Systemic Microbiology: Bacteriology || Nursing Alert ||

 

Systemic Microbiology: Bacteriology

Bacteriology is the branch of microbiology that focuses on the study of bacteria, their structure, classification, physiology, and the diseases they cause. Understanding bacteriology is essential for diagnosing and managing bacterial infections.

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Characteristics of Gram-Positive and Gram-Negative Bacteria

The distinction between gram-positive and gram-negative bacteria is based on their cell wall structure, which is determined by the Gram staining method. This method was developed by Hans Christian Gram and remains one of the fundamental techniques in microbiology.

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1. Gram-Positive Bacteria:

• Cell Wall Structure:
  • Thick peptidoglycan layer (20-80 nm) that retains the crystal violet stain, giving them a purple color under the microscope.
  • Teichoic acids are present in the cell wall, contributing to rigidity and aiding in ion transport.
  • Absence of an outer membrane.
• Periplasmic Space:
  • Generally absent or minimal.
• Lipid Content:
  • Low lipid content in the cell wall.
• Examples:
  • Staphylococcus aureus
  • Streptococcus pyogenes
  • Bacillus anthracis

2. Gram-Negative Bacteria:

• Cell Wall Structure:
  • Thin peptidoglycan layer (2-3 nm) that does not retain the crystal violet stain; instead, they appear pink after counterstaining with safranin.
  • Outer membrane composed of lipopolysaccharides (LPS), phospholipids, and proteins.
  • Presence of porins in the outer membrane, allowing selective permeability.
• Periplasmic Space:
  • Prominent, containing enzymes and transport proteins.
• Lipid Content:
  • High lipid content due to the outer membrane.
• Examples:
  • Escherichia coli
  • Pseudomonas aeruginosa
  • Neisseria gonorrhoeae

Key Differences:

Feature	Gram-Positive Bacteria	Gram-Negative Bacteria
Cell wall thickness	Thick peptidoglycan layer	Thin peptidoglycan layer
Outer membrane	Absent	Present
Teichoic acids	Present	Absent
Lipopolysaccharides (LPS)	Absent	Present
Gram stain color	Purple	Pink

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Important Bacterial Diseases

Many bacteria are pathogenic, causing diseases in humans. Here are some of the most important bacterial diseases: 

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1. Tuberculosis (TB):

• Causative Agent: Mycobacterium tuberculosis
• Transmission: Airborne droplets from an infected individual.
• Clinical Features:
  • Persistent cough, often with blood-stained sputum.
  • Fever, night sweats, weight loss.
  • Can affect the lungs (pulmonary TB) or other organs (extrapulmonary TB).
• Diagnosis:
  • Chest X-ray, sputum smear microscopy, and culture.
  • Tuberculin skin test or interferon-gamma release assay (IGRA).
• Treatment:
  • Multi-drug regimen (e.g., isoniazid, rifampin, ethambutol, and pyrazinamide) for 6-9 months.

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2. Diphtheria:

• Causative Agent: Corynebacterium diphtheriae
• Transmission: Respiratory droplets or contact with infected surfaces.
• Clinical Features:
  • Sore throat, fever, and swollen neck ("bull neck").
  • Formation of a thick, gray pseudomembrane in the throat, leading to breathing difficulties.
• Diagnosis:
  • Throat swab culture on selective media.
• Treatment:
  • Antitoxin and antibiotics (e.g., erythromycin or penicillin).
  • Preventive vaccination (DPT or Tdap vaccine).

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3. Tetanus:

• Causative Agent: Clostridium tetani
• Transmission: Entry of spores through wounds, especially in anaerobic conditions.
• Clinical Features:
  • Muscle stiffness and spasms, starting in the jaw (lockjaw).
  • Severe cases involve generalized muscle spasms and respiratory failure.
• Diagnosis:
  • Clinical symptoms and history of wound infection.
• Treatment:
  • Tetanus immune globulin (TIG) and antibiotics (e.g., metronidazole).
  • Supportive care for muscle spasms.
  • Vaccination with tetanus toxoid.

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4. Typhoid Fever:

• Causative Agent: Salmonella typhi
• Transmission: Fecal-oral route via contaminated food or water.
• Clinical Features:
  • High fever, abdominal pain, diarrhea or constipation.
  • Rose-colored spots on the abdomen.
• Diagnosis:
  • Blood culture during the febrile phase.
• Treatment:
  • Antibiotics (e.g., ceftriaxone, azithromycin).
  • Improved sanitation and vaccination.

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5. Cholera:

• Causative Agent: Vibrio cholerae
• Transmission: Contaminated water or food.
• Clinical Features:
  • Profuse watery diarrhea ("rice water stools").
  • Severe dehydration and electrolyte imbalance.
• Diagnosis:
  • Stool culture on selective media.
• Treatment:
  • Rehydration therapy (oral or intravenous).
  • Antibiotics in severe cases (e.g., doxycycline).