Pathogenesis of Bacterial Infection

Learning Outcomes

  1. Differentiate the normal flora (microbiome) from virulent & opportunistic pathogens.

  2. Explain how pathogens are transmitted.

  3. Describe how pathogens gain entry into the body and cells.

  4. Explain how pathogens avoid being killed by the immune system.

  5. Describe how pathogens spread in the body.

  6. Differentiate between endotoxins and exotoxins.

  7. Discuss host risk factors for infection.

Pathogenesis

Definitions

  • Pathogen: A microorganism that is able to cause disease (pathology).

  • Pathogenicity: Indicates whether a pathogen can cause disease or not.

  • Virulence: Indicates the degree of damage a pathogen can cause.

Normal Flora (Microbiome) Vs. Pathogens

The terms Normal Flora, Commensals, and Microbiota all refer to the microorganisms—mainly bacteria—that reside on or inside your body (like in your gut or on your skin) without causing disease.

1. Essential Characteristics

  • Mostly Non-Pathogenic: This is the most important characteristic. It means they do not cause disease when they are healthy and staying in their usual location. They live in a balanced relationship with the host (you).

2. Causes of Infection (When Good Bugs Go Bad)

While generally harmless, these good microbes can turn into infectious agents if certain conditions are met. This happens when the balance or barrier is compromised:

  • Escape from their usual location: If a microbe from the gut, for example, enters a normally sterile area of the body (like the bloodstream or a surgical site), it can become pathogenic (disease-causing).

  • Barrier is breached: If a natural physical barrier, such as the skin or the lining of the gut, is damaged (e.g., by a cut, burn, or surgery), the flora can gain entry to underlying tissues and cause an infection.

  • Host is Immunosuppressed: If the host's immune system is weakened (due to illness, medication, or age), it cannot properly control the commensal organisms. This allows the microbes to multiply unchecked, leading to a serious infection, often referred to as an opportunistic infection.

Virulent Bacteria

Virulent Bacteria are the microbes that are specifically equipped to cause disease, contrasting them with the Normal Flora.

1.Virulent Bacteria

  • Increased ability to invade or damage the host: These bacteria have special features that make them highly capable of getting past your body's defenses, multiplying aggressively, and actively harming your cells and tissues. This ability to cause damage is called pathogenicity.

2. Primary Pathogens

  • Able to cause disease because of their presence within the host: A Primary Pathogen is a microbe that, unlike the normal flora, is inherently capable of causing disease in a healthy person. They don't need a weakened immune system or a broken barrier to start an infection.

3. Colonization

  • Can live among normal flora without causing disease in host ("Colonized"): Sometimes, a primary pathogen can live on or in a person (often the nose, throat, or gut) without actually causing an active illness. When this happens, the host is said to be colonized. The bacteria are present, but the host's immune system is strong enough to keep them controlled, preventing the disease from starting.

4. Virulence Factors

  • Genetic/biochemical/structural features of a pathogen that enable it / enhance its ability to produce disease: These are the specific "weapons" or special tools that a pathogenic bacterium possesses. Virulence Factors are what give the bacteria their strength and help them cause illness.

    • Examples: Toxins that poison your cells, capsules that hide the bacteria from your immune system, or enzymes that help them break through tissue barriers. The presence of these factors directly increases the bacteria's virulence (or deadliness).

Methicillin Resistant Staphylococcus Aureus (MRSA)

MRSA is a specific type of Staphylococcus aureus bacteria that has developed resistance to common antibiotics. Its interaction with a patient can be divided into two states:

1. Colonization (The Bacteria are Present) 🦠

  • What happens: MRSA lives harmlessly on the patient's body, most commonly in the moist areas like the nostrils (nares), on the skin, or in the throat.

  • Why no infection: The MRSA bacteria stay on the body's surface barriers and do not actively invade tissues. The patient's immune system is intact and effective; it is strong enough to keep the MRSA population in check, preventing it from crossing the body's defenses to cause disease.

  • Result: The patient is a carrier. They have the virulent bacteria but remain perfectly healthy.

2. Infection (The Bacteria Cause Disease) 💥

  • What happens: The natural barrier is breached (e.g., a surgical incision, a break in the skin, or an intravenous line).

  • Why infection occurs: The MRSA gains access to deeper, normally sterile tissues (like the bloodstream or bone). Once inside the body, the bacteria's virulence factors (its "weapons") allow it to multiply rapidly and overwhelm the local tissue defenses.

  • Result: The patient develops an active illness, which can range from a minor skin boil to life-threatening conditions like sepsis.

Opportunistic Bacteria

  • Description: Bacteria with low intrinsic virulence.

  • Infection Potential:

    • Typically do not cause infections in immunocompetent patients.

    • Can cause serious infections in immunocompromised patients or with foreign body present.

Reasons for Occurrence of Infection

  • Factors Influencing Infection:

    • The organism is virulent.

    • The size of the inoculum.

    • The portal of entry.

    • Survival in the host.

    • The state of the host.

Pathogen Transmission Overview

  • From the Pathogen's Perspective:

    1. Get in – Portal of Entry

    2. Attach to cells

    3. Defeat/evade the immune system

    4. Cause damage to host cells

    5. Get out and spread further

Portals of Entry

  1. Ingestion: e.g., Salmonella food poisoning.

  2. Inhalation: e.g., Tuberculosis (droplets/aerosols).

  3. Penetration/Inoculation: e.g., Staphylococcus aureus.

  4. Sexual: e.g., Chlamydia.

  5. Vertical Transmission: e.g., Group B Streptococcus (mother to newborn).

Bacterial Adhesion

  • Role of Adhesion: Necessary for establishing a stable population of bacteria within the host and preventing clearance by mechanical defenses.

