Salmonella

Ms. Neha Sharma

Introduction

  • Salmonella is a clinically important Gram-negative bacterium belonging to the family Enterobacteriaceae.

  • It is a major cause of food-borne illnesses and systemic infections worldwide.

  • The genus Salmonella includes numerous serovars, with Salmonella Typhi, Salmonella Paratyphi, Salmonella Typhimurium, and Salmonella Enteritidis being most significant in humans.

  • Infection occurs primarily through contaminated food or water, especially poultry, eggs, meat, and dairy products.

  • Salmonella causes a wide spectrum of diseases ranging from acute gastroenteritis to enteric (typhoid) fever and septicemia.

  • The organism possesses multiple virulence factors that enable invasion, intracellular survival, and systemic spread.

  • Laboratory diagnosis relies on culture, biochemical tests, and serotyping.

  • Increasing antibiotic resistance among Salmonella strains poses a major public health challenge.

  • Prevention depends on food safety measures, vaccination (for typhoid), and public health surveillance.

  • Continuous research and monitoring are essential to control emerging resistant strains and outbreaks.

General Characteristics

  • Genus: Salmonella

  • Species: Salmonella enterica

    • Includes multiple serovars such as S. Typhi, S. Paratyphi, S. Typhimurium, and S. Enteritidis

  • Family: Enterobacteriaceae

  • Gram Staining:

    • Gram-negative bacteria

    • Appear pink on Gram stain due to a thin peptidoglycan layer and presence of an outer membrane

  • Shape: Rod-shaped (bacilli)

  • Arrangement:

    • Commonly seen as single bacilli

    • May occasionally appear in pairs or short chains

  • Oxygen Requirement:

    • Facultative anaerobes

    • Capable of growth in both aerobic and anaerobic environments

 

Morphology

Cell Wall Structure

  • Composed of:

    • Thin peptidoglycan layer

    • Outer membrane containing lipopolysaccharide (LPS)

  • LPS acts as endotoxin, playing a key role in:

    • Fever

    • Inflammation

    • Septic shock in severe infections

Flagella

  • Most Salmonella species are motile

  • Possess peritrichous flagella

  • Flagella enhance:

    • Motility

    • Invasion of intestinal mucosa

    • Spread within host tissues

 

Cultural Characteristics

Growth Media

  • MacConkey Agar

    • Selective for Gram-negative bacteria

    • Salmonella produces colourless colonies (non-lactose fermenter)

  • XLD Agar (Xylose Lysine Deoxycholate)

    • Produces red colonies with black centres

    • Black colour due to hydrogen sulfide (H₂S) production

  • Hektoen Enteric Agar

    • Colonies appear green with black centres

    • Useful for differentiating Salmonella from Shigella

Colony Morphology

  • Pale or colourless colonies

  • Absence of lactose fermentation

  • Black centres indicate H₂S production

Growth Conditions

  • Optimal temperature: 37°C

  • pH range: 6.0 – 7.5

 

Biochemical Reactions

 

Test Reaction
Catalase Positive
Oxidase Negative
Lactose fermentation Negative (rare exceptions)
Indole test Variable (some strains positive)
Methyl Red Positive
Voges–Proskauer Negative

These biochemical reactions help in laboratory confirmation and differentiation from other enteric bacteria.

Pathogenicity

Virulence Factors

  • Type III Secretion System (T3SS):

    • Injects bacterial proteins into host cells

    • Facilitates invasion and intracellular survival

  • Adhesins:

    • Enable attachment to intestinal epithelial cells

  • Endotoxin (LPS):

    • Triggers strong inflammatory and immune responses

Mechanism of Infection

  • Entry through contaminated food or water

  • Invasion of intestinal mucosa

  • Survival within macrophages

  • Systemic dissemination in invasive strains

 

Clinical Infections

Gastroenteritis

  • Commonly caused by S. Typhimurium and S. Enteritidis

  • Symptoms:

