gram-positive bacteria Flashcards
outline the structural difference between gram-positive or negative bacteria.
Gram-positive bacteria have a thick peptidoglycan cell wall that stains with crystal violet stain. Gram-negative bacteria do not have a thick peptidoglycan cell wall or stain with crystal violet stain but will stain with other stains
Describe how Gram-positive and negative pathogens are identified in the laboratory.
Initially, positively charged crystal violet is added to the cells, which binds to negatively charged cell components.
Iodine is then added, which forms large molecular complexes with crystal violet. This stains the cell blue/purple.
A decolouriser such as acetone or methanol is then added to attempt to remove these large complexes from the cell. If the cell wall has a thin layer of peptidoglycan, these complexes pass out through the cell wall, removing the blue colouration.
The cells are then stained red with safranin.
Gram-positive organisms have a thick cell wall of peptidoglycan and so retain the crystal violet stain when washed with acetone/methanol. When safranin is added, it is retained but obscured by crystal violet. Therefore these cells stain purple.
In contrast, gram-negative organisms have an outer lipopolysaccharide layer. When acetone is added these lipids dissolve, exposing the relatively thin peptidoglycan membrane. Crystal violent/iodine complexes are able to exit which decolourises the cell. Therefore when the red counterstain is added, gram-negative bacteria stain red.
what are the 2 main gram-positive bacteria?
Streptococci & Staphylococci
how does staphylococcus aureus spread and state is virulence factors?
Spread by aerosol and touch
carriers & shedders
Virulence factors
Pore-forming toxins (some strains)
a - haemolysin & Panton-Valentine Leucocidin
Proteases
Exfoliatin
Toxic Shock Syndrome toxin (stimulates cytokine release)
Protein A (surface protein which binds Ig’s in wrong orientation)
MRSA – resistant to
-lactams
gentamicin, erythromycin,
tetracycline
what conditions can staphylococcus aureus lead to.
Pyogenic:
Wound infections
Abscesses (boils, carbuncles)
Impetigo
Septicaemia
Osteomyelitis
Pneumonia
Endocarditis
Toxin mediated:
Scalded skin syndrome
Toxic shock syndrome
Food poisoning
if someone has coagulase-nehgative staph should you be worried.
no bc its on skin so would have been picked up during culture
if they have an artificial prosthetic or anything then yes becaus eit can form a biofilm
what are the 2 main coag-neg staph?
S.epidermidis
Infections in debilitated, prostheses (opportunistic)
Main virulence factor - ability to form persistent biofilms
S.saprophyticus
Acute cystitis
haemagglutinin for adhesion
urease
what classification are staphylococcus and streptococcus?
- cocci
- gram positive
- aerobic
when looking at bacteria on an agar plate, how can you tell if its staph or strep?
- chains = strep
- clusters = staph
what are the main types of staph which we encounter and how to we distinguish these during identification?
- we use to coagulase or DNase test to distinguish the 2
- positive reuslt = S. aureus
- negative result = coagulase negative staph
once strep has been identified, what further tests can we do to find out which type of strep it is?
- haemolysis on blood agar
- if its beta = beta haemolytic strep
- antigenic group A<B<C<G is then identified
- if its alpha = alpha haemolytic strep
- the optochin test can then be conducted
- if its resistant then its viridans strep, if its sensitive, then its S. pneumoniae
what are the results of a haemolysis test and why?
Alpha- haemolysis is due to the production of hydrogen peroxide, which reacts with haemoglobin to form the green compound met-haemoglobin
Beta-haemolysis is due to the production of two pore-forming toxins – streptolysin O and S. Streptolysin O is oxygen sensitive and is very antigenic
Gamma-haemolysis is a miss-noma and is an old term that still gets used in some texts
what is antigen / sero / lancefield grouping?
- classifying the bacteria by their carbohydrate cell surface antigens
- Antiserum to each group added to a suspension of bacteria
- clumping indicates recognition
what is the optochin test?
- used to differenciate S. pneumoniae from other alpha maemolytic strep
- based on their sensitivity to the chemical optochin
where is S. pneumoniae found, causes and predisposing factors.
Normal commensal in oro-pharynx ~ 30% of population
Causes - pneumonia, otitis media, sinusitis, meningitis
Predisposing factors
impaired mucus trapping (e.g. viral infection)
hypogammaglobulinaemia
asplenia
