Microbial Disease Study Notes

Introduction to Microbial Disease and Learning Objectives

Instructor: Micah Willis, PhD

Course Agenda:

Announcements
Quiz $2$ (Scheduled for next week)
Microbial Disease (Categorized by Organ Systems) — This material will not be on the upcoming quiz.
Skin and Eye Infections (Chapter $21$ )
Respiratory System Infections (Chapter $22$ )

Clinical Learning Objectives:

Name bacterial organisms associated with diseases of the different body systems.
Compare and contrast bacterial diseases of different body systems, noting mode of transmission, organs affected, and typical treatment strategies.
Discuss the differences between intoxications and infections and note the organisms associated with each.
Define the terms antibody and antigen.
Compare the primary and secondary immune responses.
Perform techniques used to identify bacteria and describe how they work.
Name the cell types and organs in the immune system.
Identify immune system disorders.
Discuss specimen collection and transport for different body systems.
Name parasites/protozoa associated with diseases of the different body systems.
Perform laboratory techniques used in the identification of clinically relevant microorganisms.
Identify cells that are important to the immune system in the laboratory.

Anatomy and Physiology of the Skin

Human skin is a critical component of the immune system, providing both physical and chemical barriers to prevent microbial invasion.

The Three Layers of Human Skin

Epidermis: The outermost layer and is relatively thin. * Stratum Corneum: The exterior surface of the epidermis is rich in the protein keratin. * Desquamation: This is the process where dead skin cells on the surface are shed, helping to prevent infections by removing potential pathogens.
Dermis: A thicker layer located beneath the epidermis. * It consists of connective tissue embedded with blood vessels, nerves, muscles, and hair follicles. * It contains two primary glands: sweat glands and sebaceous glands (which produce sebum).
Hypodermis: The third layer, composed of fibrous and adipose (fatty) tissue.

Microbiological Influences on the Skin

Microbial communities are influenced by the different properties of specific body regions:

Dry Regions (arms, legs, hands): Primarily inhabited by Betaproteobacteria.
Sebaceous Regions (sides of the nose, back): Primarily inhabited by Propionibacteria.
Moist Regions (nostrils, underarms): Inhabited by Corynebacterium and Staphylococcus.

Key Microbiota Facts:

Viruses and fungi are also found among the skin’s normal microbiota.
The most common bacterium found on the skin is $Staphylococcus\,epidermidis$ .

Medical Terminology for Skin Lesions and Rashes

Specific clinical terms are used to describe the morphology of skin infections and injuries:

Abscess: A localized collection of pus.
Bulla (pl. bullae): A fluid-filled blister that is no more than $5\,mm$ in diameter.
Carbuncle: A deep, pus-filled abscess generally formed from multiple furuncles.
Crust: Dried fluids from a lesion remaining on the skin surface.
Cyst: An encapsulated sac filled with fluid, semi-solid matter, or gas, typically located just below the upper layers of skin.
Folliculitis: A localized rash due to the inflammation of hair follicles.
Furuncle (Boil): A pus-filled abscess resulting from an infection of a hair follicle.
Macules: Smooth spots of discoloration on the skin.
Papules: Small raised bumps on the skin.
Pseudocyst: A lesion that resembles a cyst but has a less defined boundary.
Purulent: Producing pus; also referred to as suppurative.
Pustules: Fluid- or pus-filled bumps on the skin.
Pyoderma: Any suppurative (pus-producing) infection of the skin.
Suppurative: Producing pus; synonymous with purulent.
Ulcer: A break in the skin or an open sore.
Vesicle: A small, fluid-filled lesion.
Wheal: Swollen, inflamed skin that itches or burns, such as reactions from an insect bite.

Bacterial Infections of the Skin

Major genera responsible for skin infections include $Staphylococcus$ , $Streptococcus$ , $Pseudomonas$ , and $Bacillus$ .

