Chapter IV 



— 43 



Methods of Enumeration 



DAMM (1932), Gee (19326), and many others for the cultivation of marine 

 bacteria contain nitrate. Using concentrations of potassium nitrate rang- 

 ing from 0.005 to o.io per cent, ZoBell (1941a) was unable to demon- 

 strate that nitrate had any effect, either beneficial or detrimental, on the 

 number or kinds of bacteria which developed in nutrient media. 



Following the methods of soil bacteriologists, Duggeli (1924), Snow 

 and Fred (1926), Henrici and McCoy (1938), and others have featured 

 the use of sodium caseinate in media designed for the cultivation of fresh- 

 water bacteria. Sodium caseinate or Nahrstoff Heyden has also been 

 recommended for the cultivation of marine bacteria by Gazert (1906&) 

 and others. ZoBell (1941a) found, however, that caseinate tends to make 

 sea-water agar opaque, and there is no evidence that it promotes the 

 growth of any bacteria when used in combination with Bacto-peptone. 



]\Iedium 2216 is recommended for general plate counts of aerobic 

 heterotrophs occurring in the sea. Selective or differential media must 



50 



5.5 



6.0 



6.5 



70 

 pH 



7.6 



8.0 



05 



90 



Fig. 5. — Curve showing the effect of />H of nutrient sea-water agar on the 

 number of colonies of marine bacteria which develop {jrom ZoBell, 19410). 



be used when looking for special kinds or types of microorganisms. Par- 

 allel direct microscopic counts and successive dilution methods of estimat- 

 ing bacterial populations indicate that the best general-purpose plating 

 medium detects only one to ten per cent of the bacteria present in marine 

 materials, but Medium 2216 does give maximum and reproducible counts. 

 The small number of bacteria detected by plate counts is largely attribut- 

 able to the occurrence of many bacteria in clumps, chains, zooglea, or at- 

 tached to particles of detritus which give rise to single colonies. Direct 

 microscopic studies show that marine bacteria rarely occur as solitary 

 individuals. 



An amazingly large number of workers have used distilled water or 

 tap water, sometimes with and sometimes without the addition of salts, 

 for the preparation of nutrient media for marine microorganisms, thereby 

 following standard methods recommended for the cultivation of fresh- 

 water bacteria. Some workers justified their use of fresh-water media 

 for the cultivation of marine bacteria by finding nearly as many bacteria 



% 



