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Page number:180 
Remarks (internal):Species description based upon Thom's notes on the type strain, his No. 45, isolated as a contaminant from a culture of another Penicillium obtained from Dr. Wehmer's Laboratory in Hanover, Germany, and upon observations made since that time and in the current study on a large number of strains having essentially the same cultural and morphological characteristics. NRRL 1750 may be regarded as representative. This culture stems from a transplant of Thom's No. 45 made in 1930 by Dr. H. C. Greene and subsequently maintained at the University of Wisconsin, Madison, until the spring of 1941 when it was returned to us for the NRRL Collection. Thom's type, as maintained in his collection in Washington, D. C. until 1940, and subsequently at the Northern Regional Research Laboratory in Peoria, no longer represents the species adequately. It is now essentially floccose and almost non-sporulating but still produces penicilli (usually small) and conidia characteristic of Penicillium spinulosum. This tendency toward sterility in strains long maintained in artificial culture is frequently encountered in this and certain other species, e.g. P. oxalicum and P. purpurogenum. The following may be listed as additional representative strains: NRRL 724 received in 1940 from Professor E. M. Gilbert, University of Wisconsin; NRRL 728 from Biourge as his P. roseo-maculatum (Thom No. 4733.107); and, NRRL 2051, a strain received in February 1946, from the Centraalbureau as Biourge's culture of his P. flavo-cinereum.
Penicillium spinulosum is world-wide in distribution and is especially common in soil, as evidenced by its frequent isolation from this source. Although confirmation has not been possible, there appears to be some evidence that this species is near, if not identical with, Citromyces pfefferianus Wehmer (Beitr. Kennt. Einh. Pilze I: 22-24; Taf. I, figs. 1-13. 1893).
Within the species Penicillium spinulosum, individual strains vary appreciably in cultural characteristics, particularly with reference to depth, general colony texture, and amount of sporulation. Strains also differ in the degree of roughness shown by conidia and conidiophores. A few strains produce conidia more or less elliptical in contrast to the usual and more typical forms with conidia globose or subglobose.
Representative strains of Penicillium frequentans Westling and P. spinulosum Thom are fairly distinct, and these can usually be identified or separated without serious difficulty. Nevertheless, strains of somewhat intermediate character are often encountered among large groups of isolates, and these tend to bridge between the two species. Such variation may occur either in colony texture and character, in details of structure, or in the colony reverse. Individual strains are placed in either one species or the other upon the basis of their more obvious relationships. Variation from P. spinulosum may also occur in the direction of the P. lividum series. Penicillium trzebinskii Zaleski, characterized by deeply velvety colonies and conspicuously elliptical conidia, is regarded as representing an extreme example of such variation. While this species is currently placed in the P. lividum series (see p. 189), its possible relationship to P. spinulosum is well recognized.
Description type:Non-original description 
Description:Penicillium spinulosum Thom, in U. S. Dept. Agr., Bur. Anim. Ind., But' 118, p. 76, fig. 32. 1910. See also Thom, The Penicillia, pp. 183-184, fig. 21. 1930.
Colonies on Czapek's solution agar growing fairly rapidly, 4.5 to 5.5 cm. in diameter in 12 to 14 days at room temperature, consisting of a loose-textured felt partially submerged, but predominantly aerial (fig. 51A), bearing conidiophores mostly as branches from loosely interlacing hyphae but commonly from submerged mycelia in marginal areas, more or less radiately wrinkled, rarely zonate, up to 1 or 2 mm. deep in central areas, heavily sporing in some strains, lightly in others (tending toward sterility under long cultivation), in dull green shades from sage green through slate olive to deep slate olive (Ridgeway, Pl. XLVII); odor very faint; exudate lacking or very limited; reverse almost colorless to light gray shades, occasionally showing a pinkish tinge; penicilli bearing spore chains in loose columns up to 100 to 150 µm long; conidiophores varying in relation to their origin, usually fairly long, mostly 80 to 100 µm but up to 200 to 300 µm when arising from the substratum, to quite short, 25 to 50 µm when arising as branches from aerial hyphae (fig. 51C), mostly 2.5 to 3.0 µm in diameter, with walls almost smooth in some strains to definitely roughened in others (fig. 48C), with apices vesicular up to 5.0 µm in diameter, bearing penicilli generally strictly monoverticillate but with an occasional branch; sterigmata comparatively few in the verticil, about 6 to 10, measuring mostly 6 to 9 µm by 2.2 to 3.3 µm (fig. 51D); conidia sub-globose to globose, rarely somewhat elliptical, mostly 3.0 to 3.5 µm in diameter, definitely roughened, spinulose, or in some strains showing winding color bars.
Colonies on steep agar growing more rapidly, about 6.0 to 6.5 cm. in diameter in 12 to 14 days, with mycelial felt somewhat heavier and more floccose, generally heavier sporing but in the colors listed above (fig. 51B); reverse in dull gray or drab shades; penicilli as on Czapek except sterigmata more numerous in the verticil; conidia as on Czapek but showing less irregularity in size.
Colonies on malt extract agar growing even more rapidly than on steep agar, plane, but usually showing a loose flocculent felt 2 to 3 mm. deep, heavily sporing throughout; drops numerous, enmeshed in the felt; penicilli as above but with chains of conidia showing a greater tendency to form columns.
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