Oxygen transfer performances in intensive microbial cultures are compared with those occuring in coalescing and non-coalescing mineral media. E. coli fed-batch cultures are carried out in a 22 L bioreactor. Biomass concentrations of 80 gDW L−1 are reached, with oxygen consumption rates of up to 0.6 mol L−1 h−1. To achieve these high transfer performances, dissipated power e reaches 35 kW m−3. The hold-up in the culture broth and in the corresponding supernatant matches the non-coalescing mineral medium. Oxygen transfer coefficients, K L a in mineral media, and K T in the culture broth, are compared. K T, calculated online from a gas balance method, excesses 1 s−1. Yet, for given values of e, K T is 4–8 times lower than K L a determined in the non-coalescing mineral medium. The cell activity modifies the chemical medium properties and reduces the oxygen transfer conductance, as in a non-coalescing ionic medium containing surfactant.