Introduction

Pathogenicity is the ability of a pathogen to produce a disease by overcoming the defenses of the host.

Virulence is the degree of pathogenicity.

How Microorganisms Enter A Host

The specific route by which a particular pathogen gains access to the body is called a portal of entry.

Portals of Entry

Mucous membranes

Respiratory tract

• Microorganisms that are inhaled with droplets of moisture and dust particles gain access to the respiratory tract.
• The respiratory tract is the most common portal of entry.

Genitourinary tract

• Microorganisms that gain access via the genitourinary tract can enter the body through mucous membranes.

Gastrointestinal tract

• Microorganisms enter the gastrointestinal tract via food, water, and contaminated fingers.

Conjunctiva

Skin

• Most microorganisms cannot penetrate intact skin; they enter hair follicles and sweat ducts.
• Some fungi infect the skin itself.

Parenteral

• Some microorganisms can gain access to tissues by inoculation through the skin and mucous membranes in bites, injections, and other wounds.

The Preferred Portal of Entry

Many microorganisms can cause infections only when they gain access through their specific portal of entry.

Numbers of Invading Microbes

Virulence can be expressed as LD₅₀ (lethal dose for 50% of the inoculated hosts) or ID₅₀ (infectious dose for 50% of the inoculated hosts).

Adherence

Surface projections on a pathogen called adhesins (ligands) adhere to complementary receptors on the host cells.

Ligands can be glycoproteins or lipoproteins and are frequently associated with fimbriae.

Mannose is the most common receptor on host cells.

Biofilms are communities of microorganisms which, together with their extracellular products, attach to living or nonliving surfaces.

Usually forms on surfaces that are moist and contain organic substances.

Bacteria usually attach first, multiply and secrete glycocalyx, allows attachment of bacteria to each other and to the surface.

May be several layers thick .

May contain different types of microorganisms.

Resist disinfectants and antibiotics.

Biofilms may form on teeth, catheters, contact lenses, etc. - estimated that 65% of human bacterial infections involve biofilms.

Examples: Dental plaque, soap scum, algae on swimming pool walls