Penicillium citrinum
Ecology: Penicillium
citrinum is a commonly occurring filamentous fungus with worldwide
distribution. It has been isolated from
a variety of sources including soils, decaying vegetation, foodstuffs (beans,
coffee, cereals & spices) as well as a variety of indoor environments.
Pathology: While Penicillium
species are generally regarded as laboratory contaminants, or at best,
opportunists, a number of species have been implicated as being involved in the
disease process. While Penicillium
species may be isolated from clinical specimens, it is commonly believed that a
true infection can only be established by histological demonstration of tissue
invasion. With that in mind, Penicillium
citrinum has been reported in mycotic keratitis (eye), lung infections
(pneumonia), a single case of a urinary tract infection (UTI) and one of
pericarditis. Their contribution to the
disease process may be secondary an additional underlying illness. As with all fungi, immunocompromised
individuals may be at greater risk of infection including those rarely
considered as pathogenic.
Macroscopic
Morphology: Penicillium citrinum exhibits moderately slow growth on
Sabouraud-Dextrose agar (SAB) at 30ᵒC.
Surface texture is velutinous (soft, velvety surface) to floccose
(woolly tufts of soft “hairs”). The
colonial growth appears radially sulcate (narrow, deep furrows or radial
grooves –like spokes on a wheel). The
mature colony has a central greyish-turquoise to greyish-orange colour with a
white periphery (outer edge). Exudates
(extrolites) are frequently produced which appear as drops of liquid upon the
surface of the colony. These may appear
clear, to pale yellow, to a reddish-brown in colour. Some strains may also produce a soluble
pigment which can diffuse into the surrounding medium. The reverse is a pale yellow to a light
yellow-brown. Colours and growth characteristics are, of course, media and
strain dependent.
Penicillium citrinum-SAB, 14 days incubation at 30ᵒC (Nikon)
Note the drops of exudate (extrolites) which have formed on the surface.
Colour variation due to maturing of colony but also a difference in my lighting for photography.
Exudates (or
Extrolites): Some fungi can produce exudates as a by-product of their
growth, many of which can be collected for commercial use. Mycotoxins are by-products (secondary
metabolites) which are potent poisons.
Penicillium citrinum produces Citrinin, a nephrotoxic mycotoxin which
derives its name from the fungus. It may
also produce other extrolites such as tanzowaic acid A, quinolactacins,
quinocitrinines, asteric acid and compactin.
Microscopic
Morphology: Penicillium citrinum
produces septate, hyaline (clear, not pigmented) hyphae. Smooth-walled conidiophores stipes are rather
long (100 – 300 µm) and is biverticillate (see diagram at end of post). Metulae are 12 – 15 µm in length which are
found in whorls of 3 – 5 divergent structures.
Phialides are ampuliform (flask-shaped) and about 7 – 12 µm in
length. Conidia (2.2 – 3.0 µm dia.) are
globose to sub-globose (round to off-round) and are smooth or have a finely
roughened surface. Conidia resist disruption
and form rather long chains. These
characteristics: the metulae longer than the phialides and the conidia being
both spherical and produced in well-defined chains, are distinguishing features
of Penicillium citrinum.
Penicillium citrinum- distinguishing features of Penicillium 'species' can already be made out at low magnification. (250X, LPCB, DMD-108)
Penicillium citrinum- distinguishing features of Penicillium 'species' much more evident at 400X.
Typical "fingers" made up of the metulae and phialide structures from which chains of conidia extend. (400X, LPCB, DMD-108)
Penicillium citrinum- a mass of overlapping fruiting structures with copious amounts of conidia.
(1000X, LPCB, DMD-108)
Penicillium citrinum- a little less congested in this photo. Conidiophores (stipes) seen from which extend the metulae and conidia producing phialides. Conidia are globose (round) to sub-globose (somewhat off-round) in shape, (1000X, LPCB, DMD-108)
Penicillium citrinum- long metulae and the somewhat shorter phialides are clearly distinguishable in this photograph. The conidia are generally smooth or can have a finely roughened surface.
(1000+10X, LPCB, DMD-108)
Penicillium citrinum- another view.
(1000+10X, LPCB, DMD-108)
Penicillium citrinum- exhibits biverticillate branching meaning that the conidiophore can branch and the metulae & phialides extend from these branches. Triverticillate would have the conidia branching and then the branches also branching to finally produce the metulae & phialide fruiting structures.
(1000X, LPCB, DMD-108)
Penicillium citrinum- Phialides are ampuliform (flask-shaped) and about 7 – 12 µm in
length. Again, conidia (2.2 – 3.0 µm dia.) are
globose to sub-globose (round to off-round) and are smooth or have a finely
roughened surface. (1000+10X, LPCB, DMD-108)
Penicillium citrinum- Here we see the proportions of the metulae (M) and the 'flask-shaped' phialides (P) with the metulae being substantially longer than the phialides, The biverticillate structure is evident in this photo. (ie. each branch extending from the conidiophore (stipe), branches only once and then bears a fruiting structure consisting of the metulae and phialides.
(1000+10X, LPCB, DMD-108)
Penicillium citrinum- another example.
(1000+10X, LPCB, DMD-108)
Penicillium citrinum- a few more photos to finish up.
(1000+10X, LPCB, DMD-108)
Penicillium citrinum- suitable for framing!
(1000X, LPCB, DMD-108)
Penicillium citrinum
(400+10X, LPCB, DMD-108)
Penicillium citrinum
(400X, LPCB, DMD-108)
Penicillium citrinum- another colony showing the exudate (extrolites) which accumulate on the colony surface after extended incubation. These metabolites may be potent poisonous mycotoxins or might have beneficial uses in industrial or pharmaceutical applications. (Nikon)
Physiology: The spores of Penicillium citrinum fail to
germinate at 5ᵒC and may show restricted growth at 37ᵒC.
* * *
