CHAPTER XXXVI 



Common Contaminants 



THE organisms discussed in this chapter are frequently observed as con- 

 taminants on culture mediums. Their isolation, although accidental, 

 may crowd out micro-organisms of pathogenic titer; this has been a source 

 of confusion to the beginner and of annoyance to the more experienced. 



The greatest trouble is experienced from contamination with molds. Bac- 

 teria grow poorly at room temperature, and yeastlike organisms grow so 

 slowly that they may be readily discarded. Molds grow quickly, and the 

 aerial mycelium is considerable. The inside of a culture tube may be liter- 

 ally covered with the growth within two or three days. The rapidity of 

 development and the appearance of heavy pigment (green, brown, black) 

 is frequently sufficient to indicate the nature of the mold. At times the 

 growth is white, later turning to green, and unless the rate of growth is 

 known some confusion may arise. If any doubt exists, a culture mount will 

 soon reveal the diagnosis. 



It has been mentioned that various observers have credited many of 

 these fungi with having pathogenic propensities in isolated instances. We 

 urge caution in this direction; unless experimental autoinoculation is per- 

 formed it is usually best to question the diagnosis. Some reports of patho- 

 genic strains of common saprophytes have already been discussed. 



Because they are air-borne, molds may also be important in the patho- 

 genesis of asthma. This is considered elsewhere. 



Since in this text we are not primarily concerned with fungi of non- 

 pathogenic propensities, the differentiation into species will be cursory. 

 The description of the genera should be sufficient for identification. 



1. ASPERGILLUS 



The mycelium develops a stalk (conidiophore) which enlarges to form 

 a knoblikc projection (vesicle) on which regular finger-like processes 



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