The global pandemic of ESBL-producing Escherichia coli is associated with sequence type 131 (ST131). However, mechanisms of ST131 spread remain unclear. This study searched for proteins with amino acid substitutions specific for ST131 and used proteomics analysis to clarify ST131 characteristics. Five proteins had ST131-specific amino acid substitutions: uncharacterized protein YahO with E34A (m/z 7655); UPF0337 protein YjbJ with V59D, D60S and T63K (m/z 8351); uncharacterized protein YnfD with S106T (m/z 8448); and acid stress chaperone HdeA with Q92K and N94S (m/z 9714). Soluble cytochrome b562 (m/z 11783) showed seven amino acid substitutions, and the sequence differed between clade C of the pandemic clade and non-C. In silico analysis showed YahO protein-protein interaction with YjbJ, possibly related to biofilm formation. Although the function of soluble cytochrome b562 is electron transport of unknown function, its involvement in biofilm formation was predicted. HdeA was a gastric acid resistance-related protein. The function of YnfD was completely unclear. In conclusion, ST131-specific protein amino acid substitutions consisted mainly of a gastric acid resistance protein and proteins of unknown function (possibly involved in biofilm formation), which might be mechanisms for long-term colonization in the human intestinal tract.