Bacteriology

Question 1

What are the 4 common diagnostic techniques used when testing for bacterial infection? What is each used for?

Answer 1

Culture: determine antimicrobial resistance
Serology: determine body’s response to infection
Molecular techniques: detect resistance genes
Antimicrobial susceptibility test: determine antibiotic resistance

Question 2

What kind of sites may a sample be taken from?

Answer 2

Sterile: should be no bacteria present e.g. blood

Non-sterile: commensal bacteria present e.g. skin

Question 3

After a positive blood culture, what test is used to classify type of bacteria present? What are the 2 possible results?

Answer 3

Gram stain
Positive: Thick peptidoglycan cell wall, retains dye, stain purple
Negative: Thin peptidoglycan cell wall, loses dye, stain pink

Question 4

What tests can be performed on urine?

Answer 4

Bedside: Naked eye - clear, cloudy, haemorrhagic.
Dipstick tests: ketones may indicate infection. Nitrites suggest bacteriuria
Microscopy: WBC (pyuria suggests infection)
Culture on MacConkey agar (urine should be sterile so any microbial growth is potentially significant)
Antibiotic sensitivity testing of bacteria that grow

Question 5

What tests can be performed on faeces?

Answer 5

Naked eye: consistency, blood stained, colour, presence of worms 
Microscopy: ova, cysts, parasites 
Culture on inhibitory media 
Toxin detection (Clostridium difficile) 
Special stains e.g. for cryptosporidia

Question 6

List 4 common bacterial causes of diarrhoea

Answer 6

Salmonella
Shigella
Campylobacter
E coli

Question 7

How is Clostridium difficile detected?

Answer 7

Toxin detection

PCR for toxin gene

Question 8

What is the minimum inhibitory concentration?

Answer 8

Minimum dose of antibiotic required to inhibit growth in vitro

Question 9

How do you carry out a blood culture?

Answer 9

Broth at the bottom of a tube
Bacteria placed in tube + incubated
Broth colour change indicates presence of bacterium

Question 10

Describe 2 agars used to grow bacteria

Answer 10

Chocolate: Cooked blood, lets bacteria use blood nutrients to grow
MacConkey: designed to grow gram -ve bacteria

Question 11

Describe the typical staphylococci appearance

Answer 11

Clumps

Look like bunches of grapes as they divide

Question 12

What type of staphylococci tests coagulase positive and negative?

Answer 12

+: Staphylococcus Aureus

-: Common skin microbes

Question 13

Describe the typical streptococci appearance

Answer 13

Form chains in the gram stain

Question 14

What groups do streptococci separate into on a blood agar?

Answer 14

Alpha haemolysis: incomplete haemolysis, turns green e.g. Strep. pneumoniae.
Beta haemolysis: complete haemolysis, clears agar
Group A; Strep. pyogenes – skin/soft tissue infection
Group B; Strep. agalactiae – sepsis in the young

Question 15

List 3 parasites that cause diarrhoea

Answer 15

Amoeba
Giardia
Crytosporidium

Question 16

What does salmonella, campylobacter and vibrio cholerae look like on agar?

Answer 16

Salmonella: black colonies due to hydrogen sulphide produced
Campylobacter: 48hours to grow + can survive at 48 degrees so heat to kill other bacteria
Vibrio cholerae: turn agar green

Question 17

What happens in disc diffusion?

Answer 17

Use a set conc. of antibiotic in each disc + incubate for 24 hours
Measure zone diameter to determine whether a bacteria is resistant or not (large zone of inhibition= effective)