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Background   Laribacter hongkongensis  is a Gram-negative, urease-positive bacillus associated with invasive bacteremic infections in liver cirrhosis patients and fish-borne community-acquired gastroenteritis and traveler’s diarrhea. Its mechanisms of adaptation to various environmental niches and host defense evasion are largely unknown. During the process of analyzing the  L. hongkongensis  genome, a complete urease cassette and two adjacent  arc  gene cassettes were found. We hypothesize that the urease cassette and/or the  arc  gene cassettes are important for  L. hongkongensis  to survive in acidic environment and macrophages. In this study, we tested this hypothesis by constructing single, double and triple non-polar deletion mutants of the urease and two  arc  gene cassettes of  L. hongkongensis  using the conjugation-mediated gene deletion system and examining their effects in acidic environment  in vitro , in macrophages and in a mouse model.    Results  HLHK9∆ ureA , HLHK9∆ ureC , HLHK9∆ ureD  and HLHK9∆ ureE  all exhibited no urease activity. HLHK9∆ arcA1  and HLHK9∆ arcA2  both exhibited arginine deiminase (ADI) activities, but HLHK9∆ arcA1/arcA2  double deletion mutant exhibited no ADI activity. At pH 2 and 3, survival of HLHK9∆ arcA1/arcA2  and HLHK9∆ ureA/arcA1/arcA2  were markedly decreased (p < 0.001) but that of HLHK9∆ ureA  was slightly decreased (p < 0.05), compared to wild type  L. hongkongensis  HLHK9. Survival of HLHK9∆ ureA/arcA1/arcA2  and HLHK9∆ arcA1/arcA2  in macrophages were also markedly decreased (p < 0.001 and p < 0.01 respectively) but that of HLHK9∆ ureA  was slightly decreased (p < 0.05), compared to HLHK9, although expression of  arcA1 ,  arcA2  and  ureA  genes were all upregulated. Using a mouse model, HLHK9∆ ureA  exhibited similar survival compared to HLHK9 after passing through the murine stomach, but survival of HLHK9∆ arcA1/arcA2  and HLHK9∆ ureA/arcA1/arcA2  were markedly reduced (p < 0.01).    Conclusions  In contrast to other important gastrointestinal tract pathogens, ADI pathway is far more important than urease for acid resistance and intracellular survival in  L. hongkongensis . The gene duplication of the  arc  gene cassettes could be a result of their functional importance in  L. hongkongensis .

bmc microbiology

Arginine deiminase pathway is far more important than urease for acid resistance and intracellular survival in Laribacter hongkongensis: a possible result of arc gene cassette duplication