Classifications of Pathogenic Bacteria: Gram Positive Flashcards
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
What is viridans streptococci?
Another group of streptococci or alpha-hemolytic bacteria.
They typically colonise and
reside in the oral cavity.
What can viridans streptococci produce?
Bacteremias and septicemia as an immunocompromised patients.
How can β-haemolytic streptococci be further classified?
By carbohydrate surface antigens
Groups: A-G
Most important groups: A,B,F and D
Discuss Group A, β-haemolytic streptococci
Streptococcus pyogenes aka “flesh eating” bugs
An example: Necrotizing fasciitis
It can cause devastating skin and soft tissue infections and septicemia with things progressing incredibly fast.
Management of severe Group A, β-haemolytic streptococci
Antibiotics and surgery to remove effected tissue
What is the most common Group A, β-haemolytic streptococci infection?
Sore throat pharyngitis
Called streptococcus pyogenes
Discuss Group B, β-haemolytic streptococci infection
Often cause infection in neonates but also in older adults
Example: Streptococcus agalactiae
How do neonates get Streptococcus agalactiae?
Genital tract carriage in women (common in 25% of women and is harmless)
Can cause neonatal sepsis such as meningitis and bacteraemia
What would you do if a pregnant patient was positive for Group B, β-haemolytic streptococci?
Give antibiotics to prevent the transmission of the group B strep to the baby where the
baby might become infected
and subsequently develop disease.
What underlying factors can make people more at risk of developing Streptococcus agalactiae?
Diabetes
Liver disease
Alcohol abuse
CVD
Malignancy
What have group D streptococci been re-classified as?
Enterococcus
What are the two main groups of Enterococcus?
Enterococcus faecalis
Enterococcus faecium
What can Enterococcus cause?
Septicaemias and infective endocarditis
Less virulent than group A or B strep.
Often non-hemolytic although they can be beta-hemolytic.
How is gram positive bacilli classified?
Aerobic
Anaerobic
How would you describe the shape of the clostridium group of bacteria?
Large bacillli
What is a clinically important gram-positive bacillus?
Clostridiodes Difficile
aka C. diff.
Is C. diff anaerobic or aerobic?
Anaerobic
How id C. diff diagnosed?
Difficult to culture
Diagnosed based off of the antigen that’s on the surface and if it produces a toxin.
Do healthy people have C. diff?
Asymptomatic gut carriage in healthy people - 16% of adults and 66% of babies
What can C. diff cause?
Diarrrgoea, associated with with toxin production.
Lead then on to toxic megacolon and can be fatal in rare circumstances.
Pseudomembranous colitis.
What is a risk factor for C. diff?
Antibiotic use and anything that disrupts normal gut flora
Key feature of C. Diff
Produces endospores or spores.
Bacteria goes into hibernation and these spores are very resistant.
Alcohol gel does not kill them - need soap and water.
How does C. Diff spread?
When a patient has diarrhoea
they can stay on surfaces
for a prolonged period of time.
That together with hands can easily transmit infection to other patients.
What does Clostridium perfringens cause?
Can contaminate food and cause gastroenteritis (Enterotoxin producing stains)
Can cause spores
Need to go to theatre in severe circumstances
Infected wounds may cause “gas gangrene”
Where can clostridium perfringens be found?
Soil
In gut and faeces
What is available for prophylaxis of clostridium tetani?
A vaccine
Antigenetically modified toxin
What can clostridium tetani cause?
Toxin production by C. tetani causes tetanus
What can bacillus anthracis cause?
Anthrax
What bacteria can be found in botox?
Clostridium botulinum
