Normal Flora and Bacterial Pathogenesis

• Pin Ling ( 凌 斌 ), Ph.D.

Department of Microbiology & Immunology, NCKU

ext 5632

lingpin@mail.ncku.edu.tw

• References:

1. Murray, P. et al., Medical Microbiology (5th edition)

2. Samuel Baron, Medical Microbiology (4th edition)

Outline

Normal Flora

(Commensal

Microbes)

• Introduction

• Significance of the

Normal Flora

• Distribution of the

Normal Flora

Bacterial Pathogenesis

• Introduction

• Host Susceptibility

• Pathogenic

Mechanisms

• Virulence Factors

Transient colonization;

Permanent colonization;

Disease

Colonization vs. infection

Colonization: establishment of a site of reproduction of microbes on a person without necessarily resulting in tissue invasion or damage.

Infection: growth and multiplication of a microbe in or on the body of the host with or without the production of disease.

Outcomes of exposure to a microorganism:

Normal Flora and Pathogenesis

Introduction of Normal Flora

1. A diverse microbial flora => Human body

Area: the skin and mucous membranes

Time: shortly after birth until death

Number: 1014 bacteria =>1013 host cells

2. Normal flora may:

a. Aid the host

b. Harm the host (in sometimes)

c. Exist as commensals (no effect to the host)

3. Viruses and parasites => NOT normal microbial flora

Most investigators consider that they are not commensals and do not aid the host.

Significance of Normal Flora-I

1. The effect of the normal flora on the host was not well understood until germ-free animals became available.

Cesarean Section => Germ-free animals => Isolators w/o detectable pathogens (viruses, bacteria & others)

2. Two interesting observations:

a. the germ-free animals lived almost twice as long as their conventionally maintained

counterparts.

b. the major causes of death were different in the two

groups.

Germ-free Animals Regular Counterparts

Lifespan Twice One

Cause of death

Intestinal Atonia Infection

Anatomic & Physiological Changes

1. Alimentary lamina propria under-developed

2. No Ab

3. Intestinal epithelial cell renewal rate down half

Germ-free Animals vs Counterparts

Significance of Normal Flora-II

Normal flora may aid the host in several ways:• Aid in digestion of food• Help the development of mucosa immunity • Protect the host from colonization with pathogenic

microbes.

106 pathogenic microbes GI infection

w/ normal flora

GI infection

w/ reduced flora afterStreptomycin treatment

10 pathogenic microbes

Normal Flora competing w/ Invading Pathogens

Adopted from Samuel Baron “Medical Microbiology”

Normal flora may act as opportunistic pathogens

Especially in hosts rendered susceptible by:

1. Immuno-suppression (AIDS & SCID)

2. Radiation therapy & Chemotherapy

3. Perforated mucous membranes

4. Rheumatic heart disease…etc.

Respiratory tract and head

outer ear, eye, mouth, oropharynx, nasopharynx

Sterile sites: sinuses, middle ear, brain, lower respiratory tract (trachea, bronchiole, lung)

Gastrointestinal tract

esophagus, stomach, small intestine, large intestine

Genitourinary system

anterior urethra, vagina

Sterile sites: bladder, cervix, uterus

Skin

Sites of human body that the normal flora

microbes colonize

Adopted from Smaul Baron “Medical Microbiology”

Distribution of Normal Flora in Human Body

1. Local Environment (pH, temperature, redox potential, O2, H2O, and nutrient levels…).

2. Diet

3. Age

4. Health condition (immune activity…)

5. Antibiotics,…..etc

Factors Influencing Normal Flora

Outline

Normal Flora

(Commensal

Microbes)

• Introduction

• Significance of the

Normal Flora

• Distribution of the

Normal Flora

Bacterial Pathogenesis

• Introduction

• Host Susceptibility

• Pathogenic

Mechanisms

• Virulence Factors

Introduction of Bacterial Pathogenesis

1. Infection: growth and multiplication of a microbe in or on the body of the host with or without the production of disease.

2. The capacity of a bacterium to cause disease reflects its relative pathogenicity.

3. Virulence is the measure of the pathogenicity of an organism.

4. Pathogenesis refers both to the mechanism of infection and to the mechanism by which disease develops.

Host Susceptibility

1. Susceptibility to bacterial infections

=> Host Defenses vs Bacterial Virulence

2. Host Defenses:

- Barriers (skin & mucus) – first line

- Innate Immune Responses (complement, macrophages &

cytokines) – the early stage

- Adaptive Immune Responses (Ag-specific B & T cells) – the

later stage

3. Host defenses can be comprised by destructing barriers or defective immune response.

e.x. Cystic Fibrosis => poor ciliary function => NOT clear mucus efficiently from the respiratory tract => Pseudomonas aeruginosa => serious respiratory distress.

Strict pathogens

are more virulent and

can cause diseases in a

normal person.

Opportunistic

pathogens are typically

members of normal flora

and cause diseases when

they are introduced into

unprotected sites, usually

occur in people with

underlying conditions.

