3. Bacteriology lab

Question 1

List four common diagnostic techniques

Answer 1

Culture - used to test for antimicrobial resistance
Serology - used to determine the body’s response to an infection
Molecular techniques - detecting resistance genes
Antimicrobial susceptibility testing - for testing AB resistance

Question 2

What are the two different areas that cultures can be used in?

Answer 2

Sterile sites - e.g. blood and CSF

Non-sterile sites - there are ususally too many bacteria here so it is difficult to culture

Question 3

Give an example of when serology would be used

Answer 3

Measuring the body’s response to the Varicella-zoster virus by doing a blood test at the beginning and end of a chickenpox infection

Question 4

How are blood cultures carried out?

Answer 4

There is a
broth at the
bottom
of a tube and bacteria placed in the tube and incubated can
change the colour of
the broth to indicate whether a
bacterium
is present in the blood

Question 5

What would you do if a blood culture was positive for bacteria?

Answer 5

Gram testing:
Gram
POSITIVE
=
skin
and
soft
tissue
.
o
Gram
NEGATIVE
=
abdomen
and
urinary tract
.

Question 6

What are the two types of agar?

Answer 6

Chocolate
agar - cooked blood - 
let’s bacteria use the
blood nutrients
to grow.
MacConkey
agar - designed to
grow gram negative bacteria

Question 7

What is the difference between the structure of the walls of gram positive and gram negative bacteria?

Answer 7

Gram-positive bacteria do not have an outer cell membrane found in Gram-negative bacteria. The cell wall of Gram-positive bacteria is high in peptidoglycan which is responsible for retaining the crystal violet dye

Question 8

How does the cell wall of gram positive and negative bacteria affect their staining?

Answer 8

Gram +ve = thick wall, purple stain, retains dye.

Gram -ve = thin wall, pink stain, loses dye

Question 9

Why are some antibiotics only effective on gram positive bacteria? Give an example of such an antibiotic

Answer 9

Many AB target the cell wall but may not be able to get past the outer membrane in gram negative bacteria e.g. vancomycin
works only on +ve.

Question 10

What is the most common type of staphylococci and how does staph look like?

Answer 10

Gram+ cocci are the most common bacterium

Staphylococci often form clumps and look like bunches of grapes as they bud divide.

Question 11

How would you differentiate between different types of staphylococci?

Answer 11

You can do a coagulase test to test between coagulase± staphylococci:
Coagulase positive =
Staphylococci
aureus
Coagulase negative = Common skin microbes

Question 12

How do streptococci appear when gram stained?

Answer 12

Streptococci
generally form chains
in the gram stain.

Question 13

What happens to streptococci on blood agar?

Answer 13

Streptococci separate into two groups:
Alpha
haemolysis–
incomplete
haemolysis, turns
green e.g. Streptococcus
pneumoniae
Beta
haemolysis - complete haemolysis, clears
agar e.g. Group A
–
Streptococcus
pyogenes (skin/soft tissue infection) or
Group B - Streptococcus
agalactiae (sepsis in the young)

Question 14

What do gram negative bacilli appear as?

Answer 14

They do not take up stain so appear pink e.g. E.coli

Question 15

What are the possible causes of diarrhoea?

Answer 15

-Bacteria:
  Salmonella - the
salmonella colonies are black due to hydrogen sulphide
produced.
  Campylobacter - 
48hours to grow
and can
survive at 48 degrees
so
heat
to
kill other bacteria
  Vibrio cholera makes the
agar turn green
-Parasites - e.g. Amoeba, Giardia, Cryptosporidium.
-Viruses.

Question 16

How would you investigate any potential bacteria that are causing diarrhoea?

Answer 16

Culture on agar - only
salmonella, Shigella
and campylobacter
looked for routinely
as LOADS of
bacteria grow in faeces.
Clostridium difficile is difficult to culture but can use PCR to detect toxin gene

Question 17

What is the positive predictive value (PPV)?

Answer 17

Depends on the pre-test probability of the sample being positive
The more likely a patient is to have the disease, the more likely a positive test represents a TRUE positive

Question 18

What is the MIC (Minimum Inhibitory Concentration)?

Answer 18

The lowest amount of AB required

to inhibit growth of bacteria in vitro

Question 19

What are breakpoints and how are they related to the minimum inhibitory concentration?

Answer 19

Breakpoints correlate MIC with clinical success as an AB. It is simply a chosen concentration of AB that should be able to kill the bacterium
A bacterium with an MIC below the breakpoint means there is a good
chance of success with that AB
A bacterium with an MIC above the breakpoint is
RESISTANT.

Question 20

How can disk diameter be used to determine whether a bacterium is resistant to an AB or not?

Answer 20

Use a set concentration of AB in each disc and incubate for 24 hours.
The clear zone size is interpreted using the breakpoints in a AB table.
Measure zone diameter to determine whether a bacterium is resistant to an AB or not.