Microbial Mechanisms of Pathogenicity

Prof. Khaled H. Abu-Elteen

Infection and Disease

A. Definitions

B. Generalized Stages of Infection

C. Virulence Factors and Toxins

A. Definitions

Disease and Infectious Disease• Disease

• Any deviation from a condition of good health and well-being

• Infectious Disease

A disease condition caused by the presence or growth of infectious microorganisms or parasites

A. Definitions Pathogenicity and Virulence

• Pathogenicity

• The ability of a microbe to cause disease

• This term is often used to describe or compare species

• Virulence

• The degree of pathogenicity in a microorganism

• This term is often used to describe or compare strains within a species

Definitions

Acute infection vs. chronic infection • Acute Infection

• An infection characterized by sudden onset, rapid progression, and often with severe symptoms

• Chronic Infection

• An infection characterized by delayed onset and slow progression

Definitions

Primary infection vs. secondary infection • Primary Infection

• An infection that develops in an otherwise healthy individual

• Secondary Infection

• An infection that develops in an individual who is already infected with a different pathogen

Definitions

Localized infection vs. systemic infection • Localized Infection

• An infection that is restricted to a specific location or region within the body of the host

• Systemic Infection

• An infection that has spread to several regions or areas in the body of the host

Definitions

Clinical infection vs. subclinical infection • Clinical Infection

• An infection with obvious observable or detectable symptoms

• Subclinical Infection

• An infection with few or no obvious symptoms

Definitions

Opportunistic infection• An infection caused by microorganisms that are

commonly found in the host’s environment. This term is often used to refer to infections caused by organisms in the normal flora.

Definitions

The suffix “-emia”• A suffix meaning “presence of an infectious agent”

• Bacteremia = Presence of infectious bacteria

• Viremia = Presence of infectious virus

• Fungemia = Presence of infectious fungus

• Septicemia = Presence of an infectious agent in the bloodstream

Definitions

The suffix “-itis” • A suffix meaning “inflammation of”

• Examples:

– Pharyngitis = Inflammation of the pharynx

– Endocarditis = Inflammation of the heart chambers

– Gastroenteritis = Inflammation of the gastointestinal tract

Definitions Epidemiology

• The study of the transmission of disease

Communicable Disease • A disease that can be transmitted from one

individual to another

Noncommunicable Disease • A disease that is not transmitted from one

individual to another

Definitions

Endemic Disease• A disease condition that is normally found in a

certain percentage of a population

Epidemic Disease• A disease condition present in a greater than

usual percentage of a specific population

Pandemic Disease• An epidemic affecting a large geographical

area; often on a global scale

Definitions

Reservoir of Infection • The source of an infectious agent

Carrier• An individual who carries an infectious agent

without manifesting symptoms, yet who can transmit the agent to another individual

Fomites• Any inanimate object capable of being an

intermediate in the indirect transmission of an infectious agent

Definitions• Animal Vectors

• An animal (nonhuman) that can transmit an infectious agent to humans

• Two types: mechanical and biological • Mechanical animal vectors: The infectious agent is

physically transmitted by the animal vector, but the agent does not incubate or grow in the animal; e.g, the transmission of bacteria sticking to the feet of flies

• Biological animal vectors: The infectious agent must incubate in the animal host as part of the agent’s developmental cycle; e.g, the transmission of malaria by infected mosquitoes

Definitions

Direct Mechanisms of Disease Transmission• Directly From Person to Person

• Examples: Direct Skin ContactAirborne (Aerosols)

Definitions

Indirect Mechanisms of Disease Transmission• Examples:

Food & Waterborne Transmission Fomites Animal Vectors

Pathogenicity - ability to cause disease

Virulence - degree of pathogenicity

Many properties that determine a microbe’s pathogenicity or virulence are unclear or unknown

But, when a microbe overpowers the hosts defenses, infectious disease results!

Molecular Determinants of Pathogenicity

Production and delivery of various

factors

Attachment to host tissues

Replication and evasion of immunity

Damage to host tissues

Microbial Mechanisms of Pathogenicity: How Microorganisms Cause Disease

Portals of Entry

1. Mucus Membranes

2. Skin

3. Parentarel

1. Mucus Membranes

A. Respiratory Tract• microbes inhaled into

mouth or nose in droplets of moisture or dust particles

• Easiest and most frequently traveled portal of entry

Common Diseases contracted via the Respiratory Tract

Common cold Flu Tuberculosis Whooping cough Pneumonia Measles Diphtheria

Mucus Membranes

B. Gastrointestinal Tract• microbes gain entrance thru

contaminated food & water or fingers & hands

• most microbes that enter the G.I. Tract are destroyed by HCL & enzymes of stomach or bile & enzymes of small intestine

Common diseases contracted via the G.I. Tract Salmonellosis

• Salmonella sp.

