Gram-Positive Cocci Notes

Gram-Positive Cocci

Introduction

Two medically important genera: Staphylococcus and Streptococcus. This chapter focuses on Staphylococcus aureus and Streptococcus pyogenes, both significant human pathogens. These bacteria are nonmotile and do not form spores.

• Differentiation:

  • Staphylococcus: grapelike clusters microscopically, catalase-positive (degrades hydrogen peroxide: H2O2 —> O2 + H2O).

  • Streptococcus: chains microscopically, catalase-negative.

Staphylococcus

1. Staphylococcus aureus

Diseases

S. aureus causes:

• Abscesses.

• Pyogenic infections (endocarditis, septic arthritis, osteomyelitis).

• Food poisoning.

• Scalded skin syndrome (exotoxin-mediated).

• Toxic shock syndrome.

• Hospital-acquired pneumonia, septicemia, and surgical-wound infections (including cardiac pacemaker sites).

• Skin and soft tissue infections (folliculitis, cellulitis, impetigo).

• Bacterial conjunctivitis.

• Methicillin-resistant S. aureus (MRSA) is a common cause of skin abscesses in the US, pneumonia, necrotizing fasciitis, and sepsis (even in immunocompetent patients). It's also prevalent among intravenous drug users.

• Kawasaki syndrome (etiology possibly linked to certain S. aureus strains).

Important Properties

• Spherical, gram-positive cocci in grapelike clusters.

• Catalase-positive (important virulence factor by neutralizing $H<em>2O</em>2$ in neutrophils).

• Three key species: S. aureus, Staphylococcus epidermidis, and Staphylococcus saprophyticus.

  • S. aureus is the most common and virulent, differentiated by coagulase production.

  • Coagulase activates prothrombin to thrombin, which then converts fibrinogen to fibrin, resulting in clot formation.

  • S. epidermidis and S. saprophyticus are coagulase-negative.

• Table 15-1: Staphylococci of Medical Importance
  

  Species	Coagulase Production	Hemolysis	Important Features	Typical Disease
  S. aureus	+	β	Protein A on surface	Abscess, food poisoning, toxic shock syndrome
  S. epidermidis	–	None	Sensitive to novobiocin	Infection of prosthetic heart valves/hips, skin flora member
  S. saprophyticus	–	None	Resistant to novobiocin	Urinary tract infection
  S. aureus produces staphyloxanthin (golden pigment), enhancing pathogenicity by neutralizing reactive oxygen species in neutrophils. S. epidermidis produces white colonies.
  S. aureus ferments mannitol and hemolyzes red blood cells (source of iron for cytochrome enzyme synthesis), while S. epidermidis and S. saprophyticus do not.
  Over 90% of S. aureus strains carry plasmids encoding β-lactamase (degrades penicillins).
  Methicillin-resistant S. aureus (MRSA) has altered penicillin-binding proteins (PBPs) due to mecA genes on the bacterial chromosome; the most important PBP is PBP2a, which does not bind penicillins.

  • MRSA can be healthcare-acquired (HCA-MRSA) or community-acquired (CA-MRSA).

  • CA-MRSA strains commonly produce P-V leukocidin, unlike HCA-MRSA.

  • USA300 is a common MRSA strain in the United States.

• Vancomycin-intermediate S. aureus (VISA) and vancomycin-resistant S. aureus (VRSA) strains exist; resistance is encoded by genes on a transposon on a plasmid that substitutes D-lactate for D-alanine in peptidoglycan

• Cell Wall Components and Antigens:

  • Protein A: Binds to the Fc portion of IgG, preventing phagocytosis. Coagulase-negative staphylococci do not produce protein A.

  • Teichoic acids: Polymers of ribitol phosphate that mediate adherence to mucosal cells. Lipoteichoic acids induce cytokines (IL-1, TNF) from macrophages, causing septic shock.

  • Polysaccharide capsule: 11 serotypes; types 5 and 8 cause 85% of infections. Microcapsules are poorly immunogenic, hindering vaccine development.

