Formate production is dispensable for virulence in human volunteers.

is a causative agent of cutaneous ulcers in children who live in the tropics and of the genital ulcer disease chancroid in sexually active persons. In the anaerobic environment of abscesses and ulcers, anaerobic respiration and mixed acid fermentation (MAF) can be used to provide cellular energy. In , MAF produces formate, acetate, lactate, succinate, and ethanol; however, MAF has not been studied in . In human challenge experiments with 35000HP, transcripts of the formate transporter FocA and pyruvate formate lyase (PflB) were upregulated in pustules compared to the inocula. We made single and double mutants of and in 35000HP. Growth of 35000HPΔ was similar to 35000HP, but 35000HPΔ and 35000HPΔ had growth defects during both aerobic and anaerobic growth. Mutants lacking did not secrete formate into the media. However, formate was secreted into the media by 35000HPΔ, indicating that has alternative formate transporters. The pH of the media during anaerobic growth decreased for 35000HP and 35000HPΔ, but not for 35000HPΔ or 35000HPΔ, indicating that is the main contributor to media acidification during anaerobic growth. We tested whether formate production and transport were required for virulence in seven human volunteers in a mutant versus parent trial between 35000HPΔ and 35000HP. The pustule formation rate was similar for 35000HP (42.9%)- and 35000HPΔ (62%)-inoculated sites. Although formate production occurs during growth and transcripts are upregulated during human infection, and are not required for virulence in humans.