General Principles of Microbial Pathogenesis

Slide 1 · General Principles ofMicrobial Pathogenesis

Host barriers • entry • dissemination • transmission • microbial–host interactions

Core processes governing infectious disease

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes: Explain how infection is established

Identify portals of microbial entry and the defenses operating at each site.

Distinguish localized infection from lymphatic, hematogenous, and neural dissemination.

Relate transmission pathways to microbial exit and the next host’s entry.

Interpret the roles of microbiota, virulence, and immune evasion in disease.

Slide 3 · Microbial disease is an interaction—not simply exposure

Microbial exposure

→

Entry through a portal

→

Survival against defenses

→

Replication / spread

→

Host response and injury

An effective barrier or immune response may interrupt the process at multiple points.

Slide 4 · Virulence and host defenses jointly determine infection

Microbial factors

Ability to attach or penetrate surfaces

Virulence mechanisms that overcome defenses

Capacity to persist and disseminate

Host factors

Integrity of epithelial barriers

Local innate antimicrobial mechanisms

Effective adaptive immune responses

An organism can cause disease through high virulence or weakened host resistance.

Slide 5 · Infectious microbes are acquired from multiple sources

Humans

Symptomatic or asymptomatic sources

Animals

Direct zoonotic contact or bites

Arthropods

Vector-mediated introduction

Environment

Contaminated water, food, or other exposures

Slide 6 · Portals of entry are linked to particular defenses

Skin

Keratinized epithelium, low pH, fatty acids

GI tract

Acid, mucus, IgA, enzymes, microbiota

Respiratory tract

Mucociliary clearance; Alveolar phagocytes

Urogenital tract

Urine flow; Vaginal acidic microbiota

Slide 7 · Intact mucosa and injured skin present different challenges

Intact mucosal surfaces

Respiratory, GI, and genitourinary portals

Successful pathogens can adhere or penetrate

Local replication can occur despite intact epithelium

Skin after breach

Trauma, burn, ulcer, or intervention opens entry

Infections usually exploit disruption

Needlesticks and bites bypass the surface barrier

Slide 8 · Healthy skin presents mechanical and chemical defenses

Keratinized epidermis resists microbial penetration.

Surface acidity and antimicrobial fatty acids inhibit organisms.

Defensins contribute direct antimicrobial activity.

Physical injury removes multiple protective mechanisms.

Layered protection

Intact epidermis

↓

Mechanical resistance

↓

Low pH • fatty acids

↓

Antimicrobial peptides

Slide 9 · Skin infection commonly follows loss of barrier integrity

Barrier disruption

Pathogen example

Mechanistic interpretation

Burns / ulcers

S. aureus; P. aeruginosa

Exposed tissue enables microbial entry

Needlestick

HIV; HBV; HCV

Inoculation bypasses epidermis

Arthropod / animal bite

Plague; rabies; malaria

Vector or bite directly introduces agent

Slide 10 · Some pathogens penetrate skin without an obvious wound

Most pathogens require a cutaneous defect or vector-associated puncture.

Schistosoma larvae provide an important exception.

Larval enzymes disrupt adhesive proteins joining keratinocytes.

The route of entry reflects pathogen-specific capabilities.

Direct penetration — schematic

Schistosoma larva

↓

Enzymatic cleavage of adhesion

↓

Passage through epidermis

Slide 11 · The gastrointestinal tract has overlapping barriers

Chemical

Gastric acidity; Bile; Pancreatic enzymes

Surface

Mucus and epithelial barrier

Immune

Secretory IgA and defensins

Ecologic / mechanical

Microbiota and peristalsis

Slide 12 · Gastric acidity can sharply restrict infectivity

Ingested microbe

→

Encounter gastric acid

→

Viable organisms reduced

→

Lower inoculum reaching intestine

Markedly increased V. Cholerae infectivity after experimentally neutralizing gastric acid.

Slide 13 · Mucus, secretory IgA, and defensins have different roles

Local defense

Principal protective role

Superficial mucus

Physical separation of organisms from epithelium

Secretory IgA

Mucosal antibody-mediated protection

Defensins

Antimicrobial peptide activity

Epithelial integrity

Barrier to invasion

Slide 14 · Peristalsis limits microbial residence and overgrowth

Motile intestinal contents

→

Microbial clearance

→

Reduced retention

→

Reduced local overgrowth

Obstruction, ileus, or postsurgical adhesions may permit excessive growth of mixed gut bacteria.

