L-Sorbose utilization by virulent Escherichia coli and Shigella: different metabolic adaptation of pathotypes

The frequency of L-Sorbose utilization differs significantly between pathotypes of Escherichia coli and Shigella from 93% to 0%. Among 266 strains tested, this frequency increased in the order Shigella, enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), Shiga toxin-producing E. coli (STEC), enteroaggregative E. coli, enteropathogenic E. coli (EPEC), and neonatal bacterial meningitis (NBM) E. coli. This suggests an association of pathomechanism with the capability to degrade L-Sorbose. The use of a selective agar, containing L-Sorbose and antibiotics, facilitated the isolation of L-Sorbose-non-utilizing ETEC from stool specimens of patients. The sor operon, comprising seven genes in the order sorCDFBAME, confers L-Sorbose utilization. Surprisingly, L-Sorbose-non-degrading Shigella harbored all genes of the sor operon indicating L-Sorbose-utilizing E. coli as ancestor. Additionally, strains of several EIEC and STEC serotypes harbored an inactivated sor operon. These L-Sorbose-non-utilizing Shigella, EIEC, and STEC showed significantly reduced amounts of transcripts as examined for sorC and sorD. Common surface antigens, types of intimin gene, and hemolysin gene as well as use of L-Sorbose suggested the relatedness of attaching and effacing O26:H11 and O55:H7 EPEC and STEC, respectively. pepE and yibC genes flank the sor operon of E. coli and Shigella strains. Surprisingly, one O7:K1:H- NBM E. coli harbored an aroE-homologous gene between its sor operon and pepE as in Klebsiella pneumoniae suggesting a horizontal gene transfer. In conclusion, L-Sorbose utilization of virulent E. coli and Shigella is characterized by different adaptation that represents a valuable tool for evolutionary and diagnostic analysis of related patho- and serotypes.