Pathological Mechanisms of Bacterial Infections

1. Bacteria-mediated Pathogenesis

2. Host-mediated Pathogenesis

3. Bacterial virulence factors

=> bacterial factors causing diseases

Adopted from Samuel Baron “Medical Microbiology”

1. Transmissibility

2. Adherence to host cells

3. Invasion of host cells and tissue

4. Evasion of the host immune system

5. Toxigenicity

A bacterium may cause diseases by

1. Destroying tissue (invasiveness)

2. Producing toxins (toxigenicity)

3. Stimulating overwhelming host immune responses

Characteristics of Pathogenic Bacteria

Bacterial Virulence Mechanisms

Bacterial virulence factors

Adhesins Pili (fimbriae) Nonfimbrial adhesins

Invasion of host cells Tissue damage

Growth byproducts Tissue-degrading enzymes

ImmunopathogenesisToxins

Exotoxins (cytolytic enzymes and A-B toxins); enterotoxins; superantigens; endotoxin and other cell wall components

Antiphagocytic factors

Intracellular survival

Antigenic heterogeneity Antigenic variation Phase variation

Iron acquisition Siderophores Receptors for iron-containing molecules

Resistance to antibiotics

Mechanisms of acquiring bacterial virulence genes

Entry into the human body

: infection : shedding

The most frequent portals of

entry are the sites where

mucous membranes meet

with the skin. Abnormal

areas of mucous membranes

and skin are also frequent

sites of entry.

Routes:

Ingestion, inhalation, trauma,

needles, catheters, arthropod

bite, sexual transmission

Microbial defenses against host immunologic clearance

Encapsulation (Inhibition of phagocytosis and serum bactericidal effect)

Antigenic mimicry

Antigenic masking

Antigenic or phase variation

Intracellular multiplication

Escape phagosome

Inhibition of phagolysosome fusion

Resistance to lysosomal enzymes

Production of anti-immunoglobulin protease

Inhibition of chemotaxis

Destruction of phagocytes

Mechanisms for escaping phagocytic clearance and intracellular survival

Mechanisms for escaping phagocytic clearance and intracellular survival

Mechanisms for escaping phagocytic clearance and intracellular survival

Endotoxin-mediated toxicity

Fever,

leukopenia followed by leukocytosis,

activation of complement, thrombocytopenia,

disseminated intravasacular coagulation,

decreased peripheral circulation and perfusion to major organs (multiple organ system failure),

Shock and death.

Peptidoglycan, teichoic and lipoteichoic acids of gram-positive bacteria stimulate pyrogenic acute phase responses and produce endotoxin-like toxicity

Back

Lipid A of lipopolysaccharide is responsible for endotoxin activity

Pathogenesis of sepsis (septicemia)

Adopted from Samuel Baron “Medical Microbiology”

Environmental factors often control the expression o

f the virulence genes.

Common factors: temperature, iron availability, osm

olarity, growth phase, pH, specific ions, specific nutr

ient factors, bacterial cell-density, interaction with h

ost cells.

Regulation of bacterial virulence factors

The severity of a disease is determined by

the inoculum size of the pathogen and th

e host conditions, such as congenital def

ect, immunodeficiency states and other di

sease-related conditions.

The symptoms of a disease are

determined by the function of the tissue

affected.

Transmission of infection

Carrier: a person or animal with asymptomatic infection that can be transmitted to another person or animal.

The clinical symptoms of diseases produced by microbes often promote transmission of the agents.

Zoonosis: infectious diseases transmitted between animals and men.

Hospital- (nosocomial) vs. community-acquired infections

By producing asymptomatic infection or mild disease, rather than death of the host, microorganisms that normally live in people enhance the possibility of transmission from one person to another.

Many bacteria are transmitted on hands

Adhesion

Adherence of bacterium to epithelial or endothelial cell

s allow them to colonize the tissue.

Common adhesins: pili (fimbriae), slime, lipoteichoic ac

id, surface proteins or lectins.

Biofilm, formed on a surface by the bacteria that are bo

und together within a sticky web of polysaccharide, is a

special bacterial adaptation that facilitates colonization

on the surgical appliances (e.g., artificial valves or indw

elling catheters) and dental plaque. It can protect the b

acteria from host defenses and antibiotics.

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The bacteria may invade via the M cells

Back

The A-B toxins

Mode of action

Inhibition of protein synthesis

Hypersecretion

Inhibition of neurotransmitter release

A chain has the inhibitory activity against some vital function

B chain binds to a receptor and promotes entry of the A chain

In many cases the toxin gene is encoded on a plasmid or a lysogenic phage

Back

SUMMARY

1. Normal flora may aid the host in several ways:

• Aid in digestion of food

• Help the development of mucosa immunity

• Protect the host from colonization with pathogenic microbes

2. Normal flora may act as opportunistic pathogens.

3. Susceptibility to bacterial infections depends on the balance between host defenses and bacterial virulence.

4. Pathogenic mechanisms of bacterial infections include

• Bacteria-mediated Pathogenesis

• Host-mediated Pathogenesis

Back

Transfer of mobile genetic elements within or between species can result in transfer of virulence factors (e.g., pathogenicity islands).

Recombination may occur between extrachromosomal DNA and the chromosome to generate new bacterial clone.

Different clonal types of a certain species may cause different diseases.

Clonality of bacterial pathogens

Various virotypes of E. coli