Isolation and identification of Staphylococcus and Streptococcus species

Question 1

Staphylococci

Answer 1

• Gram-positive, spherical, irregular clusters
• Non-spore-forming
• Look like a “bunch of grapes”

Question 2

Staphyloccoci

Answer 2

• Normal Flora (nasal membranes, hair follicles, skin, & perineum
• Opportunistic pathogen (skin lesions, osteomyelitis, pneumonia, endocarditis)
• Nosocomial: Hosital-acquired

-Methicillin-resistant Staphylococcus aureus (MRSA) (Oxacillin, penicillin, amoxicillin, hospial- acquired, community-acquired)

Question 3

Staphylococci – Virulence Factors

Answer 3

• Highly resistant to
• Drying
• Extremes in pH
• High temperatures
• Toxins and enzymes
• Promote invasion and growth
• S. aureus – principal pathogen
• S. epidermidis – nosocomial infection in immunocompromised
• S. saprophyticus – urinary tract infections

Question 4

Streptococci

Answer 4

• Gram-positive, spherical, chains
• Non-spore-forming
• Lack catalase
  _ Degrades hydrogen peroxide
  to form water and oxygen

Question 5

Streptococci

Answer 5

• Strep throat (pharyngitis) – S. pyogenes
• Pneumonia, meningitis, endocarditis, erysipelas, necrotizing fasciitis
• Normal flora: (Commensals, Mouth, skin, upper respiratory tract)

Question 6

Streptococci - Classification

Answer 6

Hemolysis: Hemolysins: exoenzymes that lyse RBCs and degrade hemoglobin

Carbohydrate antigens: But not all have the carb Ags for classification

Identification
* Hemolysis
* Biochemical and physiological characteristics
* Lancefield identification via immunological techniques( Agglutination – serological tests, Group A)

Question 7

Lancefield Classification

Answer 7

-Group A
* strep throat, rheumatic fever, scarlet fever, necrotizing fasciitis
* S. pyogenes

-Group B
* meningitis in newborns and elderly, female reproductive tract
* S. agalactiae

• Group C
• mammal infections
• S. equi, S. equisimillis, and S. zooepidemicus

-Group D
* Reclassified into Enterococcus
* E. faecalis and E. faecium – large intestine

Question 8

Other medically important strep

Answer 8

-S. pneumoniae
* Alpha-hemolytic
* Pneumonia, meningitis, otitis media

-Viridans streptococci
* Mouth
* Tooth abscesses, meningitis, endocarditis

Question 9

Mannitol Salt Agar (MSA)

Answer 9

• Selective & Differential
• Mannitol & 7.5% NaCl
• pH indicator phenol red
• Staph only
• Fermentation of mannitol by
  certain staph produces acid
• S. aureus
  -strains of S. saprophyticus

Question 10

Streptococci: alpha-hemolysis

Answer 10

• Incomplete/Partial hemolysis
• Greenish/Brownish
• Reduction of hemoglobin

Question 11

Streptococci: beta-hemolysis

Answer 11

• Complete/true hemolysis of RBCs
• Clear zone
• Exotoxins
  -Interacts with cholesterol in membranes – beta barrel
  -Streptolysin O – lower oxygen concentration
  -Streptolysis S – ambient oxygen concentrations

Question 12

Streptococci: gamma-hemolysis

Answer 12

• Absence of hemolysis
• No color change

Question 13

Blood Agar

Answer 13

• Alpha-hemolysis (partial)
• Beta-hemolysis (complete)
• Gamma-hemolysis (none