Bacteriology lab Flashcards
What are the common diagnostic techniques used by virology labs?
Culture- sterile where (there should be no bacteria) and non sterile sites (commensal)
Serology
Molecular techniques
Antimicrobial susceptibility testing
What are bacterial cultures used to figure out?
Which antibiotic to use
Why is a culture relatively easy at sterile sites?
There shouldn’t be any bacteria in these sites e.g. CSF and blood so anything that grows is abnormal
What does serology look at?
Body’s response to infection
What are the pros and cons of molecular techniques?
Pros:
Rapid and sensitive
Useful for MRSA because resistance mechanism is encoded by MecA gene so if you do PCR for this gene you will know it’s resistant
Cons:
Myriad of resistance genes so these aren’t good for frequent use
How is antimicrobial susceptibility testing carried out?
By phenotypic methods- you impregnate agar with a microorganism and put antibiotic disc on it
How do blood cultures work?
Broth inside tube that has nutrients for bacteria and then it is incubated (around 37 degrees)- there is then an indicator at the bottom of the tube, the waste products of the bacteria will cause a change in colour of the indicator, the machine has sensors that can detect the colour change and flag it up as positive
What do you do once you confirm that blood cultures are positive?
Gram stain
Why do you do a gram stain?
Colour of gram positive
Differences in structure between positive and negative
It helps with selecting antibiotics
Gram positives – deep purple
Gram positive- thicker peptidoglycan cell wall which holds Gram stain and stains purple
Gram negative- outer membrane outside cell wall which stops them from taking up the stain, instead they take up the counter stain and stain pink LPS
– nag and nam
In what situation would you give antibiotics without checking cultures first?
Patients with meningitis or meningococcal septicaemia
What is the difference between Gram positive and negative bacteria?
Gram positive- thicker peptidoglycan cell wall which holds Gram stain and stains purple
Gram negative- outer membrane outside cell wall which stops them from taking up the stain, instead they take up the counter stain and stain pink
Why are many antibiotics ineffective on gram-negative?
They act on cell wall but outer membrane prevents them from getting there e.g. vancomycin
What is the most common type of bacteria that you find in terms of Gram and shape?
Gram positive cocci
How do staphylococci appear?
They have a particular pattern of dividing where they divide in two then daughter cells divide again to form a clump of four- look like bunches of grapes
How do streptococci appear?
They divide end on end to form chains
What is the staphylococci coagulase test used for?
To differentiate between the two types of staphylococci:
Coagulase positive–ability to form a clot
Coagulase negative
