Key Bacterial Pathogens, Virulence and Available Chemotherapy
Scallan and colleagues estimate that 37 million illnesses are acquired each year in the United States from food pathogens. An estimated 9 million of these pathogen-induced illnesses are domestically acquired . Of these, 0.2 million are attributed to parasitic infections, 3.6 million to bacterial infections and 5.5 million to viral infections . Key food-borne disease-causing bacteria include Salmonella enterica, Staphylococcus aureus, Escherichia coli, Vibrio cholerae, and Enterobacter spp.
Salmonella species are facultative anaerobic Gram-negative rod-shaped bacteria classified under the family Enterobacteriaceae. These bacteria are motile, non-spore forming, and are part of the normal flora in the gut of vertebrates . The modern nomenclature for this microorganism involves italicizing the genera and specific epithets, but not the serovars which are capitalized. Salmonella enterica serovar Typhimurium and S. enterica serovar Enteritidis cause worldwide epidemics of gastroenteritis, prominent of all human infectious diseases . Consumption of contaminated chicken, beef, pork, sausage, meat paste, unpasteurized cheese, and lettuce is a major cause of food-borne salmonellosis . The increasing incidence of antibiotic resistant clinical isolates of S. enterica enhances the risk of therapeutic failure in cases of life-threatening salmonellosis . There are two types of Salmonella infections: the very well-known typhoidal Salmonella (caused by serotypes S. Typhi and S. Paratyphi A, B or C) and the less serious and generally self-limiting non-typhoidal Salmonella (caused by serotypes other than S. Typhi and S. Paratyphi). Both types of Salmonella infections are acquired by the fecal-oral transmission mode, but typhoid fever is closely associated with incidences direct human contamination of foods or sewage contamination of crops, meats, and water, and humans are the only known reservoirs of typhoidal Salmonella. Further, non-typhoidal Salmonella are widely distributed in nature and can be found in animal meat, poultry, eggs, seafood, etc. .
Chemotherapy used to treat S. enterica is less common since a study by Wiström and Norrby in 1995 found little to no benefit in the administration of fluoroquinolones . Only early on, that is, within 48 h of the onset of symptoms, was there a benefit in taking norfloxacin . It was once thought that providing antimicrobials to patients would shorten the duration of a S. enterica infection; however, nowadays, electrolyte and fluid replacement is the best treatment since the majority of infections are self-limiting . In contrast, antibiotics are often indicated in patients who are severely ill, and the following chemotherapeutics are most often prescribed: the fluoroquinolones, trimethoprim sulfamethoxazole (TMP-SMZ), ampicillin, or extended-spectrum cephalosporins (e.g., ceftriaxone or cefixime). Regretfully, multidrug resistance has already been documented in a large number of S. Typhimurium isolates, particularly to TMP-SMZ and ampicillin .
Staphylococcus aureus is a significant contributor to food contamination being the fourth largest cause of domestically acquired foodborne illnesses in the United States and affecting about 240,000 people annually . S. aureus is a Gram-positive, non-spore forming, catalase positive, non-motile coccus which is ubiquitous in humans and in the environment, i.e., soil, water and air. The ubiquitous nature of this microbe is no doubt attributable to its robust nature. S. aureus can survive under very dry conditions, under high osmotic pressure, in a wide range of pH conditions (4.5 to 9.3) as well as in a wide range of temperatures (6 °C to 48 °C) . S. aureus causes gastroenteritis by production of heat-stable enterotoxins which induce staphyloenterotoxicosis or staphyloenterotoxemia within 1 to 7 h after consumption of contaminated food . Affected humans will be ill for about a day and very rarely have serious complications other than dehydration associated with their symptoms of diarrhea and vomiting. There is an impressive array of virulence factors contributing to the pathogenesis of S. aureus, summarized by Murray, et al.  and Foster .
Pathogenic strains of E. coli bacteria are well known food pathogens acquired from traveling, best known as traveler’s diarrhea, but surprisingly not uncommon in the United States . E. coli is a Gram-negative rod shaped bacterium belonging to the Enterobacteriaceae family, a large group of medically important microorganisms. The wild-type E. coli bacterium is a normal part of the gut microbiome of humans and other warm blooded animals. However, the enterovirulent E. coli are particularly problematic in human clinical medicine. Enterovirulent E. coli are distinctly named based upon the production of their associated virulence factors. There are six different E. coli pathotypes: enterotoxigenic E. coli (ETEC), enteropathogenic E. coli (EPEC), enterohemorrhagic E. coli (EHEC), enteroinvasive E. coli (EIEC), enteroaggregative E. coli (EAEC), and diffusely adherent E. coli (DAEC). In addition, each type has a different pathogenic scheme. The types of pathogenic E. coli most commonly involved in causing foodborne illnesses are ETEC, EPEC, EHEC, and EIEC . Carcinogenic E. coli NC101 are discussed elsewhere .
