Mechanisms of Infection: Colonisation, Invasion, Virulence, Immune Evasion, Tissue Injury and Sepsis
Mechanisms of infection describe how an infectious agent reaches a host, attaches to a suitable site, survives local defences, multiplies, invades or disseminates, damages tissue and interacts with the immune system. Infection is therefore a dynamic host–pathogen process—not merely the presence of a microorganism in a specimen.
Learning outcomes
- Describe the stages from exposure to clinically significant infection.
- Explain adhesion, colonisation, invasion, replication, tissue tropism, biofilm formation and dissemination.
- Compare toxin-mediated, immune-mediated and direct cytopathic tissue injury.
- Explain innate and adaptive immune evasion, latency, intracellular survival and persistence.
- Recognise local, systemic, chronic, latent and opportunistic patterns.
- Connect mechanisms to specimen choice, diagnosis, treatment, source control and sepsis recognition.
1. Infection is an interaction, not a culture result
| Term | What it means | Clinical example |
|---|---|---|
| Exposure | Contact with an infectious agent through a portal of entry. | Inhalation of respiratory particles, ingestion of contaminated food, needlestick or mosquito bite. |
| Colonisation | Adhesion and multiplication without tissue injury or symptoms. | Nasal Staphylococcus carriage or intestinal microbiota. |
| Infection | Agent survives, multiplies or interacts with host tissues, with or without symptoms. | Asymptomatic malaria parasitaemia or early viral infection. |
| Disease | Clinical dysfunction or tissue injury due to pathogen and/or host response. | Pneumonia, meningitis, cellulitis or diarrhoeal disease. |
| Invasion | Crossing epithelial barriers, entering tissue, lymphatics or blood. | Cellulitis progressing to bacteraemia; intestinal pathogen entering mucosa. |
| Dissemination | Spread from a primary focus to remote sites through blood, lymph, nerves, airways or tissue planes. | Miliary TB, meningococcaemia, disseminated herpes or metastatic abscesses. |
2. Stage 1—entry and overcoming barriers
2.1 Physical and chemical barriers
- Intact skin, mucus, cilia, cough, tears and urine physically remove agents.
- Acid in the stomach, bile, lysozyme, defensins and antimicrobial peptides inhibit organisms.
- Normal microbiota compete for nutrients and attachment sites.
- Macrophages, dendritic cells and epithelial pattern-recognition receptors detect danger rapidly.
2.2 How pathogens breach barriers
| Strategy | Examples |
|---|---|
| Mechanical/traumatic entry | Needle, bite, wound, surgery, burns, catheter or insect proboscis. |
| Adhesion to mucosa | Respiratory viruses, uropathogenic E. coli, Vibrio, Neisseria and enteric pathogens. |
| Enzyme-mediated penetration | Proteases, hyaluronidase, collagenase, neuraminidase or phospholipases disrupt tissue/secretions. |
| Vector or vehicle delivery | Mosquito inoculates Plasmodium; contaminated water delivers enteric agents. |
| Intracellular entry | Viruses use receptors/endocytosis; bacteria may induce phagocytosis or invade epithelial cells. |
3. Stage 2—adhesion and colonisation
To establish a focus, the agent must remain at a site long enough to resist flow, mucus, peristalsis, urine, cilia and immune clearance. Adhesins bind host receptors, often in a tissue- and species-specific manner. This determines tropism.
| Mechanism | Pathogen advantage | Clinical consequence |
|---|---|---|
| Fimbriae/pili/adhesins | Specific attachment to epithelial receptors. | Urinary tract infection, pharyngeal colonisation or enteric disease. |
| Viral receptor binding | Determines susceptible cell type and organ. | Respiratory, hepatic, neural or immunologic tropism. |
| Capsule/slime layer | Reduces phagocytosis and retains water/nutrients. | Persistent colonisation and invasive disease. |
| Biofilm matrix | Anchors communities to tissue or devices and slows antimicrobial penetration. | Chronic wound, catheter, prosthetic-joint or dental infection. |
| Antigenic variation | Changes exposed proteins and delays adaptive clearance. | Recurrent or chronic infection. |
Colonisation may be protective or harmful depending on site. A commensal organism can cause infection after barrier breach, aspiration, immunosuppression or movement to a normally sterile compartment.
