Removal of RcsB/RcsA-dependent inhibition of curli expression improves autoaggregation in the <i>Escherichia coli</i> O157:H7

Enterohemorrhagic Escherichia coli O157:H7 (E. coli O157) strains do not produce curli and do not form biofilms, but they retain their ability for autoaggregation. In our study, we investigated whether curli expression would impact E. coli O157 autoaggregation. Curli-expressing strain CPM1 was derived from E. coli O157 strain ATCC 43890 as a clone forming red colonies on Congo red (CR)-agar. To quantitatively evaluate autoaggregation we applied a recently developed experimental system based on magnetic levitation. The efficiency of autoaggregation was evaluated by the geometry of macroautoaggregates and by relative amounts of aggregated and free-swimming bacteria. The curli producing CPM1 strain's autoaggregates had a volume 3.4 times smaller than that of ATCC 43890, despite the number of autoaggregated CPM1 bacteria being higher. Curli proteins were incorporated into matrix, making CPM1 autoaggregate more compact. Curliated CPM1 bacteria adhered better to vertical surfaces reducing the number of free swimmers. The Ser206Phe substitution in the transcriptional regulator RcsB was responsible for CPM1 curli-expressing phenotype. The mutation affected RcsB interactions with the accessory protein RscA. RcsA hyperexpression inhibited curli production and decreased efficiency of autoaggregation. Taken together, the obtained results demonstrated that removal of RcsB/RcsA-dependent inhibition of curli expression improves autoaggregation in the E. coli O157.