Microbiology Study Guide: Fungi, Protists, Viruses, Immunity, and Antimicrobial Medicine

Fungi

General Characteristics

Fungi are a diverse group of eukaryotic organisms that include yeasts, molds, and mushrooms. They play important roles in decomposition, nutrient cycling, and as pathogens.

• Yeasts: Unicellular fungi that reproduce by budding or fission.

• Molds: Multicellular, filamentous fungi composed of hyphae.

• Mushrooms: Fruiting bodies of certain fungi, often visible above ground.

Diseases Caused by Fungi

• Superficial: Affect outer layers of skin, hair, or nails (e.g., ringworm).

• Intermediate: Affect deeper skin layers or subcutaneous tissues.

• Systemic: Affect internal organs, often in immunocompromised hosts.

Medically Important Genera: Aspergillus, Candida, Cryptococcus, Histoplasma, Coccidioides, Blastomyces, Microsporum, Trichophyton, Epidermophyton

Structure and Cell Wall

• Fungal cell walls contain chitin and glucans.

• Cell membranes contain ergosterol (target for antifungal drugs).

Reproduction

• Fungi reproduce by spores, which may be sexual or asexual.

Example: Candida albicans can cause superficial infections like oral thrush and systemic infections in immunocompromised patients.

Protists

General Characteristics

Protists are a diverse group of mostly unicellular eukaryotes, including protozoa and algae. Many protozoa are motile and can cause human disease.

• Motility: Achieved by flagella, cilia, or pseudopodia.

• Nutrition: Heterotrophic (protozoa) or autotrophic (algae).

Types of Asexual Reproduction

• Binary fission: Cell divides into two identical daughter cells.

• Multiple fission: Nucleus divides several times before the cell splits.

Medically Important Genera: Plasmodium (malaria), Giardia, Entamoeba, Trypanosoma

Example: Plasmodium species cause malaria, transmitted by Anopheles mosquitoes.

Viruses

Structure and General Properties

Viruses are acellular infectious agents composed of nucleic acid (DNA or RNA) surrounded by a protein coat (capsid). Some have an outer lipid envelope.

• Obligate intracellular parasites: Require host cells for replication.

• Genome: DNA or RNA, single- or double-stranded.

• Capsid: Protein shell protecting the genome.

• Envelope: Lipid membrane derived from host cell (in some viruses).

Virus Replication Cycles

• Lytic cycle: Virus replicates and lyses host cell.

• Lysogenic cycle: Viral genome integrates into host DNA (prophage in bacteriophages).

Types of Viruses

• DNA viruses: Replicate using host or viral DNA polymerase.

• RNA viruses: Replicate using RNA-dependent RNA polymerase.

• Retroviruses: Use reverse transcriptase to convert RNA to DNA (e.g., HIV).

Important Terms

• Prions: Infectious proteins causing neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease).

• Viroids: Infectious RNA molecules, mostly affecting plants.

Example: Influenza virus is an enveloped RNA virus with a segmented genome.

Innate (Nonspecific) Immunity

Overview

The innate immune system provides the first line of defense against pathogens, using physical, chemical, and cellular mechanisms.

• Leukocyte Types: Granulocytes (neutrophils, eosinophils, basophils) and lymphocytes (B cells, T cells, NK cells).

• Physical Barriers: Skin, mucous membranes.

• Chemical Barriers: Lysozyme, defensins, acidic pH.

Pattern Recognition Receptors

• Toll-like receptors (TLR): Recognize pathogen-associated molecular patterns (PAMPs).

• NOD-like receptors (NLR): Detect intracellular pathogens.

• RIG-I-like receptors (RLR): Detect viral RNA.

Complement System

• Series of proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.

• Three activation pathways: classical, alternative, lectin.

Phagocytosis

• Process by which phagocytes (e.g., neutrophils, macrophages) engulf and destroy pathogens.

Example: Macrophages use TLRs to detect bacteria and initiate an inflammatory response.

Antimicrobial Medicine

Overview

Antimicrobial drugs are used to treat infections by inhibiting or killing pathogens. Selective toxicity is a key principle, aiming to harm microbes without damaging host cells.

• Types of Antimicrobials: Antibiotics (bacteria), antifungals, antivirals, antiparasitics.

• Spectrum of Activity: Broad-spectrum (many organisms) vs. narrow-spectrum (specific organisms).

• Mechanisms of Action: Inhibit cell wall synthesis, protein synthesis, nucleic acid synthesis, or metabolic pathways.

• Bactericidal vs. Bacteriostatic: Bactericidal drugs kill bacteria; bacteriostatic drugs inhibit growth.

Antibiotic Resistance

• Resistance can arise via mutation or acquisition of resistance genes.

• Mechanisms include drug inactivation, target modification, efflux pumps, and reduced permeability.

Therapeutic Index

• Ratio of toxic dose to therapeutic dose; higher index indicates greater safety.

Example: Penicillin inhibits bacterial cell wall synthesis and is more effective against Gram-positive bacteria.

Review Questions (Sample Topics)

• Differences between fungal, bacterial, and human cells.

• Stages of protozoan life cycles and why anti-protozoal drugs are limited.

• Basic virus structure and replication cycles.

• Differences between innate and adaptive immunity.

• Mechanisms of antibiotic action and resistance.

Table: Comparison of Microbial Groups

Feature	Bacteria	Fungi	Protozoa	Viruses
Cell Type	Prokaryotic	Eukaryotic	Eukaryotic	Acellular
Cell Wall	Peptidoglycan	Chitin, glucans	Absent	Absent
Reproduction	Binary fission	Spores (sexual/asexual)	Binary/multiple fission	Host-dependent
Genetic Material	DNA	DNA	DNA	DNA or RNA

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology curricula.