Gram positive cocci Flashcards
Aerobic gram positive bacteria can be of 2 shapes, which are they?
Either bacilli or cocci
How are the gram-positive cocci further categorised?
Via the catalase test, they can either be catalase-positive, in which case they are the staphylococcus
Or they can be catalase-negative, in which case they are the streptococci
What is the catalase reaction?
catalase is an enzyme produced by the bacteria, and this enzyme catalyses the reaction of Hydrogen Peroxide to Water & Oxygen
H2O2 –>
Then, which test is used to differentiate amongst the bacteria within the staphylococcus?
The coagulase test is used
Coagulase positive = S. aureus
Coagulase negative = CoNS (coagulase negative staplococcus)
like the S. epidermidis
How is the coagulase test performed?
A loop of S. aureus is taken, it is inoculated in plasma and incubated
The S. aureus produces a coagulase enzyme which coagulates the plasma (it becomes gloopy)
In coagulase-negative staphylococci, the plasma is not coagulated and can be spread
Which organisms are the coagulase-negative staphylococci?
CoNS are usually found on humans, as members of the normal skin flora
These are commensal, beneficial organisms
Like the S. epidermidis
These have low virulence but can be pathogenic in some settings (eg. when the skin barrier has been breached, in immunocompromised individuals (premature kids)
How is the streptococcus differentiated?
By haemolysis into the alpha, beta and gamma groups
alpha = S. pneumoniae (viridans group)
beta = S. pyogenes and S. agalactiae
Classified by the way they haemolyse blood agar
Beta = blood is dissolved, agar is transparent
Alpha = partial haemolysis, agar has olive-green tinge
Gamma = no haemolysis
S. aureus - where does it infect?
how common/important is it?
It is an important human pathogen
20 - 40% of the population are carriers
It infects the anterior nares, axilla and groin regions
It likes warm, moist areas
What types of infections does S. aureus cause?
It can cause localised or disseminated infections
it is pyogenic (pus-forming), eg. abscess infections in almost any organ
An abscess is a central area of pus, which is walled off by fibrous material
The coagulase enzyme S. aureus produces assists in causing coagulation and fibrin around the site of infection, which walls off the centre of infection
The coagulase toxin walls off the infection, whilst lytic toxins like hemolysins and proteases destroy the host tissue
There is an invasion of neutrophils, hence, pus forms in the middle
There is a fibrous wall which surrounds it, by the coagulase enzyme which walls off the infection.
What types of infections does S. aureus cause?
It can cause localised or disseminated infections
it is pyogenic (pus-forming), eg. abscess infections in almost any organ
An abscess is a central area of pus, which is walled off by fibrous material
The coagulase enzyme S. aureus produces assists in causing coagulation and fibrin around the site of infection, which walls off the centre of infection
The coagulase toxin walls off the infection, whilst lytic toxins like hemolysins and proteases destroy the host tissue
There is an invasion of neutrophils, hence, pus forms in the middle
There is a fibrous wall which surrounds it, by the coagulase enzyme which walls off the infection.
S. aureus colonization vs disease
What determines whether S. aureus is simply going to colonise, or if it is going to proceed into an invasive disease?
Quorum sensing is chemical communication between groups of bacteria that is dependent on the surrounding environmental conditions, including bacterial density (number of bacteria at a site)
This communication influences bacterial behaviour, allowing bacteria to collaborate in response to environmental conditions
The quorum-sensing system in S. aureus works through the Agr (accessory gene regulator)
Agr contributes to S. aureus pathogenicity in several infection types
including subcutaneous abscesses, endocarditis and arthritis
Endocarditis is caused by bacteria in the bloodstream multiplying and spreading across the inner lining of your heart (endocardium).
What is the staphylococcal quorum-sensing system
The agr-sensing system is an auto-activating circuit which works through cell signalling
When the population density is low, the expression of virulence factor genes is repressed, but when the population density increases, the quorum-sensing signal transduction represses the repressor of the virulence factor genes, leading to increased expression of virulence factors, and an increase in infection and pathogenicity.
Each bacterial cell secretes a small protein called AIP
Each bacterial cell also expresses the receptor for the AIP molecule on its cell membrane called AgrC
AIP binds to Agr C and the receptor phosphorylates and activates an intracellular molecule called AgrA, which is a transcription factor, which when activated, induces the expression of the quorum-sensing system in a positive feedback loop (it activates transcription of P2 and P3 (Agr promoters, which drive the autoactivation circuit)
P3 drives the transcription of RNAIII, which enhances the promotion of virulence factors (increase in exotoxin production)
AgrA represses the repressor of virulence factor genes (self-activating), positive feedback loop, strengthens the quorum sensing mechanism itself and amplifies the response
Also activates the expression of genes that encode the individual components of the quorum-sensing mechanism
AgrD encodes the AIP peptide
AgrB encodes the transport protein responsible for AIP secretion
AgrC which is the receptor, that gets activated by AIP binding to it
And the gene encoding AgrA
Staphlococcus aureus infections
S. aureus is a frequent cause of skin infections
like:
folliculitis furuncles (boils) carbuncles impetigo cellulitis wound infections
Other localised infections include lung abscess, brain abscess and mastitis (infection of the breast tissue in breastfeeding mothers –> often leading to premature cessation of breast feeding)
May also cause disseminated infection - including septicaemia
Where S. aureus circulates in the bloodstream
May progress to infection of a heart valve (endocarditis) which can be very destructive
S. aureus produces particular toxins (superantigens) which cause disease…
Causes non-pyogenic (non pus-forming disease) disease caused by toxins
Like SSS staphylococcal scalded skin syndrome
STSS staphylococcus toxic shock syndrome
Staphylococcal food poisoning
These diseases are caused by an enterotoxin, which is a superantigen (this gene originated in a bacteriophage, and was genetically transferred via transduction (horizontal gene transfer)
Superantigens bypass the normal antigen presentation, by directly crosslinking the T cell receptor and the MHC class II
this causes polyclonal activation of T cells hence, stimulation of a very strong and non-specific immune response
The toxin acts on the basal layer of the epidermis, and causes lysis
The superficial layers of the skin all shed off
How are the beta haemolytic streptococci further classified?
According to antigenic differences in their cell wall carbohydrates
20 different groups in the lancefield grouping system
Lancefield group A = Strep A
AKA Streptococcus pyogenes
Lancefield group B = Strep B
AKA Streptococcus agalactiae
