254 Development of a Standardised Agar Medium 



agar platings were dried for various periods before inoculation with the 

 spreading organism. Platings of sterile nutrient agar were poured and 

 were dried for periods of 14, 11, and 2 days respectively and control 

 platings were dried for two hours. Five parallel platings were prepared 

 for each period of drying. The drying took place in an incubator at 30° C. 

 All the platings were then inoculated at the same time with 0-02 c.c. of 

 a suspension of B. dendroides spores, and were incubated at 30° C. The 

 growth after 48 hours is recorded in Table III. 



Table III. 



Spreading of Bacillus dendroides. Effect of previous 

 drying of 'plates. 



It will be seen that the spreading is checked by drying of the water 

 film even when this takes place before inoculation. On the plates that 

 had been dried for two or more days, the period of motile spreading was 

 entirely inhibited, and the organism developed long chains of cells in 

 which spores were produced, the colony assuming the anthrax-like edges 

 characteristic of normal growth after spreading has ceased. On the con- 

 trol plates the spreading was quite normal. 



Experiments similar to the above have also been carried out, using 

 a "synthetic " agar medium and in this case also the limitation of spread- 

 ing was observed on platings dried previous to inoculation. 



If the retardation and cessation of spreading on platings be normally 

 due to drying of the surface moisture, we should expect that if this drying 

 were prevented, the colony would continue to spread indefinitely, in- 

 creasing in area at an even rate, without retardation, until the plate 

 was covered. This point was therefore tested by growing B. dendroides 

 on platings kept in an atmosphere saturated with moisture. 



Six sterile platings of synthetic agar medium were poured and inocu- 

 lated at the centres with spores of B. dendroides. The plates were kept at 

 room temperature, in an atmosphere saturated with moisture in a Novy 

 jar. The areas of growth of the organisms were measured at intervals. 

 Fig. 4 shows the mean areas of growth on the six parallel plates, plotted 



