pH tolerance in Bacillus: Alkaliphiles versus non-alkaliphiles

Monovalent cation/proton antiporters that catalyse electrogenic uptake of H⁺ in exchange for cytoplasmic K⁺ and/or Na⁺ are centrally involved in bacterial pH homeostasis under alkaline challenge. Systematic attempts have identified some, but not yet all of the genes encoding such antiporters that participate in pH homeostasis in the neutrophilic Bacillus subtilis and the extremely alkaliphilic Bacillus firmus OF4. In each organism there are at least three distinct antiporters involved in pH homeostasis. They differ in cation requirement, with pH homeostasis specifically utilizing Na⁺/H⁺ antiport in the alkaliphile and using either Na⁺ or K⁺/H⁺ antiport in B. subtilis. Some of the antiporters involved in pH homeostasis are constitutive and are in place to respond to sudden pH shifts, but there is also an inducible component. At least two sets of homologous antiporters (NhaC and Mrp/Pha) function in both alkaliphiles and neutrophiles. An additional antiporter of a different transport protein family, the Gram-positive tetracycline-metal/H⁺ antiporter, is important in pH homeostasis in B. subtilis but has not yet been shown to be present in any alkaliphile. There are also differences outside of the antiporters themselves that contribute to the greater capacity of the alkaliphiles for pH homeostasis, including cation re-entry capacity and possible surface properties.