Bacteriology Flashcards
4 common diagnostic techniques for bacteriology?
- CULTURE
o used to determine antimicrobrial resistance
a. sterile sites e.g. blood & CSF
b. non-sterile sites (usually have lots of commensal bacteria so difficult to identify)
- SEROLOGY
o used to determine the body’s response to an infection
e.g. doing blood at the beginning of an infection and at the end of it
- MOLECULAR TECHNIQUES
o detect resistance genes - ANTIMICROBIAL SUSCEPTIBILITY TESTING
o used to test AB resistance (takes LONG)
What can happen to commensal bacteria?
Can become pathogenic e.g. when have a cannula or catheter
How do blood cultures work?
Broth at the bottom of the tube
- Bacteria is placed in the tube
- Incubated (dependent on bac)
- If colour changes = indicates presence of bacterium
What happens if the blood culture is positive?
Do gram-testing!
Gram-POSITIVE = skin & soft tissue
Gram-NEGATIVE = abdomen & urinary tract
What are different types of agar you can use for gram-testing?
- Non-selective agar plates
- Chocolate Agar
o COOKED blood
o lets bacteria use the BLOOD NUTRIENTS to grow (as some bac. might NOT be able to lyse RBCs) - Macconkey Agar
o grows gram -VE organisms
Difference between gram +ve and -ve bacteria?
Gram +VE
o thicker peptidoglycan cell wall
o purple
o RETAINS dye
Gram -VE o thinner pepti. cell wall o have an OUTER MEMBRANE (so renders some AB ineffective as cannot get past it) o pink stain o LOSES dye
Gram+ve cocci in lumps?
Most COMMON bacterium
e.g. Staphylococci - form CLUMPS (divide in 2 then daughter cells divide to form clump of 4)
How can you differentiate between different Staphylococci?
COAGULASE TEST!! If:
o Coagulase POSITIVE
- it is STAPHYLOCOCCUS AUREUS
o Coagulase NEGATIVE
- common SKIN MICROBES
- tend to NOT cause infection unless there are some opportunistic circumstances e.g. central lines
- if found in blood culture, probably contaminants from taking blood
Coagulase and Staphylococci?
STAPHYLOCOCCUS AUREUS!
Coagulase is a VIRULENCE FACTOR which helps it to cause infection!
Coagulase?
a bacterial enzyme which brings about the coagulation of blood or plasma (fibrinogen to fibrin) and is produced by disease-causing forms of staphylococcus.
What is different about coagulase NEGATIVE saphylococcus to coagulase POSITIVE?
o tend to NOT cause infection
- unless there are some opportunistic circumstances e.g. central lines
- can infect prosthetics e.g. in hip replacements
- if found in blood culture, probably contaminants from taking blood
Gram+VE cocci in chains?
STREPTOCOCCI!
How can Streptococci be separated?
On BLOOD agar!
Into 2 groups:
- Alpha haemolysis
- INCOMPLETE haemolysis
- turns GREEN
- e.g. streptococci pneumoniae - Beta haemolysis
- COMPLETE haemolysis
- turns CLEAR
- e.g. Group A = streptococcus PYOGENES (skin/soft tissue infection)
- e.g. Group B = streptococcus AGALACTIAE (sepsis in young)
Gram-negative Bacilli?
e.g. E-coli!
Stain PINK as do NOT take up gram stain
Possible causes of Diarrhoea?
- Bacteria
e. g. salmonella, shigella, campylobacter, E-coli, C.difficile, Cholrea - Parasites
e. g. amoeba, diardia, cryptosporidium - Viruses
Possible investigations for diarrhoea?
Stool sample!
Bacteria - culture on agar
- ONLY salmonella, shigella and campylobacter looked for routinely (as TOO MANY bacteria grow in faeces)
Parasites - concentration & special stain
How did C.difficile get its name?
DIFFICULT to grow in culture
SO tend to detection of its TOXIN or TOXIN GENE as the marker (via. antigen & toxin gene PCR)
Salmonella on XLD agar?
XLD agar is SPECIFIC for salmonella
Salmonella produces Hydrogen Sulfide so produces:
- BLACK CIRCLES
Does NOT ferment XLD so goes REDDISH (whilst normally bac. on this agar go YELLOW)
Campylobacter on agar?
Takes LONG to grow (48hours)
Can survive at HIGH temperatures (42oC) so incubate at this to kill other bac. on stool sample
Can then put the remaining campylobacter on a selective agar dish
Cholrea on TCBS agar
This agar is SPECIFIC for cholera
SO if try to grow cholera on other agar will NOT work
PPV?
Positive predictive value
What does PPV depend on?
Depends on pre-test probability of the sample being positive
i.e. the more likely a patient is to have a disease, the more likely a positive test represents a TRUE positive
What does the PPV mean ultimately?
i.e. DO NOT test everybody for everything, send tests for things it is most likely to be
How is ‘Sensitivity Testing’ carried out?
MIC - minimum inhibition concentraion
- the LOWEST amount of AB required to inhibit the growth of bacteria in vitro