  • Surface receptors: Allow phage typing for epidemiological purposes.

  • Peptidoglycan: Has endotoxin-like properties, stimulating macrophages (cytokine production) and activating complement and coagulation cascades.

Transmission

• Humans are the reservoir. The nose is the primary colonization site (approximately 30% colonized).

• Skin colonization is common among hospital personnel and patients.

• Hand contact is a major transmission mode; handwashing is crucial.

• S. aureus is found in the vagina of ~5% of women, predisposing them to toxic shock syndrome.

• Other sources: Shedding from human lesions and fomites.

• Disease susceptibility: Heavily contaminated environments, compromised immune systems, reduced humoral immunity, and diabetes.

• Intravenous drug users are at increased risk, especially with MRSA.

• Chronic granulomatous disease (CGD) patients are prone to S. aureus infections.

Pathogenesis

• S. aureus causes disease through toxin production and pyogenic inflammation.

• Abscesses are typical lesions.

• Coagulase promotes fibrin clot formation, walling off bacteria and preventing neutrophil access.

• Exotoxins:

  1. Enterotoxin: Causes food poisoning (vomiting, watery diarrhea). Acts as a superantigen stimulating IL-1 and IL-2 release. Vomiting is due to serotonin release triggering the vomiting center. Heat-resistant and resistant to stomach acid/enzymes. Types A–F exist.

  2. Toxic shock syndrome toxin (TSST): Causes toxic shock syndrome in tampon-using women or wound infections. TSST enters the bloodstream, causing toxemia. Blood cultures are often negative. Superantigen stimulating IL-1, IL-2, and TNF release. Occurs in individuals lacking TSST antibodies. 5-25% of S. aureus isolates carry the TSST gene.

  3. Exfoliatin: Causes scalded skin syndrome in young children. Protease that cleaves desmoglein in desmosomes, leading to epidermal separation. Localized exfoliatin production results in bullous impetigo.

  4. Leukocidins (alpha toxin and P-V leukocidin): Kill leukocytes and cause tissue necrosis.

     • Alpha toxin: Causes skin necrosis and hemolysis by forming holes in the cell membrane.

     • P-V leukocidin: Pore-forming toxin targeting white blood cells; important for CA-MRSA virulence and severe skin/soft tissue infections. Causes necrotizing pneumonia. ~2% of clinical isolates produce P-V leukocidin.

  5. Enzymes (coagulase, fibrinolysin, hyaluronidase, proteases, nucleases, lipases):

     • Coagulase: Clots plasma, walling off the infected site.

     • Staphylokinase: A fibrinolysin, lyses thrombi.

• Coagulase-negative staphylococci do not produce exotoxins, but cause pyogenic infections.

Clinical Findings

• Pyogenic (pus-producing) and toxin-mediated diseases.

• Major cause of skin, soft tissue, bone, joint, lung, heart, and kidney pyogenic infections.

• Table 15-2: Important Features of Pathogenesis by Staphylococci

• Staphylococcus aureus: Pyogenic Diseases

  1. Skin and soft tissue infections: Abscess, impetigo, furuncles, carbuncles, paronychia, cellulitis, folliculitis, necrotizing fasciitis, hidradenitis suppurativa, conjunctivitis, eyelid infections, and postpartum breast infections. Severe infections are caused by MRSA strains producing P-V leukocidin, typically community-acquired; common among the homeless, intravenous drug users, and athletes. HCA-MRSA causes ~50% of nosocomial S. aureus infections.

  2. Septicemia (Sepsis): Originates from localized lesions or IV drug abuse; similar to sepsis caused by gram-negative bacteria (e.g., Neisseria meningitidis).

  3. Endocarditis: Occurs on normal or prosthetic heart valves, especially right-sided endocarditis in IV drug users (prosthetic valve endocarditis often caused by S. epidermidis).

  4. Osteomyelitis and septic arthritis: Arise from hematogenous spread or local wounds; common in children.

  5. Postsurgical wound infections: Important cause of morbidity/mortality in hospitals; common at cardiac pacemaker sites.

