5.3 Staphylococcus

General Overview and Microbiological Characteristics of Staphylococcus

• Nature and Commensalism:

  • Staphylococcus is a commensal bacterium present on all humans.

  • In a commensal state, it neither harms nor benefits the host.

  • Transition to pathogen occurs when there are breaks in the skin or mucous membranes, or when a host becomes immunocompromised.

• Anatomical Sites and Systemic Infections:

  • Associated with cutaneous (skin), central nervous system, and gastrointestinal infections.

  • Associated with pneumonia.

  • Capable of giving rise to systemic infections affecting the cardiovascular and lymphatic systems.

• Microbiological Profile:

  • Morphology and Arrangement: Spherical cocci occurring in clusters.

  • Gram Stain: Gram-positive.

  • Motility: Non-motile.

  • Respiration: Facultative anaerobe.

  • Catalase Activity: Catalase-positive.

  • Oxidase Activity: Oxidase-negative.

  • Coagulase Activity: Coagulase-positive.

  • Special Culturing Requirements: Selective via bile salts.

  • Hemolytic Activity: Exhibits beta ($\beta$) hemolysis.

Virulence Factors of Staphylococcus aureus

• Coagulase:

  • Coagulase-positive nature allows the bacteria to clot blood.

  • Enables the pathogen to hide from the host immune system.

• Catalase:

  • Degrades reactive oxygen species, specifically breaking down hydrogen peroxide ($H_2O_2$).

  • Protects the pathogen because host immune cells, such as phagocytes, utilize peroxides to destroy engulfed microbes.

• Staphylokinase and Fibrinolytic Enzymes:

  • Produced by specific strains to break down blood clots.

  • Facilitates the release of bacteria from localized wound sites, promoting the spread of infection through tissue and the body.

• Toxins:

  • Pyrogenic Toxin: Induces fever and systemic responses.

  • Exfoliative Toxin: Responsible for staphylococcal scalded skin syndrome.

• Hyaluronidase:

  • Degrades the extracellular matrix of connective tissues, facilitating tissue invasion.

• Hemolysins:

  • Lyses red blood cells, giving specific strains their beta ($\beta$)-hemolytic activity.

• Lipase:

  • Digests lipids within host tissues.

Cutaneous Staphylococcal Infections

• Pathogenesis:

  • Staphylococcus aureus is typically part of the normal skin microbiome.

  • Infection is triggered by shifts in strain populations or damage/disruption to the skin.

• Folliculitis and Associated Cutaneous Lesions:

  • Folliculitis: Infection of a hair follicle by Staphylococcus, causing the site to become red, swollen, or filled with pus.

  • Stye: Folliculitis occurring specifically on the eyelid.

  • Furuncle (Boil): Infection that spreads from an infected hair follicle into the surrounding tissue.

  • Carbuncle: Formed when several furuncles coalesce or join together. Extends to deeper tissues and triggers a systemic immune response with fever and chills.

• Staphylococcal Scalded Skin Syndrome (SSSS):

  • Target Population: Typically observed in infants.

  • Etiology: Caused by exfoliative toxin released by the bacteria on the skin.

  • Clinical Manifestations: The skin turns red and develops blisters containing clear fluid.

  • Progression: Within approximately $2$ days, the skin peels off in sheets.

  • Complication: Destruction of the skin's protective barrier creates a critical risk for secondary bacterial infections.

• Infantigo:

  • Target Population: Childhood infection occurring typically in children aged $2\text{ to }5$ years.

  • Clinical Manifestations: Reddened, itchy skin characterized by pus-filled vesicles appearing on the face or extremities.

  • Transmission: Rupturing or scratching of vesicles releases fluid that spreads the infection to other body regions or to other individuals.

  • Treatment: Penicillin antibiotics.

Ocular Staphylococcal Infections and Related Pathophysiology

• Conjunctivitis (Pink Eye):

  • Anatomic Definition: Inflammation or infection of the conjunctiva, which is the transparent layer of tissue covering the sclera (white of the eye) and the inner surfaces of the eyelids.

  • Etiology: Caused by staphylococci, streptococci, or viral agents.

  • Pathophysiology of Reddening: Inflammation causes dilation of conjunctival blood vessels, making them visible upon examination.

  • Differential Diagnosis by Discharge Type:

  • Viral Origin: Produces a watery discharge from the eyes.

  • Bacterial Origin: Produces a sticky, pus-like discharge that can cause eyelids to adhere together after sleep or form a crust-like substance along the eyelashes.

• Ophthalmia Neonatorum:

  • Definition and Etiology: Conjunctivitis occurring in newborns exposed during birth to Chlamydia or Neisseria gonorrhoeae bacteria present in the birth canal.

  • Complications: Threatens permanent eye damage if left untreated.

  • Prophylaxis: Routine administration of prophylactic antibiotic drops into the eyes of all newborns.