Introduction

The neglected tropical disease mycetoma presents itself as a subcutaneous chronic granulomatous inflammatory disease and is characterized by tumorous lesions and grain formation [1,2]. This disease can be caused by more than 70 different micro-organisms and is categorized into actinomycetoma (caused by bacteria) and eumycetoma (caused by fungi). Treatment is dependent on the causative agent.

Diagnosis of mycetoma is often only made clinically in endemic areas due to the scarcity of facilities, expertise and financial capacity. Eumycetoma and actinomycetoma can be easily distinguished from each other by the texture and colour of the grains. However, species identification based on the texture and colour of the grain is not possible as many fungal species can produce similar-looking black-grains. Therefore, identification of causative agents is usually done by histopathological examination and culturing of grains. Unfortunately, that often leads to misidentifications [3]. Currently, molecular identification is the most reliable method to identify eumycetoma causative agents down to the species level and the most commonly used technique is amplifying the Internally Transcribed Spacer (ITS) region and sequencing [4]. However, many endemic areas lack the ability to perform DNA sequencing.

In 1999, a species-specific PCR primer based on the internal transcribed spacer (ITS) was developed for Madurella mycetomatis, the most common causative agent of mycetoma [5]. This PCR is currently used at the Mycetoma Reference Center in Khartoum, Sudan. It is performed on DNA obtained from cultured clinical material or directly from grains obtained from patients. Unfortunately, this M. mycetomatis specific PCR primer pair 26.1a and 28.3a was only recently discovered to cross-react with Madurella pseudomycetomatis [6]. Madurella pseudomycetomatis was not yet described at the time when the M. mycetomatis specific PCR was developed [7]. All four Madurella species (Madurella fahalii, Madurella tropicana, M. mycetomatis and M. pseudomycetomatis) are known to cause black grain eumycetoma and have different susceptibilities to antifungal agents. For instance, M. fahalii is not inhibited by itraconazole in vitro, which could have consequences for treatment strategy [8]. This makes identification of causative agents to the species level a must. Since all four Madurella species share a very conserved ITS region, this has made designing PCR primers specific for M. mycetomatis based on that region difficult [8,9]. To circumvent this difficulty, we took a comparative genome approach to design a new set of specific primers for the identification of M. mycetomatis by PCR.

Here, we report a new set of diagnostic DNA primers for M. mycetomatis identified from the genome of M. mycetomatis [10].

Materials and methods

Primer design and PCR conditions

Forward and reverse PCR primers were designed according to the nucleotide sequence of the PPCS of interest. Primer sequences are depicted in Table 1. PCR reaction contained 0.6 units of Super Taq HC DNA polymerase (Sphaero Q), 0.1 nM/μl DNTP (Thermo Fisher Scientific) and 0.5 pmol/μl of each forward and reverse primer. PCR conditions were as follows: initial denaturation at 94°C for 10 min; 40 cycles of amplification with various annealing temperatures (95°C for 1 minute, 55–59°C for 1 minute, and 72°C for 1 minute); and a final extension step of 10 seconds at 72°C. The PCR reaction products were visualized in 2% agarose gel (Sphaero Q).

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Table 1. The sixteen predicted protein sequences with their corresponding size, primer sequences and annealing temperatures.

https://doi.org/10.1371/journal.pntd.0008897.t001

Results and discussion

Since we have demonstrated that the currently used M. mycetomatis specific PCR cross-reacted with M. pseudomycetomatis, there was a need to develop a new M. mycetomatis specific PCR for proper species identification. From the genome of M. mycetomatis, 350 predicted protein-coding sequences (PPCS) were randomly selected and analysed. We chose to analyse PPCS because these protein-coding sequences are likely to be more stable than non-coding sequences [15,16]. To ensure that they can be easily amplified through PCR, we preferentially chose PPCS with sizes between 400 and 1100 bp. From the initial 350 PPCS, the top 16 candidates that fitted our requirement based on specificity and size were chosen for PCR development.

