2.2 Flashcards
Blood agar
- A nutritious solid culture medium containing 5% intact mammalian red blood cells, usually sheep or horse
○ Bacteria, when inoculated into an agar plate, will grow to form colonies
○ Some bacteria produce haemolysins - substances which digest red blood cells
○ These haemolysins diffuse from the colonies out into the agar
- Beta haemolysis
○ Some haemolysins completely digest the red blood cells in the agar, producing complete haemolysis, a transparent zone of agar surrounding each colony
- Alpha haemolysis
○ Partially digest the red blood cells, leaving an olive green colouration of the agar around each colony
○ Sometimes called viridans streptococci due to the green colour
viridans streptococci
alpha haemolytic
B-haemolytic streptococci classified by
- Using the Lancefeild classification system streptococci can be grouped into 20 serotypes
lancefeild classfication system
- In the lap, beta haemolytic strep is mixed with individual Lancefeild antisera
○ latex particles coated with antibodies against the individual carbohydrate antigens
○ Check which antisera the streptococcal bacteria agglutinates with
lancefeild group A
- Streptococcus pyogenes
lancefeild group B
streptococcus agalactiae
one Alpha haemolytic streptococci
Streptococci pneumoniae
alpha haemolytic strep characteristics
- Many of these streptococci are part of the normal flora of the oropharynx, gastrointestinal tract and vagina
- May become pathogens when they gain access to sites in the body where they don’t belong
Streptococci pneumoniae
○ Highly virulent
○ May also be commensal of the human nasopharynx
○ Virulence factor - polysaccharide capsule
○ >90 serotypes based on antigenic differences in the capsule
○ Appears as a diplococcus
2 enterococci
E. Faecalis
E. Faecium
enterococci serotype with
group D antigen
enterococci morphology
gram positive cocci in short chains
enterocci are commensals of
the human bowel
