Saturday 23 June 2012

Fusarium oxysporum

Fusarium oxysporum (Fungus)

Fusarium oxysporum has widespread distribution and can be isolated from soil and plants and decaying vegetative material (saprobe).  It is a phytopathogen (plant pathogen) of many species.

Macroscopic Morphology:
On Sabouraud-Dextrose (SAB) media (30oC) this isolate of F.oxysporum grew rather rapidly to produce of-white floccose (cottony) colonies with the aerial mycelia becoming tinged in purple.  The reverse was a rather non-descript pale to yellow.  Other sources have described the reverse as having a purple colour, particularly on Potato-Dextrose agar, a characteristic not seen on the isolate presented here and grown on SAB.

Microscopic Morphology:
Hyphae are hyaline (clear/non-pigmented) and are septate (show divisions or walls within the hyphae).  Conidiophores are rather short (8 - 14 µm) and usually non-septate when compared to other Fusarium species. The conidiophores have a somewhat inflated appearance as their sides aren’t parallel but slightly bulge out slightly in the middle.  These conidiophores (phialides, or more accurately monophialides) are produced singly as they extend from the aerial mycelium.  Microconidia (5 - 12 X 2.3 – 3.5 µm) are usually non-septate, ellipsoidal and are straight or slightly curved in shape as they are abundantly produced from the tip of these phialides.  Microconidia are produced singly and never in chains.  These microconidia may accumulate around the tip of the phialide if not dispersed (false head).
Macroconidia (23-54 X 3.0 – 4.5 µm) are fusiform in shape (hence the genus name), and have a slightly pointed apical tip with a basal ‘foot’ cell (pedicellate) at the opposite end.  The fusiform macroconidia are also somewhat curved making it appear sickle-shaped or perhaps canoe-shaped in profile.  They usually contain 3-5 compartments or divisions within the macroconidium.
Smooth-walled terminal or intercalary chlamydospores (5 – 13 µm dia.) may be found.

 Fusarium oxysporum - First look at low power (X100 LPCB)

Fusarium oxysporum (X400 LPCB: DMD-108)

Fusarium oxysporum (X400 LPCB: Nikon)
Microconidia can be seen accumulating around the tips of the phialides (see below).

Fusarium oxysporum (same photo as above but cropped)
Arrows point to tips of phialides where microconidia are produced and accumulate

Fusarium oxysporum - Microconidia and a few Macroconidia (X100 LPCB: Nikon)

Fusarium oxysporum - again showing hyphae from which monophialides extend, producing microconidia at the tips which can accumulate there unless dispersed.
 (X1000 LPCB (DMD-108)

Fusarium oxysporum - another view (as above) - Note micro bar at top of photo.
 ( X1000 LPCB: DMD-108)

Fusarium oxysporum - yet another view of two parallel hyphae from which the phialides extend producing micro (&/or macro) conidia.  Conidia vary in shape from the rather straight fusiform (lens shaped) to the curved banana or canoe shape.
 (X100 LPCB: DMD-108)

Fusarium oxysporum - microconidia accumulating at the tips of phialdes
(X1000 LPCB: DMD-108)

Fusarium oxysporum - microconidia in various stages of development (arrows) at the tips of the monophialides from which they are produced.
 (X1000+10* LPCB: DMD-108)

Fusarium oxysporum - (yeah, I like photos) -again, microconidia accumulating around tips of phialides (X1000+10* LPCB: DMD-108)

Fusarium oxysporum - somewhat inflated (sides not parallel) phialides extending from hyphae where conida are produced (arrows).  The one on left has separated from the tip of the phialide whild the one on the right is young and still developing.
  (X1000+10* LPCB: DMD-108)

Fusarium oxysporum - micro & macro conidia accumulated along side of hyphae.
(X1000+10* LPCB: DMD-108)

Fusarium oxysporum - slightly curved micro & macro conidia pictured
(X1000+10* LPCB: DMD-108)

Fusarium oxysporum - chlamydospore present (arrow).  Difficult to see from the orientation but I believe this was an intercalary chlamydospore (growing between the hyphae and not a terminal chlamydospore at the end of a hyphae.
  (X1000+10* LPCB: DMD-108)

Fusarium species - Just for comparison of the shape.  The photo on the left is of micro (and a few macro) conidia produced by Fusarium oxysporum while the one on the right is of microconidia produced by Fusarium dimerum.  The F.dimerum is showing a greater curved "banana" or "canoe" shaped microconidia thatn the F.oxysporum. (Magnification not noted: Nikon for both)

Infections with Fusarium species (fusariosis) may be localized or become disseminated.  Fusarium species are frequent agents of mycotic eye infections, particularly the cornea (keratomycosis, endopthalmitis).  They have also been implicated in onychomycosis (nail infections), catheter infections, peritonitis, sinusitis and septic arthritis.  As with many other fungi immunocompromised and neutropenic patients may be at greater risk.  Fusarium may contaminate stored grain where some species can produce potent mycotoxins.  Food prepared from these contaminated grains may cause illness on ingestion.  Fusarium species may also be found as laboratory contaminants but must not be dismissed outright without further investigation.

Fusarium oxysporum can be differentiated from F.solani complex which produce thick, blunt macroconidia and long, narrow mono-phialides as well as numerous rough-walled chlamydospores.  The colonies, however, may appear brownish, particularly on Potato-Dextrose agar   F.oxysporum can be differentiated from F.verticillioides as this species has candle-shaped mono-phialides as well as baton-shaped microconidia in chains that are not easily disrupted.  Colonies however may appear purplish as those of F.oxysporumF.proliferatum also is similar in appearance however produces polyphialides (many phialides as opposed to the single phialide in monophialides) from which delicate chains of microconidia can extend.  Macroconidia are seldom seen in most F.verticilliodes or F.proliferatum species unless induced by special media or exposed to UV light.

*DMD-108 microscope/camera is capable of taking photos with 1000X optical magnification plus and additional 10% digital magnification.