Changes in free amino acid, ammonium ion and urea contents in the culture medium were investigated during controlled acetification in the presence of acetic acid bacteria. The bacteria were found to use L-proline, L-leucine and ammonium ion as their primary sources of nitrogen; in fact, these three species accounted for 68.1% of their total nitrogen uptake. Concentration changes were very similar for all amino acids and consistent with changes in cell concentrations. Thus, cell concentration in the medium decreased through bacterial autolysis as the amino acid concentration increased. By contrast, when a cellular growth is produced the amino acid concentration was decreased. The concentration of acetic acid bacteria oscillated widely during the last fermentation stage. This can be ascribed to the bacteria using an apoptosis-like way in response to the stressing conditions produced by the high concentration of acetic acid present in the medium. (C) 2008 Elsevier Ltd. All rights reserved.