Abstract
BackgroundEscherichia coli O157:H7 (O157) strain 86–24, linked to a 1986 disease outbreak, displays curli- and biofilm-negative phenotypes that are correlated with the lack of Congo red (CR) binding and formation of white colonies (CR−) on a CR-containing medium. However, on a CR medium this strain produces red isolates (CR+) capable of producing curli fimbriae and biofilms.ResultsTo identify genes controlling differential expression of curli fimbriae and biofilm formation, the RNA-Seq profile of a CR+ isolate was compared to the CR− parental isolate. Of the 242 genes expressed differentially in the CR+ isolate, 201 genes encoded proteins of known functions while the remaining 41 encoded hypothetical proteins. Among the genes with known functions, 149 were down- and 52 were up-regulated. Some of the upregulated genes were linked to biofilm formation through biosynthesis of curli fimbriae and flagella. The genes encoding transcriptional regulators, such as CsgD, QseB, YkgK, YdeH, Bdm, CspD, BssR and FlhDC, which modulate biofilm formation, were significantly altered in their expression. Several genes of the envelope stress (cpxP), heat shock (rpoH, htpX, degP), oxidative stress (ahpC, katE), nutrient limitation stress (phoB-phoR and pst) response pathways, and amino acid metabolism were downregulated in the CR+ isolate. Many genes mediating acid resistance and colanic acid biosynthesis, which influence biofilm formation directly or indirectly, were also down-regulated. Comparative genomics of CR+ and CR− isolates revealed the presence of a short duplicated sequence in the rcsB gene of the CR+ isolate. The alignment of the amino acid sequences of RcsB of the two isolates showed truncation of RcsB in the CR+ isolate at the insertion site of the duplicated sequence. Complementation of CR+ isolate with rcsB of the CR− parent restored parental phenotypes to the CR+ isolate.ConclusionsThe results of this study indicate that RcsB is a global regulator affecting bacterial survival in growth-restrictive environments through upregulation of genes promoting biofilm formation while downregulating certain metabolic functions. Understanding whether rcsB inactivation enhances persistence and survival of O157 in carrier animals and the environment would be important in developing strategies for controlling this bacterial pathogen in these niches.
Highlights
Escherichia coli O157:H7 (O157) strain 86–24, linked to a 1986 disease outbreak, displays curli- and biofilm-negative phenotypes that are correlated with the lack of Congo red (CR) binding and formation of white colonies (CR−) on a CR-containing medium
Congo red-positive isolates were recovered at low frequency Streak-plating of the overnight cultures of strain 86–24 on Yeast extract casamino acid (YESCA)-Congo red plates resulted in recovery of about 3 Congo red-positive colonies (CR+) per 106 white colonies after 48 h of incubation at 28 °C
Increased Congo red binding of the CR+ isolates correlated with the production of higher biofilm biomass and curli fimbriae Fig. 1a affirms increased Congo red-binding ability of strain NADC 6565 that was originally selected as red colonies among the majority of the white colonies (NADC 6564) produced by plating strain 86–24 on a Congo red agar medium
Summary
Escherichia coli O157:H7 (O157) strain 86–24, linked to a 1986 disease outbreak, displays curli- and biofilm-negative phenotypes that are correlated with the lack of Congo red (CR) binding and formation of white colonies (CR−) on a CR-containing medium. The formation of biofilms represents a survival strategy involving intricate network of regulatory circuits controlling induction of various pathways conducive for biofilm formation [7, 8]. Some of these pathways encode structural elements such as curli fimbriae, cellulose and colanic acid that play specific roles at various stages of biofilm formation [9, 10]. Curli fimbriae, which are highly adhesive equivalents of functional amyloids and encoded by the divergently transcribed csgBAC and csgDEFG operons, are important in biofilm formation by promoting initial bacterial-substratum interactions and subsequent cell-cell aggregation [7]
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