Table 1. Bacterial strains and plasmids used in this study.

Strains or plasmids	Description of strains or plasmids	Source
Strains
E. coli BW 20767 λpir	RP4 2tet: mu-1kan::Tn7integrant leu 63::IS10 rec A1 cre(510 hsdR17 end A1 Zbf-5 uid) (ΔMluI): pir thi	Gift from Dr. William Metcalf, University of Illinois
WT	V. cholerae O139 isolated from a patient in HUSM	Hospital University Sains Malaysia
VCUSM14	V. cholerae O139 ALA auxotroph derived from VCUSM2; has 1 copy of Δctx operon; 1 copy of Δctx operon::ΔaphA; ΔhemA	Universiti Sains Malaysia
VCUSM20	V. cholerae O139 ALA auxotroph strain that was constructed from VCUSM14. It has 1 copy of Δctx operon, 1 copy of Δctx operon::ΔaphA, rtxC::aphA and ΔhemA genes.	This study
VCUSM21	V. cholerae O139 ALA auxotroph strain that was constructed from VCUSM20. It has 1 copy of Δctx operon, 1 copy of Δctx operon::ΔaphA, ΔrtxC/A and ΔhemA genes.	This study
VCUSM21P	V. cholerae O139 ALA non-auxotroph strain that was constructed from VCUSM21. It has 1 copy of Δctx operon, 1 copy of Δctx operon::ΔaphA, ΔrtxC/A and hemA genes.	This study
Plasmids
pWM91-rtxC::aphA	pWM91 with rtxC::aphA fragment	USM
pWM91-ΔrtxC/A	pWM91 with ΔrtxC/A fragment	USM
pCVD-hemA/M	pCVD442 with hemA-hemM fragment	USM