  6. Pneumonia: Occurs postoperatively or after viral respiratory infection/influenza; often leads to empyema or lung abscess; a common cause of nosocomial pneumonia. CA-MRSA causes severe necrotizing pneumonia.

  7. Conjunctivitis: Unilateral burning eye pain, hyperemia, and purulent discharge; often transmitted by contaminated fingers. S. aureus is the most common cause overall; Streptococcus pneumoniae and Haemophilus influenzae are more common in children. Gonococcal and chlamydial conjunctivitis is acquired during birth.

  8. Abscesses: Can occur in any organ due to bacteremia; called “metastatic abscesses” (spread from the original site of infection).

• Staphylococcus aureus: Toxin-Mediated Diseases

  1. Food poisoning (gastroenteritis): Caused by preformed enterotoxin in foods; short incubation period (1-8 hours). Vomiting is more prominent than diarrhea.

  2. Toxic shock syndrome: Fever, hypotension, diffuse macular rash (desquamation), and involvement of multiple organs.

  3. Scalded skin syndrome: Fever, large bullae, and erythematous macular rash; skin sloughing, serous fluid exudation, and electrolyte imbalance can occur. More common in young children; recovery within 7-10 days.

  4. Bullous impetigo: Vesicles containing clear fluid coalesce to form bullae; caused by localized exfoliatin production.

• Staphylococcus aureus: Kawasaki Disease

  • Kawasaki Disease (KD) is a vasculitis involving small and medium-size arteries (especially coronary arteries) that resembles toxic shock syndrome.

  • It is the most common cause of acquired heart disease in children in the United States.

  • It involves high fever, nonpurulent conjunctivitis, lesions of lips/oral mucosa (strawberry tongue, edema), cervical lymphadenopathy, diffuse rash, and erythema/edema of hands/feet with desquamation.

  • The most characteristic sign: cardiac involvement (myocarditis, arrhythmias, regurgitation).

  • The main cause of morbidity and mortality: aneurysm of the coronary arteries.

  • Common in children of Asian ancestry, suggesting MHC allele predisposition. Disease of children under 5 occurring in mini-outbreaks.

  • No definitive lab test; effective therapy includes high-dose intravenous immune globulins (IVIG) and aspirin to reduce fever and other symptoms, and it significantly reduces the occurrence of aneurysms.

Laboratory Diagnosis

• Smears: Gram-positive cocci in grapelike clusters.

• Cultures: Golden-yellow, usually β-hemolytic colonies. Coagulase-positive.

• Mannitol-salt agar: S. aureus ferments mannitol, turning the agar yellow, while S. epidermidis does not (agar remains pink).

• Toxic shock syndrome diagnosis: Clinical criteria are used as isolation of S. aureus is not required to make a diagnosis.

  • Supporting lab findings: Isolation of TSST-producing strain and development of antibodies to the toxin during convalescence.

• Epidemiologic subgrouping: Based on susceptibility to lysis by bacteriophages.

Treatment

• Drainage (spontaneous or surgical) is the cornerstone of abscess treatment; incision and drainage(I&D) is often sufficient for skin abscesses.

• Antibiotics are usually unnecessary unless systemic signs are present (oral trimethoprim-sulfa or intravenous vancomycin).

• Previous infection does not provide complete immunity to reinfection.

• 90%+ of S. aureus strains are penicillin G-resistant (produce β-lactamase).

  • Treat with β-lactamase-resistant penicillins (nafcillin or cloxacillin), some cephalosporins, or vancomycin.

  • Combination therapy of β-lactamase-sensitive penicillin (amoxicillin) and a β-lactamase inhibitor (clavulanic acid). Useful as well.

• ~20% of S. aureus strains are methicillin/nafcillin-resistant (altered penicillin-binding proteins); MRSA or NRSA.

  • Drug of choice is vancomycin, sometimes combined with gentamicin, or daptomycin.

  • Trimethoprim-sulfamethoxazole or clindamycin for non-life threatening infections.

    • Ceftaroline fosamil: First β-lactam effective against MRSA infections.

