ZoBeU 



— 122 



Marine Microbiology 



XXXIII show that some marine bacteria are quickly injured at 35° C. 

 However, it appears doubtful whether enough bacteria may be injured by 

 the plating temperature of agar (40° to 45° C.) to contra-indicate the use 

 of agar plates for estimating the abundance of viable bacteria. 



Table XXXIII. — Relative numbers of colonies developing from sea water and marine mud 

 plated with nutrient sea-water gelatin at dijferent temperatures and cooled immediately to i2°C.: — 



While brief warming during plating at temperatures ranging from 30° 

 to 40° C. does not destroy the viability of large numbers of marine bac- 

 teria, the majority of them are killed by exposure to this temperature for 

 ten minutes, as shown by the following results obtained by ZoBell and 

 Conn (1940) who plated in duplicate ten samples of sea water and ten 

 samples of marine mud : 



Samples held lo minutes at 

 Per cent survival in sea water 

 Per cent survival in mud 



Comparing these results with those reported by Frankland and Feank- 

 LAND (1894) for the percentage survival of bacteria in river water reveals 

 that, whereas the majority of marine bacteria succumb when held for 

 10 minutes at 30° to 40° C, the average thermal death point of fresh- 

 water bacteria is about 10 degrees higher: 



Samples held 15 minutes at 



Per cent survival in Seine River water 



There is no evidence to suggest that any of the bacteria are killed by pro- 

 longed maintenance at 20° to 25° C, although some marine bacteria are 

 killed in ten minutes at 28° C. The following number of pure cultures 

 of marine bacteria survived ten minutes in broth at the stated temper- 

 atures : 



Cultures held lo minutes at 

 25 cultures from sea water 

 25 cultures from marine mud 

 78 marine stock cultures 



20° C. 30° C. 40° C. 50° C. 60° C. 80° C. 100° C. 



25 24 9 3 o o o 



25 21 II 5 I I o 



78 78 36 14 8 6 2 



Bedford (19336) found that 37° C. was lethal for 40 of the 71 cultures of 

 marine bacteria with which he was working. These thermal death points 

 are considerably lower than those of most terrestrial or fresh-water 

 bacteria. 



Thermotolerant spore-formers are in the minority, although large num- 

 bers of them occur in the sea as attested by the isolation of 80 such cultures 

 from marine materials by Newton (1924). 



Optimum temperatures for the growth of the 71 cultures of marine 

 bacteria studied by Bedford (19336) ranged from 20° to 25° C. The max- 

 imum temperatures at which they grew were as follows: 5 at 25° C, 36 at 

 30° C, 13 at 37° C, 5 at 40° C, 2 at 42.5° C, and 4 at 45° C. All of Bed- 

 ford's cultures grew at 15° C, 67 of them grew at 10° C., 53 at 0° C, 22 

 at — 5° C, and 10 at — 7.5° C. 



