PREPARATION OF POURED PLATES. 



105 



The agar may be poured at 42 C. in case of organisms whose thermal death- 

 point is known to be high (50 C. or above). For all others it must be cooled 

 carefully to 40 C. before inoculating for poured plates. This requires five or six 

 minutes in the water bath at 40 C. Even this temperature is too high for some 

 organisms and then gelatin at 30 C. may be used. When ready to pour, take a 

 clean absorbent cloth and carefully wipe all water from the outside of the tube (the 

 lips of which have been previously flamed gently with a rotation of the tube on its 

 long axis), lift the cover of the dish only as much as is necessary, hold the cover aver 

 the dish (not at one side), pour quickly but gently, and re-cover, tilting the dish 

 about quickly but gently, if the fluid has not already covered the bottom. To en- 

 tirely cover the bottom sometimes requires a smart little jerk, if the agar is not 

 very fluid. The student must learn to work rapidly and dextrously, then there 

 will be no complaint that the agar has solidified before the plates are poured. The 



plates should be set on a level shelf while the agar or 

 gelatin is hardening, or, if the colonies per square 

 centimeter are to be determined, a nivelling appa- 

 ratus such as that shown in fig. 66 must be used, 

 and the dishes should have flat bottoms. When 

 plates have been inoculated too abundantly to secure 

 subcultures from single colonies, these may some- 

 times be obtained from the traces of agar or gelatin 

 left in the tubes from which the plates were poured. 

 With this end in view, these tubes should be re- 

 plugged and laid away, for a few days, the lips and 

 top of the tube which were wet by the agar or gelatin 

 being first heated hot in the flame, care being exer- 

 cised not to crack the tubes. 



All tubes containing fluids should be opened and 

 inoculated in a position as nearly horizontal as their 

 contents will permit, and tubes of solid media, such as 

 agar, may be held level or inverted for inoculation. A convenient block for holding 

 test-tube cultures during examination is shown in fig. 89. It is usually best to 

 flame the plugs slightly before their removal, particularly if they have been exposed 

 to the air for some days. As an additional precaution the transfers should be made 

 under a glass hood, or in a special culture-chamber. If sterilized needles, loops, 

 knives, forceps, pipettes, or anything else designed to be used in making the transfers 

 have accidentally touched anything wJwtsoever, they are presumably contaminated 

 and must be rejected or reflamed. Do not handle the lips of test-tubes containing 

 gelatin or agar from which plates are to be poured. Your hands may be con- 

 taminated by resistant spores. Take hold of the tubes lower down. To economize 

 gas and avoid heating the air of the small work-chamber to an uncomfortable degree, 

 small, cut-off, constant-flame burners are very convenient (fig. 90). 



*Fic. 90. A constant Bunsen burner with cut-off flame. Very useful for the laboratory table 

 and the culture room. About two-fifths actual size. 



