72 A MANUAL OF THE PENICILLIA 



ment and a finely pointed needle fashioned from nichrome or platinum- 

 iridium wire, conidia from a single or a few selected penicilli can be easily 

 removed. 



Slanted Plates: By using slanted plates one can, in a single culture, study 

 the effects of varying depths of agar as this influences the rate of evapora- 

 tion, the concentration of nutrients, and other factors which markedly 

 effect the rate and pattern of colony growth. Biourge recommended 

 the use of slanted plate cultures for some types of observation. Thom 

 likewise used this type of culture in certain of his observations and called 

 attention to the advantages and disadvantages inherent in it. The use of 

 such non-uniform agar layers necessitates careful evaluation of the in- 

 fluence of all environmental factors. 



Dilution Cultures: Dilution cultures are extremely useful in certain types 

 of work, particularly the isolation of strains from soil or other natural 

 substrata. They are equally useful for separating two or more Penicillia 

 which may be growing in close association as a result of contamination. 

 Two general techniques may be employed, both of which represent com- 

 mon bacteriological practices. 



The first of these consists of suspending the inoculum or sample of 

 natural material in a sterile water blank and diluting this progressively 

 by the serial transfer of aliquots of specified amount, usually 1 cc, from 

 one to another in a series of similar water blanks. Samples are removed 

 from the dilutions selected as probably most suitable and placed in sterile 

 petri dishes followed by the addition of a melted agar medium at about 

 45°C. In cultures developing from such dilution plates it is desirable to 

 have not less than 3 colonies or more than 10 or 12. 



The second method of preparing dilution cultures consists of adding the 

 inoculum or sample to a tube of melted agar and carefully mixing the 

 added material throughout the agar mass. By means of a pipette or loop, 

 a small amount of this suspension is then transferred to a second agar 

 tube, a portion of the second to the third, etc., to secure the desired dilu- 

 tions of the original sample. The melted agar containing such dilutions 

 is then poured into petri dishes and allowed to solidify. The second 

 method is often equally as satisfactory as the first. 



Streak Cultures: For isolating Penicillia from natural materials, or for 

 separating two or more strains growing together in culture, streak plates 

 are often quite satisfactory and are more easily prepared than either of the 

 above types of dilution plates. In the preparation of streak cultures 

 care should be taken in selecting the inoculum so that a minimum of ex- 

 traneous material is included and the streaking process should be continued 

 through a distance sufficient to allow the development of separate colonies. 

 Oftentimes this requires that streaking from an original loopful of inocu- 



