Introduction to microbial diseases

Why are some microbes pathogenic for humans?

They have evolved virulence factors, such as toxins and surface coats, that allow them to overcome the human immune system and cause disease

What mechanisms do microbes employ to enable their virulence?

Adherence, invasion, toxin production, immune evasion

Adherence

Attachment: microbes use structures such as pili, fimbriae, and flagella to attach to host cells - foothold for infection

Invasion

Can produce enzymes like collagenase and hyaluronidase to break down host tissue and barriers, and produce toxins to damage host cells, allowing them to move through and penetrate tissues

Immune evasion

Destroy antibodies

Resist phagocytosis

Release molecules that suppress immune responses

Toxin production

Damage to host cellsSecrete toxins to kill host cells or disrupt function

Endotoxins: Gram-negative bacteria contain endotoxins in cell wall - released upon. Cell death → severe inflammatory response → septic shock

Competition for resources

• Iron acquisition: Microbes often secrete molecules to bind and scavenge for iron, which is essential for their growth and can be a limiting nutrient in the host.

• Nutrient acquisition: They may break down host molecules to obtain other necessary nutrients for metabolism and growth

What is the course of an acute infectious disease?

Incubation period → prodromal phase → acute phase/illness period → recovery phase → convalescence

Incubation period

The time from exposure to the pathogen until symptoms begin to appear

The virus is multiplying and the host may be contagious, but no symptoms and don’t feel sick

Prodromal period (early symptomatic)

Onset of general, nonspecific symptoms such as low-grade fever, fatigue or muscle aches

Pathogen actively multiplying

Immune system has begun responding

Acute/illness period

The peak of infection - disease is fully developed with characteristic symptoms

Pathogen exists in high numbers and causes specific signs and symptoms of disease - typically most contagious in this phase

Decline / recovery period

Number of pathogens in body decreases as immune response or medical treatment begins to control and clear the pathogen

Symptoms start to subside, but the host may still feel unwell, and secondary infections can sometimes occur

Convalescence period

Final stage where symptoms disappear, and the body recovers, heals, and returns to its normal state

Repair of tissue damage; immune memory established