has microbe mutated gene dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA dppA host E. coli pathogen interaction mouse host pathogen interaction E. coli dppA mutant interaction with mouse