Fungi and Infectious Diseases: Key Concepts and Pathogens

L12 - Fungi

Hyphae

The long filamentous structures of mold.

Spores

Reproductive structures produced by fungi.

Budding

Asexual reproduction in yeast where a small cell forms from parent cell.

Saprophytic

Feeds on decaying organic matter.

Chitin

Polysaccharide found in fungal cell walls.

Thallus

The body of a fungus.

Septate hyphae

Hyphae divided by cross-walls.

Aseptate hyphae

Hyphae without cross-walls.

Dimorphic fungi

Fungi that can exist as yeast or mold depending on temperature.

Mycelium

Mass of hyphae.

Sporangium

Structure that forms sporangiospores.

Sporangiospores

Spores formed inside a sporangium.

Conidiospores

Asexual spores formed at the tips of hyphae.

Mycoses

Fungal infections.

Dermatophytes

Fungi that infect skin, hair, nails.

True fungal pathogen

Fungi that cause disease in healthy individuals.

Opportunistic infection

Infection occurring when host defenses are weakened.

Superficial mycosis

Fungal infection of outer epidermis.

Cutaneous mycosis

Fungal infection of deeper epidermis.

Systemic mycosis

Fungal infection that begins in lungs and spreads.

Three clinical manifestations of fungal disease

Allergy, toxicosis, fungal infection.

Predisposing factors

Weakened immunity, diabetes, broad-spectrum antibiotics, long-term corticosteroids.

Patient history

Used to correlate symptoms with fungal exposure.

Sabouraud dextrose agar

Selective medium for fungi.

KOH prep

Dissolves human tissue to reveal fungal cells.

GMS stain

Special stain that highlights fungal cell walls.

Two reasons fungi are difficult to treat

Fungal cells resemble human cells; slow growth.

Two antifungal targets

Ergosterol and cell membrane.

Amphotericin B

Binds ergosterol → membrane pores.

Azole drugs

Inhibit ergosterol synthesis.

True fungal pathogens infection site

All begin infection in lungs.

Mississippi River/Great Lakes

Histoplasma region.

Desert Southwest

Coccidioides region.

Mexico & Central America

Paracoccidioides region.

Spherule

Form of Coccidioides in human body.

Bird/bat droppings

Source of Histoplasma spores.

Steering wheel formation

Characteristic of Paracoccidioides yeast.

Aspergillus

Disease: aspergillosis; acquired by inhalation; diagnosed by imaging/KOH; treated with azoles.

Candida location

Normal microbiota of skin, mouth, vagina, GI tract.

Candida transmission

NOT spread person to person; overgrowth causes disease.

Dermatophyte diagnosis

KOH prep of skin/nail scrapings.

L13 - Infectious Disease

Human microbiota

Microbes living on/in the body; acquired at birth; can become opportunistic.

Infection

Microbe enters and multiplies.

Disease

Results in tissue damage.

Sign

Objective indicator (fever, rash).

Symptom

Subjective feeling (pain, fatigue).

Pathogenicity

Ability to cause disease.

Virulence

Severity of disease a microbe can cause.

Virulence factor

Trait that helps a microbe cause disease.

Infectious dose

Number of microbes needed to cause disease.

Asymptomatic

Infected but no symptoms.

Syndrome

Set of signs and symptoms.

Five steps of infection

Entry, adhesion, invasion, colonization, exit.

Four portals of entry

Skin, mucous membranes, placenta, parenteral route.

Fimbriae

Attachment structures.

Glycocalyx

Sticky coating aiding adhesion.

Adhesins

Attachment proteins.

Capsule

Protects from phagocytosis.

Leukocidin

Kills phagocytes.

Phagocyte

Immune cell that engulfs microbes.

Phagocytosis

Engulfing and destroying microbes.

Hyaluronidase

Breaks down connective tissue.

Collagenase

Breaks collagen.

Coagulase

Clots blood to hide bacteria.

Kinase

Breaks clots to spread bacteria.

Exotoxin

Secreted protein toxin.

Endotoxin

LPS; released when Gram-negative bacteria die.

Enterotoxin

Toxin affecting intestines.

Neurotoxin

Toxin affecting nerves.

Portals of exit

Respiratory droplets, feces, urine, semen, blood.

Stages of disease

Incubation, prodromal, illness, decline, convalescence.

Reservoir

Habitat of pathogen.

Three reservoirs

Human, animal, nonliving.

Zoonosis

Animal-to-human disease.

Three zoonotic routes

Direct contact, eating animal products, vectors.

Five carrier types

Asymptomatic, incubatory, convalescent, chronic, passive.

Three transmission modes

Contact, vehicle, vector.

Fomite

Contaminated object involved in indirect contact.

Incidence

New cases.

Prevalence

Total cases.

Endemic

Constant level in region.

Epidemic

Sudden increase in cases.

Pandemic

Epidemic on multiple continents.

Nosocomial infection factors

Microbes in hospital, immunocompromised patients, invasive procedures.

L14 - Gram Positive Pathogens

Staphylococcus Gram stain/shape/arrangement

Gram positive cocci in clusters.

Staph survival traits

Salt tolerant, desiccation resistant.

Two important Staph species

S. aureus (most important), S. epidermidis.

Staph attachment factor

Protein A or adhesins.

Staph antiphagocytic factors

Capsule, Protein A, leukocidin, slime layer.

Staph enzymes

Coagulase, hyaluronidase, staphylokinase, lipase, β-lactamase, DNase, catalase.

Staph toxins

Enterotoxin, exfoliative toxin, TSST, cytolysins.

Staph disease categories

Noninvasive, cutaneous, systemic.

Strep Gram stain/shape/arrangement

Gram positive cocci in chains.

Test differentiating Staph vs Strep

Catalase test.

Lancefield antigens

Carbohydrates on Strep; Groups A & B important.

Group A strep species

Streptococcus pyogenes.

Group A strep hemolysis

β-hemolytic; clear zone on blood agar.