52 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



one or more brooms or brushes (whence the name). Very few species 

 are known to produce asci, hence these are rarely encountered. The 

 conidial form continues for an indefinite number of generations, there- 

 fore all the activities of the genus are associated with this form. 



Cultural Considerations. — Among the numerous species and races, 

 some of the green forms are widely distributed and almost omnivorous 

 in habit. Other species are closely restricted to particular substrata. 

 Starches and sugars appear to be especially favorable components of 

 nutrient media for members of the group. The larger number of the 

 species grows best at temperatures from 15° to 30°; a very few of them 

 reach their optimum at 37°, but many species are entirely inhibited 

 and some killed at blood-heat. Vegetative mycelium begins to be 

 produced at temperatures very close to freezing, but colored conidia 

 are produced slowly or not at all at low temperatures. The species of 

 Penicillium thrive through a wide range of concentration of culture 

 media, though perhaps the most characteristic growths are produced 

 in media high in water content. The common species of each genus will 

 grow in all the standard bacteriological media. With few exceptions 

 the species grow well in synthetic media composed of assimilable car- 

 bohydrates and inorganic salts. A few species require the presence of 

 some one of the higher nitrogenous compounds, but many species refuse 

 to produce typically colored fruit without some form of starch or sugar 

 in addition to ordinary peptone and beef-extract. Very few species 

 grow well in alkaline media, but most species are tolerant of organic 

 acids at the concentrations found in fruits and vegetables. 



Some Common Species. — Penicillium roqueforti, Thom, is a green 

 form constantly found in pure culture in Jloquefort cheese, frequently 

 also in ensilage. It is widely distributed and grows ,under many sets 

 of conditions. 



Penicillium camemberti, Thom, is the chief organic agent in ripening 

 Camembert cheese. Cultures of this species are floccose orcottony, 

 at first white, later gray-green. 



Penicillium expansum, Link, is a green form, always obtainable from 

 apples decaying in storage, upon which it frequently produces large 

 coremia. It is one of the most abundant species of the genus, widely 

 distributed in different countries. In cultures, colonies produce a 

 characteristic odor, suggestive of its common habitat, decaying apples. 



Penicillium brevicaule, Saccardo, is a form with rough or spiny brown 