4. Stage 3—multiplication and nutrient acquisition
Successful pathogens obtain carbon, nitrogen, iron and other nutrients while coping with temperature, pH, oxygen tension, osmotic stress and immune attack. Bacteria may secrete siderophores, viruses redirect host metabolism, fungi use hyphae and parasites alter host nutrient pathways.
4.1 Microbial growth and inoculum
- A high inoculum may overwhelm local defences before adaptive immunity develops.
- Growth rate and generation time influence incubation and acute severity.
- Slow-growing or dormant organisms may produce chronic disease and require prolonged treatment.
- Quorum sensing coordinates biofilm, toxin or virulence-gene expression in some bacteria.
4.2 Host factors that alter multiplication
- Diabetes, malnutrition, neutropenia, HIV, steroid therapy, pregnancy and extremes of age.
- Acid suppression, broad-spectrum antibiotics, surgery, burns and devices.
- Obstruction/stasis in urinary, biliary, respiratory or gastrointestinal systems.
- Prior immunity, vaccination and protective microbiota.
5. Stage 4—local invasion and tissue spread
Invasion occurs when organisms or their products cross an epithelial barrier and enter tissue. Spread may follow planes, lymphatics, blood, nerves, airways or contiguous extension.
| Spread pattern | Mechanism | Example |
|---|---|---|
| Contiguous extension | Enzymes, pressure, necrosis or tissue planes carry infection locally. | Cellulitis, necrotising fasciitis, sinus infection spreading to orbit. |
| Lymphatic spread | Organisms/cells enter lymphatics and regional nodes. | Lymphangitis and tender regional lymphadenopathy. |
| Bloodstream spread | Transient or sustained bacteraemia/viraemia/fungaemia. | Sepsis, endocarditis, metastatic abscesses or organ seeding. |
| Neural spread | Some viruses move along peripheral nerves. | Herpesviruses and rabies-related neuroinvasion. |
| Airway/aspiration spread | Secretions reach lower airways or adjacent structures. | Aspiration pneumonia and endobronchial dissemination. |
| Transplacental or perinatal | Agent crosses placenta or contacts infant during birth/feeding. | Congenital infection or neonatal sepsis. |
6. Stage 5—virulence factors
| Virulence factor | Mechanism | Pathology |
|---|---|---|
| Adhesins | Bind host receptors and resist clearance. | Stable colonisation of mucosa or devices. |
| Capsules | Block phagocytosis/complement and promote persistence. | Invasive meningitis, pneumonia or sepsis. |
| Enzymes | Break extracellular matrix, clots or antimicrobial molecules. | Deep invasion, abscess extension and tissue destruction. |
| Secretion systems | Inject or release effector proteins into host cells. | Cytoskeletal manipulation, intracellular survival and inflammation. |
| Biofilms | Organised matrix protects a community and creates slow-growing subpopulations. | Device-associated, chronic and relapsing infection. |
| Iron acquisition | Siderophores and receptor systems overcome host iron sequestration. | Growth in blood and inflamed tissue. |
| Antigenic variation | Changes surface antigens or switches expression. | Persistent/recurrent infection and vaccine escape in selected settings. |
| Intracellular residence | Survives in epithelial cells, macrophages or other protected niches. | Chronic infection, poor antibiotic access and relapse. |
7. Toxin-mediated injury
7.1 Exotoxins
Exotoxins are microbial proteins released or injected into host cells. Some inhibit protein synthesis, disrupt membranes, alter neurotransmission, activate signalling pathways or cause intense cytokine release.
| Toxin effect | Pathophysiologic result | Clinical pattern |
|---|---|---|
| Neurotoxicity | Blocks neurotransmitter release or causes neuronal dysfunction. | Flaccid/spastic paralysis or altered autonomic function. |
| Enterotoxicity | Alters ion transport and secretion in intestinal epithelium. | Watery diarrhoea, dehydration and electrolyte loss. |
| Cytotoxicity | Damages membranes, ribosomes or intracellular targets. | Necrosis, ulceration and organ dysfunction. |
| Superantigen activity | Non-specific T-cell activation causes a large cytokine release. | Fever, rash, hypotension and multiorgan inflammation. |
7.2 Endotoxin and inflammatory cell-wall products
Lipopolysaccharide and other microbial molecules activate pattern-recognition receptors, complement, coagulation and endothelial cells. Cytokines such as TNF, IL-1 and IL-6 increase fever, vascular leak and leukocyte recruitment. A disproportionate response causes hypotension, microvascular thrombosis, metabolic failure and organ dysfunction.
