Multiple roles for iron in microbial physiology: Bacterial oxygen sensing by heme-based sensors

Bacterial oxygen sensing embodies a fascinating interplay between evolutionary pressures and physiological adaptations to varying oxygen levels. Throughout Earth's history, the composition of the atmosphere has undergone significant changes, from anoxic conditions to the gradual accumulation of oxygen. In response, microbial life has evolved diverse strategies to cope with these shifting oxygen levels, ranging from anaerobic metabolism to oxygen-dependent pathways crucial for energy production and cellular processes typical for eukaryotic, multicellular organisms. Of particular interest is the role of iron in bacterial oxygen sensing systems, which play pivotal roles in adaptation to changing oxygen levels. Only free iron, heme-iron, and non-heme iron directly sense oxygen. These iron-containing proteins, such as heme-containing sensors and iron-sulfur cluster proteins, regulate the expression of genes and activity of enzymes involved in oxidative stress defence, virulence, and biofilm formation, highlighting their significance in bacterial pathogenesis and environmental adaptation. Special attention in the review is paid to the mechanisms of oxygen detection and signal transduction from heme-containing sensing to functional domains in the case of bacterial heme-based oxygen sensors.