Antimicrobial Resistance of Shiga Toxin-Producing Escherichia coli

Jinru Chen,    Department of Food Science and Technology, The University of Georgia, Griffin, GA, USA

Keywords

Shiga toxin-producing E. coli; enterohemorrhagic E. coli; E. coli O157:H7; antimicrobial resistance; antibiotic resistance; mobile DNA elements; oxidative stress; osmotic stress; acid stress

Chapter Outline

Introduction

Shiga Toxin-Producing E. coli and Enterohemorrhagic E. coli

Shiga toxin-producing E. coli (STEC) is a group of E. coli that is capable of producing at least one of the potent, proteinous cytotoxins known as Shiga toxins. The structure and biological activity of Shiga toxins of E. coli share striking resemblance to the cytotoxin produced by Shigella dysenteriae serotype 1 (O’Brien et al., 1982). A subset of STEC with great clinical significance and public health impact is enterohemorrhagic E. coli (EHEC), which causes severe clinic manifestations such as hemorrhagic colitis, hemolytic-uremic syndrome, and thrombotic thrombocytopenic purpura. Since its debut as a notorious foodborne pathogen in 1982, EHEC has caused numerous outbreaks of infections worldwide. EHEC serotype O157:H7 is responsible for the majority of the outbreaks (Siegler, 1995) although non-O157 EHEC has also been involved, especially in Australia, Germany, Austria, and the United States (Elliott et al., 2001; Gerber et al., 2002).

Cattle have been identified as the symptomless carrier and primary reservoir of STEC, and calves seem to carry EHEC O157:H7 more frequently than adult cattle. As a result, foods originating from a bovine source have most frequently been implicated in outbreaks of EHEC infections. In addition to foods of bovine origin, other foods have also been linked to outbreaks of EHEC infections. EHEC outbreaks associated with fresh produce, such as spinach, lettuce, and alfalfa sprouts have been traced to contamination at farm level (CDC, 2007).

STEC has been isolated from different types of meat products including beef, lamb, pork, and poultry (Brooks et al., 2001; Doyle and Schoeni, 1987). It has also been isolated from unpasteurized cheese, raw and pasteurized milk, as well as mayonnaise (Watanabe et al., 1999; Werber et al., 2006). Vegetables and fruits have been contaminated with STEC during cultivation, harvesting, handling, processing, and distribution (CDC, 2007). Apple drops which are commonly used to make cider have been contaminated with STEC through contact with animal manure on the ground or during cider processing (CDC, 1997).

Antibiotic and Antimicrobial Use During Food Production and Processing

Antibiotics are natural or synthesized substances that have the ability to kill or inhibit the growth of bacteria. Antimicrobials are, nevertheless, products that act against a wide range of organisms including bacteria, viruses, fungi, protozoa, and helminths. Understandably, antibiotics are one type of antimicrobial, but not all antimicrobials are antibiotics.

The introduction of antibiotics into clinical practice in the 1930s and 1940s revolutionized human medicine (Cohen, 2000). Unfortunately, clinical success in the treatment of infectious diseases was soon followed by the emergence of antibiotic-resistant pathogens. The use of antibiotics in food animal production started shortly after World War II. It was suggested at the time that the addition of antibiotics into feed or water would promote faster growth in chickens (Moore et al., 1946). Since then, sub-therapeutic levels of antibiotics have been used in the production of many farm animals (Landers et al., 2012). The mechanism on how antibiotics improve animal growth has never been fully understood, although several theories have been proposed (Cromwell, 1991; John, 2006).

Antibiotics are used, during animal production, for different purposes, for example, therapeutic treatment of existing bacterial infections, metaphylactic prevention of infectious diseases, prophylactic prevention of infectious diseases during high-risk periods, or growth promotion (Schwarz and Chaslus-Dancla, 2001). Antibiotics important for human medicine, including tetracycline, penicillin, erythromycin, and other important therapeutic drugs, have been used extensively in food animal production. Some studies suggest that antibiotic use in animal production selects antibiotic-resistant commensal bacteria and zoonotic enteropathogens, which will eventually diminish the therapeutic value of antibiotics in human medicine (Endtz et al., 1991; Levy et al., 1976; Linton et al., 1975; Low et al., 1997). Other studies, however, indicated that antibiotic resistance was easier to acquire in some bacterial species and did not have a convincing link to antibiotic use in food animals (Dargatz et al., 2000; Wells et al., 2001). Regardless of the connection between the two issues, epidemiological data have revealed an increased antibiotic resistance in many different pathogens that are threats to human health (CDC, 2013).

Various antimicrobial substances and interventions have been used to inhibit microbial growth and extend the shelf life of food products. As osmolytes, salt and sugar increase the osmotic pressure and reduce the water activity of food, therefore inhibiting the growth of various microorganisms. In addition to elevating the acidity of food, either naturally or artificially, organic acids have been used as cleaners to disinfect foods such as fresh produce and animal carcasses. Oxidizing agents, such as hydrogen peroxide and sodium hypochlorite, are among the most commonly used sanitizers to disinfect food, or food contact, surfaces. Adaptive exposure of bacterial cells to sub-lethal levels of an antimicrobial agent increases their resistance to a higher level of the same antimicrobial agent and offers cross-protection to bacteria against different types of antimicrobial intervention. Bacterial resistance to stress is also affected by other factors such as their physiological state as well as their ability to express extracellular substances and form biofilms.

Resistance of E. coli O157:H7 to Antibiotics

Antibiotic Resistance and Mobile DNA Elements

Bacteria susceptible to antibiotics usually gain their resistance through genetic mutation or genetic material exchange with an antibiotic-resistant donor. There are three different approaches for horizontal gene transfer, that is, transformation, conjugation, and transduction (Thomas and Nielsen, 2005), and conjugation is believed to be the major mechanism of antibiotic resistance gene exchange under in vivo conditions (Schwarz et al., 2006). Mobile DNA elements play an essential role in the dissemination of antibiotic resistance genes in natural environments.

As mobile DNA elements, temperate bacteriophage (Brabban et al., 2005) and transmissible plasmid (Makino et al., 1999; Miwa et al., 2002) have both served as carriers for antibiotic resistance genes in E. coli O157:H7. E. coli O157:H7 Sakai strain was found to carry 18 prophages that had the genes for virulence as well as multidrug resistance (Asadulghani et al., 2009). Many of the prophages were defective but inducible from E. coli O157:H7 chromosomes and some could even be transferred to other E. coli strains. E. coli O157:H7 strains isolated from a mass outbreak in Obihiro-City, Hokkaido, Japan in 1996 had two distinct tetr R plasmids with...