Slide 15 · Commensal microbiota constrain opportunistic colonization

Resident gut organisms competitively inhibit pathogens.

Broad-spectrum antibiotic exposure can disrupt colonization resistance.

Clostridioides difficile is a central clinical example.

Thus antimicrobial treatment can indirectly favor infection.

Loss of colonization resistance

Resident microbiota

↓

Antibiotic-associated disruption

↓

C. Difficile overgrowth

Slide 16 · Enteric pathogens succeed by distinct barrier-evasion routes

Route / defense defeated

Examples

Pathogenic advantage

Attachment and proliferation

V. cholerae; Giardia

Persistence at epithelial surface

Local invasion

Shigella; Salmonella

Passage into mucosal tissue

Acid-resistant forms

Protozoal cysts; helminth eggs

Survival through stomach

Resistance to luminal factors

Norovirus

Survival despite bile / enzymes

Slide 17 · Gastrointestinal entry: Differentiate exposure from disease

Two individuals ingest an enteric pathogen; One has recently received broad-spectrum antibiotics.

Interpretation question

Which local mechanism may differ even when exposure is similar?

Mechanistic answer

Disruption of colonization resistance can permit pathogen expansion, particularly for C. Difficile.

Slide 18 · Respiratory entry begins with inhaled particles

Inhaled particles

→

Upper airway trapping or distal deposition

→

Ciliary transport or phagocytosis

→

Microbial elimination / persistence

Particle size influences deposition; Failure to clear organisms permits establishment.

Slide 19 · Mucociliary clearance protects the conducting airways

Larger inhaled particles are trapped in airway mucus.

Coordinated ciliary activity moves mucus toward the pharynx.

Swallowing removes trapped microorganisms from the airway.

Organisms that inhibit ciliary activity evade this defense.

Conducting airway protection

Mucus traps inhaled particles

↓

Ciliary transport toward pharynx

↓

Swallowing / clearance

Slide 20 · Particles reaching alveoli confront phagocytes

Fine particles can penetrate to distal airspaces.

The source describes particles <5 μm reaching alveoli.

Alveolar leukocytes and macrophages phagocytose organisms.

Intracellular survival becomes an effective evasion strategy.

Alveolar defense

Distal particle deposition

↓

Alveolar phagocyte encounter

↓

Killing or microbial persistence

Slide 21 · Respiratory viruses can exploit epithelial cells directly

Influenza virion

→

Attachment to respiratory epithelial cell

→

Entry and replication

→

Epithelial infection

Successful local entry can overcome clearance even without an overt barrier wound.

Slide 22 · Bacterial toxins can undermine mucociliary protection

Bordetella pertussis produces factors that impair ciliary function.

Reduced clearance increases airway retention of organisms.

Mucociliary dysfunction therefore aids establishment of infection.

This is a functional barrier failure rather than a skin-like breach.

A microbial countermeasure

Toxin action

↓

Impaired ciliary activity

↓

Reduced microbial clearance

Slide 23 · Phagocytosis does not ensure microbial elimination

Usual outcome

Alveolar uptake of an organism

Phagocyte antimicrobial activity

Elimination of the microbe

Pathogen survival

Mycobacteria resist intracellular killing

Persistence in macrophages

Potential progression of infection

Slide 24 · Impaired airway clearance predisposes to secondary infection

Influenza

Damages airway defenses

Smoking

Compromises clearance

Cystic fibrosis

Defective mucociliary clearance

Ventilation

Altered airway protection

Slide 25 · Systemic immune impairment changes pulmonary susceptibility

Intact host defense

Alveolar phagocyte activity

Mucociliary removal

Systemic antimicrobial immunity

Immunodeficient host

Opportunistic respiratory pathogens emerge

Examples include Pneumocystis jirovecii

Aspergillus spp. May cause disease

Slide 26 · Urinary flow mechanically limits colonization

Organisms reach urethra

→

Potential ascent into urinary tract

→

Micturition flushes lumen

→

Adherent bacteria may persist

Bacterial attachment to urothelium counteracts repeated washout.

Slide 27 · Adhesion is central to ascending urinary infection

Most urinary tract pathogens enter through the urethra.

Escherichia coli is a representative organism.

Attachment to urothelial surfaces resists urinary flushing.

Local proliferation may follow successful adherence.

