (14). In contrast, a large set of clinical M. mycetomatis isolates obtained from Sudan showed little genetic variation based upon classical high-throughput RAPD tests and PCR-restriction fragment length polymorphism analysis tests, and the species was identified as a clonal organism (2). Neither of these studies presented genetic markers which could be used to generate epidemiologically relevant information or taxonomic frameworks. Another microbial DNA-based typing method is the amplification of restriction fragments (AFLP) technique, a selective restriction fragment amplification method which establishes the absence or presence of DNA restriction sites by means of selective PCR (19, 21). Genomic DNA is completely digested with two restriction enzymes, after which double-stranded adaptors are ligated to the resulting DNA fragments. The resulting fragments are then amplified by using primers complementary to the adaptor and restriction site sequences. To limit the number of amplified fragments, selective nucleotides can be added at the 3' ends of the primers (19, 21). AFLP is a useful technique for the differentiation of strains within a species, even when they are clonal (19). The aim of the current study was to test whether the AFLP technique could differentiate clinical M. mycetomatis isolates obtained from mycetoma patients in various regions of Sudan. Another important question addressed was whether genetic differences among M. mycetomatis isolates could be used to link isolates with demographic and clinical characteristics.
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Antifungal susceptibility. MICs were determined after 7 days by using the colorimetric Sensititre YeastOne method (Trek Diagnostic Systems, Ltd., East Grinstead, England), as reported elsewhere (20).
DNA isolation. DNA was isolated as described before (2).
AFLP analysis. AFLP analysis was performed as described before (21). In short, DNA was restricted with the endonucleases EcoRI and MseI. After restriction, adaptors were ligated to the resulting fragments. The resulting fragments were preamplified with primers E (5'-GACTGCGTACCAATTC-3') and M (5'-GACGATGAGTCCTGAGTAA-3'), after which a selective PCR was performed. The selective primers were identical to primer E or M but were extended with selective dinucleotides at the 3' terminus. Two primer combinations were used: primers E12 and M12 and primers E20 and M12. Primers E12 and M12 were extended with AC, and primer E20 was extended with GC. Primers E12 and E20 were radioactively labeled, and the amplified material was analyzed on 4.5% polyacrylamide slab gels. The presence or absence of markers is scored in a table, which could be transformed into a dendrogram with the program NTsys (Exeter Software, Sekautet, N.Y.).
Sequencing and primer design. Selected markers were excised from the gel and reamplified with the following primers: 5'- AGCGGATAACAATTTCACACAGGACACACTGGTATAGACTGCGTACCAAT-3' and 5'-GACGATGAGTCCTGAGTAA-3'. These PCR fragments were sequenced, aligned, and compared to each other and to other sequences in the National Center for Biotechnology Information data bank by use of the BLASTN 2.2.8 and BLASTX 2.2.8 programs (5). Internal primers were designed for screening purposes (Table 1).
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TABLE 1. Madurella mycetomatis polymorphisms detected by AFLP and the PCR primers under condititions designed for detection of each individual polymorphism
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Statistical analysis. Associations between fungal DNA polymorphisms, demographics, and disease characteristics were tested for significance by Fisher's exact test (two sided). The association of genetic features with the geographic origin of the strain was studied by comparing each region with all others. The association with the size of the lesion (small, medium, or large) was tested by the chi-square test for trend. Correlations with age, disease duration, and MICs to antifungals were tested by the Mann-Whitney test (two tailed). All statistical calculations except for the adjustment of the lesion size for disease duration were performed with GraphPad InStat, version 3.00 (GraphPad InStat Software, Inc., San Diego, Calif.). Adjustment of the lesion size for disease duration was done by linear regression analysis with SPSS, release 10.1.0 (SPSS, Inc., Chicago, Ill.).
Nucleotide sequence accession numbers. The sequences reported here are deposited in the GenBank database under accession numbers AY918172 (fragment A10), AY918173 (fragment A4), AY918174 (fragment A5), AY918175 (fragment A7), AY918176 (fragment A11), AY918177 (fragment A12), AY918178 (fragment B3), and AY918179 (fragment B4).
