Escherichia coli - Allied Guru

Table of contents

Introduction

Escherichia coli (E. coli) is a Gram-negative, rod-shaped bacterium belonging to the family Enterobacteriaceae.
It is a normal commensal organism found in the intestinal tract of humans and warm-blooded animals.
Most strains of E. coli are harmless and play an important role in vitamin K production and gut health.
Some strains are pathogenic and can cause intestinal and extra-intestinal infections.
E. coli is a facultative anaerobe, capable of growing in the presence or absence of oxygen.
It is non-spore forming and usually motile due to peritrichous flagella.
Pathogenic strains are responsible for diseases such as diarrhea, urinary tract infections (UTI), septicemia, and neonatal meningitis.
E. coli is widely used as a model organism in microbiology, genetics, and biotechnology.
In medical laboratories, E. coli is an important indicator organism for fecal contamination of water and food.

General Character

Genus: Escherichia
Species: Escherichia coli (E. coli)
Family: Enterobacteriaceae
Gram Staining: E. coli is a Gram-negative bacterium, appearing pink due to its thin peptidoglycan layer and outer membrane.
Shape and Arrangement:
- Shape: E. coli are rod-shaped (bacilli).
- Arrangement: Typically found as single cells, but can also occur in pairs or chains.
Oxygen Requirements: E. coli is a facultative anaerobe, capable of growing in aerobic and anaerobic conditions.

Morphology

Cell Wall Structure:
- It comprises a thin layer of peptidoglycan, surrounded by an outer membrane containing lipopolysaccharides (LPS), contributing to its pathogenicity and immune evasion.
Flagella:
- Many strains possess flagella, which provide motility, allowing E. coli to move toward favourable environments (chemotaxis).
Capsule: Some strains have a polysaccharide capsule that helps protect against phagocytosis.

Cultural Characteristics

Growth Media
- MacConkey Agar: Selective and differential medium that inhibits Gram-positive bacteria; E. coli ferments lactose, producing pink colonies.
- Blood Agar: Supports growth and allows observation of hemolytic activity.
- XLD Agar (Xylose Lysine Deoxycholate): Useful for distinguishing E. coli from other enteric pathogens.
Colony Appearance
- On MacConkey agar, E. coli typically produces pink colonies due to the fermentation of lactose.
Temperature and pH Range
- The optimal growth temperature is around 37°C, with a pH range of 6.0 to 7.5 being favourable.

Biochemical Reactions

Catalase Test: E. coli is catalase-positive, producing bubbles when added hydrogen peroxide.
Oxidase Test: E. coli is oxidase-negative, distinguishing it from other Gram-negative bacteria.
Lactose Fermentation: E. coli ferments lactose, producing acid and gas, which can be detected on lactose-containing media.
Indole Production: Most E. coli strains are indole-positive, indicating the ability to convert tryptophan into indole.
Methyl Red Test: Positive, indicating mixed acid fermentation.
Voges-Proskauer Test: Negative, indicating the absence of acetoin production.

Pathogenicity

Virulence Factors
- Adhesins: Enable adherence to the intestinal epithelium, facilitating colonization.
- Toxins:
  - Shiga Toxin (produced by E. coli O157): Can cause severe gastrointestinal disease and hemolytic uremic syndrome (HUS).
  - Enterotoxins (e.g., heat-labile and heat-stable toxins) can cause diarrhoea, especially in enterotoxigenic E. coli (ETEC).
- Capsule and Biofilm Formation: Help protect against host immune responses.
Clinical Infections
- Gastroenteritis: Caused by pathogenic strains such as enterotoxigenic E. coli (ETEC), enteropathogenic E. coli (EPEC), and enterohemorrhagic E. coli (EHEC).
- Urinary Tract Infections (UTIs): Uropathogenic E. coli (UPEC) is a leading cause of UTIs.
- Sepsis: This can occur in immunocompromised patients.
- Meningitis: Particularly in neonates, caused by certain serotypes.

