UNLABELLED

The HUα(E38K, V42L) mutant of the bacterial histone-like protein HU causes a major change in the transcription profile of the commensal organism Escherichia coli K-12 (Kar S, Edgar R, Adhya S, Proc. Natl. Acad. Sci. U. S. A. 102:16397-16402, 2005). Among the upregulated genes are several related to pathogenic interactions with mammalian cells, as evidenced by the expression of curli fibers, Ivy, and hemolysin E. When E. coli K-12/ HUα(E38K, V42L) was added to Int-407 cells, there was host cell invasion, phagosomal disruption, and intracellular replication. The invasive trait was also retained in a murine ileal loop model and intestinal explant assays. In addition to invasion, the internalized bacteria caused a novel subversion of host cell apoptosis through modification and regulation of the BH3-only proteins Bim(EL) and Puma. Changes in the transcription profile were attributed to positive supercoiling of DNA leading to the altered availability of relevant promoters. Using the E. coli K-12/HUα(E38K, V42L) variant as a model, we propose that traditional commensal E. coli can adopt an invasive lifestyle through reprogramming its cellular transcription, without gross genetic changes.

IMPORTANCE

Escherichia coli K-12 is well established as a benign laboratory strain and a human intestinal commensal. Recent evidences, however, indicate that the typical noninvasive nature of resident E. coli can be reversed under specific circumstances even in the absence of any major genomic flux. We previously engineered an E. coli strain with a mutant histone-like protein, HU, which exhibited significant changes in nucleoid organization and global transcription. Here we showed that the changes induced by the mutant HU have critical functional consequences: from a strict extracellular existence, the mutant E. coli adopts an almost obligate intracellular lifestyle. The internalized E. coli exhibits many of the prototypical characteristics of traditional intracellular bacteria, like phagosomal escape, intracellular replication, and subversion of host cell apoptosis. We suggest that E. coli K-12 can switch between widely divergent lifestyles in relation to mammalian host cells by reprogramming its cellular transcription program and without gross changes in its genomic content.