Staphylococcal Infections

Relevant Species: $S.\,epidermidis$ , $S.\,hominis$ , and $S.\,aureus$ .
Staphylococcus aureus: Highly contagious; many individuals are chronic nasal carriers. It is often associated with pyoderma. It produces leukocidins, which kill white blood cells, leading to pus formation.
MRSA (Methicillin-resistant Staphylococcus aureus): A significant concern in hospital settings. Suspected infections are treated with drugs like tetracycline.
Virulence Factors: Hemolysins, staphylolysins, and leukocidins (cytotoxic agents).
Specific Conditions: * Folliculitis: Inflammation of follicles, often treated with topical antibiotics. * Furuncles and Carbuncles: Boils and deeper, multi-furuncle lesions. * Staphylococcal Scalded Skin Syndrome (SSSS): Caused by bacterial exotoxins that lead to severe skin peeling; most common in children and infants and treated with intravenous antibiotics. * Impetigo: Fluid-filled blisters around the nose and mouth. Highly contagious. Can be caused by $S.\,aureus$ , $S.\,pyogenes$ , or both. Treated with topical or oral antibiotics.

Streptococcal Infections

Most Relevant Species: $S.\,pyogenes$ .
Characteristics: Produces enzymes aiding transmission and causing inflammation; can produce a capsule.
Specific Conditions: * Cellulitis: A painful red rash. * Erysipelas: A raised rash with clear borders. * Erythema Nodosum: Red lumps or nodules, usually on the lower legs. * Necrotizing Fasciitis: Known as "flesh-eating bacterial syndrome." Rare and life-threatening, it infects the fascia layer. Treatment requires surgical debridement, antibiotics, and sometimes amputation.

Pseudomonas Infections

Most Relevant Species: $P.\,aeruginosa$ .
Clinical Presentation: Opportunistic infections of wounds or burns; cause of otitis externa (swimmer's ear).
Distinctive Features: Wounds produce a blue/green pigment and a distinctive odor resembling grape soda or corn tortillas.
Virulence Factors: Hemolysins, adherence to epithelial cells, exotoxins causing tissue necrosis, and a slime layer.
Resistance: Resistant to many antibiotics via beta-lactamases, altered porins, and pumps. Wounds are often treated with anti-biofilm agents.

Cutaneous Anthrax

Pathogen: $Bacillus\,anthracis$ .
Transmission: Zoonotic disease; transmitted by contact with infected animals, wool, or hides. It survives as endospores in the soil.
Portal of Entry: Approximately $99\%$ of cases occur when endospores enter a skin abrasion (cutaneous anthrax).

Viral, Fungal, and Parasitic Infections of the Skin and Eyes

Viral Skin Infections

Papillomas (Warts): Caused by Human Papillomavirus (HPV) through direct contact. Presented as common, plantar, flat, or filiform warts. Treated with liquid nitrogen, cutting, laser, or topical salicylic acid/cantharidin.
Oral Herpes: Caused by Herpes Simplex Virus 1 (HSV-1). Highly contagious via direct oral contact; manifests as cold sores/blisters around the lips. Treated with acyclovir, penciclovir, famciclovir, or valacyclovir.
Fifth Disease (Parvovirus B19): Symptoms include a "slapped cheek" rash. Highly contagious via respiratory secretions.
Roseola: Caused by HHV-6 or HHV-7. High fever followed by a macular/papular rash. Ganciclovir may be used for immunocompromised patients.
Herpes Keratitis: Caused by HSV-1. Inflammation of the conjunctiva and cornea; can lead to blindness.

Fungal Skin Infections (Mycoses)

Tineas (Ringworm): Cutaneous mycoses caused by dermatophytes (molds requiring keratin). Localized as itchy, ring-like lesions. Varieties include: * Tinea barbae: Barber’s itch (face). * Tinea pedis: Athlete’s foot. * Tinea corporis: Body ringworm.
Common Genera: $Trichophyton$ , $Epidermophyton$ , and $Microsporum$ .
Treatments: Antifungal azoles (e.g., miconazole, clotrimazole), terbinafine, or griseofulvin.
Cutaneous Aspergillosis: Caused by $Aspergillus\,fumigatus$ or $A.\,flavus$ ; hallmarks include distinctive eschars at the infection site.
Candidiasis: Caused by $Candida\,albicans$ ; results in intertrigo or yellowing of nails.

Parasitic Infections

Loiasis (African Eye Worm): Caused by the helminth $Loa\,Loa$ . Endemic to West and Central Africa. Spread by deerflies that deposit larvae during a blood meal. Adult worms migrate across the conjunctiva of the eye. Some are surgically removed; chemical treatment includes dimethylcarbamazime, though it has severe side effects.

Anatomy and Defenses of the Respiratory System

Humans take approximately $20,000$ breaths daily, exposing the body to millions of microbes.

