Chapter 19: Pathogenic Gram Positive Cocci Bacteria

I. Staphylococcus

A. Structure and Physiology
• 0.5-1.0 μm in diameter clustered in Staphylococcal
arrangement
• The species more commonly associated with
human infections are:

1) Staphylococcus aureus (main pathogenic species)
2) S. epidirmidis (causes infections mainly in
immunocompromised patients)

I. Staphylococcus

1. Staphylococcus aureus (Plan A for wounds)

B. Pathogenicity

1. Enzymes
a) Beta-lactamase ( penicillinase): breaks down
penicillin

2. Toxins
a) Cytolytic toxins: hemolysins and leukocidins

What is MRSA and why is it so clinically significant? Why is it important that healthcare providers are well informed about MRSA?

Methicillin-resistant Staphylococcus aureus (MRSA) is a type of bacterium that is genetically resistant to beta-lactam antibiotics, which include methicillin, penicillin, amoxicillin, and cephalosporins.

MRSA is highly significant in clinical settings because it transforms a common, easily treatable bacterial infection into a resilient, potentially life-threatening condition. Healthcare providers must be deeply informed about MRSA to prevent transmission, select appropriate alternative therapies, and curb the broader crisis of antimicrobial resistance.

What is Methicillin Resistant Staphylococcus aureus (MRSA) ?

→is a dangerous strain of S. aureus that emerged in the
mid 80’s as one of the so called “superbugs”

—>It’s a multi-resistant organism that poses an
epidemiological problem in the hospital setting
(nosocomial infections), and now also in the
community

—>Vancomycin resistant MRSA strains are also emerging

II. Streptococcus
The Beta (β) Hemolytic Streptococcus

What is group A?

Streptococcus pyogenes
• 0.5-1.2 μm generally cocci in pairs or chains
• S. pyogenes is a strict parasite generally involving the
throat and nasopharynx

What is the main virulence factor that allow this organism to cause necrotizing fasciitis?(Group A)

Two main structural features enable cells of S. pyogenes to evade phagocytosis:

• M protein. A membrane protein called M protein destabilizes complement, thereby interfering with opsonization and lysis.

• Hyaluronic acid capsule. Because hyaluronic acid is normally found in the body, white blood cells may ignore bacteria “camouflaged” by this type of capsule.

II. Streptococcus
The Beta (β) Hemolytic Streptococcus

What is Group B Streptococcus?

Streptococcus agalactiae`

What is the treatment of choice for infections with β-hemolytic Streptococcus infections?

The U.S. Centers for Disease Control and Prevention (CDC) recommends prophylactic administration of penicillin at birth to children whose mothers’ urinary tracts are colonized with group B streptococci. Implementation of this guideline in 1996 reduced early-onset disease morbidity and mortality by 70% by 2001.

How is this treatment protocol different from the treatment protocol of S. aureus? (Group B)

The treatment protocols differ substantially because of how S. aureus produces resistance enzymes (beta-lactamases) and the high prevalence of resistant strains like MRSA:

• Spectrum of Resistance & β-Lactamase Production:

  • β-hemolytic Streptococcus remains universally and predictably susceptible to standard penicillins without developing resistance via beta-lactamase production.

  • The vast majority of S. aureus strains produce beta-lactamase, rendering regular penicillin ineffective. Treating S. aureus requires penicillinase-resistant penicillins (such as nafcillin or oxacillin) or first-generation cephalosporins (such as cefazolin) for methicillin-susceptible S. aureus(MSSA

III. Alpha (α) Hemolytic Streptococci

1. Streptococcus pneumonia (The Pneumococcus)

• There are more than 92 strains of S. pneumonia

Describe the key gram stain morphology of S. pneumoniae in a sputum of a patient with pneumonia.

1) Gram stain on sputum: gram (+) diplococci,
lancet shaped and encapsulated

IV. Enterococcus (plan b for wounds)

• Gram (+) cocci in pairs and chains, non-hemolytic and
non-encapsulated

• Main species are E. faecalis and E. faecium

What is VRE?

Strains that constitute the superbug Vancomycin
Resistant Enterococcus (VRE) are resistant to
many commonly used antibiotics

Why is Vancomycin Resistant Enterococcus (VRE) such an important strain?

• Limited Treatment Options: Enterococcus species are already naturally resistant to many common antibiotics, and acquiring resistance to vancomycin—traditionally a powerful "last-line" drug for Gram-positive infections—leaves clinicians with very few effective alternative drugs (such as linezolid or daptomycin). [1, 2, 3]

• Healthcare-Associated Spread: VRE primarily spreads in hospitals and long-term care facilities via contaminated surfaces, medical equipment, or the hands of healthcare workers. [1, 2, 3]

• High-Risk Populations: It predominantly affects vulnerable or critically ill patients—such as those in intensive care units, patients with weakened immune systems, or individuals with invasive devices like urinary catheters or central lines. [1, 2]

• Gene Transfer and Persistence: VRE can transfer vancomycin-resistance genes (via plasmids) to other bacteria and can survive for long periods on environmental surfaces. The World Health Organization (WHO) and CDC have classified VRE as a major public health and clinical threat. [1, 2, 3, 4]