Microbiology: Gram Positive Bacteria

Gram-Positive Bacteria

• Firmicutes (Low GC) and Actinobacteria (High GC) are two categories of gram-positive bacteria
• Non-spore forming bacteria include Lactic Acid Bacteria (Streptococcus, Lactobacillus, etc.), Nocardia, Corynebacteria, Mycobacteria, and Mycoplasma
• Spore-forming bacteria include Clostridium and Bacillus

Staphylococcus

• Gram-positive, non-motile cocci (>30 species)
• Facultative anaerobes capable of fermentation and respiration
• Ferment sugars with lactic acid as a major product
• G + C content (30-40%) similar to Lactic Acid Bacteria
• Less restrictive growth requirements
• Grape-like clusters of cocci (Staphyle - Greek for "bunch of grapes")

Staphylococcus aureus

• Catalase positive (distinguishes from Streptococcus)
• Coagulase positive (distinguishes from S. epidermidis and Micrococcus)
• Causes plasma to clot by converting fibrinogen to fibrin
• Beta-hemolysis on blood agar
• Part of normal flora - skin and nose
• Causes pneumonia and septicemia in newborns and immunocompromised patients
• Causes skin infections, impetigo, furuncles, and carbuncles
• Causes septic arthritis, osteomyelitis, and endocarditis

Staphylococcal Infections

• Scalded Skin Syndrome (SSSS) - toxin produced by phage group 2 S. aureus
• Initial infection in mouth, nasal cavities, throat, or umbilicus
• Lytic toxin (exfoliatin A or B) affects skin at remote sites, leading to desquamation
• Mostly affects young children, especially neonates
• Heals within weeks

Pathogenicity

• Hemolysins - lyse erythrocytes and damage various cell types
• Leukocidin - toxin acts on polymorphonuclear leukocytes and macrophages
• Exfoliatin - plasmid-encoded skin toxin causing wrinkling and peeling of epidermis
• Enterotoxins A, B, and D - exotoxins causing food poisoning (severe diarrhea and vomiting)
• Heat-stable and resistant to boiling
• TSST-1 - superantigen stimulating T-cells to activate macrophages, releasing TNF and causing shock
• Lipases - lipid-hydrolyzing enzymes allowing invasion of tissues
• Fibrolysin - dissolves fibrin clots, enabling spread
• Extracellular coagulase - may be involved in forming fibrin wall of abscess

Panton-Valentine Leukocidin (PVL)

• Produced by strains carrying lukF and lukS genes on a phage
• Associated with community-acquired infections in the young and healthy
• Acts with other leukocidins to lyse host cell membranes, leading to invasive soft tissue infections

Food Poisoning

• Caused by ingestion of preformed toxin from contaminated food
• Toxin is heat-stable and resistant to boiling
• Onset and recovery occur within a few hours
• Inhibits water absorption, leading to explosive diarrhea
• Not a human infection, but a toxin-mediated illness

Staphylococcus aureus Treatment and Resistance

• Produces penicillinase (β-lactamases)
• Usually sensitive to synthetic penicillins (e.g., oxacillin, methicillin)
• Methicillin-resistant S. aureus (MRSA) strains produce altered penicillin-binding proteins and penicillinase
• MRSA strains are resistant to multiple antibiotics, but can be treated with vancomycin
• Community-acquired MRSA (C-MRSA) strains are becoming increasingly prevalent

Staphylococcus epidermidis

• Non-hemolytic, coagulase-negative staphylococcus
• Major component of skin flora, also found in gut and respiratory tract
• Opportunistic infections, less common than S. aureus
• Major cause of nosocomial infections (e.g., catheters, shunts, and prosthetic heart valves)

Gram-Positive Bacteria

• Firmicutes (Low GC) and Actinobacteria (High GC) are two categories of gram-positive bacteria
• Non-spore forming bacteria include Lactic Acid Bacteria (Streptococcus, Lactobacillus, etc.), Nocardia, Corynebacteria, Mycobacteria, and Mycoplasma
• Spore-forming bacteria include Clostridium and Bacillus

Staphylococcus

• Gram-positive, non-motile cocci (>30 species)
• Facultative anaerobes capable of fermentation and respiration
• Ferment sugars with lactic acid as a major product
• G + C content (30-40%) similar to Lactic Acid Bacteria
• Less restrictive growth requirements
• Grape-like clusters of cocci (Staphyle - Greek for "bunch of grapes")

Staphylococcus aureus

• Catalase positive (distinguishes from Streptococcus)
• Coagulase positive (distinguishes from S. epidermidis and Micrococcus)
• Causes plasma to clot by converting fibrinogen to fibrin
• Beta-hemolysis on blood agar
• Part of normal flora - skin and nose
• Causes pneumonia and septicemia in newborns and immunocompromised patients
• Causes skin infections, impetigo, furuncles, and carbuncles
• Causes septic arthritis, osteomyelitis, and endocarditis

Staphylococcal Infections

• Scalded Skin Syndrome (SSSS) - toxin produced by phage group 2 S. aureus
• Initial infection in mouth, nasal cavities, throat, or umbilicus
• Lytic toxin (exfoliatin A or B) affects skin at remote sites, leading to desquamation
• Mostly affects young children, especially neonates
• Heals within weeks

Pathogenicity

• Hemolysins - lyse erythrocytes and damage various cell types
• Leukocidin - toxin acts on polymorphonuclear leukocytes and macrophages
• Exfoliatin - plasmid-encoded skin toxin causing wrinkling and peeling of epidermis
• Enterotoxins A, B, and D - exotoxins causing food poisoning (severe diarrhea and vomiting)
• Heat-stable and resistant to boiling
• TSST-1 - superantigen stimulating T-cells to activate macrophages, releasing TNF and causing shock
• Lipases - lipid-hydrolyzing enzymes allowing invasion of tissues
• Fibrolysin - dissolves fibrin clots, enabling spread
• Extracellular coagulase - may be involved in forming fibrin wall of abscess

Panton-Valentine Leukocidin (PVL)

• Produced by strains carrying lukF and lukS genes on a phage
• Associated with community-acquired infections in the young and healthy
• Acts with other leukocidins to lyse host cell membranes, leading to invasive soft tissue infections

Food Poisoning

• Caused by ingestion of preformed toxin from contaminated food
• Toxin is heat-stable and resistant to boiling
• Onset and recovery occur within a few hours
• Inhibits water absorption, leading to explosive diarrhea
• Not a human infection, but a toxin-mediated illness

Staphylococcus aureus Treatment and Resistance

• Produces penicillinase (β-lactamases)
• Usually sensitive to synthetic penicillins (e.g., oxacillin, methicillin)
• Methicillin-resistant S. aureus (MRSA) strains produce altered penicillin-binding proteins and penicillinase
• MRSA strains are resistant to multiple antibiotics, but can be treated with vancomycin
• Community-acquired MRSA (C-MRSA) strains are becoming increasingly prevalent

Staphylococcus epidermidis

• Non-hemolytic, coagulase-negative staphylococcus
• Major component of skin flora, also found in gut and respiratory tract
• Opportunistic infections, less common than S. aureus
• Major cause of nosocomial infections (e.g., catheters, shunts, and prosthetic heart valves)