Bacteriology Flashcards
Common diagnostic techniques for bacteria
Cultures
Serology
Molecular techniques
Antimicrobial susceptobility testing
2 different types of culture
Sterile- like blood and CSF
Non sterile sites- sites of infection
NOTE- skin is not sterile as million of commensal bacteria here
How are blood cultures tested as positive
The blood is put in tubs containing nutrients for bacteria and intubated at 37C- they are left and after time the bacteria produce waste products that change the colour of indicator at the bottom of the tubs
What happens to tubs containing blood cultures after their intubation
They are put in a machine with sensors to tell if there is a change in the colour of the indicator
What do you do if blood tests positive for bacteria and why
Carry out gram staining to work out what antibiotics would be best appropriate
Gnerally speaking where do gram positive infect
Skin and soft tissue
Generally speaking where do gram negative bacteria infect
abdomen and urinary tract
Cases where should give antibiotics regardless of if tests have come back
Meningitis and meningococcoal spticaemia
Why can you use non-selective agar plates for blood cultures
Blood should be sterile anyway
What is chocolate agar
Where the blood on agar is cooked
Why is chocolate agar cooked
Some bacteria are unable to lyse to RBCs to gain nutrients so cooking blood does this for them
Common example of bacteria growing on chocolate agar
Haemophilus influenzae
What grows on macconkey agar
Gram negative bacteria
Problem of gram negative bacteria
Most antibiotics target cell wall which in gram negative bacteria antibiotics are unable to reach because of its lipopolysaccaride outer membrane
Most common bacteria seen
gram positive cocci- easy to identify on cultures as have distinguishable growing pattens which can help drive antibiotic treatment
Growing pattern of staphylococci
Divide in 2 and from clumps of four- they look like a bunch of grapes
Growing pattern of streptococci
Divide end on end so form chains
Test carried out on staphylococci bacteria
Coagulase test
What is purpose of coagulase test
Distinguish between virulent staphylococci and commensal bacteria
Positive coagulase test
This indicates virulence as coagulase is virulence factor
Negative coagulase tests
Sign of commensal bacteria
Test carried on streptococci bacteria
Haemolytic test- depends on how they appear on blood agar
Potential results of haemolytic agar tests
Can either be alpha or beta haemolysis
Alpha haemolysis test result
Appear green as there is incmplete haemolysis