Ascending infection — schematic

Urethral entry

↓

Urothelial adhesion

↓

Retention / proliferation

Slide 28 · Obstruction and reflux compromise urinary defense

Effective urine flow

Regular emptying of the urinary tract

Washout of nonadherent microbes

Reduced opportunity for growth

Impaired drainage

Obstruction slows flushing

Vesicoureteral reflux favors persistence / spread

Predisposition to urinary tract infection

Slide 29 · Vaginal lactobacilli maintain a protective acidic niche

Resident lactobacilli

→

Lactic acid production

→

Low vaginal pH

→

Suppression of some pathogens

Antibiotic-associated disruption of vaginal microbiota may permit Candida overgrowth.

Slide 30 · Portal-specific failures predict likely entry mechanisms

Portal

Protective mechanism

Representative failure

Skin

Epidermal integrity

Trauma or vector puncture

Gut

Acid; motility; microbiota

Acid resistance; ileus; antibiotics

Airway

Mucociliary clearance; phagocytes

Toxins; intracellular persistence

Urogenital

Urination; lactobacilli

Adhesion; obstruction; antibiotic use

Slide 31 · Local infection and dissemination are distinct outcomes

Entry and local growth

→

Local containment OR invasion

→

Access to transport route

→

Distant tissue seeding

Successful entry need not imply systemic infection; The capacity to spread is pathogen-dependent.

Slide 32 · Contiguous spread follows direct tissue invasion

Some pathogens expand from the initial focus into adjacent tissue.

Microbial enzymes can disrupt tissue barriers.

No bloodstream step is necessary for direct extension.

Anatomic continuity explains the distribution of lesions.

Local extension

Initial focus

↓

Adjacent tissue infiltration

↓

Contiguous expansion

Slide 33 · Lymphatic channels can transport microorganisms

Local infection

→

Entry into lymphatic drainage

→

Transport to regional sites

→

Possible subsequent blood access

Organisms may transit lymph freely or in inflammatory cells.

Slide 34 · Bloodstream dissemination provides widespread access

Local tissue invasion

→

Access to bloodstream

→

Systemic circulation

→

Potential seeding of multiple organs

The bloodstream as the most common and efficient long-distance route.

Slide 35 · Bacteremia, tissue seeding, and sepsis are not identical

Distribution and seeding

Microbes enter blood

Organ-specific vascular delivery

Focal tissue infection may develop

Systemic clinical syndrome

Disseminated infection can trigger sepsis

Fever, hypotension, and coagulopathy

Severe cases can progress to organ failure

Slide 36 · Nerves permit directed dissemination by selected microbes

Some viruses exploit axonal transport.

Rabies virus and varicella-zoster virus are textbook examples.

Neural routes differ from hematogenous distribution.

The route of transport helps explain target-organ disease.

Neural dissemination — schematic

Peripheral inoculation

↓

Axonal transport

↓

Central nervous system / ganglia

Slide 37 · Transport routes provide different anatomical opportunities

Pathway

Mechanism

Consequence

Contiguous

Local enzymatic invasion

Extension to adjacent structures

Lymphatic

Transport in lymph or cells

Regional spread; potential blood access

Hematogenous

Circulation in blood

Distant organ seeding

Neural

Axonal transport

Spread along neuronal pathways

Slide 38 · Transmission completes the pathogen life cycle

Infected source host

→

Microbial exit / shedding

→

Exposure of another host

→

Entry through susceptible portal

Shedding and successful entry are both needed for host-to-host propagation.

Slide 39 · Microbial shedding varies in timing and duration

Brief

Short period of release

Intermittent

Release during disease flares

Persistent

Long-duration shedding

Asymptomatic

Transmission without overt illness

Slide 40 · Respiratory transmission requires release into air

Infected hosts expel pathogens in respiratory particles.

Particle characteristics influence travel and deposition.

Influenza and tuberculosis illustrate different patterns.

Respiratory release links directly to airway entry in the recipient.

Airborne exposure cycle

Infected airway

↓

Droplets / aerosols

↓

Recipient respiratory tract

Slide 41 · Fecal–oral transmission joins excretion and ingestion

Enteric infection

→

Pathogen shed in stool

→

Food or water contamination

→

Ingestion by new host

Examples include hepatitis A/E viruses, rotavirus, cholera, Shigella, and Salmonella.

