Enteric Gram-Negative Bacilli Part-9

Gram-Negative Bacilli Related to the Gastrointestinal Tract Several primary bacterial pathogens that belong to different families genera of Gram-negative bacilli, infect humans through the digestive tract. Infection takes place due to the ingestion of contaminated food or drinks. Enteric pathogen: it is any microbial pathogens that infect humans only through the digestive tract. Enteric microbial pathogens can be classified into: Enteric pathogens that cause disease only within the digestive tract (Gastroenteritis). Enteric pathogens that cause disease within the digestive tract and then spread to other body locations (gastroenteritis + infections out side the digestive tract). Enteric pathogens that infect humans through the digestive tract without causing disease within the digestive tract, however, they cause disease in other body locations (use the digestive tract as only a portal of entry.

Primary Pathogens That Belong to the Family Enterobacteriaceae: Common Features to all members of the family Enterobactericae: Gram-negative bacilli Facultative anaerobes Catalase positive (but oxidase negative) Most of them are motile Main surface antigens that are common to most members of the family Enterobacteriaceae : The somatic, or O, antigen: which is the outer polysaccharide portion of the lipopolysaccharide. The H antigen: which is on the flagellar protein The K antigen: which is capsular polysaccharides Virulence factors Pili: common pili (all have common pili that play role in attachment and can interfere with phagocytosis) Capsule: some pathogenic species that belong this family are capsulated (recall that in general. bacterial capsules are anti-phagocytic) Endotoxin (LPS/ all have LPS)(recall that LPS is a strong inflammatory agents as mentioned before) Exotoxins: (some of the primary pathogens that belong to produce different types of exotoxins such as AB toxins ) Cell invasiveness: (some of the primary pathogens that belong to this family have the ability to invade cells (such as intestinal mucosal cells and macrophages) and replicate within them Type III secretion system: some of the primary pathogens that belong to this family have Type III secretion system that plays an important role in their pathogenesis

Primary enteric Pathogens that belong to the family Enterobacteriaceae: I- Enteropathogenic E. coli: it is a group that includes five strains of E coli that cause gastroenteritis and other clinical conditions. These E coli strains of this group are not like the regular E coli strains that are members of the intestinal microbiota. These E coli strains have gained virulence genes (by horizontal gene transfer mechanisms discussed before) that enable them able to cause disease once they are ingested along with contaminated food or drinks (exogenous infection). In other words, these five strains of E coli are primary pathogens and are not part of the human intestinal microbiota. five pathogenic either produce exotoxins or have the Examples: Enterohemorrhagic strains (EHEC) Enteroinvasive E. coli (EIEC) Enterotoxigenic E. coli (ETEC)

Enterohemorrhagic strains (EHEC) (E. coli, O157:H7): Clinically, it’s the most important strain of this group. It belongs to intestinal microbiota of animals (mainly cows). Human are infected upon the ingestion of contaminated raw milk, undercooked contaminated meat or contamination of hands while dealing with animals. Diseases caused by E. coli, O157:H7: 1- Gastroenteritis: The main virulence factor of EHEC is Shiga-like toxin (because it is similar to the toxin produced by Shigella spp). Shiga-like toxin of E. coli, O157:H7 is an AB toxin, in which the A subunit inhibits the ribosomes of intestinal mucosal ells (thus it inhibits the translation process in these cells). This will cause death of mucosal cells (apoptosis), and consequently ulceration of the intestinal mucosa resulting in Gastroenteritis that is associated abdominal pain , fever and bloody diarrhea. Inflammatory WBCs and red blood cells can be seen once feces from the patient is examined microscopically)

2- Hemolytic uremic syndrome (HUS): It is a potentially fatal condition that may happen after absorption of shiga-like-toxin form the intestinal tract into the into the blood stream (toxemia). Shiga-like toxin affect the endothelial cells of blood capillaries (such as those found in kidneys) and small blood vessels. HUS is characterized by hemolytic anemia, thrombocytopenia, stroke, heart problems and acute renal failure, coma and death

