Comparison of the inhibitory effect of sorbic acid and amphotericin B on Saccharomyces cerevisiae: is growth inhibition dependent on reduced intracellular pH?
D. Bracey et al., Comparison of the inhibitory effect of sorbic acid and amphotericin B on Saccharomyces cerevisiae: is growth inhibition dependent on reduced intracellular pH?, J APPL MICR, 85(6), 1998, pp. 1056-1066
The effects of sorbic acid and amphotericin B on the growth and intracellul
ar pH (pH(i)) of Saccharomyces cerevisiae were studied and compared. Past e
vidence has suggested that the inhibitory action of sorbic acid on yeast is
due to reduction of pH(i) per se. However, using a novel method to measure
pH(i) in growing cells, little correlation was found between reduced growt
h rate on exposure to sorbic acid and reduction of pH(i). In fact, growth i
nhibition correlated with an increase in the intracellular ADP/ATP ratio du
e to increased ATP consumption by the cells. This was partly attributed to
the activation of protective mechanisms, such as increased proton pumping b
y the membrane H+-ATPase, which ensured that pH(i) did not decline when cel
ls were exposed to sorbic acid. Therefore, the available evidence suggested
that the inhibitory action of sorbic acid nias due to the induction of an
energetically expensive protective mechanism that compensated for any disru
ption of pH(i), homeostasis but resulted in less available energy for norma
l growth. In contrast to sorbic acid, with amphotericin B there was a direc
t correlation between growth inhibition and reduction of pH(i) due to the u
ncoupling effect of this compound on the plasma membrane. The inhibitory ef
fect of amphotericin B was consistent with membrane disruption, or 'proton-
uncoupling' leading to growth inhibition due to proton influx, decline in p
H(i) and partial dissipation of the proton gradient.