Laboratory Diagnosis

Specimen Collection: Commonly involves stool samples for gastroenteritis or urine samples for UTIs.
Microscopic Examination:
- Gram staining reveals Gram-negative bacilli.
Culture Techniques:
- Inoculation on selective media like MacConkey or XLD agar, followed by incubation.
Biochemical Testing:
- Confirmatory tests for lactose fermentation, indole production, and other biochemical reactions to identify E. coli.
Molecular Methods: PCR and other nucleic acid amplification techniques can identify specific pathogenic strains, especially in outbreaks.

Antibiotic Resistance

Emergence of Resistance: E. coli has shown increasing resistance to multiple antibiotics, including penicillins and fluoroquinolones.
Multidrug-Resistant Strains: Especially concerning hospital-acquired infections and among strains producing extended-spectrum beta-lactamases (ESBLs) or carbapenemases.
Treatment Options: The selection of antibiotics based on susceptibility testing is crucial due to varying resistance patterns.

Prevention

Hygiene Practices: Proper handwashing and food safety measures (cooking food thoroughly, avoiding cross-contamination) are vital to prevent E. coli infections.
Vaccination: Currently, no vaccines exist for E. coli; however, research is ongoing, particularly for pathogenic strains such as EHEC.
Surveillance: Monitoring food sources and outbreaks can help manage risks associated with E. coli infections.

MCQs

Section A: General Characteristics

1. Escherichia coli belongs to which family?
A. Vibrionaceae
B. Enterobacteriaceae
C. Pseudomonadaceae
D. Neisseriaceae

2. E. coli is best described as:
A. Gram-positive cocci
B. Gram-negative bacilli
C. Acid-fast bacilli
D. Gram-variable bacilli

3. The natural habitat of E. coli is:
A. Soil
B. Water
C. Human intestine
D. Skin

4. E. coli is a:
A. Obligate aerobe
B. Obligate anaerobe
C. Facultative anaerobe
D. Microaerophile

5. Which structure is responsible for motility in E. coli?
A. Pili
B. Capsule
C. Flagella
D. Fimbriae

Section B: Morphology & Staining

6. E. coli shows which arrangement microscopically?
A. Chains
B. Clusters
C. Single rods
D. Palisades

7. E. coli is:
A. Spore forming
B. Non-spore forming
C. Acid fast
D. Encapsulated always

8. The Gram reaction of E. coli is due to:
A. Thick peptidoglycan
B. Absence of cell wall
C. Thin peptidoglycan with outer membrane
D. Teichoic acid

9. Capsule in E. coli is:
A. Always present
B. Never present
C. Present in some strains
D. Only in laboratory strains

10. The flagellar arrangement in E. coli is:
A. Monotrichous
B. Lophotrichous
C. Amphitrichous
D. Peritrichous

Section C: Culture Characteristics

11. E. coli grows best at:
A. 25°C
B. 30°C
C. 37°C
D. 45°C

12. On MacConkey agar, E. coli produces:
A. Colorless colonies
B. Pale colonies
C. Pink colonies
D. Black colonies

13. Lactose fermentation by E. coli indicates:
A. Gas production only
B. Acid production
C. Protein digestion
D. Spore formation

