Problems of adverse pH and bacterial strategies to combat it; overview of bacterial pH homeostasis; receptor-mediated signalling; inducible acid tolerance mechanisms in enteric bacteria; genes regulated by acids and bases - a global view; acid tolerance induced by metabolites and secreted proteins, and how tolerance can be counteracted; acid tolerance in root nodule bacteria; how can archea cope with extreme acidity?; pH homeostasis in acidophiles; pH tolerance in bacillus: alkaphile vs. non-alkaphile; the molecular mechanisms of the regulation of the escherichia coli NHaA Na+/H+ antiporter - a paradigm for and adaptation to Na+ and H+; energetics of Na+/H+ movements at alkaline pH; proton ATPases in bacteria - comparison to escherichia coli F1F0 as the prototype; cation movements at alkaline pH in bacteria growing without respiration.