Presumably, ethanol then is more damaging for the cytoplasm than glycerol.
(It may be noted thataw¼0.973 caused by ethanol corresponds to 7%(w/w)
of ethanol in water.) But also substances that can be kept out of the cell, like
polyethylene glycol and sucrose, show a significant difference. Table 8.1 also
shows considerable difference between the two bacteria, even in the order of
inhibitory activity of the components.
Moreover, other conditions determine the lowestawtolerated. Figure
8.13 illustrates how two inhibiting components can reinforce each other’s
effect. How this quantitatively works out greatly depends on the
components and the organism, and the curves can be markedly asymmetric.
Nevertheless, the trend illustrated in the figure is often obeyed. One may
also determine contour lines for equal rate of growth. Other conditions,
notably temperature and the kinds and concentrations of nutrients present,
also affect the result. Actually, one could in principle ‘‘construct’’ n-
dimensional graphs, in which the combined effect of all factors inhibiting
and promoting growth for a certain organism would be given. The mutual
enhancement of inhibiting factors is at the basis of ‘‘mild conservation,’’
FIGURE8.13 Growth of a microorganism as a function of the concentration of
two components X and Y, say, a sugar and an acid. The full contour line marks the
boundary between growth and no growth at optimum conditions (say, optimum
temperature), the broken one at some suboptimum condition. Only meant to
illustrate trends.