universe84a

General Characteristics of Fusarium species

Fusarium species are a large group of filamentous fungi belonging to the hyphomycetes. They are commonly distributed in the soil they are saprophytic fungi and are plant pathogens and thus causing a wide range of plant diseases. This is because of their ability to produce mycotoxins especially in cereal crops, that can cause disease in human and animal hosts if ingested. Fusarium species majorly produces fumonisins and trichothecenes mycotoxins. Infections due to Fusarium species are collectively referred to as fusariosis. Fusarium species do not commonly cause diseases in humans because some exist as commensals in the skin, but it has been found to cause opportunistic infections in immunocompromised individuals with clinical manifestation of

• Endophthalmitis
• Sinusitis
• Pneumonia
• Skin Involvement
• Fungemia
• Disseminated Infection
• Fusarium is one of the emerging causes of opportunistic mycoses.

Whereas in pant, plant diseases are as follows-Fusarium head blight (FHB, Footrot (FR), root rot (RR), crown rot (CR), Fusarium wilts, Pokkah Boeing on sugarcane, and bakanae disease of rice.

Animal diseases are equine leukoencephalomalacia (ELEM): It is a disease of the central nervous system that affects horses, mules, and donkeys and is also called “Moldy Corn Poisoning”.  Abdominal distress, diarrhea, cardiac insufficiency, emesis, and even death in pigs due to consumption of myotoxin.

Medically Important Fusarium species are Fusarium solani is the most frequent species, accounting for about 50% of all infections, followed by F. oxysporum (~20%), F. verticillioidis and F.  moniliforme. Other species include Fusarium dimerum, F.  proliferatum, F.  chlamidosporum, F.  sacchari, F.  nygamai, F. napiforme, F.  antophilum and F.  vasinfectum.

Scientific Classification of Fusarium species

Kingdom: Fungi

Division: Ascomycota

Class: Sordariomycetes

Order: Hypocreales

Family: Nectriaceae

Genus: Fusarium

Species: F. solani

Habitat of Fusarium species

Fusarium species are commonly found in soil and environmental habitats, with many growing and thriving in tropical and temperate regions and even in desert regions, the alpine, the arctic regions with harch cold conditions, they seem to prevail. It is also found in normal mycoflora of commodities, such as rice, bean, soybean, and other crops. Due to widely distribution and having efficient dispersal mechanisms, they became able to grow in a wide range of substrates as well.

Morphology of Fusarium species

Fusarium species reproduce asexually and produces three kinds of fungal spores known as macroconidia, microconidia, and chlamydospores. Some species of Fusarium produce all three types of spore while others produce singularly. These spores especially the microconidia (2-4 x 4-8 µm)  are held by microconidiophores. These conidiophores may be either mono-phialides only or both mono-phialides and poly-phialides in a given species producing microconidia. Macroconidia (3-8 x 11-70 µm) are produced in a sporodochium, which is an erumpent crowded cluster of conidiophores arising from stroma to form a cushion-like mass that supports the macroconidia. These macroconidia are also produced on mono-phialides (a conidiophore with a single opening through which an endoconidia is released) and poly-phialides (two or more openings or pores from which the endoconidia are forced out) on aerial mycelium. The macroconidia vary in size and shape. The Major producers of macroconidia are F. semitectum, F. avenaceum and F.  suglutinans. Microconidia are produced in the aerial mycelium. The microconidia can be produced on false heads or false chains on mono-phialides or poly-phialides. False Heads are a result of moisture drops on the conidiophore and they contain the endoconidia as they are produced. Microconidia have different shapes and sizes. The microconidia produced in chains have a truncate base. Chlamydospores are thick-walled spores filled with lipid-like material that carries the spores overwinter in the soil. Chlamydopsores are sometimes airborne occurring in pairs, in clumps, or in chains. They have an outer wall that can be smooth or rough.

Cultural Characteristics of Fusarium species

 Sabouraud Dextrose Agar (SDA)

Fusarium spp. grow rapidly on Sabouraud dextrose agar at 25°C and produce woolly to cottony, flat, spreading colonies. The only slow-growing species is F. dimerum. From the front, the color of the colony may be white, cream, tan, salmon, cinnamon, yellow, red, violet, pink, or purple. From the reverse, it may be colorless, tan, red, dark purple, or brown.

Carnation Leaf Agar

Carnation Leaf Agar promotes sporulation and suppresses mycelial growth. It produces conidia and conidiophores in large numbers and specialized morphologies of the spores are distinct. Carnation leaf agar has low carbohydrates with complex substances that provide a natural environment that promotes Fusarium growth.

