PHYSIOLOGICAL AND BIOCHEMICAL TECHNICS 173 



determined, a standard curve is often constructed, relating dry weight to 

 optical density (see below). 



Turbidity. Any of the available photometers are suitable for turbidity 

 determination, although the type (Evelyn) which utilizes the sample tube 

 as a lens for concentrating the transmitted light on a single phototube is 

 probably the more accurate. Turbidity is measured most accurately at 

 420 m/x, providing the suspending fluid or medium is colorless. For 

 yellow or brownish media, a wavelength of 660 uifi is employed. Turbid- 

 ity changes during growth are often expressed simply as changes in 

 optical density. However, at the higher values of optical density (0.4- 

 2.0) turbidity is not a linear function of other expressions such as dry 

 weight, total nitrogen, or cell numbers. Therefore, the culture or suspen- 

 sion should be suitably diluted before measurement, in order to fall 

 within the range 0.0-0.4 optical density unit. 



If turbidity is selected as the method of determination of growth, 

 number of cells, dry weight, or cell N, a standard curve relating optical 

 density to the desired unitage is constructed. Determinations of both 

 optical density (optical density = 2 — logio per cent transmission) and, 

 for example, dry weight are performed on aliquots of a given cell suspen- 

 sion. The standard curve then consists of a plot of optical density vs. 

 dry weight. Since the optical density of a given cell suspension varies 

 with the wavelength employed, separate standard curves must be con- 

 structed for each wavelength which is to be used. 



The measurement of turbidity during growth may be accomplished by 

 growing the culture directly in colorimeter tubes or in specially fabricated 

 flasks fitted with side arms which may be inserted in a colorimeter 

 (Wiame and Storck, 1953). 



Production of acid or alkali. The growth of several acid-forming bac- 

 teria may be followed by simple titration of the culture, using standard 

 alkali. For example, nutritional studies or microbiological assays with 

 certain lactic acid bacteria are commonly recorded in terms of milliliters 

 of O.OIA^ NaOH required to titrate the culture to the bromthymol blue 

 end point. Conversely, during grow^th of some Mycobacterium species, 

 acid disappears from the medium and growth may be recorded as milli- 

 liters of O.OIA'' HCl required to reach the indicator end point. 



One limitation of this method is obvious. After maximum growth is 

 attained, metabolism (e.g., acid formation) continues; thus, the proper 

 incubation time must be selected in order to determine true growth rather 

 than true acid production. In addition, some species of bacteria accumu- 

 late acid in the early stages of growth but subsequently metabolize the 

 acid. 



Total nitrogen. Add a 2.0-ml ahquot (10-100 jug of total N) of the 

 washed bacterial suspension to a 30-ml micro-Kjeldahl flask. Add 2.0 ml 



