Microorganisms, including bacteria, viruses, fungi, and parasites, are an integral part of our environment. While many play beneficial roles in human health, a subset can cause disease — these are known as pathogens.

The pathological effects of microorganisms refer to the harmful changes they cause in the host body, leading to tissue damage, dysfunction, or systemic illness. These effects result from complex interactions between the microbe and the host immune system, and they vary depending on the type of microorganism and the system involved.

1. Mechanisms of Microbial Pathogenicity

To understand the pathological effects, we must examine the mechanisms by which microorganisms cause damage:

a) Invasion and Colonization

• Microorganisms first enter the body through portals like the respiratory tract, gastrointestinal tract, skin, or urogenital tract.

• They may adhere to host cells using surface proteins, pili, or biofilms (e.g., Streptococcus mutans in dental plaques).

• Some microbes invade deeper tissues using enzymes like hyaluronidase and collagenase (e.g., Clostridium perfringens in gas gangrene).

b) Toxin Production

• Exotoxins: Secreted proteins that directly damage cells (e.g., botulinum toxin blocks nerve signals leading to paralysis).

• Endotoxins: Found in the outer membrane of Gram-negative bacteria; trigger massive immune responses (e.g., E. coli endotoxin can cause septic shock).

c) Immune System Evasion

• Microbes may evade phagocytosis using capsules (e.g., Streptococcus pneumoniae).

• Others hide inside host cells (e.g., viruses, Mycobacterium tuberculosis).

• Some constantly change their surface antigens (e.g., Trypanosoma brucei), confusing the immune system.

d) Immune-Mediated Damage

• Sometimes, it is the host’s immune response that causes pathology, not the microbe itself.

  • Example: Post-streptococcal glomerulonephritis results from immune complex deposition in kidneys after throat infection.

2. Organ-System-Based Pathological Effects

A. Respiratory System

Case Example: Mycobacterium tuberculosis

• Pathogen invades alveoli and is engulfed by macrophages but survives inside them.

• Causes granuloma formation, tissue necrosis, and cavitary lesions.

• Leads to chronic cough, hemoptysis, and weight loss.

Other Pathogens:

• Streptococcus pneumoniae: Causes lobar pneumonia with alveolar exudates.

• Viruses (e.g., influenza): Destroy respiratory epithelium, increasing risk of secondary infections.

B. Gastrointestinal System

Case Example: Vibrio cholerae

• Produces cholera toxin that triggers excessive secretion of electrolytes and water, leading to watery diarrhea and dehydration.

• No invasion of tissue; pathology is purely toxin-mediated.

Other Examples:

• Salmonella typhi: Invades intestinal lining, causes ulcers, enters bloodstream, leading to typhoid fever.

• Helicobacter pylori: Disrupts gastric mucosa, causes gastritis and peptic ulcers.

C. Nervous System

Case Example: Clostridium tetani

• Produces tetanospasmin, a neurotoxin that inhibits inhibitory neurotransmitters.

• Leads to spastic paralysis (muscle rigidity, lockjaw).

Viral Effects:

• Herpes simplex virus: Causes encephalitis, leading to inflammation and necrosis of brain tissue.

• Poliovirus: Destroys motor neurons in the spinal cord, causing flaccid paralysis.

D. Cardiovascular System

Case Example: Staphylococcus aureus

• Can cause infective endocarditis — infection of heart valves.

• Leads to valve destruction, embolism, and heart failure.

Other Effects:

• Treponema pallidum (syphilis): Causes aortitis and aneurysm formation in tertiary stage.

• Viral myocarditis: Direct attack on heart muscle by viruses like Coxsackie B, leading to arrhythmias.

E. Skin and Soft Tissue

Case Example: Streptococcus pyogenes

• Causes necrotizing fasciitis, rapidly destroying skin, fat, and muscle tissues.

• Toxins and enzymes spread infection quickly.

Other Pathogens:

• Fungi like Candida albicans: Cause skin rashes, especially in moist areas.

• Viruses (e.g., varicella): Cause skin eruptions and vesicles.

F. Genitourinary System

Case Example: Neisseria gonorrhoeae

• Adheres to mucosal cells, causes inflammation and urethritis.

• In females, can ascend to cause pelvic inflammatory disease (PID), risking infertility.

Other Pathogens:

• Escherichia coli: Major cause of urinary tract infections (UTIs), causes dysuria and pyelonephritis.

• Schistosoma haematobium: A parasite linked to chronic inflammation and bladder cancer.

3. Systemic and Chronic Effects

a) Sepsis and Septic Shock

• Occurs when microorganisms or their toxins spread in the bloodstream.

• Trigger widespread inflammation, vasodilation, capillary leakage, and organ failure.

• Common culprits: E. coli, Staphylococcus aureus, Klebsiella.

b) Chronic Infections and Granulomas

• Chronic infections like TB or leprosy persist due to incomplete immune clearance.

• Body walls off the infection in granulomas, but tissue damage continues over time.

c) Oncogenic Effects

Some microorganisms have been linked to cancer development:

• Human papillomavirus (HPV) → cervical cancer

• Helicobacter pylori → gastric cancer

• Hepatitis B and C viruses → liver cancer

4. Host Factors Influencing Pathology

Not every infection results in disease. Pathological effects depend on:

• Host immunity (e.g., HIV patients are prone to opportunistic infections).

• Nutritional status

• Age (elderly and children are more susceptible).

• Co-morbidities (e.g., diabetes increases risk of fungal infections).