Bacterial Diseases: Cocci of Medical Importance

Bacterial Diseases: Cocci of Medical Importance

1. Gram Positive Cocci

A. Staphylococci

  • General Information

    • Species: Over 30 species recognized, commonly found on the skin and mucous membranes of warm-blooded animals, including humans.

    • Characteristics: Gram-positive cocci in grape-like clusters; facultative anaerobes; catalase positive; resistant to high salt concentrations (7.5 - 10%), extremes in pH, drying, and withstands high temperatures (60 °C for 60 minutes).

    • Key Pathogen: Staphylococcus aureus is the most significant human pathogen in this group.

B. Virulence Factors

  • Coagulase: Causes coagulation of blood plasma to form a layer of fibrin around cells, shielding them from phagocytosis. Present in S. aureus and S. intermedius (pathogen in dogs).

  • Staphylokinase: Activates plasminogen to plasmin, which digests fibrin in clots.

  • Hyaluronidase: Digests hyaluronic acid in connective tissue, allowing for tissue invasion.

  • Nucleases: Digest DNA and RNA.

  • Lipase: Facilitates colonization of oily skin surfaces.

  • Penicillinase: Destroys penicillin, contributing to antibiotic resistance.

  • Hemolysins: Lyse red blood cells (RBCs).

  • Leukocidin: Lyse white blood cells (WBCs).

  • Enterotoxins: Damage intestinal epithelial cells, causing secretory diarrhea due to loss of water and electrolytes.

  • Exfoliative Toxin: Causes the loss of the epidermal layer of skin.

  • Protein A: Binds antibody molecules (IgG) non-specifically, incapacitating them.

  • Toxic Shock Syndrome Toxin: Some strains produce a toxin that enters the blood, leading to fever, rash, vomiting, liver damage, and renal failure.

C. Types of Disease

  1. Cutaneous Infections: Associated with skin, hair, or nails (e.g., boils, furuncles, carbuncles).

    • Superficial infections like staphylococcal impetigo may lead to scalded skin syndrome.

  2. Systemic Infections: Can spread from a skin infection to blood (bacteremia) and then to various organs, including bones (osteomyelitis), heart (endocarditis), lungs (pneumonia), CNS (meningitis), etc.

  3. Food Poisoning: Commonly contaminates foods like custards and processed meats; heat-resistant enterotoxins can cause cramping, diarrhea, and nausea. Symptoms manifest within 2-6 hours.

  4. Immunization and Chemotherapy: Currently, no vaccine available. Most strains are resistant to penicillin (95%) due to penicillinase; methicillin was once effective but MRSA (methicillin-resistant S. aureus) is now prevalent. Vancomycin is typically the effective treatment.

D. Other Pathogenic Staphylococci

  • Coagulase Negative Staphylococci: Species like S. epidermidis and S. saprophyticus can cause disease in immunocompromised individuals, especially in nosocomial settings.

2. Streptococcus

A. Characteristics

  • Morphology: Gram-positive cocci typically found in chains (or pairs); facultative anaerobes; catalase negative, with peroxidase to inactivate H2O2.

  • Classification: Separated based on serological differences and type of hemolysis (Beta-hemolytic vs Alpha-hemolytic).

B. Key Pathogen

  • S. pyogenes: Most serious pathogen in group A.

    • Location: Found in throat, nasopharynx, and sometimes skin of humans.

C. Virulence Factors

  • Capsule: Protects against phagocytosis and aids in adherence.

  • M Protein: Provides resistance to phagocytosis and enhances adherence.

  • Hemolysins: Damage RBCs, WBCs, liver, and heart muscle.

  • Erythrogenic Toxin: Associated with scarlet fever, produces red rash and fever.

  • Streptokinase: Activates plasminogen to plasmin, digesting fibrin in clots.

  • Hyaluronidase: Facilitates tissue invasion by digesting connective tissue.

  • Nucleases: Digest DNA and RNA.

D. Types of Disease

  1. Cutaneous Infections:

    • Streptococcal Impetigo: Superficial skin condition.

    • Erysipelas: A deeper skin infection.

    • Necrotizing Fasciitis: A severe, invasive infection destroying skin and muscle tissues.

  2. Throat Infections:

    • Streptococcal Pharyngitis: Common in children, can cause tonsillitis.

    • Scarlet Fever: If lysogenized strains are involved, can cause systemic symptoms with rash.

  3. Systemic Infections:

    • Includes septicemia, pneumonia, toxic shock, and puerperal fever, which can be fatal if uncontrolled.

E. Complications

  • Rheumatic Fever & Acute Glomerulonephritis: Autoimmune responses triggered by similarities between M protein and human tissues, leading to inflammation and potential damage, particularly to the heart and kidneys.

F. Other Pathogenic Streptococci

  • S. agalactiae: Opportunistic infections in immunocompromised, can cause wound, skin, and neonatal infections.

  • Enterococcus faecalis: Causes infections of the urinary tract and wounds.

  • Viridans Group: Causes dental caries and endocarditis, typically originating from normal flora in the mouth.

  • S. pneumoniae: Major cause of bacterial pneumonia.

    • Immunization: Effective vaccines available, especially for vulnerable populations.

3. Gram-Negative Cocci: Neisseria

A. Characteristics

  • Structure: Gram-negative diplococci; pathogenic strains are usually encapsulated and require enriched media for growth.

B. Virulence Factors

  • Fimbriae and Capsules: Assist in adherence and resistance to phagocytosis.

  • LPS (Endotoxin): Causes severe inflammatory responses upon release.

