Subtilase Cytotoxin-Encoding subAB Operon Found Exclusively among Shiga Toxin-Producing Escherichia coli Strains

Abstract

The presence of subAB was investigated for 3,453 Escherichia coli strains of various pathogenic categories. The occurrence of other virulence genes in subAB-positive strains was investigated. The subAB operon was detected among some Shiga toxin-producing E. coli (STEC) serotypes devoid of eae and carrying ehxA. Most subAB-positive strains also harbored stx 2 , iha, saa, and lpfA O113 .Subtilase cytotoxin, a new member of the AB 5 toxin family, was identified for the first time in 2004 in a virulent O113:H21 Shiga toxin-producing Escherichia coli (STEC) strain that caused an outbreak of hemolytic-uremic syndrome in South Australia (16,18). The presence of subAB genes was further detected in other STEC strains belonging to different serotypes (19). Subsequently, subAB genes were identified among STEC strains isolated in other countries (3,8,9,14,25).To evaluate how widely distributed the subAB operon is, we studied a large collection of STEC serotypes from nonhuman sources and E. coli strains of different pathogenic categories associated with human infections. The subAB-positive strains were further characterized regarding the presence of other virulence genes.A total of 2,255 E. coli strains isolated from humans and belonging to enteropathogenic E. coli (EPEC), enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), enteroaggregative E. coli (EAEC), extraintestinal pathogenic E. coli (ExPEC), and E. coli strains not belonging to the diarrheagenic categories described so far were randomly selected. STEC strains isolated in Brazil from humans have previously been tested for the presence of subAB by our group (3). The 1,198 STEC strains from nonhuman sources were isolated from dairy cattle, beef cattle, buffaloes, and goats. Overall, 109 different STEC serotypes were tested. An STEC strain of serotype O113:H21 (3) was used as a reference strain for subAB, cdt-V, and lpfA O113 , and E. coli strain DH5␣ was used as a negative control.The strains were screened for the presence of the subAB operon (encoding subtilase cytotoxin) using colony hybridization assays (21). The 1,823-bp subAB-specific DNA probe was derived from the STEC serotype O113:H21 (3) strain by PCR as previously described (19). Hybridization assays were performed under stringent conditions, and the probe was labeled with [␣-32 P]dCTP (Amersham), using the Ready-To-Go DNA labeling kit (Amersham). All strains which yielded a positive or weak signal in hybridization assays with the subAB probe were retested by PCR (18,19), and only those confirmed by PCR were considered to be carrying this sequence.The genetic profiles of the subAB-positive strains were determined using our previously reported data for the same strains (6,7,12,13,20,24) regarding the presence of the ehxA, eae, stx 1 , stx 2 , and adhesin-encoding genes (1,10,11,15,17,22,23).A total of 130 STEC strains carrying the subAB operon, representative of each serotype and isolated from different animals, were analyzed by PCR for the presence of genes encoding Lpf O113 and cytolethal distendi...

Virulence Characterization of Shiga-Toxigenic Escherichia coli Isolates from Wholesale Produce

“…Results showed that only six STEC strains carried the saa gene but that all six also had subAB, stx 2 , and ehxA and expressed both Stx2 and EhxA (Table 1). This is consistent with the findings from the analysis of a large panel of STEC isolates in Brazil, which showed that strains that carry saa and subAB often also have stx 2 and ehxA (7). Two of the six STEC strains from produce that had all four of these virulence genes were of the O116:H21 serotype.…”

Prevalences of Shiga Toxin Subtypes and Selected Other Virulence Factors among Shiga-Toxigenic Escherichia coli Strains Isolated from Fresh Produce

“…The saa and subAB genes are putative virulence factors that are found mostly in eae-negative STEC strains (10,23,24). Consistently, neither gene was present in produce STEC strains that had eae, and among the eae-negative strains, 35% and 32% had the saa and subAB genes, respectively.…”

“…There were three O113:H21 strains that were isolated from spinach, and all three carried both saa and subAB (32). Studies of STEC isolated from animals and ground meats showed that O116:H21 strains often also carried subAB (20,23). There were three O116:H21 strains, all isolated from spinach, and two carried subAB, so this gene does appear to be fairly prevalent in this serotype as well.…”

“…The prevalence of the subAB gene among STEC strains varies greatly. A study of environmental STEC strains in Brazil showed that 25% of the ϳ1,200 strains tested had subAB, and the prevalence rates ranged from 3% in isolates from goats to 44% in those from dairy cattle (23). Another study from Brazil tested 121 STEC strains isolated from clinical and animal samples and found subAB in 48% of the strains but not in the 49 clinical isolates (30).…”

Molecular Characterization of Shiga Toxin-Producing Escherichia coli Strains Isolated in Poland

“…Only four STEC non-O157 strains isolated from cattle were included in the LEE -negative group (not carry eae gene). LEE-negative STEC non-O157 strains have been isolated from human and animals from several years (Galli et al, 2010;Irino et al, 2010). Those isolates may carry other adhesion factors as LpfA, Iha and Saa which play role in pathogenesis of human infection of this bacteria (Galli et al, 2010;Irino et al, 2010).…”