ZoBeU 



44 



Marine Microbiology 



from "marine" materials developing on fresh- water media as on parallel 

 sea-water media. However, most if not all of the workers in this category 

 were plating material from bays, estuaries, and near-shore bodies of brack- 

 ish or salt water which probably contained large numbers of bacteria from 

 the land. Workers who have plated samples of sea water or marine mud 

 collected at places remote from possibilities of terrigenous contamination 

 have found many more bacteria developing on sea-water media than on 

 fresh-water media. The relative numbers of bacterial colonies developing 

 on media similar in all respects except salt content when inoculated with 

 material from different sources are summarized in Table VIII. Similar 

 results have been obtained by Berkeley (1919), Korinek (1926), and 

 LiPMAN (1926), who recommend sea water for the growth of marine 

 microorganisms. 



Table VIII. — Relative numbers of bacteria from marine and terrestrial sources which de 

 velopcd on nutrient agar prepared with various salt solutions: — 



There is not enough difference in the salinity of sea water collected 

 from different parts of the ocean to influence plate counts materially, but 

 there are detectable differences in the growth-promoting properties of 

 sea water collected at different seasons, stations, and depths (Matudaira, 

 1939). These differences in the growth-promoting property of sea water 

 disappear when the sea water has been stored or aged at room temper- 

 ature for a few weeks. Presumably the organic fractions which are respon- 

 sible for the differences are oxidized by bacteria during storage. 



Special media must be employed when examining water or mud 

 samples for the presence of yeasts, molds, autotrophic bacteria, and other 

 physiological types of microorganisms. 



Solidifying agents for plating media : — Gelatin has only a limited 

 usefulness as a solidifying agent in media for marine microorganisms. It 

 may not congeal in the tropics or in the hot hold of a research vessel lack- 

 ing adequate facilities for refrigeration. If it escapes these rigors, plates 

 of gelatin will be liquefied by the actively proteolytic marine microflora, 

 causing colonies to merge before many of the more slowly growing bac- 

 teria have had time to develop into macroscopically visible colonies. 



An appreciable number of the bacteria found in the sea digest agar, but 

 rarely do they liquefy plates of nutrient agar sufficiently to invalidate 

 plate counts. The principal disadvantage of agar is the relatively high 

 temperature at which it commences to congeal, i.e. about 42° C. Many 

 marine bacteria are injured by exposure to this temperatureTalthough the 

 injurious effects can be largely minimized if precautions are taken to cool 

 the agar rapidly immediately after inoculation. 



Being cognizant of the limitations of gelatin and agar, Bertel (1936) 

 has recommended the use of organic skeletal material of sponges. While 