8. Direct cytopathic injury and immune-mediated injury
| Mechanism | How injury occurs | Examples |
|---|---|---|
| Direct cytopathic effect | Replication, toxins, enzymes or metabolic competition kill or disable cells. | Viral lysis, bacterial necrosis, fungal hyphal invasion. |
| Immune-mediated damage | Neutrophils, antibodies, complement, T cells or cytokines injure host tissues while eliminating the agent. | Hepatitis, immune-complex disease, severe pneumonitis. |
| Granulomatous response | Macrophage/T-cell containment surrounds difficult-to-eradicate organisms. | TB, fungal disease and some parasites. |
| Fibrotic repair | Chronic inflammation activates fibroblasts and remodels tissue. | Liver fibrosis, pulmonary fibrosis, urethral or intestinal strictures. |
| Vascular injury | Endothelial activation, thrombosis, vasculitis or shock reduce perfusion. | Sepsis, meningococcaemia, severe malaria and viral haemorrhagic syndromes. |
The most severe pathology can occur after microbial load falls, because inflammatory and coagulation cascades continue. Clinical deterioration therefore requires treatment of both the pathogen and the host response.
9. Immune evasion and persistence
- Capsules and complement resistance: limit opsonisation and phagocytosis.
- Intracellular survival: blocks phagolysosome fusion, resists oxidative killing or moves between cells.
- Antigenic variation and mimicry: changes or disguises targets recognised by antibodies/T cells.
- IgA protease and mucosal evasion: weakens secretory antibody protection.
- Latency: genome persists with limited expression and reactivates when immunity falls.
- Immune modulation: cytokine interference, T-cell exhaustion or destruction of immune cells supports chronicity.
- Biofilm and spatial protection: matrix, slow growth and persister cells reduce antibiotic/immune access.
- Antigen sequestration: hiding in privileged sites such as CNS, eye, intracellular compartments or cysts.
10. Host response and containment
| Host response | Protective function | Possible pathology |
|---|---|---|
| Innate recognition | Pattern-recognition receptors detect microbial motifs and activate barriers/cytokines. | Excess cytokines, fever, capillary leak and shock. |
| Neutrophils | Phagocytose and kill bacteria/fungi. | Enzymes/NETs cause collateral tissue damage and abscess. |
| Macrophages | Clear organisms/debris and coordinate repair. | Persistent activation causes granuloma, fibrosis or chronic inflammation. |
| Complement | Opsonisation, chemotaxis and membrane attack complex. | Inflammatory vascular injury; deficiencies increase susceptibility. |
| Antibodies | Neutralise toxins/viruses, opsonise and activate complement. | Immune complexes or antibody-dependent tissue injury. |
| T cells | Kill infected cells and activate macrophages. | Immunopathology, granulomas and cytotoxic tissue damage. |
11. Biofilms and device-associated infection
A biofilm is a structured microbial community attached to a surface and enclosed in an extracellular matrix. Organisms within it may grow slowly, exchange genes and display altered metabolic states.
- Initial attachment is followed by matrix production, maturation and dispersal.
- Biofilms occur on central lines, urinary catheters, prosthetic joints, heart valves, endotracheal tubes, chronic wounds and teeth.
- Planktonic cultures may be negative while organisms remain attached to the device.
- Antibiotics alone may fail; source control or device removal can be decisive.
- Prevention uses aseptic insertion, minimal device duration, closed systems, maintenance bundles and hand hygiene.