Shigellosis• Shigella sp.

Cholera• Vibrio cholorea

Ulcers• Helicobacter pylori

Botulism• Clostridium botulinum

Clostridium botulinum

Fecal - Oral Diseases

These pathogens enter the G.I. Tract at one end and exit at the other end.

Spread by contaminated hands & fingers or contaminated food & water

Poor personal hygiene.

Mucus Membranes of the Genitourinary System - STD’s

Gonorrhea

Neisseria gonorrhoeae

Syphilis

Treponema pallidum

Chlamydia

Chlamydia trachomatis

HIV

Herpes Simplex II

Mucus Membranes

D. Conjunctiva –• mucus membranes that

cover the eyeball and lines the eyelid

Trachoma• Chlamydia trachomatis

2nd Portal of Entry: Skin

Skin - the largest organ of the body. When unbroken is an effective barrier for most microorganisms.

Some microbes can gain entrance through openings in the skin: hair follicles and sweat glands, wound …etc

3rd Portal of Entry: Parentarel

Microorganisms are deposited into the tissues below the skin or mucus membranes

Punctures and scratches injections bites surgery

Preferred Portal of Entry

Just because a pathogen enters your body it does not mean it’s going to cause disease.

pathogens - preferred portal of entry

Preferred Portal of Entry

Streptococcus pneumoniae • if inhaled can cause pneumonia• if enters the G.I. Tract, no disease

Salmonella typhi • if enters the G.I. Tract can cause Typhoid Fever• if on skin, no disease

Number of Invading Microbes

LD50 - Lethal Dose of a microbes toxin that will kill 50% of experimentally inoculated test animal

ID50 - infectious dose required to cause disease in 50% of inoculated test animals• Example: ID50 for Vibrio cholerea 108 cells

(100,000,000 cells)• ID50 for Inhalation Anthrax - 5,000 to 10,000

spores ????

How do Bacterial Pathogens penetrate Host Defenses?

1. Adherence - almost all pathogens have a means to attach to host tissue

Binding Sites

adhesins

ligands

Some cells use fimbriae to adhere.

Fimbriae can play a role in tissue tropism. Example - attachment of Candida to vaginal epithelial cells

Adhesins and ligands are usually on Fimbriae Neisseria gonorrhoeae ETEC

(Entertoxigenic E. coli)

Bordetello pertussis

Bacteria typically employ proteins known as Adhesins to attach to host tissues, which usually are located on ends of fimbriae.Alternatively, adhesins can consist of glycocalyx.

2. Capsules Prevent phagocytosis attachment Streptococcus

pneumoniae Klebsiella pneumoniae Haemophilus

influenzae Bacillus anthracis Streptococcus mutans

K. pneumoniae

Avoidance of Phagocytosis

Capsules are Involved in avoidance of

phagocyte-mediated recognition and

attachment.

Cell Wall ComponentsM protein: Found on cell surface and fimbriae of Streptococcus pyogenes. Mediates attachment and helps resist phagocytosis. M-protein is heat and acid resistantWaxes [ Mycolic Acid]: In cell wall of Mycobacterium tuberculosis helps resist digestion after phagocytosis and can multiply inside WBC.

3. Enzymes

Many pathogens secrete enzymes that contribute to their pathogenicity

Enzymes and toxins that harm eukaryotic cells.

A. Leukocidins

Attack certain types of WBC’s

1. Kills WBC’s which prevents phagocytosis 2. Releases & ruptures lysosomes

• lysosomes - contain powerful hydrolytic enzymes which then cause more tissue damage

B. Hemolysins - cause the lysis of RBC’s

Streptococci

1. Alpha (α) Hemolytic Streptococci

- secrete hemolysins that cause the incomplete lysis or RBC’s

IncompleteLysis of RBC

2. Beta (β) Hemolytic Streptococci

- secrete hemolysins that cause the complete lysis of RBC’s

CompleteLysis of RBC

3. Gamma (γ) Hemolytic Streptococci - do not secrete any hemolysins

C. Coagulase - cause blood to coagulate

Blood clots protect bacteria from phagocytosis from WBC’s and other host defenses

Staphylococcus aureus - are often coagulase positive

Fibrinogen ----------------- Fibrin ( Clot)

D. Kinases - enzymes that dissolve blood clots 1. Streptokinase - Streptococci 2. Staphylokinase - Staphylococci

Helps to spread bacteria - Bacteremia

Streptokinase - used to dissolve blood clots in the Heart (Heart Attacks due to obstructed coronary blood vessels)

E. Hyaluronidase

Breaks down Hyaluronic acid (found in connective tissues)

“Spreading Factor” mixed with a drug to help spread the drug

through a body tissue Streptococci, Staphylococci, Clostridia and

pneumococci.