2- Salmonella enterica Salmonella enterica is a Gram-negative bacilli that belongs to the family Enterobacteriaceae. It has been found that this species has more that 2000 strains. Most of these strains are members of animal intestinal normal flora, and thus most human infections occur due to ingestion of food and drinks contaminated with animal feces, such as ingestion of contaminated raw milk or undercooked meat, and contamination of hands while dealing with animals Infection with most of these Salmonella enterica strains result in gastroenteritis that can be associate with bloody or watery diarrhea (some of them with fever). However, most infections are self-limiting Example: Salmonella enterica serovar Typhimurium

Salmonella enterica serovar Typhimurium:

Salmonella enterica serovar Typhi (Old name: Salmonella typhi) Clinically, it is the most important strain of Salmonella enterica stains It infects humans only (it is a primary human pathogen) The infection with Salmonella enterica serovar Typhi is transmitted from human to human by fecal-oral route Virulence factors and pathogenesis: Its ability to grow intracellularly (cell invasiveness) : It can do so by its ability to it inhibits fusion of the its phagosome with lysosome by a process that is mediated by its type III secretion system LPS

Pathogenesis of Salmonella enterica serovar Typhi

Clinical symptoms: Initially: abdominal pain, fever and constipation (no obvious gastroenteritis) Later on: Systemic infection that is characterized by sustained high fever that reaches above 40 °C. Leucopenia and anemia (due infection of the bone marrow) Skin Rash may also occur (Rose spots). Potentially fatal complications: Septicemia that may lead to septic shock may develop) Meningitis Pneumonia Elevated liver enzymes die to liver infection Infection of the intestinal wall to cause intestinal perforation and bleeding The disease may begin to resolve by the third week, but life-threatening complications may occur

Shigella spp: Epidemiology: The genus Shigella belongs to the family Enterobacteriaceae. This genus includes four medically important species that infect humans to cause severe gastroenteritis known as Shigellosis or Bacillary Dysentery (thus they are considered as primary enteric pathogens) These four species are: Shigella dysenteriae Shigella flexneri Shigella sonnei Shigella boydii Notes: Most Shigella species are non-motile Dysentery means severe bloody diarrhea Transmission: Fecal-oral route (human feces from infected individuals contaminate water, food, fruites and vegitables irrigated by sewege.

Pathogenesis: Mainly attributed to the destruction of mucosal cells . During infection with Shigella spp, destruction of intestinal mucosal cells is mediated by: 1- Invasiveness and replication within mucosal cells: Upon reaching the intestinal tract, Shigella species invades intestinal mucosal cells to replicate within mucosal cells resulting in their destruction. Upon being internalized by mucosal cells by phagocytosis, Shigella spp escape from the phagosome to replicate in the cytosol. Intracellular replication within mucosal cells results in the destruction of these cells 2-Production of a cytolethal exotoxin that kills intestinal mucosal cells: Shigella spp produce an AB exotoxin known as Shiga toxin. The A subunit inhibits the ribosomes of the intestinal mucosal cells (inhibition of translation means inhibition of protein synthesis in these cells. This leads to apoptosis of intestinal mucosal cells and ulceration of the intestinal mucosa. Notes: LPS of Shigella spp is also involved in the inflammation process The mechanism of action of the AB toxin of Enterohemorrhagic E. coli O157: H7 (known as Shiga-like toxin) is similar to the mechanism of action of Shigella AB exotoxin