• VISA and VRSA strains exist; Daptomycin or Quinupristin-dalfopristin can be used in these cases. Often methicillin/nafcillin-resistant.

• Toxic shock syndrome treatment: Correction of shock (fluids, pressor/inotropic drugs), nafcillin, and removal of tampon or debridement of the site. Pooled serum globulins (anti-TSST antibodies) may be useful.

• Mupirocin: Topical antibiotic effective for skin infections and nasal carriage reduction; add topical antiseptics (e.g., chlorhexidine).

• Staphylococcal tolerance: Treat with drug combinations.

Prevention

• No vaccine exists.

• Cleanliness, frequent handwashing, and aseptic management of lesions help control the spread.

• Persistent nasal colonization can be reduced by intranasal mupirocin or oral antibiotics (ciprofloxacin or trimethoprim-sulfamethoxazole) and shedder removed from high-risk areas (OR/newborn nurseries).

• Cefazolin: Often used perioperatively to prevent surgical-wound infections.

2. Coagulase-Negative Staphylococci (Staphylococcus epidermidis & Staphylococcus saprophyticus)

Diseases

• S. epidermidis: Prosthetic valve endocarditis, prosthetic joint infections, CNS shunt infections, sepsis in newborns.

• S. saprophyticus: Urinary tract infections, especially cystitis.

Important Properties

• Spherical gram-positive cocci in grapelike clusters; catalase-positive.

• Coagulase-negative.

• Do not synthesize staphyloxanthin produce white colonies.

• Do not produce protein A. Less virulent than S. aureus.

• S. aureus usually ferments mannitol and hemolyzes red blood cells, whereas S. epidermidis and S. saprophyticus do not.

Transmission

• Humans are the reservoir.

• S. epidermidis: Primarily on human skin; enters bloodstream via intravenous catheters.

• S. saprophyticus: Primarily on the genital tract mucosa of young women; ascends into the bladder causing UTIs.

• S. epidermidis infections are almost always hospital-acquired, whereas S. saprophyticus infections are almost always community-acquired.

Pathogenesis

• Do not produce exotoxins.

• S. epidermidis: Prominent cause of pyogenic infections on prosthetic implants.

• Strains of S. epidermidis that produce a glycocalyx are more likely to infect implants.

• S. saprophyticus: Causes pyogenic urinary tract infections, especially cystitis.

• Hospital personnel can often carry antibiotic-resistant strains of S. epidermidis.

Clinical Findings

• S. epidermidis: Normal flora on skin and mucous membranes, but can cause bacteremia, and metastatic infections (especially at implants). Commonly infects IV catheters and prosthetic implants (heart valves, vascular grafts, joints). Major cause of sepsis in neonates and peritonitis in peritoneal dialysis patients. Common cause of CSF shunt infections.

• S. saprophyticus: Causes UTIs, mainly in sexually active young women (usually within 24 hours of intercourse). Second to Escherichia coli as a cause of community-acquired UTIs in young women.

• Staphylococcus lugdunensis: Uncommon, causes prosthetic valve endocarditis and skin infections.

Laboratory Diagnosis

• Smears: Gram-positive cocci in grapelike clusters.

• Cultures: White, nonhemolytic colonies; Novobiocin sensitivity differentiates them: S. epidermidis is sensitive, S. saprophyticus is resistant.

Treatment

• S. epidermidis: Highly antibiotic resistant. Most strains produce β-lactamase. Some are methicillin/nafcillin-resistant (MRSE). Treat MRSE with vancomycin, rifampin, or an aminoglycoside can be added. Catheter removal is usually needed.

• S. saprophyticus: UTIs can be treated with trimethoprim-sulfamethoxazole or a quinolone (ciprofloxacin).

Prevention

• No vaccine.

• Prompt removal of IV catheters reduces the risk of infection by S. epidermidis.

Streptococcus

Table 15-3: Streptococci of Medical Importance

Diseases

Streptococci cause diverse infections:

• S. pyogenes: Pharyngitis, cellulitis, impetigo, erysipelas, necrotizing fasciitis, scarlet fever, streptococcal toxic shock syndrome, rheumatic fever, acute glomerulonephritis.