F. Collagenase

Breaks down collagen (found in many connective tissues)

Clostridium perfringens - Gas Gangrene• uses this to spread through muscle tissue

Severe gangrene caused by Clostridium perfringens.Source: Tropical Medicine and Parasitology, 1997

Tissue Damage Caused by Microbial Enzymes of Clostridium perfringens

G. Necrotizing Factor

- causes death (necrosis) to tissue cells

“Flesh Eating Bacteria”

Necrotizing fasciitis

H. Lecithinase

Destroys lecithin ( phosphatidylcholine) component of plasma membrane.

Allowing pathogen to spread Clostridium perfringens

Summary of How Bacterial Pathogens Penetrate Host Defenses 1. Adherence 2. Capsule 3. Enzymes

• A. leukocidins B. Hemolysins• C. Coagulase D. Kinases• E. Hyaluronidase F. Collagenase• G. Necrotizing Factor H. Lecithinase

4. Toxins Poisonous substances produced by

microorganisms toxins - primary factor - pathogenicity 220 known bacterial toxins

• 40% cause disease by damaging the Eukaryotic cell membrane

Toxemia • Toxins in the bloodstream• Toxigenicity: Capacity of microorganisms to

produce toxins.

Two Types of Toxins

1. Exotoxins• secreted outside the bacterial cell

2. Endotoxins• part of the outer cell wall of Gram (-)

bacteria. ??

Exotoxins versus Endotoxins

I- Exotoxins

Mostly seen in Gram (+) Bacteria

Most gene that code for exotoxins are located on plasmids or phages

Three Types of Exotoxins

1. Cytotoxins• kill cells e.g. Diphtheria toxin

2. Neurotoxins• interfere with normal nerve impulses.e.g.

Botulinum Toxin 3. Enterotoxins

• effect cells lining the G.I. Tract. e.g. Cholera toxin or choleragen.

Response to Toxins

If exposed to exotoxins: antibodies against the toxin (antitoxins)

Exotoxins inactivated ( heat, formalin or phenol) no longer cause disease, but stimulate the production of antitoxin• altered exotoxins - Toxoids

Toxoids - modified toxin by heat, chemical, radiation, that have lost their toxicity. Injected to stimulate the production of antitoxins and provide immunity.

Example: DPT Vaccine D - Diphtheria

• Corynebacterium diphtheriae

P - Pertussis• Bordetello pertussis

T - Tetanus• Clostridium tetani

DPT - Diphtheria Toxoid

Pertussis Antigen

Tetanus Toxoid

Required Immunizations in Jordan 1. Diphtheria 2. Pertussis 3. Tetanus 4. Measles 5. Mumps 6. Rubella

• German Measles

7. Polio 9. Hepatitis B

Corynebacterium diphtheriae Bordetello pertussis Clostridium tetani Measles virus Mumps virus Rubella virus

Polio virus Hepatitis B Virus

Most genes that code for exotoxins - plasmids or phages

Lysogenic convergence Diphtheria Cytotoxin inhibits

protein synthesis - resulting in cell death

Pseudomembrane• fibrin, dead tissue,

bacterial cells

Lysogenic Convergence

Scarlet Fever Streptococcus pyogenes

• lysogenic convergence cytotoxin - damages blood capillaries and results in a

skin rash

• Strep Thoat with a rash

Rash of Scarlet Fever Caused by Erythrogenic Toxins of Streptococcus pyogenes

Diseases Caused by Staphylococcal Toxins

Scalded Skin Syndrome Toxic Shock Syndrome

Diseases caused by Neurotoxins Botulism

• Clostridium botulinum• Gram (+), anaerobic, spore-forming rod, found in

soil

• works at the neuromuscular junction• prevents impulse from nerve cell to muscle cell• results in muscle paralysis

Tetanus (Lock Jaw) Clostridium tetani Gram (+), spore-forming, anaerobic rod neurotoxin acts on nerves, resulting in the

inhibition of muscle relaxation tetanospasmin - “spasms” or “Lock Jaw”

Neonatal Tetanus (Wrinkled brow and risus sardonicus)Source: Color Guide to Infectious Diseases, 1992

Muscle Spasms of Tetanus are Caused by Neurotoxin of Clostridium tetani

Diseases caused by Enterotoxins

Cholera• Vibrio cholerae

• Gram (-) comma shaped rods

Cholera toxin

Converts ATP into cAMP causes cells to excrete Cl- ions and inhibits

absorption of Na+ ions Electrolyte imbalance H2O leaves by osmosis H2O Loss (Diarrhea)

Two polypeptides: A (active) and B (binding). The A subunit of enterotoxin causes epithelial cells to discharge large amounts of fluids and

electrolytes.

