Classification of pathogenic bacteria-gram positive Flashcards
Differences between gram positive cell wall and gram negative?
Gram negative:
more complex cell wall
same cytoplasmic membrane
periplasmic space in between the peptidoglycan thin layer and cytoplasmic membrane
outer membrane (contains lipopolysaccharide which is responsible for endotoxic shock in gram negative)
What is peptidoglycan?
sugar network
composed of two major sugars
n-acetylmuramic acid and n-acetylglucosamine
What do purple and pink stains from gram stain show?
purple bacteria- gram positive
pink bacteria-gram negative
How would a gram stain be conducted?
heat fix bacteria to slide and flood with crystal violet
then add iodine
decolourisation step using alcohol
use counter stain using alcohol
Why is gram positive purple?
thick layer of peptidoglycan can form complexes within the peptidoglycan and stain retained
What forms do gram positive aerobic cocci take?
chains- streptococci
clusters-staphylococci
How are streptococci classified?
according to their hemolysis- appearance on blood agar
betahemolytic- complete detsruction of surrounding red blood cells
alphameolytic- partial
non-hemolytic- no effect
How can staphylococci be classified?
on whether can produce enzyme coagulase.
Coagulase positive- pathogens
coagulase negative- less pathogenic
How do we classify gram positive bacilli?
size of bacillus
large - bacillus
small- corynebacetrium or listeria
How do we classify anaerobes?
cocci or bacilli
What is staphylococcus aureas?
- Commensal organism carried in nose, axilla, perineum. (can live harmlessly in these)
- A major human pathogen!
- Wide range of disease from boils/abscesses and soft tissue
infections to septicaemia and osteomyelitis
Antimicrobial patterns with staphylococcus aureas?
- Commonly penicillin resistant due to production of penicillinase.
- By a different mechanism, some strains are “methicillin
resistant”. - Methicillin resistant Staph aureus (MRSA) poses major problems
for infection prevention and control in hospitals.
What are some characteristics of coagulase negative staphylococci?
Many different species e.g. S. epidermidis, S. haemolyticus, S.
saprophyticus.
* Beware of S. lugdunensis.
* Mainly skin commensals.
* Form biofilms and may be significant pathogens in the
presence of foreign bodies/prostheses e.g. prosthetic heart
valves, prosthetic hip joints, pacemaker wires etc or
immunocompromised e.g. neonates / BMT with central
venous catheters
How do you classify streptococci based of f of hemolysis?
alpha haemolytic streptococci- partial haemolysis and turns blood agar green
beta haemolytic- complete haemolysis and turns blood agar clear
non haemolytic- no difference
What does streptococcus pneumoniae cause? (alpha haemolytic)
pneumonia, meningitis, septicaemia
has a capsule that helps protect it from immune system
What do viridans streptococci cause? (alpha haemolytic streptococci)
typically colonise and reside in the oral cavity.
How can beta haemolytic streptococci be further classified?
further identified by carbohydrate surface antigens (Lancefield group)
Groups A-G
A,B,F and D clinically most important
Describe group A strep?
- Streptococcus pyogenes
- Major pathogen-Pharyngitis, cellulitis, necrotising fasciitis (“flesh
eating bug”)
Describe group B streptococci?
- Streptococcus agalactiae
- Neonatal sepsis– Meningitis– Bacteraemia
- Genital tract carriage common (around 25% women).
- Also responsible for invasive infections in adults (often older and
underlying factors e.g. diabetes, liver disease / alcohol abuse, CVD,
malignancy).
Describe Group D streptococcus?
- Now re-classified as Enterococcus spp.
E.g. Enterococcus faecalis, Enterococcus faecium - Often non-haemolytic.
- Found in the gut as a normal commensal.
- Cause of urinary tract infection and infective endocarditis
How do we classify gram positive bacilli?
aerobic
anaerobic
Describe clostridiodes difficile? (gram positive bacilli)
- “Difficult” to culture.
- Asymptomatic gut carriage in healthy people – up to 16% of adults and
66% of babies. - Important cause of diarrhoea, associated with toxin production and
potentially fatal. - Increased risk with antibiotic use and anything else that disrupts the
normal gut flora. - Pseudomembranous colitis.
- Spread / transmitted via spores.
- Detect antigen & toxin in stool sample by ELISA
Describe clostridium perfringens?
- Found in soil and normal commensal in human and animal
gut/faeces. - Can contaminate food and cause gastroenteritis (enterotoxin
producing strains). - Infects wounds may can cause “gas gangrene”
Describe clostridium tetani?
Toxin production by C. tetani cause tetanus.
Uncontrolled muscle spasm due to loss of inhibition at neuro
muscular junction.
Antigenically modified toxin (toxoid) used for immunisation.
vaccine preventable disease
What is the source of botox?
clostridium botulinum
what is cause of anthrax?
bacillus anthracis
