Fungi as pathogens

True pathogens

• Infect healthy hosts

• Adapted to:

  • High temperature

  • Low oxygen tension

• Thermal dimorphism: Switch form based on temperature

• Restricted to specific geographical regions

• Usually asymptomatic in healthy individuals

• Not obligate parasites

Thermal dimorphism

• Environment: Natural habitat, Fungal form: hyphal, Reproduction: Spores, Nutrition mode: Saprophytic

• Environment: Animal host, Fungal form: yeast, Reproduction: Budding/endospores, Nutrition mode: Parasitic

Opportunistic Pathogens

• Require immunocompromised host

• Worldwide distribution

• Many species, numbers increasing

• No specific adaptations to host but tolerate it well

• Cause superficial to systemic infections

• Often poor prognosis

Dermatophytes

• Cause superficial infections (non-invasive)

• Infect healthy hosts

• Worldwide

• Not life-threatening, but cause discomfort

• Target keratinized tissue → “Keratin Lovers”

• e.g. ringworm

Control of Human Mycoses

• Fluorocytosine inhibits RNA synthesis

• Polyenes bind membrane - cytosol leaks

PHYTOMYCOSIS

Plant Pathogenic Fungi

Pathogens of Immature Tissue

• Broad host range

• Typically soil-borne

• Invade root tips aggressively

• Thrive in water-logged soils

Pathogens of Mature Tissue

• Usually host-specific

• Necrotrophic Pathogens

• Biotrophic Pathogens

Necrotrophic Pathogens

• Kill host cells → then invade

• Secrete toxins and cell wall-digesting enzymes

• Induce host resistance mechanisms

Biotrophic Pathogens

• Keep host alive

• Limited tissue invasion

• Don’t trigger strong host defences

• Life cycle depends on host survival

Haustorial Biotrophs

• Develop haustoria to extract nutrients from living host cells

• Haustoria act as nutrient sinks:

  • Penetrate cell wall (not membrane)

  • Siphon nutrients without killing host

• Puccinia graminis enter through stomata and target mesophyll cells