Severe cases, 12 - 20 liters of liquid lost in a day

Untreated cases - Mortality Rate about 50%

Mortality may be reduced to about 1%• administering fluids and electrolytes

Rice-water stool of cholera. The A subunit of enterotoxin causesepithelial cells to discharge large amounts of fluids and electrolytes.Source: Tropical Medicine and Parasitology, 1995

Vibrio Enterotoxin Causes Profuse Watery Diarrhea

EHEC (Enterohemorrhagic E. coli) E. coli (0157:H7) enterotoxin causes a hemolytic inflammation

of the intestines results in bloody diarrhea

• Toxin• alters the 60S ribosomal subunit

• inhibits Protein Synthesis

• Results in cell death

• lining of intestine is “shed”

• Bloody Diarrhea (Dysentary)

Mor

e on

Tox

ins

II- Endotoxins• Part of outer membrane surrounding gram-negative

bacteria.

• Endotoxin is lipid portion of lipopolysaccharides (LPS), called lipid A.

• Effect exerted when gram-negative cells die and cell walls undergo lysis, liberating endotoxin.

• All produce the same signs and symptoms:

• Chills, fever, weakness, general aches, blood clotting and tissue death, shock, and even death. Can also induce miscarriage.

• Fever: Pyrogenic response is caused by endotoxins.

Exotoxins vs. Endotoxins

Endotoxin is LPS

Endotoxins (Continued)• Endotoxins do not promote the formation of

effective antibodies.• Organisms that produce endotoxins include:

• Salmonella typhi

• Proteus spp.

• Pseudomonas spp.

• Neisseria spp.

• Medical equipment that has been sterilized may still contain endotoxins.

• Limulus amoebocyte assay (LAL) is a test used to detect tiny amounts of endotoxin.

Events leading to fever:• Gram-negative bacteria are digested by

phagocytes.

• LPS is released by digestion in vacuoles, causing macrophages to release interleukin-1 (IL-1).

• IL-1 is carried via blood to hypothalamus, which controls body temperature.

• IL-1 induces hypothalamus to release prostaglandins, which reset the body’s thermostat to higher temperature.

Microbial Mechanisms of Pathogenicity: How Microorganisms Cause Disease

III. B. The Normal Flora of Humans

Types of Symbiosis• Mutualism

• A symbiotic relationship in which both species benefit

• Commensalism

• A symbiotic relationship in which one species benefits, and the other species is neither helped nor harmed

III. B. The Normal Flora of Humans

Types of Symbiosis (cont.)• Parasitism

• A symbiotic relationship in which one species benefits, and the other species is harmed

• Generally, the species that benefits (the parasite) is much smaller than the species that is harmed (the host)

III. B. The Normal Flora of Humans

Normal flora is present in • skin

• upper respiratory tract

• oral cavity

• intestine, especially large intestine

• vaginal tract

Very little normal flora in eyes & stomach

III. B. The Normal Flora of Humans

Notably absent in most all internal organs• Absent in:

• lower respiratory tract

• muscle tissue

• blood & tissue fluid

• cerebrospinal fluid

• peritoneum

• pericardium

• meninges

III. B. The Normal Flora of Humans

Benefits of the normal flora • Nutrient production/processing

eg Vitamin K production by E. coli

• Competition with pathogenic microbes

• Normal development of the immune system

Normal flora and opportunistic infections

III. C. Generalized Stages of Infection

1. Entry of Pathogen • Portal of Entry

2. Colonization • Usually at the site of entry

3. Incubation Period• Asymptomatic period

• Between the initial contact with the microbe and the appearance of the first symptoms

III. C. Generalized Stages of Infection

4. Prodromal Symptoms• Initial Symptoms

5. Invasive period• Increasing Severity of Symptoms

• Fever

• Inflammation and Swelling

• Tissue Damage

• Infection May Spread to Other Sites

III. C. Generalized Stages of Infection

6. Decline of Infection

5. Convalescence

Course of Infectious Disease

Incubation period is the interval

between exposure and illness onset.

Convalescence is a time of

recuperation and recovery from

illness.

Depending on various factors an individual may still be infectious during

either incubation or convalescence.