Mucorales: Comparison of Phenotypic and Molecular Identification methods
November 26 2015
Mucormycosis is an emerging opportunistic infection caused by fungi belonging to the order Mucorales and is frequently fatal in immunocompromised individuals.
Once rare, it is now one of the most common invasive fungal infections in patients with hematologic malignancies second only to aspergillus. Symptoms are varied and include, a one-sided headache behind the eyes, facial pain, fever, and acute sinusitis with eye swelling, there may be blood clots, leading to the necrosis of surrounding tissue. The treatment of mucormycosis relies on the exact identification of the Mucorales fungi, however; it can be difficult to differentiate between different Mucorales species and Aspergillus on the basis of imaging studies and histopathological analysis alone. This can result in delayed diagnosis and a poor prognosis.
A recent study (Yang et al 2015) was conducted to compare phenotypic and molecular identification methods in order to improve the early diagnosis and treatment of Mucormycosis. Twelve Mucorales isolates, collected between 2010 and 2014 from Samsung Medical Center, Seoul, Korea were used to compare the two methods. Included in the analysis were isolates that were repeatedly obtained from a patient or patients with suspected invasive fungal infection that were morphologically identified as Mucorales. Phenotypic identification methods included identifying characteristics such as, growth rate, colony morphology and reproductive structures. For the molecular identification method, DNA was extracted using the MagNa Pure LC DNA Isolation Kit (Roche Diagnostics, Mannheim, Germany) or the I-genomic BYF DNA Extraction Mini Kit for fungi (iNtRON Inc., Seongnam, Korea). Isolates were considered to be incorrectly identified, if their molecular identification was different from their phenotypic identification at the genus level.
Four isolates were considered to be misidentified by phenotypic methods. Of these, three isolates identified as Rhizomucor based on phenotypic analysis were re-classified as Lichtheimia (L. ramosa and L. corymbifera) and Mucor (M. fragilis) on the basis of molecular analysis. The fourth isolate, initially identified as Mucor, was re-classified as Rhizomucor (R. pusillus). One isolate that was unidentifiable by morphology was subsequently classed as Rhizopus microsporus by molecular identification.
Phenotypic identification methods were found to be inadequate, for example, the absence of rhizoids is considered the key to differentiating between Mucor spp. and other Mucorales fungi. However, despite missing rhizoids, molecular analysis identified two isolates as Rhizopus (R. microsporus and R. Pusillus respectively). These exceptions lead to a low concordance rate between phenotypic and molecular identification and means that molecular identification methods are more reliable than phenotypic identification. In conclusion, relying on phenotypic identification methods may lead to incorrect diagnoses and treatment because different Mucorales share similar morphological features.
However, molecular identification methods are valuable tools for correctly identifying Mucorales.