Slide 42 · Sexual transmission exploits mucosal or intimate contact

Agent group

Representative organism

Viruses

HSV; HIV; HPV

Bacteria

N. gonorrhoeae; T. pallidum

Protozoa

Trichomonas vaginalis

Arthropods

Phthirus pubis

Slide 43 · Vertical transmission has three distinct windows

In utero

Placental–fetal transmission

At delivery

Exposure during passage through birth canal

After birth

Transmission through maternal milk

Clinical implication

Timing can alter fetal or neonatal manifestations

Slide 44 · Animal reservoirs and vectors expand transmission routes

Zoonotic pathways

Direct animal contact / bites

Exposure to infected animal products

Vector-mediated transmission

Other pathways

Arthropod blood meals carry some protozoa

Saliva can transmit agents such as EBV

Not every infection requires person-to-person spread

Slide 45 · Transmission and entry must be matched

Exit / transmission

Portal in next host

Example

Respiratory particles

Respiratory mucosa

Influenza

Fecal contamination

Gastrointestinal tract

V. cholerae

Sexual contact

Genital mucosa

N. gonorrhoeae

Vector bite

Skin

Malaria

Maternal transmission

Fetal / neonatal interface

Rubella; CMV

Slide 46 · The microbiome comprises resident microbial communities

The human microbiome includes microorganisms living on or within the body.

It encompasses bacteria, fungi, viruses, and protozoa.

Communities differ across skin, gut, oral, lung, and vaginal niches.

Each broad niche contains multiple site-specific microenvironments.

Slide 47 · Microbial communities differ by anatomical niche

Oral

Distinct tooth, tongue, gingival, and salivary communities

Gut

Nutritional and metabolic interactions

Skin

Organisms adapted to cutaneous niches

Vaginal

Microbes can regulate local acidity

Slide 48 · Commensal microbes influence host physiology

Nutrition

Microbes generate some vitamins and amino acids

Metabolism

Host physiology responds to microbial processes

Immunity

Microbiota modulate host immune function

Protection

Commensals can limit pathogen colonization

Slide 49 · Microbiome–organ axes describe intersite relationships

Gut–lung

Cross-site associations with respiratory function

Gut–skin

Reported links between intestinal and cutaneous niches

Gut–brain

Investigated connections with neural function

Interpretation

Specific causal mechanisms are often uncertain

Slide 50 · Dysbiosis may be a cause, consequence, or modifier

Association observed

Microbiome differs between groups

Disease may alter diet, physiology, or ecology

Confounding can explain part of the signal

Causal role established?

Direction of effect must be tested

Specific responsible species often unknown

Association alone cannot prove causation

Slide 51 · Species-level associations demand cautious interpretation

Microorganism

Association / biological property

Interpretive limit

Ruminococcus gnavus

IBD flares

Association; causation not established

Prevotella copri

Rheumatoid arthritis

Correlation, not sole cause

Akkermansia muciniphila

Mucin degradation; barrier effects

Context-dependent host interaction

Slide 52 · Culture-independent methods extend microbiome analysis

Method

What it contributes

Constraint

Culture

Growth of viable organisms

Many organisms difficult to culture

Sequencing

Taxonomic / genomic community profiling

Requires bioinformatic analysis

Metabolomics

Microbial and host-associated metabolites

Interpreting sources and functions is complex

Slide 53 · Microbiome therapy has unequal levels of support

Textbook-supported application

Fecal microbiota transplantation can treat recurrent C. Difficile infection

Donor-stool-derived bacterial spore therapy is described

Investigational extension

Trials in chronic noninfectious diseases have mixed results

Engineered microbes, phages, and metabolites remain exploratory

Slide 54 · Outcome depends on pathogen potency and host immunity

Microbial virulence

→

Host barriers / innate defenses

→

Adaptive immune response

→

Elimination, persistence, or damage

The immune response may both protect the host and contribute to tissue injury.

Slide 55 · Protective immunity is multilayered

Physical

Intact epithelium and mechanical clearance

Innate

Phagocytes, complement, peptides, interferons

Adaptive humoral

Antibodies constrain extracellular microbes

Adaptive cellular

T cells recognize and control infected cells

Slide 56 · Antigenic variation can defeat existing antibody recognition

Antibody recognizes surface antigen

→

Microbe changes antigenic structure

→

Recognition becomes less effective

→

Variant gains selective advantage

Borrelia, trypanosomes, influenza, and other variable pathogens.

