CHAPTER III 



METHODS FOR STUDYING MOLDS, YEASTS, AND 



ACTINOMYCETES 



Culture Media. Molds may be cultivated on either solid or liquid 

 media in tubes or dishes, as bacteria are. But molds generally grow 

 more slowly than bacteria and, where both are present in the ma- 

 terial to be examined, the latter are apt to overgrow Petri plate cul- 

 tures before the former can develop. For isolation it is therefore 

 desirable to use a medium favorable to the growth of molds and un- 

 favorable for bacteria. Such a medium may be obtained by adding 

 to ordinary nutrient agar a rather large amount of sugar, and by 

 making the medium rather strongly acid, since most molds will 

 tolerate higher degrees of acidity than bacteria will. A medium con- 

 taining 5 per cent glucose and 0.5 per cent tartaric acid in addition 

 to the usual meat extract and peptone is very satisfactory for some 

 fungi but is not suitable for most pathogens of man. 



If agar is autoclaved in such an acid medium, however, it will be 

 markedly hydrolyzed and will fail to jell on cooling. This difficulty 

 is obviated by sterilizing the glucose and acid separately in concen- 

 trated solution. A solution containing 50 per cent glucose and 5 

 per cent tartaric acid may be safely autoclaved. Such a sterile solu- 

 tion may be kept on hand and, when need for a medium for isolating 

 molds arises, it can be quickly met by melting a deep tube (about 

 10 ml.) of ordinary agar (beef extract 0.3 per cent, peptone 1 per 

 cent, pH 7.4) and adding to it 1 ml. of the glucose-tartaric acid solu- 

 tion. This gives a final concentration of nearly 5 per cent glucose 

 and 0.5 per cent tartaric acid and a reaction of about pH 3.8. Less 

 of the glucose-tartaric solution may be used for less acid media. If 

 an agar medium with reduced buffer content and lower pH is used 

 (e.g., A.P.H.A.) less of the glucose-tartaric solution will be used. 

 On these media most molds and yeasts will grow luxuriantly, most 

 bacteria not at all. 



The growth on the above media is for some purposes, as in count- 

 ing molds in soil, for instance, almost too luxuriant, since some of the 

 rapid growers tend to spread over the plate and crowd out others. 

 Media less rich in nutrients may be preferred for counting molds by 



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