Respiratory Anatomy

Upper Respiratory System: Includes the nares, nasal cavity, nasopharynx, eustachian tube (connecting the middle ear), oral cavity, and pharynx.
Lower Respiratory System: Begins below the epiglottis. Includes the larynx (voice box), trachea (windpipe), bronchi, bronchioles, and alveoli (site of gas exchange).

Defense Mechanisms

Mucous Membranes: Sticky mucus with high viscosity and acidity inhibits microbial attachment.
Mucociliary Escalator: Ciliated epithelial cells move microbes up and away from the lungs.
MALT (Mucosa-associated lymphoid tissue): Constant surveillance of the upper system.
Chemical Defenses: Secreted antibodies ( $IgA$ ), lysozyme, and antimicrobial peptides (defensins).
Alveolar Macrophages: Protect the lower respiratory tract.
Normal Microbiota: Upper system is diverse (Firmicutes, Actinobacteria, Proteobacteria). $Staphylococcus\,epidermidis$ is most common. The lower tract has very few microbes.

Common Respiratory Conditions

Rhinitis: Inflammation of nasal cavities.
Otitis Media: Inflammation of the middle ear; common in children ages $3\,months$ to $3\,years$ .
Pharyngitis: Sore throat.
Pneumonia: Infection of the alveoli; leads to accumulation of pus and edema (consolidations), reducing gas exchange and causing a productive cough.

Bacterial Respiratory Diseases

Streptococcal Pharyngitis (Strep Throat)

Pathogen: $Streptococcus\,pyogenes$ .
Signs/Symptoms: Fever, throat patches of pus, swollen glands.
Complications: Can lead to acute rheumatic fever, an autoimmune inflammatory disease.

Bacterial Pneumonia Profiles

Streptococcus pneumoniae: Most common cause of community-acquired pneumonia. Symptoms include productive cough and bloody sputum. Prevented by PCV13 or PPSV23 vaccines.
Mycoplasma pneumoniae: Known as "walking pneumonia." Characterized by a low fever and persistent cough.
Klebsiella pneumoniae: Often healthcare-associated (ventilators). Causes lung necrosis and "currant jelly" sputum; frequently fatal.

Tuberculosis (TB)

Pathogen: $Mycobacterium\,tuberculosis$ .
Mechanism: Bacteria are phagocytized by macrophages but survive and multiply within them, forming lesions called tubercles.
Global Impact: $1/3$ of the world population is colonized; $1.5\,million$ deaths in $2014$ .
Treatment: Multidrug protocols are used over months or years to prevent drug resistance.

Viral Respiratory Diseases and Skin Rashes

Common Viral Infections

Common Cold: Caused by over $200$ viruses, primarily rhinoviruses, coronaviruses, and adenoviruses.
Influenza: Characterized by (H) Hemagglutinin and (N) Neuraminidase protein spikes. Evolves through antigenic shift and antigenic drift.
Respiratory Syncytial Virus (RSV): Common in infants; can be life-threatening.

Respiratory Viruses Causing Skin Rashes

Measles: Causes high fever, Koplik’s spots on oral mucosa, and a confluent macular rash. Highly contagious; prevented by the MMR vaccine.
Rubella (German Measles): Facial rash spreading to extremities. Significant risk for congenital rubella, causing birth defects if contracted during pregnancy.
Chickenpox (Varicella): Caused by Varicella-zoster virus. Characterized by a pustular rash of lesions that burst and crust.

Questions & Discussion

Case Study: Sam's Shaving Injury Sam, a college freshman, nicked himself while shaving. Two days later, the cut was red, warm, and oozing pus.

Microbe Type: The doctor suspected a bacterial infection because the wound was purulent (pus-producing).
Diagnostics: The lab technician would use cultures and specific tests to differentiate between $Staphylococcus$ and $Streptococcus$ .
Treatment Advice: The doctor prescribed an over-the-counter topical antibiotic ointment and instructed Sam to keep the wound clean and change the bandage twice daily.

General Discussion Points:

How does desquamation prevent infection? By shedding dead cells, it physically removes microbes attached to the surface.
Sweat and Sebum: These provide chemical protection through acidity and antimicrobial properties.
Antibacterial use for colds: The instructor posed the question of whether colds should be treated with antibacterials. Given colds are viral, antibacterials are ineffective.