Infections from Salmonella, Shigella, & Yersinia

Infections Due to Salmonella, Shigella, and Yersinia

• Objectives include describing bacterial agents causing diarrhea, clinical presentation of infections, recommended diagnostics and common findings, and treatment and management.
• Salmonella is a serious public health problem in both developing and developed countries.
• Salmonella is a Gram-negative rod-shaped, facultative bacterium, belonging to the Enterobacteriaceae family.
• It is catalase-positive, oxidase-, and lactose-non-fermenting.
• It typically resides in the large intestine.
• Salmonella morphology includes Gram-negative rod shape, facultative bacterium, belonging to the Enterobacteriaceae family, catalase-positive, oxidase-positive, and H2S+. Colonies appear as blue-colored on certain agars (e.g., Hectoen-Enteric agar, Bismuth sulfide agar).
• Salmonella is classified into 2500 serotypes, with two main species: S. enterica and S. bangori.
• Clinical Salmonella serotypes include Paratyphi A (serogroup A), Paratyphi B (serogroup B), Typhi (serogroup D), Choleraesuis (serogroup C1), and Typhimurium (serogroup B).
• Typhoid fever (caused by S. Typhi) is a life-threatening infection affecting 17 million people annually, resulting in 600,000 deaths. S. Paratyphi causes milder infection.
• Typhoid fever is common in children under 5 in developing countries.
• Transmission occurs through contaminated food and water, specifically via feces and urine of infected patients or carriers. Food handlers can act as carriers.
• The pathogen invades the intestines, causing intracellular multiplication in cells of the reticuloendothelial system, eventually leading to systemic infection and/or localized infection.
• Laboratory diagnosis for Enterocolitis involves stool culture, identification by biochemical tests (seroagglutination), and phage typing. Enteric fever diagnosis includes blood and urine (2-3 weeks post infection) culture, stool culture (1 week post infection). Serology (Widal test) can be unreliable.
• Prevention strategies include controlling the human reservoir, improving water and food sanitation, and vaccination (though currently licensed Vi polysaccharide vaccines are not highly effective).
• Therapy for typhoid fever involves antibiotics like chloramphenicol, ampicillin, or TMP-SXT. If resistance is observed, quinolone or 3rd-generation cephalosporins can be used.
• Therapy for Enterocolitis often does not require antibiotics, but antibiotics like ciprofloxacin might be required for severe cases, especially in immunocompromised or elderly individuals.

Shigella

• Classified into serogroups A, B, C, and D, differentiated by biochemical tests and antigenic characteristics.
• Shigella is a strictly human pathogen, unlike Salmonella which has animal reservoirs.
• Transmission is fecal-oral, with a low infective dose (< 103 organisms).
• It causes bacillary dysentery (shigellosis), characterized by bloody diarrhea, intestinal pain, and pus in feces.
• Clinical presentation is very similar to salmonella infection but can involve severe complications.
• Laboratory diagnosis involves culturing fresh stool samples on selective media (MacConkey agar, SS agar) and microscopic examination looking for lactose-negative (except S. sonnei) and deoxycholate-resistant colonies.
• Treatment involves managing dehydration and using antibiotics such as TMP-SXT or quinolones (for adults), Ceftriaxone .

Yersinia

• Yersinia is an animal parasite transmissible to humans directly or via insect vectors.
• It grows at 14-37°C (27°C) and is non-haemolytic.
• Three human pathogens within the Yersinia genus are Y. pestis, Y. enterocolitica and Y. pseudotuberculosis.
• Y. enterocolitica causes Enterocolitis, usually from contaminated, refrigerated food and blood products.
• Y. pseudotuberculosis causes a less severe disease, sometimes mimicking acute appendicitis.
• Y. pestis is the causative agent of plague (Black Death). It is endemic in some parts of the world, transmitted via infected fleas to humans, resulting in bubonic (painful, swollen lymph nodes), pneumonic (highly contagious droplets, infectious), and septicemic (bloodstream, fatal) forms.
• Laboratory diagnosis involves culturing at 27°C on blood agar or MacConkey agar using methylene blue stain for visualization.
• Treatment involves using antibiotics like tetracycline immediately upon suspicion of plague.