14. On EMB agar, E. coli shows:
A. No growth
B. Blue colonies
C. Green metallic sheen
D. Yellow colonies

15. E. coli is an indicator of:
A. Air pollution
B. Water hardness
C. Fecal contamination
D. Chemical toxicity

Section D: Biochemical Reactions

16. Indole test of E. coli is:
A. Negative
B. Weak positive
C. Positive
D. Variable

17. Methyl red test of E. coli is:
A. Negative
B. Positive
C. Variable
D. Delayed

18. Voges–Proskauer test of E. coli is:
A. Positive
B. Weak positive
C. Negative
D. Variable

19. Citrate utilization test of E. coli is:
A. Positive
B. Negative
C. Variable
D. Weak

20. Which IMViC pattern is typical for E. coli?
A. ++–
B. –++
C. +-+-
D. ++++

Section E: Antigenic Structure

21. O antigen of E. coli is part of:
A. Capsule
B. Flagella
C. Lipopolysaccharide
D. Pili

22. H antigen is associated with:
A. Capsule
B. Flagella
C. Cell wall
D. Ribosomes

23. K antigen represents:
A. Flagellar antigen
B. Capsular antigen
C. Somatic antigen
D. Enzyme

24. Endotoxin of E. coli is:
A. Exotoxin
B. Protein toxin
C. Lipopolysaccharide
D. Enzyme

25. Endotoxin mainly causes:
A. Paralysis
B. Fever and shock
C. Muscle spasm
D. Diarrhea only

Section F: Pathogenic Types

26. Which strain causes traveler’s diarrhea?
A. EHEC
B. ETEC
C. EIEC
D. EAEC

27. Hemolytic uremic syndrome is associated with:
A. EPEC
B. ETEC
C. EHEC
D. EIEC

28. Enteropathogenic E. coli mainly affects:
A. Adults
B. Neonates and infants
C. Elderly only
D. Pregnant women

29. Enteroinvasive E. coli resembles infection caused by:
A. Vibrio cholerae
B. Salmonella
C. Shigella
D. Campylobacter

30. Which toxin causes watery diarrhea?
A. Shiga toxin
B. Heat-labile toxin
C. Endotoxin
D. Cytotoxin

Section G: Clinical Significance

31. Most common cause of UTI is:
A. Klebsiella
B. Proteus
C. E. coli
D. Pseudomonas

32. E. coli can cause all EXCEPT:
A. UTI
B. Neonatal meningitis
C. Typhoid
D. Septicemia

33. Extra-intestinal infections of E. coli include:
A. Diarrhea only
B. Pneumonia only
C. UTI and septicemia
D. Skin infection only

34. Neonatal meningitis strain is commonly associated with:
A. O antigen
B. H antigen
C. K1 antigen
D. F antigen

35. Source of infection is usually:
A. Airborne
B. Vector-borne
C. Endogenous flora
D. Sexual contact

Section H: Laboratory Diagnosis

36. Specimen for diagnosing E. coli diarrhea is:
A. Blood
B. Urine
C. Stool
D. CSF

37. Primary culture medium for E. coli is:
A. Blood agar
B. Lowenstein–Jensen
C. MacConkey agar
D. TCBS agar

38. Gas production by E. coli is seen in:
A. Glucose fermentation
B. Lactose fermentation
C. Both A and B
D. None

39. Which test differentiates E. coli from Klebsiella?
A. Catalase test
B. Indole test
C. Oxidase test
D. Urease test

40. E. coli oxidase test is:
A. Positive
B. Negative
C. Weak positive
D. Variable

Section I: Applied & Miscellaneous

41. E. coli is widely used in research because it:
A. Is pathogenic
B. Grows slowly
C. Is genetically well studied
D. Is acid fast

42. Indicator organism for water quality is:
A. Staphylococcus
B. E. coli
C. Clostridium
D. Bacillus

43. Which enzyme test is positive in E. coli?
A. Oxidase
B. Urease
C. Catalase
D. Coagulase

44. Antibiotic resistance in E. coli is commonly due to:
A. Spore formation
B. Capsule
C. Plasmids
D. Flagella

45. E. coli is sensitive to:
A. Penicillin always
B. All antibiotics
C. Antibiotics based on sensitivity testing
D. Antifungals

Section J: True Concept MCQs

46. E. coli is part of normal gut flora.
A. True
B. False

47. All E. coli strains are pathogenic.
A. True
B. False

48. E. coli ferments lactose rapidly.
A. True
B. False

49. E. coli produces exotoxin only.
A. True
B. False

50. Presence of E. coli in water indicates fecal pollution.
A. True
B. False

Answer Key