MICROBIAL PATHOGENICITY

Factors that Influence the degree of Pathogenicity and the Progression of Infection and Disease

• •  Host factors: Age, sex, ethnicity, nutrition (diet), hormonal status; personal hygiene and immune status; Underlying disease or medical condition; Antibiotic or drug usage; Presence of foreign object (e.g., splinter, catheter, sutures, etc.); Innate differences between hosts

•  Microbial factors: Bacterial virulence factors; Inoculum size (dosage)

External factors (e.g., crowding; seasonal variations; hygiene, sanitation and public health; food processing, storage and preparation; etc.

To cause disease a pathogen must:

• •  Gain access to the host.

•  Adhere to host tissues.

•  Penetrate or evade host defenses.

•  Damage the host, either directly or accumulation of microbial wastes.

Progression of Infection and Disease

Entrance ( Portal of entry ).

• •  Mucous membrane: - is most common route for most pathogens. The mucous membranes are respiratory tract, gastrointestinal tract, urinary/genital tracts and conjunctiva.

•  Skin (keratinized cutaneous membrane):- Some pathogens infect hair follicles, sweat glands and colonize surface. But unless broken, skin is usually an impermeable barrier to microbes.

•  Parenteral route: - penetrate skin, punctures, injections, bites, cuts, surgery and deposit organisms directly into deeper tissues.

The microbes must enter through preferred portal of entry in order to cause disease. But some can cause disease from many routes of entry

Colonization (Adherence; Adhesion; Attachment)

• •  Attachment/Adherence: Close association of bacterial cells and host cells generally characterized by receptors and target sites.

•  Surface Receptors/Target Sites: Receptor sites present on both hosts (Receptor) and bacterial surfaces (Adhesins).

•  Adhesins: Bind Specific Host Receptors often involve fimbriae as structural cell component; Host cell receptors are often sugar moieties. The lectin s are adhesin specific for polysaccharide target receptor (sugar residues).

•  Fimbriae (plural): Short hair-like protein (pilin) appendages extending outward from the surface of certain bacteria.

•  Pili (plural); Pilus (singular): Short hair-like protein (pilin) appendages extending outward from the surface of certain bacteria and responsible for bacterial conjugation.

Numbers of Invading Microbes: - The chances of causing diseases increase as the numbers of invading pathogens increases. This expressed by infectious dose (ID50) and lethal dose (LD50). The ID50 (Infectious Dose) is the number of microbes required to produce infection in 50% of the population. The ID50 is different for different pathogens i.e. different ID50 for different portals of entry for the same pathogen. The LD50 (Lethal Dose) amount of toxin or pathogen necessary to kill 50% of the population in a particular time frame.