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Flashcards in Bacteriology lab Deck (52)
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What are the common diagnostic techniques used by bacteriology labs?

Culture- sterile and non sterile sites
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?

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

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