Complications that require hospitalization: Clinical symptoms: The net result of Shigella spp pathogenesis is ulceration and inflammation of the intestinal mucosa resulting in abdominal pain, fever, massive (severe) bloody diarrhea (Dysentery). In people with normal immune system, the diarrhea frequently resolves in 2 or 3 days. Complications that require hospitalization: 1- Dehydration and ketoacidosis These complications occur mainly among young children, old people and immunocompromised patients. Antibiotics and fluid and electrolytes replacement therapy has to be initiated specially 2- Hemolytic uremic syndrome: Upon reaching the blood stream, the Shigella toxin causes hemolytic uremic syndrome (HUS). (See Enterohemorrhagic E. coli O157:H7). Note: the toxin may each the blood steam BUT not the bacteria ( as mentioned earlier, Shigella spp do not cause systemic infection)

Uropathogenic E. coli: UTI: E. coli is the leading cause of urinary tract infections (UTIs) (90% of UTIs world-wide) Uropathogenic strains of E. coli are characterized by having pili with high binding affinity to certain CHO found in the surface of epithelial cells of the urinary tact. These types of pili known as Type I pilus and P pilus UTI: Ascending UTI Infections are the most common forms of UTIs. The term Ascending implies that the UTIs starts initially in the urethra (urithritis ) and then the infection ascends to the upper parts of the urinary tract, the balder (cystitis) and then the kidneys (Pyeloneohritis). Most ascending UTIs are mainly attributed to fecal contamination of the urethra. UTIs are most commonly seen among females ( this is because of the anatomical proximity of the urethral opening to the anus, and colonization of the vagina by members of the fecal microbiota) Clinical symptoms of UTI: Dysuria and burning sensation during urination (more obvious in males) Hematuria Fever and chills If left untreated, the infection may spread from infected kidneys to the blood stream resulting in septicemia. Septic shock and other systemic infections.

The next slides continue discussing diseases caused by primary enteric pathogens of Gram-negative bacilli that belong to families other than Enterobacteriaceae

Primary enteric pathogens of Gram-negative bacilli that belong to the family Vibrionaceae General features: Curved or comma-shaped Gram-negative rods Aerobic Alkaliphil Note: The word Vibrio means comma-shape. Medically important species of the family Vibrionaceae: Vibrio cholera (most important ) Vibrio parahaemolyticus Vibrio vulnificus

Vibrio cholera: Transmission: Ingestion of contaminated raw sea food Fecal-oral route (feces from infected humans) Insect vector: Flies that may have their legs heavily contaminated with Vibrio cholera because of contact contact with sewage or feces that contain this pathogen.

Pathogenesis of Vibrio cholera: Infectious dose: approximately 1 million (because Vibrio cholera is Alkaliphile, so most of the ingested Vibrio cholera are killed by Stomach acidity) Vibrio cholera that survive passing the stomach reaches small intestine and start replicating. The pathogenesis of Vibrio cholera is mainly attributed to its exotoxin, which is an AB toxin that is known as Cholera toxin or as Choleragen. The mechanism of action of Cholera toxin is similar to the LT toxin of Enterotoxigenic E. coli discussed in part 9-. The effect of the toxin results in water efflux that enters the lumen of the gut, resulting in a massive watery diarrhea.

Clinical symptoms of Cholera Includes: Abdominal pain and cramps Massive and severe watery diarrhea (no red blood cells or white blood cells are seen in stool upon microscopic examination) Rice-water (White-Turbid) stool is a very characteristic feature of the watery diarrhea seen in cholera Note: the whitish-turbid diarrheal stool of Cholera is because of the secretion of large amount of bicarbonate from intestinal cells Massive loss of fluid and electrolytes leads to: Loss of water causes loss of skin elasticity and scaphoid abdomen Loss of electrolytes affects the function of muscle cells and nerves Loss of bicarbonate and potassium results in acidosis and hypokalemia Severe dehydration Death rate: up to 40% Treatment: Water and electrolytes replacement therapy and antibiotics administration Prevention: These is an available vaccine, however, it is not a very successful vaccine. Effectiveness : 50% Protective duration : 3 to 6 months

