Identification and epidemiological description of enterohemorrhagic Escherichia coli O157 strains producing low amounts of Shiga toxin 2 in Aichi Prefecture, Japan

Abstract

Out of 68 Esherichia coli O157 field isolates tested in vitro for Shiga toxin (Six) 2 production, 12 (17.6%) produced no or a limited amount of Stx2 (Stx 2 non- or low-producing strain; TNLP) even though all 68 possessed the stx2 gene. The remaining 56 were Stx2 high-producing strains. The 12 TNLPs carried the q21 gene allele, which encodes a transcription antiterminator Q protein and is highly homologous to that of Φ21 phage. They also carried nucleotide substitutions and insertions in the promoter region of the stx2 gene compared with that of O157 EDL933, producing a considerable amount of Stx2. In contrast, the Stx2 high-producing strains carried the q933 gene allele, which was first reported on an stx2 phage (933W), but not the q21 gene allele, and did not have mutations in the promoter region of the stx2 gene. These 2 genetic characteristics, i.e., replacement of the q gene and mutation in the promoter region of the stx2 gene, seemed to determine the amount of Stx2 produced by each strain. The TNLPs were more frequently isolated from healthy carriers than from patients (P < 0.05), suggesting that TNLPs are less virulent than those with normal Stx2 production.