Fusarium oxysporum in Medical mycology

Fusarium

Taxonomy: The genus Fusarium is a filamentous fungus under the phylum Ascomycota, class Ascomycetes, order Hypocreales, while the teleomorphs of Fusarium species are mostly classified in the genus Gibberella, and smaller number of species are classified as Hemanectria and Albonectria ¹.  According to Leslie and Summerell modern taxonomy of Fusarium, there are 70 species within the genus ¹. Known as the most difficult species to distinguish by pathologists and mycologists worldwide ², the taxonomy of Fusarium is still a continuous debate. The members of this genus are difficult to identify and require specialized media to induce conidia. However positive identification can be achieved using PCR-sequencing of different genes and comparisons with the FUSARIUM-ID database ³.

Habitat: Fusarium species are ubiquitous, found widely distributed in soil, plant, organic substrates, water, and biofilms ⁴. This distribution of Fusarium is mainly attributed to its efficient mechanisms of dispersal and the ability to grow on a wide range of substrates ⁴.

Fungal Structure and Culture characteristics:
Fusarium has a distinctive canoe or banana shaped macroconidia, an asexual spore which is the hallmark character of this genus ⁵. Macroscopic features of Fusarium include a sclerotium or a sporodochium according to the growth conditions. During favourable conditions sclerotium, an organized mass of hyphae, is usually dark blue in color. The sporodochium, a cushion-like mat of hyphae bearing conidiophores over its surface, is usually absent in culture. If present, it may be cream to tan or orange color, with an exception for Fusarium solani, which gives blue-green or blue sporodochia. Microscopically, Fusarium consists of hyaline septate hyphae, conidiophores, phialides, macroconidia, and microconidia ⁶.

Diseases:

Fusarium genera is a well-known mycotoxin producer ⁷ with about 50 species ⁴ that act as opportunistic human and animal pathogens as well as a phytopathogenic genera of microfungi ⁷. In humans, it is known to cause a broad spectrum of infections, known as fusariosis, from superficial, local to disseminated infections according to the immune status of the host and the site of entry ⁴. Superficial and locally invasive diseases ^{3, 4} like onychomycosis and keratitis are seen in immunocompetent people as well as infections in people with serious burns or those receiving peritoneal dialysis ³. In immunocompromised people, it is responsible for diseases such as paronychia, invasive sinusitis and pulmonary and extra pulmonary hematogenously disseminated disease with skin lesions and fungemia³.   

The biggest population at risk are people with prolonged neutropenia and T-cell immunodeficiency, especially in hematopoietic stem cell transplant recipients with severe graft-versus-host disease ⁴. Fusarium enters the body through inhalation. Skin at site of tissue breakdown and the mucosal membranes are also other sites of entry ⁴.

Disease Diagnosis: Disseminated fusariosis is frequently identified as a combination of characteristic cutaneous lesions and positive blood cultures while lung or sinus involvement may or may not be seen ⁴. Invasive fusariosis is similar to invasive asperigillosis and other invasive infections with similar high risk patients including corticosteroids receiving and neutropenic patients⁴. The diagnosis is difficult and is determined largely by the degree of immunosuppression as well as extent of infection. Hundred percent death rate is seen in persistently neutropenic patients with disseminated disease ⁴.  Fusarium can be detected using colony morphology, microscopy, blood cultures and FUSARIUM-ID.³

Treatment: Amphotericin B, voriconazole, and posaconazole ⁴.

Fusarium oxysporum:

Habitat: Fusarium oxysporum are ubiquitous soil and plant inhabiting microbes ⁸.

Fungal structure and Culture Characteristics: With production of three types of asexual spores, microconidia, macroconidia, and chlamydospores, the fungus has the ability to survive as mycelium or as any one of these spores. Microconidia are one or two celled and are the spores produced most abundantly and frequently by the fungus under all conditions, even in infected plants. Macroconidia are three to five celled, gradually pointed and curved toward the ends and are commonly found on the surface of plants killed by this pathogen as well as in sporodochia like groups. Chlamydospores of F. oxysporum are either one or two celled and possess a two-layered wall, the outer layer representing the original hyphal wall and the inner secondary layer formed during maturation of the chlamydospore ⁹. These are produced either terminally or intercalary on older mycelium or in macroconidia⁸. The cells of hyphae of F. oxysporum are uninucleate with only the nucleus in the apical compartment being mitotically active (Acropetal nuclear pedigree) ¹⁰.  The top of the colonies start with white aerial mycelium, become purple, with discrete orange sporodochia present in some strain while the reverse hyaline  are dark blue or dark purple. Conidiophores are short, single, lateral monophialides in the aerial mycelium, later arranged in densely branched clusters ¹¹.

Diseases: With lack of good epidemiology studies, the real frequency of this pathogen is unknown and apart from keratitis, it is known as an infrequent cause of fungal infections. In the Fusarium genus, Fusarium oxysporum isthe second most frequent species to cause human infection, with F. solani being the most frequent one ¹². Multistate US and international outbreaks of keratitis with use of soft contact lenses related to Fusarium oxysporum biofilm production was seen during 2005-2006³.   Endophthalmitis, onychomycosis, cutaneous and subcutaneous infections, arthritis and mycetoma and sinusitis have been associated with F. oxysporum. Mortality close to 100% has been associated with disseminated infections in immunosuppressed patients (mainly haematological)¹².

