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Genetic Characterization of Escherichia coli O157:H7/– Strains Carrying the stx 2 Gene but Not Producing Shiga Toxin 2
Author(s) -
Koitabashi Tsutomu,
Vuddhakul Varaporn,
Radu Son,
Morigaki Tadaaki,
Asai Norio,
Nakaguchi Yoshitsugu,
Nishibuchi Mitsuaki
Publication year - 2006
Publication title -
microbiology and immunology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.664
H-Index - 70
eISSN - 1348-0421
pISSN - 0385-5600
DOI - 10.1111/j.1348-0421.2006.tb03779.x
Subject(s) - shiga toxin , stx2 , biology , escherichia coli , shiga like toxin , gene , microbiology and biotechnology , antitermination , nucleic acid sequence , virology , plasmid , genetics , bacteriophage
Nine Escherichia coli O157:H7/– strains isolated primarily from non‐clinical sources in Thailand and Japan carried the stx 2 gene but did not produce Stx2 toxin in a reversed passive latex agglutination (RPLA) assay. A strain (EDL933) bearing a stx 2 phage (933W) was compared to a strain (Thai‐12) that was Stx2‐negative but contained the stx 2 gene. To study the lack of Stx2 production, the Thai‐12 stx 2 gene and its upstream nucleotide sequence were analyzed. The Thai‐12 stx 2 coding region was intact and Stx2 was expressed from a cloned stx 2 gene using a plasmid vector and detected using RPLA. A lacZ fusion analysis found the Thai‐12 stx 2 promoter non‐functional. Because the stx 2 gene is downstream of the late promoter in the stx 2 phage genome, the antitermination activity of Q protein is essential for strong stx 2 transcription. Thai‐12 had the q gene highly homologous to that of Φ21 phage but not to the 933W phage. High‐level expression of exogenous q genes demonstrated Q antitermination activity was weak in Thai‐12. Replication of stx 2 phage was not observed in Stx2‐negative strains. The q‐stx 2 gene sequence of Thai‐12 was well conserved in all Stx2‐negative strains. A PCR assay to detect the Thai‐12 q‐stx 2 sequence demonstrated that 30% of O157 strains from marketed Malaysian beef carried this sequence and they produced little or no Stx2. These results suggest that stx 2 ‐positive O157 strains that produce little or no Stx2 may be widely distributed in the Asian environment.