An attempt was made to examine quantitatively the survival of Escherichia coli and the halotolerant Brevibacterium species, as a function of the exposure time to carbonyl cyanide m-chlorophenylhydrazone (CCCP), a proton conductor. Growth rates, viability, and protein concentrations of E. coli grown in the absence of glucose were unaffected by the addition of 100 microM CCCP. In the presence of glucose the viability was reduced after 24 h incubation with CCCP. Such a high efficiency of CCCP lethality for E. coli cells in the presence of glucose was attributed to not only the growth phase but also the acidic pH of the culture due to metabolites from glucose, mainly lactic acid. The culture of E. coli appeared to be in a syncopic state hovering between life and death when it was exposed to CCCP in the presence of more than 30 mM glucose. In contrast, growth rates of Brevibacterium were reduced in proportion to the exposure time to CCCP. The lethal effect of CCCP to Brevibacterium was slightly enhanced by the addition of glucose into the culture.