Bacterial Infections Quiz

Microbial Virulence

• Microbial virulence refers to a microbe's ability to cause damage to a host.
• A pathogen is a microbe that can cause host damage.

Human Body and Bacteria

• Humans co-exist with a vast abundance of microorganisms.
• A single adult human harbors approximately 10¹⁴ microbial cells on exposed body surfaces.
• This number is 10 times the number of human cells in the body.
• External surfaces (skin, oral cavity, conjunctiva, respiratory tract, gastrointestinal tract, vagina, urinary tract) often host normal microbiota.
• Normal microbiota is generally non-pathogenic.
• However, some strains can cause mild to life-threatening infections if they enter normally sterile sites. The colon is a significant reservoir of normal microbiota.

Virulence and Pathogens

• Obligate pathogens are always associated with disease and infection in humans; examples include M. tuberculosis, Leptospira interrogans, and Hepatitis B virus.
• Typical pathogens are highly pathogenic organisms often found as part of the normal microbiota; examples include S. pneumoniae, S. aureus, H. influenzae, and N. meningitidis.
• Opportunistic pathogens generally cause disease only in individuals with compromised immune systems or those with breaches in their normal defenses; examples include S. epidermidis and P. aeruginosa.

Bacterial Virulence

• Virulence is a relative concept ranging from non-pathogenic to highly pathogenic.
• Virulence can vary between strains due to differences in virulence factors (components necessary for infection and disease).

Encounter with Pathogen

• Humans encounter pathogens through infected individuals, contaminated food/water/vectors, aerosols, etc.
• Portals of entry for pathogens include ingestion, inhalation, sexual transmission, arthropod bites, trauma, needlestick injuries, and contaminated blood and body fluids.
• Natural surface defenses (intact skin, flushing actions, mucus, competition among microbes and antimicrobial substances) resist many pathogens.
• Intact skin, mucus membranes, and other internal surfaces often provide a formidable barrier against pathogen invasion.

Bacterial Colonization

• Pathogens can cause disease by breaking through the external surface or invading external cells, or by ingesting pre-formed exotoxins.
• Some pathogens produce exotoxins that cause disease without the organism's invasion, e.g., food poisoning from S. aureus, Bacillus cereus, and Clostridium botulinum.
• Enteric infections (e.g., from V. cholerae, enterotoxigenic E. coli, and enteropathogenic E. coli) often involve amplified pathogen multiplication and disruption of GIT function via exotoxins.

Bacterial-Host Cell Adhesion

• Pathogens must resist the flushing action of body fluids and compete with other microbiota to colonize a new site; this is mediated by adhesins.
• Adhesins (host cell and tissue binding factors) facilitate pathogen attachment to target tissues, often located on hair-like structures surrounding bacteria.
• Adhesins bind to specific host receptors to allow close contact.
• Flagella aid in movement through mucous lining of the gastrointestinal tract

Bacterial-Host Cell Invasion

• Many pathogens directly infect the cells lining external mucosal surfaces.
• Others may enter through breaks in the surface or areas of inflammation.
• Different pathogens use varied mechanisms for invasion. -Bacteria may invade directly or after colonization.

Bacterial-Host Cell Invasion

• Pathogens establish themselves securely in a new niche by resisting initial host defenses.
• Adherence, host cells, and tissue-binding factors facilitate colonization.
• Degradative enzymes that break down host tissue supply nutrients to the invading bacteria.
• Spreading to other locales occurs via local tissue planes or the systemic/lymphatic circulation.
• Microbial invasion may be confined to its primary site or it may spread.

Pathogenic Actions of Bacteria

• Bacteria multiply rapidly under favorable growth conditions.
• Degradative enzymes (proteases, collagenases, lipases, DNases) damage tissues leading to host damage.
• Cytolysins damage host cell cytoplasmic membranes, causing cell death and consequent tissue loss.
• Toxins damage tissues or interrupt key biological activities; toxins may act locally or at distant sites.

Immunity Activation and Endotoxins

• Humans have an early warning system to detect diverse bacterial cell wall components, like LPS (endotoxin).
• Conserved molecular patterns bind Toll-like receptors (on immune system cells), triggering the immune response.
• Pro-inflammatory cytokines (IL-1, TNF-α, IL-6) produce the pyrogenic acute (fever) phase response.
• Peptidoglycan and lipoteichoic acid (gram-positive bacteria) and lipid A (gram-negative bacteria, such as LPS) can trigger immune responses.
• LPS promotes a protective response at low concentrations.

Pathogenic Actions of Bacteria (Exotoxins)

• Exotoxins damage host cells by disrupting cell metabolism, and they are highly potent.
• Exotoxins may be secreted or released during bacterial lysis.
• Superantigens (Type I exotoxins) activate T cells, leading to excessive cytokine production, fever, and hypotension.
• Type II exotoxins damage host cell membranes.
• Type III exotoxins interfere with host-cell function, leading to various pathological consequences (e.g., tetanus, paralysis, cell death, massive fluid loss).

Pathogenic Actions of Bacteria (Exo+Endotoxins)

• Both exotoxins and endotoxins generate pathological systemic responses, giving rise to collateral tissue damage.
• Immune responses (complement activation, neutrophils, macrophages) contribute to tissue damage.
• Compromised tissue perfusion from inflammation can increase tissue damage.

Immunopathogenesis

• Circulating antigen-antibody complexes can pose a hazard leading to damage at distant sites (immunopathogenesis).

Pathogenic Actions of Viruses

• Direct viral cytopathogenesis inhibits protein/DNA synthesis, degrades host DNA, alters membrane structure, generates cellular inclusion bodies or is toxic to virion components, potentially triggering immune responses.
• Viral immunopathogenesis might include flu-like symptoms, delayed hypersensitivity, inflammation, immune complex disease, hemorrhagic disease, post-infection cytolysis, immunosuppression

Evasion of Host Defenses (1)

• The outcome of infections depends on the pathogen's virulence, host factors (age, nutrition, health, vaccination history, genetic background), infection site, and number of invading pathogens.
• Factors for the outcome include host's ability to mount an adequate immune response and pathogen's capacity to evade this response.
• Primary microbial defense systems include polysaccharide capsules, hiding within protected zones, producing enzymes or mediators that prevent normal immune responses (chemotaxis, opsonization, phagocytosis) and masking, shifting, or mimicking of antigens.

Evasion of Host Defenses (2)

• Some microbes survive for a host's lifetime by adopting intracellular lifestyles, inhibiting phagolysosome fusion, resisting lysosomal enzymes, and replicating inside host cytoplasm.
• Some viruses can persist indefinitely inside host cells as partial/complete virions or as chromosomally integrated proviral genomes (e.g., HIV).

Onward Transmission of a Pathogen

• Successful pathogens must amplify their numbers in the primary host, move to the next host (directly or via vectors), and survive in their new environment.
• Transmission routes include respiratory, fecal-oral, sexual, vector-borne, and vertical.
• Highly successful pathogens have widespread penetration into the human population; an example is M. tuberculosis.
• Transmission routes of pathogen include respiratory, fecal-oral, sexual, vector-borne transmission, as well as vertical transmission. (Adding details on transmission routes and specifying that they can be respiratory, fecal-oral, sexual, vector-borne, or vertical)

Test your knowledge on bacterial infections and their mechanisms. This quiz covers various aspects, including how pathogens enter the body, the role of adhesins, and the impact of bacterial byproducts. Challenge yourself and learn more about the intricate ways bacteria affect host systems.