Fungi of Medical Importance

FUNGI OF MEDICAL IMPORTANCE

• Fungi are a diverse group of organisms
• Some fungi are saprobes (living on dead matter), others are symbionts (mutual association)
• Some are commensals (one benefits, the other is unaffected)
• Some are parasites (one benefits, the other is harmed)
• Only about 300 of the 100,000 known species cause disease in animals

INTENDING LEARNING OUTCOMES

• Students should be able to describe the structure of a fungal cell
• Enumerate the major differences between fungi and bacteria
• Differentiate between fungi and bacteria
• Know the differences between molds and yeast
• Know the reason why fungi sporulate
• Classify human mycoses

WHAT ARE FUNGI?

• Visual aids (images) showcasing different types of fungi (molds, yeasts, mushrooms)

INTRODUCTION

• Molds and yeasts are widely distributed in air, dust, fomites, and normal flora
• Fungi are usually non-pathogenic, mostly plant pathogens
• Humans are relatively resistant to fungal infections
• Only a small fraction (300) of known fungi species are linked to animal diseases

INTRODUCTION CONT'D

• Fungi represent a diverse group of organisms
• Saprobes: living on dead and decaying matter
• Symbionts: mutual association between two organisms
• Commensals: a relationship where one organism benefits and the other neither benefits nor is harmed
• Parasites: an association where the host is harmed and the other organism benefits

CAUSES OF FUNGAL INFECTIONS

• Yeast infections are not caught—the yeast is already present
• Overgrowth of yeast can occur due to antibiotic use, which affects pH allowing yeast to overgrow
• Other causes include pregnancy, uncontrolled diabetes, and an impaired immune system
• Anything that alters the type and amount of bacteria present can also increase yeast growth

STRUCTURE OF FUNGI

• Fungi can exist as unicellular forms (yeasts) that reproduce asexually
• Can also be filamentous (molds) that reproduce both sexually and asexually
• Most fungi exist as either yeast or molds
• Some fungi can exist in both forms (dimorphic)

WHAT IS A DIMORPHIC FUNGUS?

• A dimorphic fungus can exist in both mold and yeast forms
• The form it takes depends on temperature—25°C is mold, 37°C is yeast
• Mold exists in soil, yeast in the lungs

STRUCTURE OF FUNGI CONT'D

• Fungi can exist as unicellular yeasts
• These yeasts reproduce asexually

GENERAL STRUCTURE OF A FUNGAL CELL

• Fungal cells are distinguished from other eukaryotes by having a rigid cell wall
• The cell wall is composed of chitin and glucan
• Next to the cell wall is the cell membrane
• The cell contains an endoplasmic reticulum, nucleus, mitochondria, and Golgi body

FUNGI AND BACTERIA DIFFERENCES

MOLDS AND YEAST

• Both yeast and molds are fungi
• Both are eukaryotic fungi
• Both are considered as parasites

DIFFERENCES BETWEEN MOLDS AND YEAST

• Yeast are unicellular organisms
• Multiply by binary fission and budding
• Molds are multicellular organisms

YEAST

• They are unicellular organisms
• Multiply by binary fission and budding

MOLDS

• Multicellular organisms that grow in a form called hyphae
• Some molds are used in production of antibiotics (e.g., Penicillin, Cephalosporins)
• Grow in form of filaments called hyphae
• Consist of aerial and vegetative hyphae
• Reproduce by spore formation (either sexually or asexually) on aerial hyphae

WHY DO FUNGI SPORULATE?

• Sporulation typically occurs under suitable nutritional and aerational conditions
• Spores are produced during both sexual and asexual reproduction
• Purpose of sporulation is dispersal, allowing the fungus to spread to a wider geographic area
• Spores also aid in preservation, allowing the fungus to survive unfavorable environmental conditions
• Spores allow for genetic variation

CLASSIFICATION OF HUMAN MYCOSES

• Two main classifications: Formal taxonomic and clinical
• Clinical classification is based on infected tissue

CLASSIFICATION OF HUMAN MYCOSES CONT'D

• Fungal infections can be classified based on the tissue infected
• Specific characteristics of the organism, including superficial, cutaneous, subcutaneous, endemic, and opportunistic mycoses

SUPERFICIAL MYCOSES

• Infections limited to the very superficial surfaces of skin and hair
• Nondestructive, cosmetic importance only

CUTANEOUS MYCOSES

• Infections of the keratinized layer of the skin, hair, and nails
• May elicit a host response and thus are symptomatic

CLASSIFICATION OF HUMAN MYCOSES CONT'D

• Subcutaneous mycoses involve deeper layers (skin, cornea, muscles, etc)
• Endemic mycoses are true pathogens, causing infections in healthy individuals
• Opportunistic mycoses involve normally commensal fungi

QUESTIONS

• Filamentous structures of fungi are called hyphae
• Fungi are eukaryotic

QUESTIONS (CONT.)

• Fungi are eukaryotic organisms