Enterobacteriaceae II Flashcards
Shigella
- Gram -
- bacilli
- facultative anaerobe
- Shigella is the classic cause of dysentery which is typically spread person-to-person under poor sanitary conditions.
- The illness begins as a watery diarrhea but evolves into an intense colitis with frequent small volume stools containing blood and pus.
- Despite the invasive properties of the organism the infection usually does not spread outside the intestinal tract.
Shigella Organism
- Four species are defined by biochemical reactions and specific O antigens organized into serogroups
- Shigella dysenteriae (serogroup A), Shigella flexneri (serogroup B), Shigella boydii (serogroup C), and Shigella sonnei (serogroup D).
- All but S. sonnei are further subdivided into a total of 38 individual O antigen serotypes specified by numbers.
Shigellosis Epidemiology
Shigellosis is a strictly human disease with no animal reservoirs. Worldwide it is consistently one of the most common causes of infectious diarrhea in both developed and developing countries.
- Readily transmitted by the fecal–oral route through person-to-person contact or by contamination of food or water.
- Spread is efficient due to the low infecting dose shown to be less than 200 organisms.
- Incidence of shigellosis is directly related to personal and community sanitary practices.
- S. dysenteriae is largely limited to underdeveloped tropical areas.
- S. dysenteriae, type 1 (Shiga bacillus) produces the most severe disease.
Shigellosis Pathogenesis
- Shigella, is acid-resistant and survives passage through the stomach to reach the intestine. Once there the fundamental pathogenic event is invasion of the human colonic mucosa. This triggers an intense acute inflammatory response with mucosal ulceration and abscess formation.
- Invasion and spread is a multistep process which is the same in Shigella and EIEC.
Shigellosis Invasion
- Adhere selectively to M cells and can transcytose through them into the underlying collection of phagocytic cells
- Inside M cells and phagocytic macrophages, the bacteria are able to cause their demise by activating normal programmed cell death (apoptosis).
- Bacteria released from the M cell initiate a multi-step invasion process mediated by a set of invasin plasmid antigens (IpaA, IpaB, etc.). injected by an injection secretion system
- Each Ipa has its individual action. These include cell attachment, cytoskeleton reorganization, actin polymerization, and induction of apoptosis.
- Bacteria orient in parallel with the filaments of the cell’s actin cytoskeleton. Once attached thy initiate a process in which their control of the polymerization of actin fibrils to creates an actin “tail” behind the bacterial cell. This tail appears to drive the microbe through the cytoplasm like a comet.
- When bacteria encounter the host cell membrane, they push it into the adjacent cell forming finger-like projections which eventually pinch off (like Listeria).
Consequences of Shigella Invasion
•This process creates focal ulcers in the mucosa particularly in the colon. The diarrhea created by this process is almost purely inflammatory consisting of small volume stools containing WBCs, RBCs, bacteria and little else.
Shigella and Stx
- Some Shigella produce Stx which is not essential for disease but does contribute to the severity of the illness.
- The most potent producer of Stx, S. dysenteriae type 1, is the only Shigella with a high mortality rate.
- Disease severity is due to systemic effects of the toxin which can be the same as described for the EHEC including HUS.
Shigellosis Manifestations
•Shigella organisms cause an acute inflammatory colitis and bloody diarrhea, which in the most characteristic state presents as a febrile dysentery syndrome—a clinical triad consisting of cramps, painful straining to pass stools (tenesmus), and a frequent, small-volume, bloody, mucoid discharge
Shigellosis Diagnosis
•Shigella is readily isolated using selective media (such as Hektoen agar) which are part of the routine stool culture in clinical laboratories.
Shigellosis Treatment
•Because the disease is usually self-limiting, the beneficial effect of treatment is in shortening the illness and the period of excretion of organisms. Resistance rates to ampicillin, once the treatment of choice, have caused a shift to other antimicrobial agents.
Shigellosis Prevention
•Standard sanitation practices such as sewage disposal and water chlorination are important in preventing the spread of shigellosis.
Salmonella
Under a single species name, Salmonella enterica, the genus has been divided into hundreds of serotypes based on antigenic differences in O, K, and H antigens. The major groupings are:
- Salmonella serotypes highly adapted to humans. The prototype is S. enterica serotype Typhi (hereafter S. Typhi), the typhoid bacillus
- Salmonella highly adapted to animals.
- Salmonella with a broad host range infecting humans and animals. Most Salmonella belong to this category and the bulk of human disease is caused by ten serotypes.
Salmonella Gastroenteritis – Salmonella enterica
- The typical example of Salmonella “food poisoning” is the community picnic or bazaar, where volunteers prepare poultry, salads, and other potential culture media to be eaten later in the day.
- Because the refrigerators are filled with beer and soda, the food is left out in covered pans. A near physiologic incubation temperature is provided by the still-warm contents and the afternoon sun. This allows the organisms to enter logarithmic growth during the softball game. The bacteria usually produce no noticeable change in the food.
- One to two days after the feast, a significant portion of the revelers develop abdominal pain, nausea, vomiting, and diarrhea lasting for 3 or 4 days. An investigation points to a particular food such as potato salad or turkey dressing, which is found to have a correlation with both attack rate and severity of illness.
Salmonella Gastroenteritis Epidemiology
S. enterica gastroenteritis is predominantly a disease of industrialized societies and improper food handling which allows the transmission from the animal reservoir to humans.
- Infecting dose is much higher than with Shigella; ingestion of 10,000 or more Salmonella bacilli is required.
- Poultry products, are most often implicated as the vehicle of infection.
- Raw milk and exotic pets such as turtles have been associated with outbreaks.
- Multistate outbreaks of salmonellosis take place through the contamination of foodstuffs during large-scale industrial production.
- Chronic carriers who are food handlers are an important reservoir.
Salmonella Gastroenteritis Pathogenesis
In Salmonella gastroenteritis the bacteria invade the enterocytes of the large and small bowel, transcytose through the basolateral membrane to enter the lamina propria, and induce a profound inflammatory response. Upon initial contact of bacteria with the intestinal brush border the normal architecture is dramatically altered within minutes.
• Surface adhesin(s) react with a host cell receptor to stimulate localized filamentous actin cytoskeletal rearrangement (membrane ruffling)
Bacteria are internalized by induced pinocytosis associated with ruffles.
• Injection (type III) secretion systems inject multiple effector proteins into host cells
Shigella vs. Salmonella serotype Typhi