Potato dextrose agar (PDA)

This is the most valuable medium for Fusarium growth-producing gross morphological appearance and colony colorations. The medium contains a high carbohydrate content which promotes sporulation, however, takes longer to grow in this medium. The conidia produced are misshapen and atypical. The colonies have a velvety or cottony surface and are white, yellow, pink, purple salmon, or gray on the surface, with a pale, red, violet, brown, or sometimes the blue reverse.

Potassium Chloride Agar (KCLA)

KCLA is used to observe the formation of microconidia in chains by species. The species that form chains of microconidia form more abundant, longer chains on this medium. The chains are easier to observe because there is less moisture on the surface of the agar and fewer droplets of moisture in the aerial mycelium.

Soil Extract Agar

Soil is a natural medium for many organisms as they provide a perennial source of organic matter and other sources of carbon, nitrogen, minerals, and vitamins required for their growth. Soil Extract Agar is a medium used for isolating soil microorganisms like bacteria, actinomycetes, fungi, algae, and protozoa.  Soil extract provides all the essential nutrients required for the growth of such microorganisms. Glucose serves as a readily metabolizable carbon source whereas dipotassium phosphate buffers the medium. Soil agar promotes rapid chlamydospore formation in a number of Fusarium species. Large inoculum with actively growing fusarium inoculates produces chlamydospores within 3-4 days but secondary inoculates produce chlamydospores in 30 days.

Pathogenesis of Fusarium

Fusarium species cause diseases in plants, animals, and human hosts, but most commonly in plants. These pathogenesis have been linked to the toxigenicity of the species associated with the production of mycotoxins such as trichothecenes (types A and B), fusaric acid, and fumonisins. Trauma is the major predisposing factor for the development of cutaneous infections whereas neutropenic and transplant hosts are for disseminated opportunistic infections. Fusarium species cause superficial, locally invasive, and diffuse infections in humans. Localized infection includes septic arthritis, Keratitis, osteomyelitis, otitis media, cystitis, onychomycosis, cutaneous infections particularly of burn wounds, mycetoma, sinusitis, pulmonary infections, endocarditis, peritonitis, central venous catheter infections, and brain abscess. Invasive infections are a result of surgery and oral antifungal therapy. Disseminated infection occurs when two or more noncontiguous sites are involved. Fungemia and outbreaks of nosocomial fusariosis have also been reported. The infections are opportunistic and they are majorly caused by F. solani complex , F. oxysporum, F. verticillioides, and F. proliferatum, F. moniliforme and F. fujikuroi species complex.

They cause opportunistic infections in immunocompromised patients. The elderly and diabetics with prevalent meningospondylodiscitis are opportunistically infected by F. oxysporum. F. sacchari, F. anthophilum, F. chlamydosporum, and F. dimerum. A perinephric abscess caused by F. chlamydosporum is common in children who have been reported before. Corneal infections (endophthalmitis) caused by F. oxysporum and F. solani also occur because of the adherence of the fungi to the corneal membrane causing eye damage. Some Fusarium species, such as F. dimerum, are associated with keratomycosis, particularly in bad hygiene conditions. Mycotoxicosis caused by Fusarium species is common in the ingestion of the mycotoxins produced by the fungi.

Clinical manifestation of fusariosis in immunocompromised hosts

• Endophthalmitis
• Sinusitis
• Pneumonia
• Skin Involvement
• Fungemia
• Disseminated Infection

Laboratory Diagnosis of Fusarium

Specimen: It depends on the nature of the infection site e.g. in case fungemia, blood, in the diagnosis of keratitis corneal scrapings (most frequent) or tissue biopsy and skin lesions (either cellulitis or metastatic lesions), and also blood for cultures for mold.

KOH mount: Presence of fungal elements

Fungal culture: To obtain growth of fungi. Growth of Fusarium oxysporum on SDA  and its LPCB preparation as shown below video-

LPCB preparation: Observation of fungal structures from culture.

Serological test: β-d-Glucan Testing Is Important for Diagnosis of Invasive Fungal Infections but cannot distinguish Fusarium from other fungal infections (Candida, Aspergillus, Trichosporon, and others) which are also detected by the assay.

• Histopathological examination:  It is also a helpful tool for confirmatory diagnosis of fusariosis. In tissue, the hyphae are similar to those of Aspergillus species, with hyaline and septate filaments that typically dichotomize in acute and right angles. Although, adventitious sporulation may be present in the tissue, and the finding of hyphae and yeast-like structures together is highly suggestive of fusariosis in the high-risk population. In the absence of fungal growth, distinguishing fusariosis from other hyalohyphomycoses (a group of opportunistic mycotic infections caused by nondematiaceous molds) may be difficult and requires the use of another technique in situ hybridization in paraffin-embedded tissue specimens.