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FIG. 1. (A) Areas of Sudan sampled for clinical Madurella mycetomatis isolates. Sudan is divided into central Sudan (C), western Sudan (W), northern Sudan (N), and Khartoum (K). (B) Phylogenetic tree based on the AFLP data obtained for 39 clinical black grain mycetoma fungal isolates. Thirty-seven of these isolates are Madurella mycetomatis. Strain mm3 represents the fungus Leptosphaeria senegalensis, and strain mm27 represents a fungal species belonging to the M. mycetomatis cluster without precise species definition. The M. mycetomatis strains are divided into clusters I, II, and III. For each isolate the geographic origin and the size of the lesion are presented. Strains originated from central Sudan (C), western Sudan (W), northern Sudan (N), Khartoum (K), and Mali (M). The lesion size is designated as small (size 1), moderate (size 2), and large (size 3). U, unknown.
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TABLE 2. Comparison of ITS sequences of mm27 and mm3 with those of true Madurella mycetomatis isolatesa
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Two of the strains used in this study, strains mm72 and mm73, were isolated from a female patient with two independent large lesions, one on the sole of the foot and one on the knee. The lesion on the sole of the foot had been there for over 13 years, while the lesion on the knee joint was just 4 years old. Figure 1 shows that both isolates are found in cluster I and were closely related but not identical. Strains mm33 and mm44 appeared to be 100% identical by AFLP analysis. Those strains originated from two different central Sudanese patients (a 24-year-old male with a moderate lesion and a 28-year-old female with a large lesion) belonging to the same tribe. They had been infected for 2 years, which could imply that they were infected with the same strain originating from somewhere in that area. In cluster II, strains mm46 and mm50 had exactly the same AFLP banding pattern. Those two strains derived from central Sudanese patients (a 28-year-old male and a 35-year-old male). Those two patients had both been infected for 1 year and had only small lesions.
The M. mycetomatis strains were divided into two main clusters: clusters I and II (Fig. 1). Minor cluster III consists of only one strain (mm83) which originated from western Sudan and which caused a large lesion. Cluster I and cluster II have several distinctive features, but the most striking one is that in cluster I, fragment A12 is largely absent and fragment B3 is largely present, while in cluster II this is the other way around. Seventy-five percent of the strains found in cluster I originated from central Sudan, while only 45.8% of the strains in cluster II originated from central Sudan. The other 54.2% of the strains encountered in this cluster were divided as follows: 16.7% of the strains originated from Khartoum, 12.5% originated from northern Sudan, 8.3% originated from western Sudan, and 4.2% (n = 1) originated from Mali. Data on lesion size are displayed next to the phylogenetic tree in Fig. 1; and it appeared that among the strains in cluster I, only one strain caused a small lesion. All other strains in cluster I caused moderate (41.7%) or massive (41.7%) lesions. Among the strains in cluster II, half of the strains caused small lesions, while 12.5% and 33.3% of the strains caused moderate and large lesions, respectively. This may point to differences in virulence between the strains from the two main clusters.
DNA sequencing of polymorphic markers. Of 93 amplimers, a total of 25 markers were polymorphic in the M. mycetomatis strains. Thirteen of these markers were only seen in one to three strains, while the rest of the markers were seen in at least 15% of the strains. Eleven of these more common polymorphic markers were reamplified and sequenced. No useful sequence could be obtained from four of these fragments, probably because mixtures of DNA fragments were excised from the gel. After BLASTN and BLASTX analysis, it appeared that the sequences obtained for fragments A12 and B3 were 97% identical. A tribase substitution and two deletions were the only differences obtained, suggestive of gene duplication or heterozygostiy. Fragment A7 showed the highest homology (66% identity) with an endo-1,4-beta-glucanse gene from Aspergillus oryzae. Fragment B4 showed similarity with a casein kinase 1 isoform delta gene. The highest homologies for this gene, namely, 55%, were obtained with the species Danio rerio. The closely related fragments A12 and B3 matched a gene for a noncharacterized hypothetical protein from Gibberella zeae. The other four fragments showed no significant homology with any other known sequence.
Development of PCR tests. Because no genome data are available for M. mycetomatis, internal PCR primer pairs were designed for all of the sequences obtained. PCR with the novel primers for fragments A4, A5, A7, A12, and B3 resulted in equally sized amplicons for all strains except mm3 and mm27. Size-variable amplicons were obtained for fragments A10, A11, and B4. As stated in Table 3, PCR for fragments A11 and B4 resulted in positive PCR signals only for those strains in which the original AFLP fragment was present. For fragment A10, all strains gave a positive PCR signal, but in the strains in which the original AFLP fragment was not present, the PCR products were smaller. This size difference still allowed the discrimination of different types.