Disease Diagnosis: Colony morphology (fast growing fungus), microscopic morphology and blood cultures ³.

Treatment: Amphotericin B. It is resistant to azoles (except for Voriconazole) and echinocandins¹².

Case study: 

1. Cutaneous disease by Fusarium oxysporum ¹³

A 67-year-old female with a 20 year old-lesion in the right ear was referred to the hospital in 2000. She had a history of type II diabetes (treated orally). Upon asking the patient, she didn’t recall any previous traumatic injury in the affected area. According to the patient history, in 1980 she was diagnosed with infectious granuloma caused by fungi or mycobacteria.  All the complementary tests for mycobacteriosis were negative at that time and no mycological cultures were done. She received treatment with rifampicin, isoniazid and ethambutol for 2 months on the basis of clinical and histological findings followed by rifampicin and isoniazid for 7 months. She received tentative cryotherapy in 1982 and in 1990, she was diagnosed with cutaneous mycosis due to the presence fungal elements in biopsy specimens. She received a treatment of itraconazole 200 mg/day for 6 months followed by terbinane® 250 mg/day for 2 months, yet the lesions were no cured.

Her lesions resembled lupus vulgaris (chronic, postprimary, paucibacillary cutaneous tuberculosis)

Routine tests were normal and a standard multi-test for cellular immunity was also normal. PCR detection as well as cultures for mycobacteria was negative. When biopsy specimen was stained with Hematoxylin-eosin, a chronic inflammatory response by epithelioid cells, eosinophils and numerous giant foreign body cells was seen. Pale structure resembling fungal hyphae were seen in the foreign body cells. Numerous hyphae sparsely distributed in the dermis and also inside giant cells were seen after Periodic acid-Schiff (PAS) staining. 

When the biopsy sample was cultured in chloramphenicol, gentamicin, SDA agar and SDA with cyclohexamide at 25 and 37^OC, the former showed velvety colonies, white to pale gray in color at 25^oC and orange at 37^oC. With time the cultures developed violet areas at both temperature. When Subcultures and slide cultures were prepared for species identification using oatmeal agar, potato sucrose and PDA, “wide vegetative aerial mycelium, with septate, branched hyphae, scarce fusiform, 3 to 5-celled, slightly curved, macroconidia, pointed at the tip, numerous, single-celled, oval/ellipsoid, non-chain microconidia developed on short lateral conidiophores and intercalary, single or paired, hyaline, smoothwalled chlamydospores” were seen. On the basis of these data, the colony was identified as F. oxysporum.  The patient was treated with oral Flucanazole for 3 months during which the patient showed a marked improvement.

References:

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4.      Nucci, M., and E. Anaissie. "Fusarium Infections in Immunocompromised Patients." Clinical Microbiology Reviews 20, no. 4 (2007): 695-704.

5.      Nelson, Paul E., and Brett A. Summerell. Fusarium: Paul E. Nelson Memorial Symposium. St. Paul, Minn.: APS Press, 2001.

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8.      Gonsalves, K., and Stephen A. Ferreira. "Fusarium Oxysporum." Fusarium Oxysporum. Accessed April 10, 2015.

9.      Zhao, Yueju, Yang Liu, Jonathan Nimal Selvaraj, Adam Driks, Yawa Minnie Elodie Folly, Lancine Sangare, Lichao Sun, Xinxin Tan, Huimin Song, Yan Wang, Lu Zhou, and Fuguo Xing. "Antagonistic Action of Bacillus subtilis Strain SG6 on Fusarium graminearum." PLoS ONE 9.3 (2014): e92486. Print.

10.  Ruiz-Roldan, M. C., M. Kohli, M. I. G. Roncero, P. Philippsen, A. Di Pietro, and E. A. Espeso. "Nuclear Dynamics during Germination, Conidiation, and Hyphal Fusion of Fusarium Oxysporum." Eukaryotic Cell 9, no. 8 (2010): 1216-224.

11.  "Fusarium Oxysporum." Mycology Online. May 15, 2001. Accessed April 12, 2015. http://www.mycology.adelaide.edu.au/Fungal_Descriptions/Hyphomycetes_(hyaline)/Fusarium/oxysporum.html.

12.  "Life." Life. Accessed April 12, 2015. http://www.life-worldwide.org/fungal-diseases/fusarium-oxysporum/.

13.  Jr, M. Pereiro, M. T. Abalde, A. Zulaica, J. L. Caeiro, A. Flórez, C. Peteiro, and J. Toribio. "Chronic Infection Due to Fusarium Oxysporum Mimicking Lupus Vulgaris: Case Report and Review of Cutaneous Involvement in Fusariosis." Acta Dermato-Venereologica 81 (2001): 51-53.