ETEC is the causative agent of the illness commonly known as traveler’s diarrhea. The virulence factors that are most notable to this type are the heat-labile (LT) toxin and heat-stable (ST) toxins. ETEC can possess LT, ST, or both . ETEC is generally a self-limiting infection but can resemble a cholera infection. Antibiotics are not required for treatment but do reduce the severity and duration of the symptoms. Mortality due to an infection with ETEC is seen mostly in children and kills about 380,000 a year in the world according to the World Health Organization . Mortality rates in the US caused by ETEC are rare since these infections normally occur in places where poor sanitation is common . Humans are the primary source of ETEC transmission and dissemination through populations via the consumption of contaminated water or food.
Members of the EPEC are identified as such if the locus for enterocyte effacement (LEE) is harbored on the pathogenicity island that is present in their genome. LEE encodes a 94 kDa outer-membrane protein called intimin . EPEC was the first enterovirulent E. coli to be described, and it is now one of the best understood of the E. coli pathogens . The infective dose of EPEC is lower in infants than in adults . Thus, it is thought to be more virulent in infants and is usually connected to a relatively high infant mortality rate due to infantile diarrhea . EPEC that are similar to ETEC are more common in underdeveloped countries where sanitary practices are less common . Contamination of foods with EPEC are apparently sporadic but have been documented in mayonnaise, pickles, lettuce, raw beef, and raw chicken but can ultimately be any food that is subjected to fecal contamination .
A notorious group of potentially fatal E. coli, the EHEC, confer hemorrhagic colitis, bloody diarrhea, and hemolytic uremic syndrome (HUS) . There are upwards of 400 serotypes of Shiga-toxin producing
MORPHOLOGY AND STRUCTURE
The Enterobacteriaceae are among the largest bacteria, measuring 2 to 4μm in length and 0.4 to 0.6μm in width, with parallel sides and rounded ends. Forms range from large coccobacilli to elongated, filamentous rods. The organisms do not form spores or demon-strate acid fastness.
The cell wall, cell membrane, and internal structures are morphologically similar for all Enterobacteriaceae, and follow the cell plan described for Gram-negative bacteria. Components of the cell wall and surface, which are antigenic, have been exten-sively studied in some genera and form the basis of systems dividing species into serotypes (Fig 21 – 1). The outer membrane lipopolysaccharide (LPS) is called the O antigen. Its antigenic specificity is determined by the composition of the sugars thatform the long terminal polysaccharide side chains linked to the core polysaccharide and lipid A. Cell surface polysaccharides may form a well-defined capsule or an amorphous slime layer and are termed the K antigen (from the Danish Kapsel, capsule). Motile strains have protein peritrichous flagella, which extend well beyond the cell wall and are called the H antigen. Many of the Enterobacteriaceae have surface pili, which are anti-genic proteins but not yet part of any formal typing scheme.
GROWTH AND METABOLISM
Enterobacteriaceae grow readily on simple media, often with only a single carbon energy source. Growth is rapid under both aerobic and anaerobic conditions, producing 2- to 5-mm colonies on agar media and diffuse turbidity in broth after 12 to 18 hours of incu-bation. All Enterobacteriaceae ferment glucose, reduce nitrates to nitrites, and are oxidase negative.
Genus and species designations are based on phenotypic characteristics, such as patterns of carbohydrate fermentation, and amino acid breakdown. The O, K, and H antigens are used to further divide some species into multiple serotypes. These types are expressed with letter and number of the specific antigen, such as Escherichia coliO157:H7, the cause of numerous food-borne outbreaks. These antigenic designations have been estab-lished only for the most important species and are limited to the structures at hand. For example, many species lack capsules and/or flagella. In recent years, DNA and RNA ho- mology data have been used to validate these relationships and establish new ones. The genera containing the species most virulent for humans are Escherichia, Shigella, Salmo- nella, Klebsiella, and Yersinia. Other less common medically important genera are Enter- obacter, Serratia, Proteus, Morganella, and Providencia.
In addition to the LPS endotoxin common to all Gram-negative bacteria, some Enter-obacteriaceae also produceprotein exotoxins,which act on host cells by damaging mem-branes, inhibiting protein synthesis, or altering metabolic pathways. The end result of these actions may be cell death (cytotoxin) or a physiologic alteration, the net effect of which depends on the function of the affected cell. For example, enterotoxins act on in-testinal enterocytes, causing the net secretion of water and electrolytes into the gut to pro-duce diarrhea. Although these toxins are most strongly associated with E. coli, Shigella, and Yersinia, others with the same or very similar actions have now been discovered in other species. When found in another species, the toxin may differ by a few amino acids in structure and in genetic regulation but has the same basic action on host cells. Details of these toxins are discussed below in relation to their prototype species.