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TABLE 3. Presence of AFLP amplimers versus PCR amplicons
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TABLE 4. Correlations of Madurella mycetomatis strain genetic clusters and polymorphic markers and various demographics, disease characteristics, and antifungal susceptibility
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FIG. 2. (A) Distributions of susceptibility to the following antifungal agents for strains in both clusters I and II: amphotericin B (AMB), fluconazole (FLZ), itraconazole (ITZ), ketoconazole (KTZ), voriconazole (VCZ), and flucytosine (FC). Each bar represents the number of strains (indicated at the top of the bar) with an MIC at the concentration listed on the x axis for the antifungal shown on the y axis. For example, in cluster I there are three strains with an MIC of 1 mg/liter for amphotericin B. (B) MIC distributions for amphotericin B for fragments A4 and B4. The numbers of strains for a certain MIC are shown for the two fragments. The strains for which a fragment is present are indicated with black bars; the strains for which this fragment is absent are indicated with white bars.
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In the past AFLP was demonstrated to be a valuable technique for the typing of various fungal species, such as Aspergillus fumigatus and Cryptococcus neoformans (12, 22). For M. mycetomatis we showed that AFLP is also valuable for the differentiation of this species. Despite the remarkable clonality found before by RAPD analysis (2), AFLP was able to discriminate these same strains into three clusters. This suggests that AFLP is a much stronger technique than RAPD analysis for the discrimination of M. mycetomatis strains. With AFLP the M. mycetomatis isolates could be divided into two main clusters and one minor cluster. Cluster I mainly consisted of strains that originated from central Sudan and that caused moderate or large lesions, while cluster II was more heterogeneous. Although there was an association with the lesion size and the duration of the disease, the association found between cluster I and larger lesions still remained intact after adjustment for disease duration. As a matter of fact, the lesion size is probably defined on the basis of individual host-pathogen interactions. With the AFLP conditions used, it was also possible to discriminate between two M. mycetomatis isolates obtained from the same patient but from different lesions.
At least 4 of the 12 polymorphic markers were actually part of coding regions, and 2 of these (A12 and B3) were not previously identified. Marker A7 was homologous to the gene for endo-1,4-beta-glucanase, an important enzyme involved in cellulose degradation (11, 18). Polymorphic marker B4, homologous to casein kinase 1 delta, was primarily detected in strains isolated from Central Sudanese patients. Casein kinase 1 is thought to play a physiological role in activating transcription of various DNA repair genes, in intracellular trafficking, and in normal cell cycle progression (10, 13, 16, 23).
In the present study, MICs were determined by using the Sensititre YeastOne system instead of using the modified CLSI (formerly NCCLS) method for M. mycetomatis. It has been demonstrated that the results obtained with this system are in good agreement with those obtained by the modified NCCLS method. The rates of agreement ranged from 91.2 to 100.0%, depending on the antifungal tested (20). The reproducibility of this test system was also satisfactory, ranging from 88.2 to 97.1% (20). We found an association between susceptibility to amphotericin B and both markers B4 and A4. The distribution of MICs of strains with and without the B4 and A4 markers differed significantly (P = 0.02), but the difference was very small. The median for the groups differed only by a single twofold dilution, which is allowed as background variation in antifungal susceptibility testing. Furthermore, the median MICs for each group were in the treatable range and are therefore probably not clinically relevant. The association found in this study could be a chance finding and not reproducible in other strain collections. Further study of the relevance of this finding is needed.
In conclusion, the AFLP method differentiates clinical isolates of M. mycetomatis. AFLP clusters I and II are associated with different clinical presentations, with cluster I strains apparently causing larger lesions. An AFLP marker sequence with casein kinase 1 homology seemed to be associated with the geographical origin of the fungal isolate. We present the first pathogenicity markers for a fungal species that still has a devastating socioeconomic effect on small communities in rural Sudan.
AFLP is a registered trademark of Keygene NV, and the AFLP technology is covered by patents (US 6,045,994 and EP 0 534 858 B1) and patent applications owned by Keygene NV.
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