12. Local, systemic, acute, chronic and latent infection
| Pattern | Mechanism/definition | Example |
|---|---|---|
| Local acute | Rapid multiplication and inflammation at one site. | Cellulitis, otitis, bacterial pneumonia. |
| Local chronic | Persistence with granuloma, fibrosis, biofilm or repeated injury. | TB, chronic osteomyelitis, chronic wound. |
| Systemic/disseminated | Organism/toxin spreads through blood, lymph, airways or multiple sites. | Meningococcaemia, miliary TB, disseminated candidiasis. |
| Latent | Organism/genome persists with limited activity and may reactivate. | Herpesviruses, latent TB. |
| Opportunistic | Normally low-virulence organism invades when host barriers/immunity are compromised. | Pneumocystis, invasive mould or Candida. |
| Reinfection/relapse | New exposure versus persistence of original organism, respectively. | Recurrent UTI, TB relapse or repeat malaria. |
13. From infection to sepsis
WHO defines sepsis as life-threatening organ dysfunction caused by a dysregulated host response to infection. It is not synonymous with bacteraemia and may arise from bacterial, viral, fungal or parasitic infection.
Mechanistic progression
- Pathogen products and tissue injury activate innate immune receptors.
- Cytokines, complement, coagulation and endothelial pathways become amplified.
- Vasodilation, capillary leak, microvascular thrombosis and maldistributed blood flow reduce effective tissue perfusion.
- Mitochondrial/metabolic dysfunction and cellular stress impair oxygen use and organ function.
- Compensatory anti-inflammatory and immune-exhaustion pathways increase susceptibility to secondary infection.
| Organ | Possible sepsis-related dysfunction |
|---|---|
| Brain | Confusion, delirium, reduced consciousness. |
| Lungs | Tachypnoea, hypoxaemia, acute lung injury. |
| Kidneys | Oliguria, rising creatinine, electrolyte/acid-base disturbance. |
| Circulation | Hypotension, mottling, prolonged capillary refill, shock. |
| Liver/coagulation | Cholestasis, hypoglycaemia, coagulopathy, thrombocytopenia. |
Emergency management requires early recognition, resuscitation, appropriate antimicrobials, cultures when feasible, source control, monitoring and prevention of further organ injury. Follow current local sepsis guidance; do not rely on one score alone.
14. Diagnostic implications
- Sample the focus: tissue or aspirate is usually more informative than a superficial swab for deep infection; CSF, blood, urine, stool or respiratory specimens match different syndromes.
- Time matters: culture yield and molecular detection change after antibiotics, partial treatment, latency or low organism burden.
- Use complementary tests: microscopy, culture, antigen/PCR, histology, serology, imaging and inflammatory markers answer different questions.
- Interpret colonisation: a positive airway, skin, stool or urine result may not explain the patient’s illness.
- Look for tissue pattern: abscess, granuloma, necrosis, viral inclusions, hyphae, parasites or vasculitis can support pathogenesis.
- Communicate urgency: suspected meningitis, sepsis, TB, high-consequence pathogen, invasive mould or prion disease requires early laboratory notification and route-specific safety.
15. Treatment and source control by mechanism
| Mechanism/clinical problem | Principle | Example |
|---|---|---|
| Extracellular bacteria | Target organism with appropriate antimicrobial based on syndrome/local resistance; drain pus. | Abscess requires drainage because antibiotics penetrate poorly. |
| Intracellular/latent pathogen | Select drugs that reach the intracellular/site compartment and use adequate duration. | TB, chlamydial infection or herpes latency require specific regimens. |
| Biofilm/device infection | Remove/replace device when indicated plus targeted therapy. | Infected catheter or prosthetic material. |
| Toxin-mediated disease | Neutralise/limit toxin, remove source, provide supportive care and treat organism where useful. | Antitoxin, wound debridement, airway support or toxin-suppressing therapy under specialist guidance. |
| Sepsis | Resuscitation, prompt antimicrobials, source control and organ support. | Do not wait for culture confirmation if shock or rapidly progressive illness is present. |
16. Integrated cases
Case 1: Abscess
Fever and a fluctuant painful collection suggest bacterial invasion with neutrophilic inflammation and a walled-off abscess. Aspirate/drain for culture, provide source control and use antibiotics according to severity, site and susceptibility.
