coli and an isogenic mutant lacking relA, the gene coding for the ppGpp synthetase gene. There was no change in β-galactosidase activity (data not EX 527 in vitro shown). This suggests that the stringent response might not be involved in aah-aidA control. Other nutrient-limitation pathways might therefore be involved. In summary, we identified,

in a wild-type pathogenic strain of E. coli, two putative promoters upstream of aah likely to drive the production of aah-aidA bicistronic transcripts. Our work shows that aah and aidA are induced at the early-stationary phase, likely because of nutrient starvation. This pattern leads us to hypothesize that RpoS is the principal regulator of the expression of the aah-aidA operon, at least in the wild-type strain and the conditions we used. This hypothesis is consistent with the consensus sequences of one promoter we identified upstream of aah. Other promoters are likely to be present upstream of aah and aidA and could play further roles in conditions not reproduced in our assays. The preferential expression of AIDA-I at a high density of bacteria and/or during nutrient starvation is consistent with the fact that AIDA-I is mediating bacterial auto-aggregation.

It would indeed make sense to turn on the gene coding learn more for AIDA-I in an environment where a high density of bacteria are present or under adverse conditions of poor nutrient availability, so as to form ‘micro-colonies’ of bacteria of the same kind and increase Acetophenone survival chances. Similar effects of nutrient starvation influencing the expression of virulence genes

in pathogenic E. coli have been observed before: for instance, in enterohemorrhagic E. coli, the expression of LEE genes is activated in response to starvation and bacterial adherence is increased (Nakanishi et al., 2006), and in uropathogenic E. coli, the entry into the stationary phase triggers the expression of fimbriae and an increase in the frequency of adherent bacteria (Aberg et al., 2006). As we were writing this report, another study was published on the regulation of aah and aidA (Benz et al., 2010). This work was performed in a laboratory strain of E. coli with the aah-aidA region cloned on a multicopy plasmid. It therefore complements well our own study performed in a wild-type background. The two aah promoters we identified were also found in this recent study and experiments showed the existence of a bicistronic message, in agreement with our conclusions. Two additional promoters were found upstream of aidA. As explained above, it is possible that the presence of a multicopy plasmid and/or the background of a laboratory strain of E. coli allowed the identification of these promoters that we failed to find.