Gram Positive/Negative

Chapter 18: The Gram-Positive and Gram-Negative Cocci of Medical Importance

Overview

  • Pathogens categorized as coccus-shaped bacteria.

  • Distinguished based on:

    • Staining properties (Gram positive vs. Gram negative)

    • Arrangement (clusters vs. chains)

    • Biochemical properties

Major Groups of Cocci

  1. Gram Positive Cocci

    • Two major genera:

      • Staphylococcus

      • Streptococcus

    • Characteristics:

      • Lack spores

      • Non-motile

      • Capsules may be present, depending on species

    • Arrangement:

      • Staphylococcus: cluster arrangement

      • Streptococcus: chain arrangement

    • Biochemical test:

      • Staphylococcus: catalase positive

      • Streptococcus: catalase negative

  2. Gram Negative Cocci

    • Neisseria genus distinguished by different gram staining patterns.

    • Gram negative, typically found in a diplococci arrangement, and catalase positive.

Staphylococcus Group

  • Of the 40 species of Staphylococcus, only five are noted as significant human pathogens:

    • Staphylococcus aureus

    • Staphylococcus epidermidis

    • Staphylococcus saprophyticus

Biochemical Testing

  • Coagulase Test

    • Differentiates between Staphylococcus species.

    • Coagulase: an enzyme stimulating blood clotting.

    • S. aureus: coagulase positive

    • S. epidermidis and S. saprophyticus: coagulase negative

Importance

  • Responsible for ~20,000 deaths annually, frequent causes of healthcare-associated infections.

Staphylococcus aureus Characteristics

  • Gram positive, circular cells in cluster arrangement.

  • Catalase positive, coagulase positive.

  • Beta-hemolytic.

  • Non-spore forming, non-flagellated.

Survival and Spread

  • Can withstand high salt, pH extremes, and high temperatures.

  • Resistant to many disinfectants; can survive in environments for months.

  • Colonizes skin and mucous membranes shortly after birth.

  • Approximately 20% of people carry this bacterium in their anterior nares.

Infection Pathways

  • Spread through poor hygiene, nutritional issues, tissue injury, or reduced immune function.

Staphylococcus aureus Virulence Factors

  • Capsule: Enables biofilm formation on medical devices and in the body, increasing resistance to antibiotics.

  • Enzymes:

    • Coagulase: differentiation tool; present in most strains.

    • Hyaluronidase: facilitates skin invasion by degrading epithelial cell cement.

    • Penicillinase: inactivates penicillin resulting in drug resistance.

Exotoxins

  • Hemolysins: Lyse red blood cells, damaging organs like heart and kidney.

  • Leucocidin: Lyse neutrophils and macrophages, leading to pus formation.

  • Enterotoxin: Found in food contamination, causes gastrointestinal distress.

  • Exfoliative Toxin: Separates epidermis from dermis.

  • Toxic Shock Syndrome Toxin (TSST): Initiates a strong immune response leading to septic events.

Staphylococcal Infections

  • Localized Infections: Often cutaneous, gained through wounds, follicles, or glands.

    • Folliculitis: Superficial hair follicle inflammation, usually resolves without complications.

    • Furuncle: Deeper inflammation of hair follicles; can progress to abscess/pustule.

    • Carbuncle: Cluster of interconnected furuncles; deeper lesions causing potential complications.

    • Impetigo: Bubbling skin that breaks; most common in newborns, can crust over.

Systemic Infections

  • Pneumonia: Risk increases in children with cystic fibrosis; aspiration into lungs.

  • Osteomyelitis: Bacteria settle in bones causing severe symptoms.

  • Endocarditis: Inflammation of heart lining; rapid damage and possible death.

  • Bacteremia: Bacteremia from localized skin infections; associated with medical devices leading to septicemia.

Toxin-Induced Infections

  • Staphylococcal Food Poisoning: Occurs when enterotoxins are ingested; common culprits include custards, meats, etc.

  • Staphylococcal Scalded Skin Syndrome (SSSS): Mainly affects newborns with skin peeling symptoms.

  • Toxic Shock Syndrome: Symptoms: fever, low blood pressure, vomiting, and possibly death.

Methicillin-resistant Staphylococcus aureus (MRSA)

  • Resistant to multiple antibiotics; presents as hospital-acquired (HA-MRSA) and community-acquired (CA-MRSA).

  • CDC estimates 11,500 deaths annually from MRSA.

  • Vancomycin: Traditionally the drug of choice against MRSA but now facing resistance with vancomycin-resistant strains (VRSA).

Other Staphylococci

Staphylococcus epidermidis

  • Normal skin microbiota; opportunistic infections in healthcare settings due to skin transfer.

  • Forms biofilms on devices leading to endocarditis, urinary tract infections, and bacteremia.

