Escherichia coli O157:H7--piecing together the jigsaw puzzle

Acid tolerance of enterohemorrhagic Escherichia coli O157:H7 strain ATCC 43894 and its relationship with a large virulence plasmid pO157

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a zoonotic pathogen that causes a severe intestinal infection including hemolytic uremic syndrome in humans. Various factors contribute to its pathogenesis, including a large virulence plasmid pO157. This F-like 92-kb plasmid is isolated in virtually all clinical EHEC isolates, and is considered a hallmark of EHEC virulence. A previous report stated that removal of pO157 from EHEC ATCC 43894 induced overexpression of GadAB that are essential in glutamate-dependent acid resistance (GDAR) system, yet the mechanism remains elusive. Based on this observation, we surmised that pO157 is involved in the regulation of GDAR system. We comparatively analyzed 43894 and its pO157-cured (ΔpO157) mutant 277 for i) their acid resistance, ii) changes in the transcriptional profiles and iii) expression of GDAR associated genes/proteins. Survivability of 43894 upon exposure to acidic conditions was significantly lower than the ΔpO157 mutant. In addition, RNA-sequencing revealed that genes involved in GDAR were significantly down-regulated in 43894 when compared to the ΔpO157 mutant. Exogenous expression of GadE in 43894 led to expression of GadAB, suggesting possible intervention of pO157 in GDAR regulation. Despite these findings, reintroduction of pO157 into 277 did not reverted Gad overexpression. Likewise, removing pO157 from 43894 using the plasmid incompatibility method did not induce Gad overexpression as shown in 277. Taken together, the results suggest that variation in acid resistance among EHEC isolates exists, and the large virulence plasmid pO157 has no effect on weak acid resistance phenotype displayed in 43894.

Prepared salads and public health

In recent years the importance of prepared salads as potential vehicles of gastrointestinal infection has been highlighted by several large outbreaks both nationally and across international boundaries. Between 1992 and 2006, 2274 foodborne general outbreaks of infectious intestinal disease were reported in England and Wales, of which 4% were associated with the consumption of prepared salads. In total, 3434 people were affected, with 66 hospitalizations and one death reported. The attribution of prepared salad types and pathogens among prepared salad associated outbreaks are presented and discussed. Findings from UK studies on salad vegetables, fruit and mixed salads from 1995 to 2007 (21 247 samples) indicate that most bacteria of concern with regard to human health are relatively rare in these products (98.6% of satisfactory quality); however, outbreaks of salmonellosis were uncovered associated with bagged salad leaves and fresh herbs during two such studies. Although it is known that fresh salad vegetables, herbs or fruit may become contaminated from environmental sources, only in recent years has the association of foods of nonanimal origin, such as salad vegetables, with foodborne illness become evident and recurrent, demonstrating that major health problems can arise from consumption of contaminated prepared salads if hygiene practices breakdown.

Update on Escherichia coli O157:H7

Escherichia coli O157:H7 infection is one of the more intriguing emerging infectious diseases of the industrialized world. The clinical importance of this organism first came to light in the 1980s and has been associated with significant morbidity and mortality in the United States. The infection is more common in industrialized countries than developing ones and is most closely associated with asymptomatic colonization of cattle. Fecal oral transmission is the rule, with the inoculum needed for infection much smaller than that required for E. coli-related travelers' diarrhea. The organism can survive for months in the environment, and cross contamination is common. Watery diarrhea that progresses to bloody diarrhea without prominent fever is the classic presentation. The classic biopsy finding is similar to that of ischemic colitis, with acute inflammation and hemorrhage involving the superficial mucosa with preservation of the deeper crypts. E. coli O157:H7 has powerful Shigella-like toxins that are encoded by bacteriophages and can trigger thrombotic complications such as the hemolytic uremic syndrome or thrombotic thrombocytopenic purpura. The very young and the elderly are most at risk for serious disease and complications. Treatment with antibiotics has been reported to increase the risk for complications, but the evidence supporting this conclusion is unconvincing, with many variables affecting outcome in any one patient.

Shiga toxin-producing Escherichia coli infection

Large-scale outbreaks of Shiga toxin-producing Escherichia coli (STEC) infection have revealed the great disease-causing potential of this organism, especially among children and elderly persons. Approximately 5%-10% of people with STEC infection will develop hemolytic-uremic syndrome (HUS), approximately 10% of those who develop HUS will die or have permanent renal failure, and up to 50% of those who develop HUS will develop some degree of renal impairment. Important concepts in understanding the pathogenesis and prevention of STEC-associated HUS are emerging, although no specific therapy yet exists. Optimal management of STEC infection includes intravenous hydration, avoidance of antimotility agents and antimicrobials, and monitoring for sequelae. Antimicrobials may have a potentially harmful role, possibly by inducing intestinal production of Shiga toxin during the diarrheal phase of illness. A recent clinical trial evaluating an intraluminal Shiga toxin-binding agent to ameliorate HUS showed no improvement in outcome. Interventions to prevent HUS from developing in STEC-infected children are under investigation. Prevention of exposure to STEC remains important, and animal vaccines to prevent stool shedding of STEC among food animals are in development.