  • Protease: Destroys secretory antibodies on mucous membranes.

  • Penicillinase: Destroys penicillin, complicating treatment efforts.

C. Key Pathogens

  1. N. gonorrhoeae: Transmitted primarily through sexual contact; causes urethritis, vaginitis, arthritis, and can lead to systemic infections.

    • Ophthalmia Neonatorum: Can cause blindness in newborns; treatment involves administering prophylactic eyedrops.

    • Immunization: Neither vaccine nor lasting immunity exists.

    • Chemotherapy: Combination of antibiotics may be needed as resistance is common.

  2. N. meningitidis: Causes meningococcal meningitis, primarily through respiratory droplets.

    • Symptoms can escalate quickly; treatment requires antibiotics and vaccination during outbreaks.

4. Gram-Positive Bacilli

A. Endospore Formers

  1. Aerobic - Bacillus:

    • Pathogens: B. anthracis (causes anthrax) and B. cereus (causes food poisoning).

  2. B. anthracis:

    • Virulence Factors: Dumbbell-shaped polypeptide capsule and exotoxins.

    • Types of Anthrax:

      • Gastrointestinal: Rare and caused by contaminated meat.

      • Cutaneous: Localized skin infection leads to necrosis.

      • Pulmonary: Inhalation of spores leads to severe, often fatal illness.

    • Immunization: Live spore vaccine for at-risk individuals; treatment includes antibiotics (penicillins, tetracyclines).

  3. B. cereus: Similar to S. aureus in causing food poisoning via enterotoxins produced in food.

B. Anaerobic - Clostridium

  1. General Characteristics: Gram-positive, catalase-negative, endospore-forming rods.

  2. Pathogenic Species:

    • Cl. perfringens: Causes gas gangrene and food intoxications, damaging tissues with exotoxins.

    • Cl. difficile: Causes antibiotic-associated colitis; can lead to severe inflammation and may require cessation of antibiotic therapy.

    • Cl. tetani: Causes tetanus; results in severe muscle contractions due to neurotoxin.

      • Immunization: DTP vaccine for prevention, with boosters recommended.

5. Non-Endospore Formers

A. Listeria monocytogenes

  • Characteristics: Short to long rods, motile; causes listeriosis from contaminated food, particularly dairy.

  • Symptoms: In healthy adults, mild symptoms; severe for immunosuppressed and pregnant women.

  • Treatment: Antibiotics typically include ampicillin.

B. Corynebacterium diphtheriae

  • Characteristics: Pleomorphic rods, often in palisades; causes diphtheria.

  • Symptoms: Localized throat infection with potential systemic effects from produced toxins.

  • Treatment: Antitoxin and antibiotics; vaccination via DTP.

C. Mycobacterium

  1. M. tuberculosis: Causes tuberculosis, primarily via respiratory droplets.

    • Symptoms: Chronic coughing, potential systemic spread.

    • Immunization: BCG vaccine is common in some regions but not widespread in the US.

  2. M. leprae: Causes leprosy; primarily spreads through skin contact.

    • Symptoms: Vary from mild to progressive forms.

    • Treatment: Long-term chemotherapeutic agents used for management.

6. Gram-Negative Bacilli

//General Note: Endotoxic nature of Gram-negative bacteria contributes to inflammatory responses, and pathogenesis often involves endotoxin release upon lysis.**//

A. Pseudomonas

  • General Information: Soil organisms; major opportunistic pathogens in hospital settings.

  • Pathogen: P. aeruginosa; resistant to several disinfectants and causes severe infections especially in immunocompromised individuals.

B. Brucella

  • Disease: Causes brucellosis primarily associated with livestock exposure; presents with prolonged fever symptoms.

C. Francisella tularensis

  • Cause: Tularemia transmitted by vectors or direct contact with infected animals.

D. Bordetella pertussis

  • Cause of Whooping Cough; vaccine is part of DTaP.

E. Legionella pneumophila

  • Causes Legionnaires’ disease; acquired through aerosolized water.

F. Yersinia pestis

  • Causes Plague; three forms: bubonic, septicemic, and pneumonic.

G. Haemophilus

  • Haemophilus influenzae: Causes bacterial meningitis; vaccines are available.

H. Enteric Bacteria

  • Group Characteristics: Includes E. coli and Salmonella; common causes of gastroenteritis and foodborne illnesses.

  • E. coli: Multiple pathogenic strains exist, responsible for causing various intestinal and systemic infections.

  • Salmonella: Important in public health, especially S. typhi causing typhoid fever.

  • Shigella: Causes bacillary dysentery.

7. Miscellaneous Bacterial Pathogens

A. Spirochetes

  1. Treponema pallidum: Causative agent of syphilis, transmitted sexually and via congenital routes.

  2. Leptospira interrogans: Causes leptospirosis; primarily acquired through contact with infected animal urine.

  3. Borrelia burgdorferi: Causes Lyme disease; transmitted through tick bites.

    • Symptoms: Characteristic bull's-eye rash; can lead to severe complications if untreated.

B. Vibrio cholerae

  • Cause of Cholera: Transmitted through contaminated water; produces potent toxin leading to secretory diarrhea; treatment includes fluid replacement and antibiotics.

C. Rickettsias and Chlamydias

  1. Rickettsia rickettsii: Causes Rocky Mountain spotted fever transmitted by tick bites.

  2. Chlamydia trachomatis: Major cause of STDs and eye infections; effective treatment with tetracycline.

D. Mycoplasma

  • Mycoplasma pneumoniae: Causes atypical pneumonia; treatment must involve antibiotics that inhibit protein synthesis.