    • Diarrhoea

    • Fever

    • Abdominal cramps

    • Vomiting

Typhoid Fever

  • Caused by S. Typhi

  • Characterized by:

    • Prolonged fever

    • Abdominal pain

    • Hepatosplenomegaly

    • Systemic toxicity

Enteric Fever

  • Caused by S. Typhi and S. Paratyphi

  • Severe systemic illness

Bacteremia and Sepsis

  • Seen in:

    • Immunocompromised patients

    • Elderly

    • Infants

 

Laboratory Diagnosis

1. Specimen Collection

The choice of specimen depends on the clinical syndrome:

  • Stool sample

    • Gastroenteritis

    • Carrier detection

  • Blood

    • Early stage of typhoid / enteric fever (1st week)

  • Urine

    • Later stages of enteric fever

  • Bone marrow

    • Most sensitive specimen in typhoid fever

  • Other samples

    • Pus, CSF, or tissue (in systemic infections)

Proper aseptic collection and rapid transport are crucial to avoid contamination.

2. Microscopic Examination

Gram Staining

  • Shows Gram-negative bacilli

  • Appear as pink, slender rods

  • Non-sporing, non-capsulated

🔹 Note: Microscopy alone is not confirmatory but helps in early suspicion.

3. Culture Techniques 

A. Primary Culture

Specimens are inoculated onto selective and differential media:

  • MacConkey Agar

    • Non-lactose fermenting → colourless colonies

  • XLD Agar (Xylose Lysine Deoxycholate)

    • Red colonies with black centres

    • Black colour due to H₂S production

  • Hektoen Enteric Agar

    • Green colonies with black centres

  • Blood Agar

    • Non-haemolytic colonies

B. Enrichment Media

Used when bacterial load is low:

  • Selenite F broth

  • Tetrathionate broth

These suppress normal gut flora and promote Salmonella growth.

4. Biochemical Identification

Isolated colonies are subjected to biochemical tests:

Test Result
Catalase Positive
Oxidase Negative
Lactose fermentation Negative
Indole Variable
Methyl Red Positive
Voges–Proskauer Negative
H₂S production Positive (most strains)

These reactions confirm genus-level identification.

5. Serological Identification (Serotyping)

  • Based on Kauffmann–White classification

  • Detection of:

    • O (somatic) antigens

    • H (flagellar) antigens

  • Helps in:

    • Identifying specific serovars (e.g., S. Typhi)

    • Epidemiological surveillance

    • Outbreak investigations

6. Serological Tests (Indirect Diagnosis)

Widal Test

  • Detects antibodies against O and H antigens

  • Useful after 1st week of illness

  • Limitations:

    • False positives

    • Prior vaccination or infection

  • Interpretation must consider baseline titres

 Widal test is supportive, not confirmatory.

7. Molecular Methods

  • PCR-based assays

    • Rapid and sensitive

    • Detect specific Salmonella genes

  • Used in:

    • Reference laboratories

    • Outbreak investigations

8. Antibiotic Susceptibility Testing

  • Performed on all confirmed isolates

  • Detects:

    • Multidrug-resistant strains

    • Fluoroquinolone resistance

  • Guides appropriate antibiotic therapy

 

Antibiotic Resistance

Emerging Resistance

  • Increasing resistance due to:

    • Overuse of antibiotics

    • Food-animal transmission

Multidrug-Resistant Salmonella

  • Resistance to:

    • Ampicillin

    • Chloramphenicol

    • Trimethoprim-sulfamethoxazole

Treatment Considerations

  • Antibiotic susceptibility testing is essential

  • Commonly used drugs:

    • Fluoroquinolones

    • Third-generation cephalosporins

 

Prevention and Control

Food Safety

  • Proper cooking of food

  • Safe water supply

  • Good personal hygiene

Vaccination

  • Typhoid vaccines available:

    • Oral (live attenuated)

    • Injectable (polysaccharide)