Slide 57 · Antigenic variation arises through several mechanisms

Mechanism

Examples

Underlying change

High mutation rate

HIV; SARS-CoV-2

Sequence variation

Genome reassortment

Influenza; rotavirus

Exchange of genome segments

Genetic rearrangement

N. gonorrhoeae; Borrelia

Altered expressed antigen

Serotype diversity

Rhinoviruses; S. pneumoniae

Pre-existing antigenic heterogeneity

Slide 58 · Antimicrobial peptide resistance counters innate killing

Usual peptide action

Host cationic peptides bind microbial surfaces

Peptide insertion can disrupt membranes

Innate peptides therefore contribute to killing

Resistance strategies

Change surface charge or hydrophobicity

Secrete peptide-inactivating enzymes

Export peptides using efflux pumps

Slide 59 · Capsules and antibody-binding proteins deter phagocytosis

Polysaccharide capsule

Inhibits phagocytic uptake

Examples: Pneumococci, meningococci, H. Influenzae

Barrier to effective opsonization / engulfment

Staphylococcal protein A

Binds antibody Fc region

Interferes with Fc-receptor-mediated uptake

Illustrates a distinct anti-phagocytic strategy

Slide 60 · Intracellular pathogens may survive after phagocytosis

Strategy

Example

Cellular consequence

Block phagosome–lysosome fusion

Mycobacteria

Reduced phagolysosomal killing

Escape phagosomal membrane

Listeria monocytogenes

Growth in cytosol

Survive within host cells

Leishmania; other protozoa

Persistence despite cell uptake

Slide 61 · Intracellular survival includes manipulation of cell fate

Apoptosis

Some viruses delay infected-cell death

Autophagy

Microbes evade degradative trafficking

Host signaling

Viral proteins alter cellular pathways

Persistence

Altered cell fate extends replication opportunities

Slide 62 · Microbes can interfere with interferon signaling

Usual interferon response

Host cells produce interferons

Receptor signaling activates JAK–STAT pathways

Antiviral effectors include PKR

Viral interference

Soluble receptor homologues act as decoys

Viral factors inhibit JAK–STAT signaling

Other factors block or inactivate PKR

Slide 63 · Antigen presentation is an immune-evasion target

Normal recognition

MHC class I presents to CD8⁺ T cells

MHC class II presents to CD4⁺ T cells

Cell-mediated immunity detects infected cells

Viral interference

Some herpesviruses alter MHC I trafficking

Other mechanisms affect MHC II

Reduced presentation impairs T-cell recognition

Slide 64 · Latency and immune-cell infection promote persistence

Latency

Few viral genes may be expressed

HSV / VZV persist in neurons

EBV can persist in B lymphocytes

Immune-cell targeting

HIV infects and destroys CD4⁺ T cells

Loss of helper function compromises immunity

Persistent infection changes host defense

Slide 65 · Different immune deficits expose distinct vulnerabilities

Defective defense

Characteristic susceptibility

Antibodies

Encapsulated / extracellular bacteria

Terminal complement (C5–C9)

Neisseria species

Neutrophil function

S. aureus; selected bacteria and fungi

T-cell immunity

Intracellular pathogens, viruses, some parasites

Slide 66 · Direct microbial and host-mediated mechanisms cause injury

Direct

Microbes infect cells and alter viability

Toxins

Secreted products cause injury at a distance

Tissue invasion

Enzymes / vascular injury produce necrosis

Immune-mediated

Antimicrobial responses damage host tissue

Slide 67 · Clinical application: Integrate portal, defense, and outcome

A patient with impaired urinary outflow develops an infection caused by an adherent strain of E. Coli.

Interpretation question

Explain the microbial and host contributions to infection.

Mechanistic answer

E. Coli attachment opposes urinary washout, while obstruction further reduces a major local mechanical defense.

Slide 68 · Clinical application: Distinguish spread from transmission

A neurotropic virus enters following an animal bite and later produces neurological disease.

Interpretation question

Which mechanism explains movement within the original host?

Mechanistic answer

Neural transport is an intrahost dissemination route; The bite was the original exposure / entry mechanism.

Slide 69 · Integrative map: Where pathogenesis can be interrupted

Exposure

→

Entry barriers

→

Replication / immune evasion

→

Spread and injury

→

Exit / transmission

Each transition requires compatible microbial traits and host conditions.

Slide 70 · Core principles to retain

Portal-specific physical, chemical, ecological, and immune defenses govern initial infection.

Virulence factors and deficient host immunity are alternative ways to overcome defenses.

Local invasion, blood, lymph, and nerves create different distributions of disease.

Microbiome associations and immune responses require careful mechanistic interpretation.

Slide 71 · Primary reference and source boundaries

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026). Chapter 8, Infectious Diseases.

General Principles of Microbial Pathogenesis and adjacent sections, printed pp. 304–308.

Original educational schematics are native PowerPoint shapes; source figures and photographs have not been reproduced.