Week 4- The Streptococci and Enterococci Flashcards
how many genera of medical importance is there In the family of streptococcaceae
two:
- streptococcus
- enterococcus
how is gram positive cocci (GPC) often arranged
in chains or pairs
what are the characteristics of streptococci
non-motile, non-sporing, fastidious facultative anaerobe
how many species of streptococci is there
less than 80
what kind of organisms are streptococci
comensal organisms
(typically of the mucous membranes of upper respiratory tract, and some species in the intestines)
how is streptococci separated into groups
- by colony appearance on horse blood agar (BA)
- by lancefield grouping
what streptococci colonies are there
- Alpha (α) haemolytic streptococci
- Beta (β) haemolytic streptococci
- non haemolytic streptococci
what are Alpha (α) haemolytic streptococci
S. pneumoniae and ‘viridans’ group streptococci (VGS)
what is Alpha (α) haemolytic streptococci
- Colonies greenish appearance, approx. 1 mm diameter
(Oxidation of haemoglobin to methaemoglobin = green) - No actual haemolysis
what do VGS tend to be
commensal, opportunistic pathogens of low virulence
what is Beta (β) haemolytic streptococci
- Complete lysis of RBC
- Zone of clearing around the colony
- Colonies approx. 1 mm diameter, no pigmentation
- GPC in chains
- catalase negative reaction
what is non-haemolytic streptococci
No effect on RBC
(also known as gamma (γ) haemolysis)
who discovered lancefield grouping
Rebecca Lancefield (1895-1981)
what is lancefield grouping
Separated streptococci on their version of a major cell wall polysaccharide antigen
what group is lancefield group clinically applied to
β-haemolytic streptococci
how do enterococcal colonies appear on BA
α-haemolytic, β-haemolytic streptococci and non-haemolytic
what catalase is enterococcal colonies
catalase negative
what antigen is used for enterococcal colonies in lancefield grouping
D antigen
what gram is Presumptive Streptococcus species
Gram positive cocci
what catalase is Presumptive Streptococcus species
catalase negative
what should a catalase test be performed for
for gram positive bacteria
how do you perform a catalase test
- Dispense 50μl of hydrogen peroxide into a bijou
- Transfer one colony of the test strain to the opposite side of the bijou
- Cap the bijou
- Tip the hydrogen peroxide over the colony and observe
- Bubbles of oxygen indicate a positive reaction, no bubbles a negative reaction
what does group specific antibody conjugated to latex beads react with
with cell wall polysaccharide
what groups are of the greatest medical importance
A, B, C, D, F, and G
what is group A β - haemolytic streptococci are sensitive to
bacitracin
what’s resistant to bacitracin
other β - haemolytic streptococci
what is bacitracin often placed on
primary plates
what lancefield groups are the MOST important medically
A, B and D
what is group A
S. pyogenes
sensitive to bacitracin
what is group B
S. agalactiae
what does group D include
S. bovis
what is viridans group streptococci (VGS)
gram positive cocci in chains or pairs
what is the appearance of VGS on BA
colonies are typically greenish, smooth, circular, approx. 1 mm
how is VGS and S. pneumoniae distinguished
optochin sensitivity
what is optochin sensitivity
- viridans group streptococci are resistance
- s. pneumoniae are sensitive
what is s. pneumoniae
- gram-positive diplococcus
- α-haemolysis on blood agar
- polysaccharide capsule
- quelling reaction
- optochin sensitivity
- bile solubility
what is quelling reaction
- Capsule – specific antibody reaction causes the capsule to appear to swell
how can u distinguish s. pneumoniae and VGS with bile solubility
- S. pneumoniae are bile soluble
- VGS are bile insoluble
What is the analytical profile index (API)?
It is used to identify based on biochemical tests
API test strips consist if wells that contain substrates to detect enzymatic activity
What is streptococcus pyogenes aka?
