Secondary structures of LptD from S. flexneri (SfLptD) are labelled at the top of the sequence alignment. Strands that consist of the β-barrel of LptD are numbered. Residues of the two distorted β-strands (β1 and β2), L4 and L8 are boxed. Identical hydrophobic residues, positively-charged residues and negatively-charged residues are highlighted in yellow, blue and red, respectively. Three proline residues Pro 231, Pro 246 and Pro 261 in SfLptD are conserved in different species. SfLptD, StLptD, PaLptD, KpLptD and AbLptD represent LtpD sequences from S. flexneri, Salmonella enteric Typhimurium str. LT2, P. aeruginosa, Klebsiella pneumoniae and Acinetobacter baumannii, respectively.