Enteroaggregative Escherichia coli (EAEC) is a common diarrhoeagenic human pathogen, isolated
from patients in both developing and industrialized countries, that is becoming increasingly
resistant to many frontline antibiotics. In this study, we screened 50 E. coli strains from children
presenting with diarrhea at the outpatients clinic of Assiut University Children’s Hospital, Egypt.
We show that all of these isolates were resistant to multiple classes of antibiotics and identified
two as being typical EAEC strains. Using whole genome sequencing, we determined that both
isolates carried, amongst others, blaCTX-M and blaTEM antibiotic resistance genes, as well as many
classical EAEC virulence determinants, including the transcriptional regulator, AggR. We demonstrate
that the expression of these virulence determinants is dependent on AggR, including aar,
which encodes for a repressor of AggR, Aar. Since biofilm formation is the hallmark of EAEC
infection, we examined the effect of Aar overexpression on both biofilm formation and AggRdependent
gene expression. We show that whilst Aar has a minimal effect on AggR-dependent
transcription it is able to completely disrupt biofilm formation, suggesting that Aar affects these
two processes differently. Taken together, our results suggest a model for the induction of
virulence gene expression in EAEC that may explain the ubiquity of EAEC in both sick and healthy