A critical examination of SEM ultrastructural features in two Penicillium thomii isolates from soil

A SEM ultrastructural study of two isolates of a soil fungus, Penicillium thomii, is presented, which compares the morphology of critical point-dried tissues with cryofixed material. The paper also reports features of the isolates which sometimes are different from the usual diagnostic morphological data for this fungus. Thus, branched penicilli have been seen in one of the isolates on MEA, yet monoverticillate penicilli are regarded as a major feature of P. thomii. In the same isolate the stipes appear non-vesiculate on both agars unlike the vesiculate stipes normally associated with this species from light microscope observations. Some wrinkling is seen in the stipes and phialides from critical point-dried specimens and this phenomenon is generally absent in cryofixed samples, although in one isolate, collapse of some of the phialides occurred. Critical point-drying results in shrinkage of conidia. Penicillium thomii is an important species in soil ecosystems and thus an accurate description of its morphological features is desirable for proper classification although it is a widely held belief that no character in Penicillium taxonomy is totally invariant.