Pathogenic Gram-Positive Bacteria: Structure, Physiology, and Pathogenicity

Pathogenic Gram-Positive Bacteria

Overview

Gram-positive bacteria are a major group of clinically significant microorganisms. They stain purple in the Gram stain due to their thick peptidoglycan cell wall. These bacteria are divided into two major groups based on their DNA content:

• Low G + C bacteria

• High G + C bacteria

Many Gram-positive bacteria are part of the normal human microbiota but can become opportunistic pathogens under certain conditions.

Staphylococcus

General Characteristics

Staphylococcus species are facultative anaerobes that commonly occur in grapelike clusters. They are nonmotile and highly salt-tolerant, allowing them to survive on human skin and environmental surfaces. These bacteria are also tolerant of desiccation, radiation, and heat.

• Facultative anaerobes: Can grow with or without oxygen.

• Salt-tolerant: Survive on skin and in salty environments.

• Tolerant of harsh conditions: Desiccation, radiation, heat.

Two species commonly associated with human disease:

• Staphylococcus aureus: More virulent, causes a variety of conditions depending on infection site.

• Staphylococcus epidermidis: Normal skin microbiota, causes opportunistic infections.

Pathogenicity

Staphylococcal infections occur when bacteria breach the body's physical barriers. Even a small number of bacteria can cause disease. Pathogenicity is due to:

• Structural defenses against phagocytosis

• Production of enzymes

• Production of toxins

Structural Defenses Against Phagocytosis

• Protein A: Coats the cell surface, binds to immunoglobulin G (IgG), inhibiting opsonization and the complement cascade.

• Bound coagulase: Converts fibrinogen into fibrin molecules, forming clots that hide bacteria from phagocytic cells.

• Polysaccharide slime layers (capsules): Inhibit leukocyte chemotaxis and phagocytosis, facilitate attachment to surfaces.

Diseases Caused by Staphylococcus

Staphylococci can cause a wide range of diseases, including:

• Skin and soft tissue infections: Impetigo, cellulitis, folliculitis, wound infections, scalded skin syndrome.

• Endovascular infections: Endocarditis, septicemia.

• Genitourinary tract infections: Urinary tract infections, lower urinary tract infections.

• Diseases caused by toxins: Scalded skin syndrome, food poisoning (gastroenteritis).

Staphylococci are normal members of every human's microbiota but can become opportunistic pathogens, especially when the immune system is compromised or physical barriers are breached.

Example: Staphylococcal Food Poisoning

Food poisoning is caused by ingestion of enterotoxin-contaminated food. Symptoms include nausea, vomiting, and diarrhea, typically appearing within hours of consumption.

Additional info: The distinction between low and high G + C Gram-positive bacteria is based on the proportion of guanine and cytosine nucleotides in their DNA, which can influence their physiology and pathogenic potential.