Classifications of Pathogenic Bacteria: Gram Positive Flashcards
(53 cards)
What does it mean for a bacteria to be gram positive?
The cell wall of the bacteria has a thick layer of peptidoglycan
What is peptidoglycan?
Sugar network made from two major sugars:
N-acetylmuramic acid
N-acetylglucosamine
Why is peptidoglycan very important?
Forms the basis of gram staining - a method used to classify bacteria
What colour do gram positive bacteria appear under the microscope?
Purple
What are the five steps to gram staining?f
Applying a primary stain (crystal violet).
Adding a mordant (Gram’s iodine).
Rapid decolorization with ethanol, acetone or a mixture of both.
Counterstaining with safranin.
Dry slides and view under microscope
Why do gram positive bacteria stain purple?
The crystal violet is able to form complexes within the peptidoglycan, so the stain is retained.
What is another way bacteria can be classified?
Aerobic - grows in presence of oxygen
Anaerobic - grows in absence of oxygen
What are the main shapes a bacteria can take?
Cocci - round
Bacilli - rods
What are the two major forms of cocci bacteria?
Streptococci - in chains
Staphylococci - in clusters
How can streptococci be further divided up?
Appearance on blood agar
Alpha - haemolytic - partial destruction of surrounding red blood cells - turn blood agar green
β -haemolytic - compete destruction of surrounding red blood cells - turn blood agar clear
Non-haemolytic - no effect and no difference on blood agar
How can staphylococci be further divided up?
Whether they produce the enzyme coagulates or not
Coagulase +ve - tend to be pathogens or disease forming organisms eg Straph Aureus
Coagulase -ve - tend to be less pathogenic
How can bacilli be further divided up?
By size
Small
Large
How can anaerobic bacilli be divided up?
Most gram +ve anaerobic are bacilli eg clostridium group - C. diff.
What is a gram positive bacteria that effects the respiratory tract?
Streptococcus pneumoniae
Alpha-haemolytic streptococci
What is an example of a coagulase +ve gram +ve bacteria and what can it be broken up into?
Staphylococcus aureus
- MSSA
- MSRA
Where is staphylococcus aureus found and what can it cause?
Found: nose, axilla, pernieum
A major human pathogen that can cause wide range of diseases from boils/abscesses and soft tissue infections to septicaemia and osteymyelitis (inflammation of bone) .
When would you decolonise staphylococcus aureus?
Only in specific circumstances such as going into surgery, as it can be harmless when just colonising
What are typical anti-microbial susceptibility patterns of staphylococcus aureus?
Commonly penicillin resistant due to production of enzyme penicillinase - destroys the beta lactam ring and renders penicillin ineffective.
Some strains are methicillin resistant - synthetic penicillin
What are examples of coagulase-negative staphylococci?
S. epidemidis - main pathogen in catheter-related bloodstream infections
S. haemolyticus
S. saprophyticus - UTI and wirses in patients
What can S. lugdunensis, a coagulase negative, gram positive, staphylococci cause?
A harmless skin commensal to a life-threatening pathogen (as with infective endocarditis).
Behaves like Staphylococcus aureus.
What is key characteristic of coagulase negative staphylococci?
Form biofilms - bacteria adheres to a surface and forms a complex microbial community and there is released biofilm matrix.
What has the risk of forming biofilms?
Anything prosthetic:
Hip joints
Pacemaker wires
Prosthetic heart valves
What can Streptococcus pneumoniae cause?
Pneumonia
Meningitis
Septicaemia (blood poising)
What is a unique feature of Streptococcus pneumoniae?
Has a capsule that helps protect it from the host immune system