Group A strep
Describe what group A strep is
it is a commensal of the nasopharynx
Transmits via direct contact with infected skin lesions and respiratory droplets
What are the virulence factors of group A strep - S pyogenes?
- Cell surface proteins/adhesins
- Toxins
- capsule
- cell envelope
- enzymes
Describe the cell surface proteins/adhesins of S pyogenes (group A strep)
M protein - Binds to many host proteins and resists phagocytosis so can avoid immune response.
F protein -
Describe the capsule of S pyogenes?
Composed of hyaluronic acid and are antiphagocytic, non immunigenic
Describe the enzymes of S pyogenes?
Streptolysins - They lyse (break down) RBCS, WBCS, platelets
C5a peptidase - Cleaves human C5a (which attracts phagocytic cells) to evade phagocytosis
Streptokinase - It aids the spread of infection through tissues by binding plasminogen activating plasmin to break down fibrin.
Describe the different toxins of S pyogenes?
Pyrogenic exotoxins - Thye generate fever, consists of SpeA, SpeB, SpeC
SpeA - Its antigen activity results in a huge cytokine release which results on inflammation, shock, organ failure
SpeB - associated with causing necrotising fascitis.
What are the different types of disease caused by S pyogenes?
Superficial infection
Deep seated infections
Toxin mediated disease
Immune mediated disease
Describe S pyogenes superficial infections
Causes infections such as Pharyngitis, Impetigo, erysipelas
What is pharyngitis?
Symptoms such as inflamed pharyngeal membrane, swelling of uvula
treated with penicillin
can lead to scarlet fever.
Describe S pyogenes deep seated infections
These are life threatening invasive infections such as Cellulitis, puerperal fever, necrotising fasciitis
Describe S pyogenes toxin mediated disease aka scarlet fever?
This is where the toxin SpeA (causes huge cytokine release) spreads throughout the body which causes a rash spreading to most of the body
White strawberry tonguw=e which turns into a red strawberry tongue
Can lead to cellulitis, sepsis
Describe S pyogenes immune mediated disease aka rheumatic fever?
This is where the antibody against the M protein of S pyogenes cross reacts with human tissue causing inflammation, rash, carditis etc
What is streptococcus agalactiae aka?
Group B strep
Describe streptococcus agalactiae (Group B strep)
It is a commensal of the colon and in the female genital tract
If a pregnant woman has group B strep whilst pregnant, what can it lead to?
Premature labour
Neonatal sepsis, meningitis
Describe S pneumoniae?
Found in the nasopharynx/upper respiratory tract.
Transmitted by direct contact, respiratory droplets
What are the virulence factors of S pneumoniae?
Toxins - pneumolysin
Polysaccharide capsule
Describe the toxin of S pneumoniae?
The toxin is pneumolysin - it generates pores in eukaryotic cell membranes, it activates the classical pathway leading to cytokine production and inflammation.
Describe the polysaccharide capsule of S pneumoniae?
It is antiphagocytic and is essential for causing invasive infections so is used to avoid the immune response
List the different S pneumoniae infections
Non invasive - Otitis media, sinusitis
Invasive Pneumococcal disease (IPD) - Pneumonia, sepsis, meningitis
Describe pneumonia
This is where the alveoli in the lungs become filled with pus and fluid resulting in painful breathing and limited oxygen intake.
Symptoms consist of: Cough, purulent sputum, pyrexia, consolidation of lung.
What is Viridans Group Streptococci (VGS)?
A type of bacteria that has low pathogenic potential because of very few virulence factors, has many species and is susceptible to most antibiotics
How do VSG species enter into the body?
Via the oral cavity and gut, they enter into the bloodstream
How are enterococci bacteria identified?
Enterococci are:
Catalase negative
Bile insoluble
Have brown/black colonies on aesculin agar
Is a lance field group D organism
Describe an enterococcal infection
It is a commensal of the intestinal tract.
Can cause endocarditis, abdominal wound infections etc
Resistant to penicillin, but sensitive to amoxycillin