  • Mechanics of Adhesion:

    • Involves receptor/ligand mediated processes through adhesins (small bacterial protein ligands) that recognize specific receptors on host cells.

Biofilm Formation

  • Definition: Adhesion of bacteria to the surface of foreign materials in the body (e.g., joint replacement, heart valve).

  • Impact: Biofilm protects bacteria and makes eradication difficult, often requiring removal of the prosthesis for eradication.

Invasion Mechanisms

  • Invasion Factors: Biochemical virulence factors termed “Invasins” facilitate pathogen invasion.

    • Invasins include proteins such as Hyaluronidase, Kinase, and Collagenase, which damage host cells and facilitate spread and invasion.

Immune Evasion Strategies

  • Purpose: Bacteria induce immune responses but develop mechanisms to prevent immune-mediated clearance:

    • Inhibition of phagocytosis (ingestion by immune cells).

    • Survival within phagocytes.

    • Immune cell destruction.

Inhibition of Phagocytosis

  • Capsule Description: Composed of polysaccharides, the capsule sits outside the cell membrane of pathogens like Neisseria meningitidis and Streptococcus pneumoniae.

Survival Within Phagocytes

  • Escape Mechanisms:

    • Certain bacteria, such as Listeria monocytogenes, can escape the phagosome.

    • Others like Mycobacterium tuberculosis prevent phagosome/lysosome fusion, while Staphylococcus aureus can survive within the phagolysosome.

Immune Cell Destruction/Evasion

  • Mechanisms:

    • Leucocidins: Pore-forming enzymes targeting phagocytes (e.g., Panton-Valentine Leucocidin of S. aureus).

    • Streptolysin: Pore-forming enzyme from Streptococci.

    • Coagulase: Produced by S. aureus, converts fibrinogen to fibrin, creating a barrier to immune cells.

Damage to Host Cells

  • Mechanisms of Damage:

    • Bacterial enzymes such as Invasins, Toxins, and Superantigens damage host cells while also playing roles in immune evasion.

Bacterial Toxins Types

  • Definition: Toxins are compounds/chemicals harmful to host cells through direct damage or interaction with immune cells.

    • Types of Toxins:

    1. Endotoxin

    2. Exotoxins

Endotoxins

  • Composition: Integral part of bacterial cell envelope; released from bacterial cells upon lysis or turnover.

  • Example: Lipopolysaccharides (LPS), notably in Gram-negative bacteria, can induce pro-inflammatory responses leading to septic shock.

Exotoxins

  • Composition: Polypeptide molecules secreted by living, usually Gram-positive, bacteria during cell lysis or replication.

  • Types of Exotoxins: Target specific cells to induce toxic effects:

    • Cytotoxins

    • Neurotoxins

    • Enterotoxins

Superantigens

  • Definition: A type of exotoxin, initially discovered in S. aureus (e.g., TSST-1).

  • Mechanism: Causes hyperactivation of the immune system through non-specific binding to T-cell receptors via MHC class II, leading to polyclonal T-cell expansion and shock syndrome.

Comparison of Exotoxins and Endotoxins

Feature

Endotoxin

Exotoxin

Source

Gram-negative bacteria

Gram-positive > Gram-negative bacteria

Chemical Composition

Lipopolysaccharides or lipooligosaccharides

Polypeptides or proteins

Mechanism of Release

Released upon bacterial cell lysis

Secreted by bacteria; released on cell lysis

Examples

Meningococcal sepsis

Cholera, Tetanus, Botulism

Pathogen Dissemination

  • Methods of Spread:

    1. Spread through tissues or tissue planes (contiguous).

    2. Spread through the bloodstream (haematogenous).

    3. Spread through the lymphatic system.

    4. Carriage within macrophages (e.g., Typhoid).

    5. Ascending/descending spread within a tract.

Pathogen Transmission Methods

  1. Person-to-Person: Direct contact (skin carriage).

  2. Respiratory: Through coughing and sneezing.

  3. Gastrointestinal: Via diarrhea.

  4. Sexually: Transmitted via genital discharge or ulcers.

  5. Vertically & Perinatally: From mother to newborn.

  6. Environmental Transmission: Through contaminated inanimate objects (fomites).

Host Risk Factors for Infection

  • Factors Increasing Susceptibility:

    • Extremes of age.

    • Diabetes.

    • Immunosuppression (disease-induced or medication-induced).

    • Major breaches in skin/mucosa (recent major surgeries).

    • Prosthetic material (portal of entry, biofilm formation with IV lines, urinary catheters).

    • Anatomical abnormalities or complications post-surgery.

    • Antibiotic therapy leading to Clostridioides difficile infections.

Summary

  • Key Points:

    • Bacteria can be part of normal flora (microbiome), cause opportunistic infections under host compromise, or display virulence as primary pathogens.

    • Various portals of entry and transmission mechanisms characterize the pathogenesis cycle of bacteria.

    • Important roles of adhesins and invasins in infection establishment and immune evasion by mechanisms like phagocytosis inhibition.

    • Endotoxins and exotoxins represent key bacterial factors contributing to host pathology.

    • Certain host risk factors elevate susceptibility to infections, warranting awareness in clinical settings.