  • Recommended for travellers to endemic areas

Public Health Surveillance

  • Monitoring outbreaks

  • Food safety inspections

  • Antibiotic resistance tracking

 

MCQs

1. Salmonella belongs to which family?

A. Vibrionaceae
B. Enterobacteriaceae
C. Pseudomonadaceae
D. Neisseriaceae

Answer: B

2. Salmonella is a:

A. Gram-positive coccus
B. Gram-negative bacillus
C. Gram-positive bacillus
D. Acid-fast bacillus

Answer: B

3. Gram staining of Salmonella shows:

A. Purple cocci
B. Pink rods
C. Blue rods
D. Red cocci

Answer: B

4. Oxygen requirement of Salmonella is:

A. Obligate aerobe
B. Obligate anaerobe
C. Facultative anaerobe
D. Microaerophile

Answer: C

5. Most common habitat of Salmonella is:

A. Soil
B. Water
C. Intestinal tract
D. Skin

Answer: C

6. Salmonella Typhi primarily causes:

A. Food poisoning
B. Dysentery
C. Typhoid fever
D. Cholera

Answer: C

7. Salmonella Typhimurium commonly causes:

A. Typhoid fever
B. Gastroenteritis
C. Pneumonia
D. Meningitis

Answer: B

8. Motility in Salmonella is due to:

A. Pili
B. Fimbriae
C. Flagella
D. Capsule

Answer: C

9. Flagella arrangement in Salmonella is:

A. Monotrichous
B. Amphitrichous
C. Lophotrichous
D. Peritrichous

Answer: D

10. Salmonella is:

A. Lactose fermenter
B. Non-lactose fermenter
C. Late lactose fermenter
D. Sucrose fermenter

Answer: B

11. On MacConkey agar, Salmonella produces:

A. Pink colonies
B. Green colonies
C. Colourless colonies
D. Mucoid colonies

Answer: C

12. XLD agar shows Salmonella colonies as:

A. Yellow colonies
B. Red colonies with black centre
C. Green colonies without black centre
D. Pink colonies

Answer: B

13. Black centre in Salmonella colonies is due to:

A. Lactose fermentation
B. Gas production
C. Hydrogen sulfide production
D. Acid formation

Answer: C

14. Salmonella is oxidase:

A. Positive
B. Negative
C. Variable
D. Weakly positive

Answer: B

15. Salmonella is catalase:

A. Negative
B. Variable
C. Weakly positive
D. Positive

Answer: D

16. Methyl red test for Salmonella is:

A. Negative
B. Weak positive
C. Positive
D. Variable

Answer: C

17. Voges–Proskauer test in Salmonella is:

A. Positive
B. Negative
C. Variable
D. Weak positive

Answer: B

18. Indole production in Salmonella is:

A. Always positive
B. Always negative
C. Variable
D. Strongly positive

Answer: C

19. Major virulence factor of Salmonella is:

A. Capsule
B. Exotoxin
C. Type III secretion system
D. Coagulase

Answer: C

20. Endotoxin of Salmonella is:

A. Peptidoglycan
B. Lipopolysaccharide
C. Teichoic acid
D. Protein A

Answer: B

21. Route of transmission of Salmonella is mainly:

A. Airborne
B. Vector-borne
C. Feco-oral
D. Sexual

Answer: C

22. Most common specimen for diagnosis of gastroenteritis is:

A. Blood
B. Stool
C. Urine
D. CSF

Answer: B

23. Best specimen for early diagnosis of typhoid fever is:

A. Stool
B. Urine
C. Blood
D. Saliva

Answer: C

24. Most sensitive specimen for Salmonella Typhi isolation is:

A. Stool
B. Blood
C. Urine
D. Bone marrow

Answer: D

25. Gold standard for diagnosis of Salmonella infection is:

A. Widal test
B. PCR
C. Culture
D. ELISA

Answer: C

26. Widal test detects antibodies against:

A. K antigen
B. O and H antigens
C. Vi antigen only
D. Capsule