Case 2: Catheter bloodstream infection
A patient with a central line develops fever and rigors. Biofilm allows organisms to persist on the catheter. Obtain appropriately paired cultures, assess for shock and discuss line removal/source control rather than relying only on prolonged antibiotics.
Case 3: Viral encephalitis
Neural tropism, intracellular replication and immune-mediated injury produce altered consciousness and seizures. CSF, MRI and molecular testing guide therapy, but treatment must not be delayed in a compatible emergency.
Case 4: Sepsis after pneumonia
Local airway infection progresses to endothelial activation, capillary leak and organ dysfunction. Treat the pathogen and the host response: oxygenation, perfusion, antibiotics, cultures, source evaluation and close reassessment.
17. Examination-ready summary
- Pathogenesis proceeds through exposure, adhesion/colonisation, multiplication, invasion/toxin effect, inflammation, dissemination and outcome.
- Adhesins, capsules, biofilms, secretion systems, toxins, iron acquisition, antigenic variation and intracellular survival are major virulence mechanisms.
- Damage may be direct, toxin-mediated, vascular, or caused by an excessive immune response.
- Biofilms and intracellular niches explain chronicity, relapse and poor antibiotic penetration.
- Sepsis is organ dysfunction caused by a dysregulated host response to infection—not simply a positive blood culture.
- Diagnosis must match the specimen to the focus and distinguish colonisation/contamination from invasive infection.
- Treatment combines appropriate antimicrobials, source control, supportive care, infection prevention and stewardship.
18. Quick self-test
- List the stages from exposure to disease.
- What is tissue tropism?
- How do adhesins and capsules support infection?
- What is a biofilm and why is it clinically important?
- Give three mechanisms of immune evasion.
- Differentiate direct cytopathic from immune-mediated injury.
- How can endotoxin contribute to shock?
- Why can a superficial swab mislead in deep infection?
- How does intracellular survival alter treatment?
- What is the difference between local, systemic, chronic and latent infection?
- Define sepsis according to WHO.
- Why may sepsis worsen after the microbial burden falls?
- What is source control? Give three examples.
- How do devices promote infection?
- What are the first emergency actions in suspected sepsis?
Answer guide
1. Exposure, adhesion/colonisation, multiplication, invasion/toxin effect, inflammation, dissemination or containment, and resolution/persistence/sepsis. 2. The preference/ability of an agent to infect particular cells or tissues based on receptors and environment. 3. Adhesins attach to host receptors; capsules reduce phagocytosis/complement clearance. 4. An attached microbial community in protective matrix with altered growth and antimicrobial resistance. 5. Capsules, intracellular survival, antigenic variation, latency, IgA protease, immune modulation and biofilm. 6. Direct injury is caused by replication/toxin/enzyme; immune injury is collateral damage from host defences. 7. It activates innate receptors, cytokines, endothelium, coagulation, vasodilation and capillary leak. 8. It samples colonising flora rather than the invasive focus. 9. Drugs need intracellular/site penetration and longer or specialised therapy. 10. Local is one focus; systemic disseminates; chronic persists with ongoing injury; latent persists with low activity/reactivation potential. 11. Life-threatening organ dysfunction caused by a dysregulated host response to infection. 12. Inflammation/coagulation/metabolic dysfunction can persist and cause organ injury. 13. Drain abscess, remove infected catheter, relieve obstruction, debride necrotic tissue. 14. They breach barriers and support biofilm. 15. Recognise, resuscitate, obtain cultures when feasible, start prompt appropriate antimicrobials, assess organs and pursue source control.
Authoritative resources and further reading
- NCBI Bookshelf: Cell Biology of Infection
- NCBI: Infectious agents and how they cause disease
- NCBI: The front line of host defence
- WHO: Sepsis
- WHO: prevention, diagnosis and clinical management of sepsis
- CDC Pink Book: pathogen attachment and toxin-mediated disease example
Clinical caution: Empiric antimicrobial choices, sepsis bundles and source-control decisions must follow current Ugandan/facility guidance and microbiology advice, with adjustment for local resistance and patient-specific factors.
