Bacteriology lab Flashcards
What are the common diagnostic techniques used by virology labs?
Culture- sterile and non sterile sites
Serology
Molecular techniques
Antimicrobial susceptibility testing
What bacterial cultures used to figure out?
Which antibiotic to use
What is the main problem with using cultures?
It takes 24hrs to grow the bacteria and another 24 to do the susceptibility testing
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
Using chicken pox as an example, what will be the difference in the blood before or at the start of the infection and when they’ve had an immunological response to chicken pox?
They will have gone from IgG negative to IgG positive
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?
It helps with selecting antibiotics because Gram positives are susceptible to certain antibiotics and Gram negatives are susceptible to others
Where do gram positive and negative generally tend to affect?
Gram positive= skin and soft tissue
Gram negative= abdomen and urinary tract
When you use blood cultures, what sort of agar plates do you use and why?
Non-Selective because there shouldn’t be any bacteria there in first place and they’re designed to grow anything
What is chocolate agar?
Cooked blood- certain bacteria will not be able to lyse blood cells so by cooking it you release some of the nutrients in the blood agar and let certain bacteria grow
What is the commonest bacteria that grows on chocolate agar?
Haemophilus influenzae
What is Macconkey agar designed to grow?
Gram-negative organisms
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
Coagulase negative
What sort of bacteria is the most important staphylococci coagulase positive?
Staphylococcus aureus (coagulase is a virulence factor which helps staphylococcus aureus to cause infection)
What sort of bacteria are coagulase negative?
Common skin microbes- don’t to cause infection unless opportunistic circumstances
What are the two groups of streptococci and what are the groups based on?
Based on how they look on blood agar:
Alpha haemolysis
Beta haemolysis
What is alpha haemolysis?
Incomplete haemolysis- turns agar a green colour
What is beta haemolysis?
Complete haemolysis- clears the agar
What is an example of alpha haemolytic streptococci?
Streptococcus pneumoniae- common cause of pneumonia and meningitis
What is an example of beta haemolytic group A streptococci?
Streptococcus pyogenes- causes skin and soft tissue infections and rheumatic fever
What is an example of beta haemolytic group B streptococci?
Streptococcus agalactaie- Important cause of sepsis in young children and involved in infections in diabetes
What are possible causes of diarrhoea?
Bacteria- Salmonella, Shigella, Campylobacter, E coli O157, C difficile and Cholera
Parasites- Amoeba, Giardia and cryptosporidium
Viruses
Who is affected by E. coli O157?
Young children and elderly people
What can E. coli O157 lead to?
Haemolytic anaemia syndrome which could lead to renal failure
What is the prevalence of c. difficile?
5% of population but more common in children
What is generally associated with D and V?
Viruses
What are the three main bacteria that stools are commonly tested for?
Salmonella
Shigella
Campylobacter
Why is testing for C. difficile difficult?
It got its name because it is difficult to grow so labs tend to use detection of toxin or the toxin gene as a marker for C. difficile- a combination of antigen and toxin gene PCR is used to diagnose
What happens to salmonella when put on XLD (xylose lysine deoxycholate) agar?
It goes pink because the salmonella can’t ferment xylose whereas most other coliform or enteric bacteria can ferment xylose so it goes a yellow colour
What does salmonella also form and what is it responsible for?
Hydrogen sulphide which forms black colonies
How do you test for campylobacter?
It takes a long time to grow but it’s able to survive at 42 degrees so you incubate the sample at 42 degrees to kill other bacteria in sample then put the remaining campylobacter on selective agar
How do you test for vibrio cholerae?
On TCBS agar, cholera makes it go green
What is the minimum inhibitory concentration (MIC)?
Lowest amount of antibiotic required to inhibit the growth of bacteria in vitro
What are breakpoints?
Markers that correlate MIC with clinical success when using antibiotic
What does it mean if the bacteria has an MIC below the breakpoint?
There is a good correlation with clinical success if you use that antibiotic
What does it mean if the bacteria has an MIC above the breakpoint?
It is resistant
How do most labs measure MIC?
Using graded MIC strips
What is the traditional way of testing antibiotic sensitivity?
Disc diffusion-
Set conc of antibiotic on each disc which is then placed on agar and incubated for 24 hours, zone size is interpreted using breakpoints and zone diameter can be used to determine whether it is sensitive or resistant
What is special about carbapenamase?
It is a type of beta lactamase which breaks down all classes of beta lactams so they are resistant to pretty much everything and untreatable