Primary enteric pathogens of Gram negative bacilli that belong to the family Campylobacteraceae Common features of the species that belong to this family: Comma- or S-shaped Gram-negative bacilli Microaerophilic Medically important species: Campylobacter jejuni (can grow at 42 °C) Campylobacter intestinalis (can not grown at 42 °C) Epidemiology: Inhabit the intestinal tract of many animals such as cattle, chickens, and dogs (intestinal microbiota) Transmission: Ingestion of food and drinks contaminated with animals feces (most commonly, contaminated poultry meat, and raw milk) Human-to-human transmission may occur (fecal-oral route)

Pathogenesis: Invasion and replication within intestinal mucosal cells. This results in their destruction and thus consequently leads to ulceration and inflammation of the intestinal mucosa. Note: Campylobacter jejuni produces a toxin (cytolethal distending toxin) that inhibit cell division. Accordingly, it contributes to mucosal cell destruction and inflammation. Clinical symptoms: mainly, gastroenteritis that is associated with abdominal pain with watery or bloody diarrhea. The infection may go beyond the intestinal mucosa: Infection of mesenteric lymph nodes: infection may reach the mesenteric lymph nodes, resulting in their inflammation that is associated with severe abdominal pain similar to that seen in case of appendicitis Invasion of the blood stream Most commonly seen in neonates or debilitated adults infected with Campylobacter intestinalis, resulting in septicemia, septic shock, meningitis…. Prevention: There is no available vaccine Proper sewage disposal and personal hygiene (hand washing) are important.

Guillain-Barré syndrome: Gastrointestinal infection with Campylobacter jejuni can be associated with Guillain-Barré syndrome, an acute neuromuscular paralysis. Guillain-Barré syndrome is an autoimmune disease: It has been found that Campylobacter jejuni has antigens that are similar to surface antigens found on the myelin sheath of motor neurons. Infection with C. jejuni may induce an immune response involves the production of cross-reacting auto-antibodies causes destruction of the myelin sheath of motor neurons. In most cases, this neuromuscular paralysis is self-liming, however, it can be permanent and in rare cases, can be fatal.

Primary enteric pathogens of Gram negative bacilli that belong to the family Helicobacteraceae: Helicobacter pylori is the most medically important species that belongs to this family. It is a curved gram-negative comma or S-shaped It is a strongly urease-positive Diseases caused by Helicobacter pylori: Gastritis and peptic ulcers It is a risk factor for gastric carcinoma In addition, it has been linked to mucosal-associated lymphoid tissue (MALT) lymphomas. Epidemiology: The natural habitat of H. pylori is the human stomach. Person-to-person transmission: mainly by kissing. About half of the world population has this bacterium in their stomach. Pathogenesis: Helicobacter pylori attaches to the mucus-secreting cells of the gastric mucosa underneath the mucous layer the covers the gastric mucosa. Ulceration of the gastric mucosa can be mediated by: Helicobacter pylori LPS-induced inflammation Vaculatizing toxin (such as Vac A) produced by Helicobacter pylori Cytotoxic effects of ammonia generated by the urease enzyme of Helicobacter Death of mucosal cells with the loss of the protective mucus coating that covers the infected region, the acid produced by the stomach to contribute to the destruction of mucosal cells and thus exacerbating mucosal ulceration.

Helicobacter pylori and cancer: Clinical Symptoms: Gastritis and peptic ulcer are characterized by recurrent pain in the upper abdomen, frequently accompanied by bleeding into the gastrointestinal tract. No bacteremia or systemic infection have been reported Helicobacter pylori and cancer: Helicobacter pylori has been associated in high rates with gastric carcinoma and mucosal associated lymphoid tissue lymphoma (MALT). Antibiotic treatment directed against the organism often causes the tumor to regress.

Diagnosis: Isolation of Helicobacter pylori from biopsies (culture) Urea breath test Serology based tests: Detection of anti-Helicobacter pylori antibodies in blood Detection of Helicobacter antigens in stool.