• S. agalactiae: Neonatal sepsis, meningitis.

• Enterococcus faecalis: Hospital-acquired UTIs, endocarditis.

• Viridans group streptococci: Endocarditis.

• Streptococcus bovis: Endocarditis.

Important Properties

• Spherical, gram-positive cocci in chains or pairs; catalase-negative (staphylococci are catalase-positive).

• Hemolysis:

  • α-hemolytic: Green zone due to incomplete RBC lysis (hydrogen peroxide oxidizes hemoglobin to biliverdin).

  • β-hemolytic: Clear zone due to complete RBC lysis (streptolysin O and streptolysin S).

  • γ-hemolysis: Nonhemolytic.

• Antigens:

  • C carbohydrate: Determines Lancefield group (A to U).

  • M protein: Main virulence factor of S. pyogenes. Antiphagocytic (inactivates C3b); determines serotype; antibody to M protein provides type-specific immunity. Some types are rheumatogenic, others are nephritogenic.

Classification of Streptococci

• β-Hemolytic Streptococci: Classified into groups A–U (Lancefield groups) based on antigenic differences in C carbohydrate.

  • Group A streptococci (S. pyogenes): Frequent bacterial cause of pharyngitis and skin infections; adhere to the pharyngeal epithelium via lipoteichoic acid and M protein. Have a hyaluronic acid capsule which prevents phagocytosis. Growth is inhibited by bacitracin.

  • Group B streptococci (S. agalactiae): Colonize the genital tract and can cause neonatal meningitis and sepsis. They are usually bacitracin-resistant and hydrolyze hippurate.

  • Group D streptococci: Includes Enterococci (E. faecalis, E. faecium) and Nonenterococci (S. bovis).

    • Enterococci: Normal flora of the colon; hardy organisms that cause UTIs, biliary, and cardiovascular infections. Resistance to penicillin G requires synergistic treatment with an aminoglycoside (gentamicin). Vancomycin is useful, however, VREs have emerged, requiring the use of linezolid or daptomycin.

    • Nonenterococcal group D streptococci, such as S. bovis, can cause similar infections. Easily inhibited by 6.5% NaCl and killed by penicillin G.

  • Groups C, E, F, G, H, and K–U: Infrequently cause human disease.

• Non-β-Hemolytic Streptococci:

  • Some produce no hemolysis (γ-hemolysis), others produce α-hemolysis.

  • The principal α-hemolytic organisms are S. pneumoniae (pneumococci) and the viridans group of streptococci.

  • Distinguished in the clinical laboratory by two main criteria:

    • The growth of pneumococci is inhibited by optochin, whereas the growth of viridans streptococci is not inhibited.

    • Colonies of pneumococci dissolve when exposed to bile (bile-soluble), whereas colonies of viridans streptococci do not dissolve.

  • Viridans streptococci: Part of the normal flora of the human pharynx; S. mutans synthesizes polysaccharides (dextrans) found in dental plaque and cause dental caries. Streptococcus intermedius and Streptococcus anginosus (also known as the S. anginosus-milleri group) are found primarily in the mouth and colon.

• Peptostreptococci:

  • Grow under anaerobic or microaerophilic conditions and produce variable hemolysis.

  • Members of the normal flora of the gut, mouth, and female genital tract.

  • Participate in mixed anaerobic infections.

Transmission

• Most streptococci are part of the normal flora of the human throat, skin, and intestines. Disease happens when they gain access to tissues or blood.

• Viridans streptococci and S. pneumoniae are found mainly in the oropharynx; S. pyogenes in the skin and oropharynx, S. agalactiae in the vagina and colon; enterococci and anaerobic streptococci in the colon.

Pathogenesis

• Group A streptococci (S. pyogenes) cause disease by three mechanisms:

  1. Pyogenic inflammation: induced locally near organisms in tissue (i.e., an abscess).

  2. Exotoxin production: widespread systemic symptoms in areas of the body where there are no organisms.

  3. Immunologic: antibody against a component of the organism cross-reacts with normal tissue or forms immune complexes that damage normal tissue.