Molecular test:  PCR-based method, using sequencing identification as a gold standard but why this, it verifies as identification of Fusarium species is often difficult due to the variability between isolates and because not all features required are always well developed (e.g. the absence of macroconidia in some isolates after subculture). It is possible to identify the genus Fusarium by several methods. On culturing, hyaline, banana-shaped, and multicellular macroconidia are very common; however, to identify them at the species level is not easy. Therefore, molecular methods are needed. Some of the most commonly used molecular methods are genus-specific PCR, 28 s rRNA gene sequencing, sequence-based PCR, multiplex tandem PCR, and automated repetitive sequence-based PCR.

Treatment

Useful antifungal drugs are-

• Itraconazole
• Voriconazole
• Amphotericin B and
• Posaconazole.

Prevention and Control and Fusarium species

General

• Prognosis in fusariosis is poor and the limited susceptibility of Fusarium spp. to antifungal agents and therefore prevention of infection remains the cornerstone of management. Reducing immunosuppression should be attempted in patients with an earlier history of Fusarium infection and can be achieved by a reduction in or discontinuation of immunosuppressive agents, shortening the duration of neutropenia.
• Skin evaluation is also mandatory before giving immunosuppressive therapy because the skin may be the source for disseminated and frequently life-threatening fusarium infections.
•   Any area of tissue breakdown should be identified and suspicious skin lesions cultured and biopsied. Local debridement should be performed and topical antifungal agents e.g. Natamycin, amphotericin B considered if Fusarium species are identified.

Antifungal Agent Prophylaxis
There are no recommendations for antifungal prophylaxis against Fusarium species either as primary prophylaxis or as secondary prophylaxis (patients with prior fusariosis who will be exposed to periods of prolonged neutropenia or will undergo an allogeneic HSCT). However, the use of an antifungal agent should be considered for secondary prophylaxis, and the choice should be based on the Fusarium species causing infection and/or the results of in vitro antifungal susceptibility testing (AFST) if available.

Infection Control
Since the airways are the principal portal of entry for Fusarium species, the placement of patients at high risk (prolonged and profound neutropenia and allogeneic HSCT recipients) in rooms with HEPA filter and positive pressure may decrease the risk of nosocomial acquisition of fusariosis. In addition, since the water may be a source of Fusarium species in the hospital, every effort should be made to prevent patient exposure (e.g., by avoiding contact with reservoirs of Fusarium spp., such as tap water, and/or cleaning showers prior to use by high-risk patients during periods at risk. For human and animal infections can be treated with intravenous administration of itraconazole, oral amphotericin B.

Others attempts

• Fungicides have very minimal effects with likely resistance development on most of the Fusarium species groups.
• Resistant cultivars(a plant variety that has been produced in cultivation by selective breeding) can be used to control Fusarium Head Blight by controlling the plant lines that allow the release of mycotoxins. This reduces fungal growth and lowers mycotoxin contamination.

Key Notes on Fusarium species

1. Fusarium differs from comparatively matching some fungi like Acremonium, Lecythophora, and Phialemonium by having macroconidia. It differs from Cylindrocarpon by having macroconidia with foot cells and pointed distal ends.
2. Fusarium oxysporum: In contrast to Fusarium solani the phialides are short and mostly non-septate. Fuasrium solani growth on PDA and its LPCB mount under the microscope as shown in this video-

• Macroscopic and microscopic features, such as the color of the colony, length and shape of the macroconidia, the number, shape and arrangement of microconidia, and presence or absence of chlamydospores are key features for the differentiation of Fusarium species.
• Molecular methods, such as 28S rRNA gene sequencing, may be used for the rapid identification of Fusarium strains to species level.
• Plant diseases are Fusarium head blight (FHB) caused by F. graminearum, which contributed to the loss of starch and proteins in cereals, Footrot (FR) and root rot (RR), crown rot (CR), Fusarium wilts a destructive disease in bananas caused by F. oxysporum whereas Pokkah Boeing on sugarcane and bakanae disease of rice.

References

1. http://www.antimicrobe.org/new/f15.asp
2. https://cmr.asm.org/content/20/4/695
3. https://www.scielo.br/scielo.php?script=sci_arttext&pid=S0037-86822020000100250
4. https://academic.oup.com/cid/article/35/8/909/329202
5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2646427/
6. https://www.clinicalmicrobiologyandinfection.com/article/S1198-743X(15)30058-6/pdf
7. https://www.sciencedirect.com/science/article/pii/S1198743X15300586
8. https://en.wikipedia.org/wiki/Fusarium
9. http://www.himedialabs.com/TD/M455.pdf
10. https://www.sciencedirect.com/science/article/pii/S0254629912001573
11. Clinical Microbiology Procedure Handbook, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
12. A Text-Book of Medical Mycology. Editor: Jagdish Chander.  Publication Mehata, India.
13. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC492390/