O'Donnell Gram Positive Pathogens

Streptococcus Bacteria

Gram positive cocci, usually in chains

Facultative anaerobic

Characterized by Lancefield carbohydrates and hemolytic activity

Characterization of Streptococcus bacteria by Lancefield carbohydrates

Surface carbohydrates

• Groups A-W

• Groups A, B, and D are the most important clinically

• A few cannot be classified by their carbohydrates and are considered “ungroupable”

  • Ex. S. pneumoniae

Characterization of Streptococcus bacteria by hemolytic activity

Ability to lyse red blood cells

• Alpha-hemolytic → partial hemolysis (Ex. S. pneumoniae)

• Beta-hemolytic → complete hemolysis (Ex. S. pyogenes)

• Gamma-hemolytic → no hemolysis (Ex. Group D strep)

Group A Streptococcus

Most common bacterial cause of sore throat

Also causes toxic shock syndrome and necrotizing fasciitis (flesh-eating disease)

Necessary to quickly identify

• Bacitracin sensitivity

• Rapid Strep test

  • Tests for Group A Lancefield carbohydrates

Streptococcus pyogenes

Group A Streptococcus

Beta-Hemolytic

F protein (an adhesin)

• Attaches to fibronectin on the surface of epithelial cells

M protein

• Part of pili

• Also important for attachment

• Site of extensive antigenic variation

Infections caused by Group A Streptococcus

Pharyngitis (Strep throat)

• Most common infection

• If left untreated can progress to:

  • Scarlet fever

  • Rheumatic fever

Skin Infections including:

• Impetigo

• Necrotizing fasciitis

Streptococcal Toxic Shock Syndrome

Scarlet Fever

Rash all over body

Caused by a superantigen

High fever

Rare if treated with antibiotics

Rheumatic Fever

Life-threatening inflammatory disease

Occurs weeks to years after sore throat

• Group A Strep infection is no longer active

Characterized by fever, endocarditis (inflammation of the endocardium in the heart), joint pain

Why is there a delay between Group A Strep infection and rheumatic fever?

Cell wall is poorly broken down by our enzymes

• Cell wall fragments persist in the body for years after infection and can retrigger the immune response

Antibodies against M-protein cross-react with cardiac myosin, a protein in our heart muscle

• Immune system mistakenly targets myosin in the heart

Impetigo

Highly contagious skin infection

Mostly infants and school-age kids

Open sores around mouth and face

Easily treatable with antibiotics

Necrotizing fasciitis

Typically after a wound in the skin

Can become invasive

Produces superantigens that atack and kill fascia

Can also be caused by other kinds of bacteria

Streptococcal Toxic Shock Syndrome

Rapid, systemic infection

Follows skin infections, rarely follows strep throat

Superantigens are almost always expressed

Rare, but extremely life threatening

Streptococcus pneumoniae

Gram positive diplococci

Ungroupable, but genetically related to Group A

Alpha-hemolytic

A leading cause of pneumonia in the US

• Usually occurs after a cold or other viral infection in healthy people

Has a capsule → can lead to more invasive infections (meningitis, bacteremia (blood infection), sinus infection)

Over 90 serotypes, vaccine provides protection against 23 of them

Antibiotic resistance is common

Teichoic acid in the cell wall attracts a large number of inflammatory cells in the lung

Group B Streptococcus (GBS)

Streptococcus agalactiae

Causes sepsis, meningitis, and pneumonia in newborns

• GBS is in the lower intestine and vaginal tract of 15-40% of healthy adult women

• Passed to newborn during delivery

  • Symptoms can occur weeks later

  • 1 in 2000 births in the US

• Pregnant women are screened for GBS during weeks 35-37 → if positive, deliver antibiotics during labor

Also causes invastive infections in the elderly

Group D Streptococcus

Enterococci (Gut)

Most common ones are Enterococcus faecalis and Enterococcus faecium

• Causes UTIs and soft tissue infections almost exclusively in hospitalized patients with trauma/surgery or that are immunocompromised 

High levels of antibiotic resistance

Why do Group D Streptococcus bacteria have high levels of antibiotic resistance?

Efficient at acquiring plasmids and transposons from other species of bacteria

Complete resistance to cephalosporins, high resistance to most β-lactams and aminoglycosides

• Ampicillins were the most consistently active antibiotic

• If ampicillin-resistant, vancomycin was used

VRE (Vancomycin Resistant Enterococci) are resistant to both vancomycin and ampicillin

Staphylococcus Bacteria

Gram positive grape-like clusters of cocci

Major component of normal skin flora

• Potent opportunist - can cause disease in many ways

Produces IgG binding protein

MSSA and MRSA

Complications of Staphylococcus aureus

Skin lesions (boils, abscesses)

Osteomyelitis (bone infection)

Arthritis

• Most common cause of joint infection

Toxic Shock Syndrome

• Historically associated with tampon usage

• Caused by a superantigen

Scalded Skin Syndrome (under age 6)

• Exfolatin exotoxin (“exfoliate”)

• Epidermis splits from the other layers of the skin

Clostridia Bacteria

Gram positive bacilli

Spore-forming

• Spores last for years

Produce exotoxins:

• C. perfringens → gas gangrene

• C. tetani → tetanus toxin (lockjaw)

• C. botulinum → botulism toxin (flaccid paralysis)

• C. difficile → antibiotic-associated diarrhea

  • Associated with hospital stays and long-term care facilities

  • More common and severe in the elderly

  • Expresses Toxin A and Toxin B

    • Toxin A increases fluid release from the bowel

    • Toxin B kills cells in mucosa

Clostridium botulinum

Pennsylvania is part of the “Botulism Belt”

• Spores of C. botulinum are found in the soil → most likely source of most US cases

• Spores are also found in raw honey and improperly canned foods

In infants, treatment involves BabyBIG

• Human-derived antibodies against the exotoxin