PCR primers for the 16 candidates were then designed (Table 1). To ascertain that these primers would amplify their targets in all M. mycetomatis isolates, they were evaluated in 60 M. mycetomatis isolates from different geographical origins, genotypic backgrounds and phenotypic appearance. Out of the 16 primer sets tested, 13 were positive in all 60 M. mycetomatis isolates tested (Fig 1). Primer sets 4, 5 and 12 were present in 58, 4 and 59 isolates, respectively (Fig 1). To determine the specificity of the 13 positive primer sets to M. mycetomatis, they were tested against other fungal mycetoma causative agents and close relatives of M. mycetomatis. As seen in Table 2, only primer set 11 –later renamed as Mmy-Fw and Mmy-Rv—was found to be specific for M. mycetomatis. Primer set 2, 4, 8 and 9 were not able to discriminate between the different Madurella species while 5 and 7 could discriminate between the four Madurella species but cross-reacted with at least one other mycetoma causative agent. The amplicon generated by Mmy-Fw and Mmy-Rv appears to be a putative single-copy gene. To determine if these PCR primers were as sensitive as the currently used ones, we compared the two PCRs head-on. Mmy-Fw and Mmy-Rv were able to detect DNA concentrations as low as 5 pg. This is only slightly less sensitive compared to the currently used diagnostic PCR primer pair 26.1a and 28.3a that is able to detect DNA at 0.5 pg.

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Fig 1. Presence of the 16 PCR amplicons in 60 M.

mycetomatis isolates tested. Most PCR reactions resulted in amplification in all isolates tested except PCR 4, 5 and 12.

https://doi.org/10.1371/journal.pntd.0008897.g001

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Table 2. Presence or absence of PCR amplicons of the sixteen primer sets and PCR primers developed in 1999 [5] in the other eumycetoma causing agents and close relatives of M. mycetomatis.

No amplicons were observed in all species tested here using Mmy-Fw and Mmy-Rv. Only PCR with bands of the same sizes to M. mycetomatis is considered specific to M. mycetomatis.

https://doi.org/10.1371/journal.pntd.0008897.t002

Primers Mmy-Fw and Mmy-Rv were also tested on DNA extracted from grains obtained from eumycetoma patients. As shown in Fig 2, amplicons were only observed when DNA from M. mycetomatis grains were present. The primers did not cross-react to DNA obtained from F. senegalensis or F. tompkinsii grains (Fig 2). Our findings show that these primers are sufficiently sensitive to be used in diagnosis directly from clinical specimens.

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Fig 2. The specificity of Mmy-Fw and Mmy-Rv on DNA isolated from eumycetoma grains.

Lane 1, 100 bp DNA ladder; Lane 2, negative control; Lane 3, 4, 6, 8, 11 and 12, Madurella mycetomatis DNA extracted from grains; Lane 5 and 7, Falciformispora senegalensis DNA extracted from grains; Lane 9 and 10, Falciformispora tompkinsii DNA extracted from grains; Lane 13, Madurella mycetomatis DNA from isolate as a positive control. Presence of amplicons on lane 3, 4, 5, 6, 8, 11 and 12 and none on the other lanes confirms the specificity of Mmy-Fw and Mmy-Rv towards M. mycetomatis.

https://doi.org/10.1371/journal.pntd.0008897.g002

One of the advantages of using this comparative genome approach is that primer designs are less constrained since the targeted genes are unique. With this method, we were able to design primers that can distinguish between M. mycetomatis and M. pseudomycetomatis. Other studies have also succeeded in designing specific primers for their organism of choice using this approach [17–19]. In a study by Withers et al, a similar genome comparison method was performed on Pseudoperonospora cubensis and Pseudoperonospora humuli [19]. The comparison was first performed in silico and subsequently in vitro. Using this approach, they were able to identify and determine a large number of specific markers for their organism of interest while reducing the number of diagnostic candidates to validate with PCR [19]. However, a similar in silico approach could not be performed in our study because at the time of data analysis and the preparation of this manuscript, only the genome of one M. mycetomatis isolate and none of M. fahalii, M. tropicana and M. pseudomycetomatis was sequenced.

In conclusion, since cross-reactivity occurs with the current M. mycetomatis specific PCR primer pair 26.1a and 28.3a, we have used a comparative genome approach to identify and designed new M. mycetomatis species-specific PCR primers. Since new fungi causing eumycetoma are still being discovered, proper identification of its causative agents can help to fully understand the epidemiology and global burden of this disease. Thus, there is clearly a need for a specific PCR marker to identify its causative agents. We recommend reference centers such as the WHO collaborative Mycetoma Reference Center in Khartoum, Sudan and yet to be established reference laboratories in other endemic countries to use the new PCR primers Mmy-Fw and Mmy-Rv to identify M. mycetomatis to the species level. Furthermore, this comparative genome approach may also be used to design markers for other eumycetoma agents and also other fungi that share conserve ITS region within its genus.