Answer: B

27. Limitation of Widal test is:

A. Low sensitivity
B. False positivity
C. Cannot detect antibodies
D. Always negative

Answer: B

28. Serotyping of Salmonella is based on:

A. Sugar fermentation
B. Antibiotic resistance
C. O and H antigens
D. Capsule

Answer: C

29. Kauffmann–White scheme is used for:

A. Antibiotic testing
B. Serotyping
C. Culture identification
D. Molecular diagnosis

Answer: B

30. Salmonella causes bacteremia commonly in:

A. Healthy adults
B. Immunocompromised patients
C. Athletes
D. Pregnant women only

Answer: B

31. Enteric fever is caused by:

A. Salmonella Typhimurium
B. Salmonella Enteritidis
C. Salmonella Typhi
D. Salmonella Arizonae

Answer: C

32. Incubation period of typhoid fever is approximately:

A. 1–2 days
B. 3–5 days
C. 7–14 days
D. 1 month

Answer: C

33. Chronic carrier state in typhoid is associated with:

A. Kidney
B. Gall bladder
C. Intestine
D. Liver

Answer: B

34. Multidrug-resistant Salmonella shows resistance to:

A. Penicillin only
B. Ampicillin, chloramphenicol, cotrimoxazole
C. Vancomycin
D. Linezolid

Answer: B

35. Preferred treatment for MDR typhoid fever is:

A. Penicillin
B. Aminoglycosides
C. Fluoroquinolones / Cephalosporins
D. Tetracycline

Answer: C

36. Antibiotic susceptibility testing is important because of:

A. High virulence
B. Emerging resistance
C. Slow growth
D. Capsule formation

Answer: B

37. Typhoid vaccine is available against:

A. Salmonella Typhimurium
B. Salmonella Enteritidis
C. Salmonella Typhi
D. All Salmonella species

Answer: C

38. Oral typhoid vaccine is:

A. Killed vaccine
B. Subunit vaccine
C. Live attenuated vaccine
D. Toxoid

Answer: C

39. Injectable typhoid vaccine is:

A. Live vaccine
B. Polysaccharide vaccine
C. DNA vaccine
D. Vector vaccine

Answer: B

40. Main source of Salmonella infection is:

A. Air
B. Soil
C. Contaminated food and water
D. Insects

Answer: C

41. Common food associated with Salmonella is:

A. Fruits
B. Milk products
C. Poultry and eggs
D. Cereals

Answer: C

42. Salmonella survives inside host cells mainly in:

A. Neutrophils
B. Macrophages
C. RBCs
D. Platelets

Answer: B

43. Fever in typhoid is due to:

A. Exotoxin
B. Endotoxin
C. Capsule
D. Enzyme

Answer: B

44. Rose spots in typhoid are due to:

A. Allergy
B. Endotoxin-induced vasodilation
C. Platelet destruction
D. Hemorrhage

Answer: B

45. Relapse in typhoid fever occurs due to:

A. Reinfection
B. Incomplete treatment
C. New strain
D. Vaccine failure

Answer: B

46. Which test differentiates Salmonella from E. coli?

A. Gram stain
B. Lactose fermentation
C. Catalase test
D. Capsule stain

Answer: B

47. Salmonella is non-spore forming and:

A. Capsulated
B. Acid-fast
C. Non-capsulated
D. Branching

Answer: C

48. Public health control of Salmonella includes:

A. Isolation only
B. Vaccination and food hygiene
C. Chemoprophylaxis
D. Sterilization

Answer: B

49. Most important preventive measure for Salmonella infection is:

A. Antibiotics
B. Vaccination alone
C. Food safety and hygiene
D. Hospitalization

Answer: C

50. Salmonella remains a public health problem mainly due to:

A. Fastidious nature
B. Antibiotic resistance and foodborne spread
C. Capsule formation
D. Lack of diagnosis

Answer: B