Salmonella, Shigella, and Yersinia (Ditki Notes)

• Salmonella, Shigella, and Yersinia belong to the Enterobacteriaceae family, which also includes Escherichia coli, Citrobacter, Serratia, Proteus, Klebisiella, and Morganella.
• Salmonella, Shigella, and Yersinia enter the host by using Peyers' patch M-cells to cross the intestinal epithelium.

SALMONELLA

• The nomenclature for Salmonella is complex and somewhat disputed.
• The complete nomenclature for a Salmonella isolate includes italicized genus and species, along with the non-italicized serovar type.
• Salmonella enterica serovar Typhi, often shortened to Salmonella Typhi, causes typhoid fever.
• Salmonella causes foodborne infections.
• Unlike other Enterobacteriaceae members, Salmonella does not ferment lactose but produces hydrogen sulfide.
• Salmonella infects both humans and non-human animals.
• Typhoid-causing strains are strictly human pathogens.
• Salmonella are intracellular pathogens and can invade all tissue types, including the brain.
• Virulence genes are encoded on Pathogenicity islands I and II.
• Type three secretion systems inject effector proteins into host cells.
• Salmonella-secreted invasion proteins facilitate entry into host cells.
• Immune evasion proteins are important for Salmonella survival.
• Infection is usually acquired via consumption of contaminated foods, especially eggs, poultry, and dairy.

Salmonellosis

• Characterized by inflammation of the intestinal lining, gastroenteritis is the most common form of Salmonellosis in the U.S.
• Symptoms include nausea, vomiting, diarrhea, fever, and cramps.
• Gastroenteritis is usually self-limiting, requiring only supportive care: water and ion replacement.
• Salmonella Typhimurium, often associated with gastroenteritis, resides in Salmonella-containing vacuoles (SCVs) within phagocytes; this protective endosome allows the bacteria to survive and replicate safely inside host cells.
• Septicemia causes typical bacteremia symptoms like fever, chills, and low blood pressure.
• Localized infections can develop in a small subset of patients, especially in the joints and cardiovascular endothelium.
• Salmonella septicemia is most common in young children, older adults, and in individuals with HIV or other immune deficiencies.
• Recurrent septicemia is an AIDS-defining condition.
• Treatment includes Fluoroquinolone or Cephalosporin antibiotics.
• Enteric fevers are associated with Salmonella Typhi.
• Paratyphoid fever, a milder form, is associated with Salmonella serotypes Paratyphi A, Paratyphi B, and Paratyphi C.
• S. Paratyphi B is also referred to as Salmonella Schottmeulleri, and S. Paratyphi C is also referred to as Salmonella Hirshfeldi.
• Symptoms & Pathogenesis: Approximately 10 days after ingestion of the bacteria, individuals experience fever, fatigue, and other non-specific symptoms; some patients develop "rose spots," which are small red macules on the trunk.
• Roughly 20 days after ingestion, patients have gastrointestinal symptoms like constipation or diarrhea, nausea, and vomiting.
• Typhoid-associated strains like Salmonella Typhi move through the intestinal lining and are engulfed by macrophages.
• Macrophages transport the microbes to the bone marrow, liver, and spleen, where bacterial replication occurs.
• Infection increases splenic cellularity and leads splenomegaly in some patients; liver enlargement can occur.
• Bacteria colonize the gallbladder and re-infect the intestines, producing gastrointestinal symptoms.
• Treatment includes fluoroquinolones or ciprofloxacin.
• Asymptomatic carriers maintain typhoid-associated strains within populations; enteric fever is endemic in Southeast Asia and parts of Africa.

SHIGELLA

• So closely related to Escherichia coli, especially Enteroinvasive E. coli, that some authors consider Shigella an E. coli biogroup.
• Shigella is an intercellular pathogen that only infects humans.
• Asymptomatic carriers are a crucial reservoir.
• Virulence factors are encoded on a plasmid, but regulated by chromosomal genes.
• Type three secretion systems inject effectors Ipa's A-D, facilitate invasion of epithelial cells and macrophages.
• Shigella lyses phagosomes and replicates within the cytoplasm, unlike Salmonella, which replicates within special vacuoles.
• Actin tails help Shigella migrate to adjacent cells, allowing spread without exposure to innate immunity.

Shigellosis

• Primarily affects young children and outbreaks are often linked to daycares and preschools, where fecal-oral transmission is more likely to occur.
• Shigella has a low infectious dose.
• Upon infection, Shigella enterotoxin initially causes watery diarrhea.
• As bacteria invade the colon mucosa, bloody diarrhea with pus, cramping, and fever occur.
• The presence of neutrophils, red blood cells, and mucus in the stool can be a diagnostic aid.
• Most patients with Shigellosis recover on their own with supportive care.
• In severe cases, antibiotics can be administered.
• Multiple pathogenic strains of Shigella exist.
• Shigella sonnei is responsible for most cases of Shigellosis in the U.S.
• Shigella flexneri accounts for most cases of Shigellosis in developing countries.
• Shigella dysenteriae causes dysentery, the most severe form of Shigellosis.
• These strains produce Shiga exotoxin, which impairs protein synthesis, resulting in damaged intestinal epithelium and bloody diarrhea.
• Damaged renal endothelial cells can lead to Hemolytic uremia syndrome (HUS), most often in infected children.

YERSINIA

• Zoonotic infections.
• Short rods, coccobacilli with Bipolar Gram-staining: their tips stain darker than their middles.
• Type three secretion systems secrete immune-suppressing effectors into host cells, allowing Yersinia to survive and replicate.

Plague

• Yersinia Pestis causes plague.
• Infected fleas and lice transmit the bacteria to humans.
• Virulence factors and their genes:
  • Antiphagocytic protein capsule (Factor 1 gene).
  • Outer membrane proteins degrade complement proteins C3b and C5a, preventing bacterial opsonization and phagocyte migration (Plasminogen activator gene).
  • Plasminogen activator proteases also degrade fibrin clots, which may facilitate bacterial spread in the host (Plasminogen activator gene).
• Bubonic plague is characterized by fever, headache, chills, and weakness.
• Replication in the lymph nodes causes swelling and tenderness.
• Buboes are the visible lymph node swellings that typically appear in the axilla or groin.
• Septicemic plague occurs when infection spreads via the bloodstream or lymph.
• Symptoms include fever, chills, and extreme weakness, as well as abdominal pain, and, possibly, shock or bleeding into the skin or other organs.
• Pneumonic plague can quickly lead to respiratory failure and death.
• Because is spread in respiratory droplets, transmission is human-to-human.
• Treatment includes streptomycin or gentamycin.
• Because disease progression is rapid, and infection can spread to the lungs, early administration of these antibiotics is crucial.

Yersiniosis

• Yersinia enterocolitica and Yersinia pseudotuberculosis cause enterocolitis, especially in children, associated with undercooked pork.
• Young children tend to experience bloody diarrhea, cramps, and fever.
• Older children tend to experience fever and pain in the right side of the abdomen that may be confused with appendicitis.
• In most cases, supportive care is sufficient.