6. Salmonella, Yersinia

Salmonella

History

  • Daniel Elmer Salmon: American microbiologist noted for contributions to microbiology.

  • Karl Joseph Eberth: First detection of Salmonella.

General Characteristics

  • Motile, gram-negative, facultative rods.

  • Non-lactose fermenting.

  • Resistant to bile salts.

  • Hydrogen sulfide (H2S) producing.

Classification and Taxonomy of Salmonella

Old Classification

  • Previously used serotyping and biochemical assays for naming species:

    • Examples: Salmonella enteritidis, S. typhimurium, S. choleraesuis, S. typhi.

    • Bioserotyping: O:K:H antigens.

    • Over 2400 serotypes referred to as species (Kauffman-White antigenic scheme).

New Classification

  • Modern DNA homology shows only two species:

    • Salmonella enterica (six subspecies).

    • S. bongori.

  • Most pathogens found in S. enterica subspecies.

Notable Subspecies

  • S. enterica subspecies enterica serovar typhi (or S. Typhi) is particularly significant.

Salmonella Pathogenicity

Different Serotypes

  • S. Paratyphi B: Strict human pathogen, enteric fever

  • S. Typhimurium: Affects multiple host species, gastroenteritis

  • Differences in single antigenic mosaic can result in varied pathotypes and clinical outcomes.

Salmonellosis

Definition

  • General term for the disease caused by Salmonella.

Clinical Syndromes

  • Enteritis: Acute gastroenteritis.

  • Enteric fever: Prototype is typhoid fever, includes milder paratyphoid fever.

  • Septicemia: Exhibits features of Gram-negative sepsis.

  • Asymptomatic carriage: Gall bladder reservoir (especially for Salmonella Typhi).

Enteritis

Overview

  • Most common form of zoonotic salmonellosis.

  • Linked to major foodborne outbreaks and sporadic diseases.

  • High infectious dose: Approximately 10^5 CFU.

Common Sources

  • Poultry, eggs, swine, etc.

  • 24-72 h incubation period

  • Symptoms include nausea, vomiting, diarrhea, fever, cramps, myalgia, and headache.

  • Common bioserotypes: S. Enteritidis and S. Typhimurium.

Virulence Factors

Key Properties

  • Invasiveness and intracellular survival/multiplication.

  • Presence of endotoxin and exotoxins.

  • Injectosome: Complex that facilitates virulence.

Specific Virulence Factors

  • Endotoxin: Found in the LPS layer and contributes to fever.

  • Enterotoxin: Causes diarrhea.

  • Type 1 fimbriae: Aids in adherence.

  • Virulence plasmids: Responsible for adherence, invasion, and serum resistance.

Clinical Progression of Salmonella Enteritis

  • Ingestion leads to absorption by epithelial cells in the small intestine.

  • Bacteria penetrate cells and migrate to lamina propria.

  • Multiply in lymphoid follicles, leading to inflammatory responses.

  • Symptoms include diarrhea due to active fluid secretion.

Enteric Fever

Global Impact

  • 27 million cases and 200,000 deaths annually.

  • Strictly human hosts for S. Typhi and milder forms caused by S. Paratyphi A, B, and C.

Transmission

  • Fecal-oral route; person-to-person transmission possible through chronic carriers.

  • Infectious dose: As low as 10^3 CFU.

Clinical Progression

  • Initial signs of bacteremia, sustained fever, followed by gastrointestinal symptoms after 10-14 days.

Septicemia

Overview

  • Can be caused by all Salmonella species.

  • More commonly associated with S. Choleraesuis and S. Dublin.

  • Increased risk in older individuals, young children, and immunocompromised patients.

Asymptomatic Carriage

Characteristics

  • Chronic carriage occurs in 1-5% of cases following S. Typhi or S. Paratyphi infection.

  • Gall bladder is often the reservoir.

  • chronic carriage w/ other salmonella spp. occurs in «1% cases

Diagnosis

Gastroenteritis

  • Diagnosis through feces and food samples using culture methods.

Enteric Fever

  • Blood and urine cultures during the feverish phase and reconvalescence, respectively

  • Serological testing (e.g., Gruber-Widal reaction).

Septicemia

  • Blood cultures plus samples from focal infections if present.

Treatment, Prevention, and Control

General Approaches

  • Enteritis: Antibiotics not recommended unless in high-risk groups as they prolong excretion

    • AB to use if needed: cirpoflocacin and ceftriaxone

    • Control measures: Proper food handling, preparation, and sanitation.

  • Enteric fever: Antibiotics (ciprofloxacin, ceftriaxone) to prevent carrier states.

    • macrolides in children

    • cholecystectomy

    • Vaccination for travelers in endemic areas → attenuated strain/extract

Septicemia

  • Treatment typically involves parenteral antibiotics.

Yersinia Species

Overview

  • Gram-negative facultative anaerobic rods.

  • Includes pathogenic species (Y. enterocolitica, Y. pseudotuberculosis, Y. pestis).

Pathogenicity

  • Y. enterocolitica and Y. pseudotuberculosis are enteropathogenic and can cause zoonotic diseases.

  • They can multiply at refrigeration temperatures (4°C).

Diagnosis and Therapy for Yersinia

Diagnosis

  • Selective fecal cultures, biochemical identification, and serological methods → MALDI-TOF

Therapy

  • Recent recommendations include fluoroquinolones until antibiograms are available.

Y. pestis: Plague

Overview

  • Causative agent of plague, characterized by predominantly gram-negative coccobacilli.

Important Factors

  • Virulence Factors: Include capsule, phospholipase D, and plasminogen activator for extreme invasiveness.

Diagnosis and Treatment

  • Diagnosis through direct smear, Gram stain, and cultures from relevant sites.

  • Treatment: Streptomycin, gentamicin, tetracycline, chloramphenicol.

Case Study

Overview

  • Family of four experience diarrhea. Symptoms included leukocytes in stool but no bacterial pathogens found.

Insights

  • Pet hamsters can be sources of zoonotic infections (Yersinia). Conservative approach to antibiotic treatment resulted in eventual recovery.

Questions for Consideration

  1. Discuss host-range aspects of Salmonella pathogens.

  2. Outline the infectious process of typhoid fever.

  3. Identify cases for antibiotic therapy in Salmonella gastroenteritis.

  4. Assess food handling mistakes leading to salmonellosis.

  5. Discuss characteristics of enteropathogenic Yersinia relevant to food storage.

  6. Examine reactive complications following Yersinia infections.

  7. Outline clinical forms of plague and transmission mechanisms.

  8. Identify the Y. pestis virulence factor behind tissue invasiveness.