Table 15-4: Important Features of Pathogenesis by Streptococci

• M protein has antiphagocytic properties. Antibodies aren't made against hyaluronic acid capsule since it's generally tolerated by the body.

1. Hyaluronidase degrades hyaluronic acid in subcutaneous tissue, facilitating rapid spread in skin infections (cellulitis).

2. Streptokinase (fibrinolysin) activates plasminogen forming plasmin; used to lyse thrombi in heart attack patients.

3. DNase (streptodornase) degrades DNA in exudates or necrotic tissue. Used for diagnostic purposes

4. IgG degrading enzyme is a protease that specifically cleaves IgG heavy chains reducing opsonization and complement activation
   *Group A streptococci produce five toxins, and hemolysins, including:

5. Erythrogenic toxin - causes rash of scarlet fever - its mechanism of action is similar to that of the TSST of S. aureus (i.e., it acts as a superantigen).

6. Streptolysin 0 - a hemolysin causes β-hemolysis. It is antigenic.

7. Streptolysin S - hemolysin that is not inactivated by oxygen (oxygen-stable)

8. Pyrogenic exotoxin A - toxin responsible for most cases of streptococcal toxic shock syndrome.

9. Exotoxin B- is a protease that rapidly destroys tissue produced in large amounts by “flesh-eating” streptococci.
   *

• Pathogenesis by group B streptococci (S. agalactiae): Based on induction of inflammatory response.

• Group B streptococci have an antiphagocytic polysaccharide capsule, effective anticapsular antibody.

• Pathogenesis by S. pneumoniae and viridans streptococci is uncertain.

• S. pneumoniae's main virulence factor is its antiphagocytic polysaccharide capsule. Viridans streptococci produce a glycocalyx, that enables adherence to heart valves.

3Clinical Findings

• S. pyogenes causes:

  1. Pyogenic diseases: pharyngitis and cellulitis.

  2. Toxigenic diseases: scarlet fever and toxic shock syndrome.

  3. Immunologic diseases: rheumatic fever and acute glomerulonephritis (AGN).

Streptococcal pharyngitis (strep throat) is characterized by a sore throat, fever, headache, and purulent tonsils.

Erysipelas: An acute infection of the skin characterized by a raised, well-defined, intensely erythematous area with sharp borders. It often occurs on the face and is caused by S. pyogenes.

Cellulitis: A deeper infection of the skin than erysipelas, involving subcutaneous tissues. It presents as a red, swollen, painful area of skin without distinct borders. Cellulitis can be caused by various bacteria, including S. pyogenes and S. aureus.

Impetigo: A superficial skin infection characterized by vesicles, pustules, and crusted lesions. It is highly contagious and commonly occurs in children. S. pyogenes is one of the main causative agents of impetigo.

Necrotizing Fasciitis: A severe, rapidly progressing infection of the fascia (the tissue beneath the skin) that leads to tissue destruction and systemic toxicity. S. pyogenes is a common cause of necrotizing fasciitis, often referred to as "flesh-eating bacteria."

Scarlet Fever: A systemic illness that occurs as a result of S. pyogenes infection that produces erythrogenic toxin. Scarlet fever is characterized by a diffuse, erythematous rash that typically starts on the chest and spreads to the extremities. Other symptoms include a high fever, sore throat, and strawberry tongue (red and swollen tongue).

• Toxic Shock Syndrome: A severe condition associated with certain strains of S. pyogenes that produce exotoxins, leading to rapid onset of fever, rash, and multi-organ failure.

S. agalactiae:

• Neonatal sepsis, meningitis.

Enterococcus faecalis:

• Hospital-acquired UTIs, endocarditis.

Viridans group streptococci:

• Endocarditis.

Streptococcus bovis:

• Endocarditis.

Staphylococcus aureus: Kawasaki Disease: Vasculitis, fever, conjunctivitis, rash, cardiac aneurysm