Staphylococcus saprophyticus

  • Present in sexually active young women; second most common cause of UTIs in this demographic.

  • Requires antibiotic susceptibility testing due to increased resistance.

Streptococcus Group

Characteristics

  • Gram-positive, circular to ovoid shaped cells forming chains.

  • Catalase negative, do not form spores, non-flagellated.

  • Some can form capsules/slime layers.

Classification Methods

  • Lancefield Classification: Based on carbohydrates in the cell wall, labeled by letter.

  • Hemolytic Patterns:

    • Beta-hemolytic: White colonies with cleared zones.

    • Alpha-hemolytic: Greenish coloration around colonies.

  • Growth requires enriched medium due to fastidiousness.

Beta-Hemolytic Streptococci

  • Important species include:

    • Streptococcus pyogenes (Group A)

    • Enterococcus faecalis (Group D)

    • Streptococcus agalactiae (Group B)

Streptococcus pyogenes

  • Reservoir: Throat and nose; 5-15% carry virulent strains.

  • Transmission: Direct contact, respiratory droplets, food, fomites.

Virulence Factors

  • Capsule: Biofilm formation.

  • Cell Surface Antigens: Help attachment and immune evasion.

  • Exotoxins:

    • Streptolysin: Cytotoxic effects.

    • Erythrogenic Toxin: Causes fever and the rash of scarlet fever.

  • Exoenzymes:

    • Hyaluronidase: Tissue invasion facilitation.

Streptococcus pyogenes Infections

  • Localized Diseases:

    • Impetigo: Highly contagious, symptoms of burning/itching.

    • Erysipelas: Deeper skin infection with fever and pain.

    • Necrotizing fasciitis (“flesh-eating disease”): Rapidly spreading severe infection causing severe tissue damage.

    • Strep Throat (Pharyngitis): Identified via rapid tests, usually treated with penicillin.

Complications

  • Scarlet Fever: Systemic reaction to erythrogenic toxin.

  • Sequelae:

    • Rheumatic Fever: Can lead to heart damage.

    • Acute Glomerulonephritis: Inflammation of the kidneys impacting filtration.

Other Beta-Hemolytic Streptococci

  • Streptococcus agalactiae (GBS): Can cause neonatal infections; pregnant women screened for prevention.

  • Enterococcus faecalis (GDS): Opportunistic infections in immunocompromised individuals; associated with VRE.

Alpha-Hemolytic Streptococci

  • Viridans Group: Includes Streptococcus mutans and S. sanguinis.

    • Streptococcus mutans: Causes tooth decay; produces slime layers.

    • Streptococcus sanguinis: Can cause endocarditis in individuals with preexisting heart conditions.

  • Streptococcus pneumoniae: Causes pneumonia, meningitis; virulent strains possess a capsule.

Diseases Associated with Streptococcus pneumoniae

  • Commonly remembered by acronym MOPS:

    • Meningitis: Fever, stiff neck, disorientation.

    • Otitis Media: Ear infections, especially in young children.

    • Pneumonia: Major cause of bacterial pneumonia.

    • Sinusitis: Inflammation of sinuses with similar symptoms to colds.

Treatment and Prevention

  • Streptococcus pneumoniae: Treatment involves antibiotics, though drug resistance is an emerging issue.

  • Vaccination: Pneumovax and Prevnar 13 vaccines recommended for certain populations.

Gram Negative Cocci

Genus Neisseria

  • Characterized by:

    • Gram negative, typically diplococci arrangement.

    • Normal residents of mucous membranes; short survival outside the host.

    • Catalase positive.

Important Pathogens

  1. Neisseria gonorrhoeae (Gonococcus)

    • Causes gonorrhea; prevalent among 15-24-year-olds.

    • Symptoms in males: urethritis, painful urination, yellowish discharge; scarring and infertility risks.

    • Symptoms in females: painful urination, vaginal discharge; complications lead to PID and infertility.

    • Transmitted during vaginal birth; newborns at risk for gonococcal eye infection.

    • Diagnosis via gram stain indicating gram-negative diplococci; treatment constrained by antibiotic resistance.

  2. Neisseria meningitidis (Meningococcus)

    • Second leading cause of bacterial meningitis in the U.S.

    • Asymptomatic carriers in 10% of the population.

    • Symptoms of meningococcemia: fever, sore throat, stiff neck, potentially leading to shock and death.

    • Treatment involves rapid antibiotics; preventive vaccination recommended for preteens.

Acinetobacter

  • Acinetobacter baumannii: Known for survival on fomites causing nosocomial infections.

    • Issues with multidrug resistance; outbreaks traced back to combat injuries.

Note: The virulence factors, infections, symptoms, and treatment options presented highlight the critical nature of understanding gram-positive and gram-negative cocci in medical microbiology.