Bacteriology [Complete]

Question 1

State some common diagnostic techniques used in a bacteriology lab.

Answer 1

Culture:

1) Sterile site (e.g blood/CSF) 2) Non-sterile sites

Can be used to amplify amount of bacteria for further testing.

Serology

Molecular techniques

Antibmicrobial sensitivity testing

Question 2

What are molecular techniques used for?

Answer 2

Detect resistance genes

Alternatively do antimicrobial susceptibility testing but this takes a long time

Question 3

Where are cultures extracted from to determine antimicrobial resistance?

Answer 3

Sterile sites e.g. blood and CSF

Non-sterile sites (difficult due to great numbers of bacteria)

Question 4

What type of culture is found in the abdomen and urinary tract

Answer 4

gram -ve

Question 5

Define serology.

Answer 5

Scientific study of bodily fluids (usually to detect presence of antiboides)

Question 6

What is serology done for?

Answer 6

Determine body’s response to an infection

Question 7

Give examples of ways the body prevents commensal bacteria from becoming pathogenic.

Answer 7

Question 8

What type of culture is found on skin and soft tissue?

Answer 8

Gram +

Question 9

Describe how blood cultures are run

Answer 9

Different bottles for collection (some are anaerobic and some aerobic)

Stored in conditions which promote growth (e.g 37 degress and bottles desgined to negate effects of antibiotics).

pH indicator on bottom of bottle changes colour as pH lowered from bacteria producing CO2

Takes about 18 hours

Question 10

Describe antimicrobial susceptibility testing

Answer 10

After blood culture, collect sample and place on agar plates.

Place different ABs and see whether there is still growth or inhibition.

Question 11

What are the complications of starting patients on antibiotics before sending off samples?

Answer 11

The antibiotics can interfere with the test results - potential for false negatives as the dose administered may not have a clinical effect but it might have an effect at the cellular level on the agar In meningitis or sepsis you administer anyway

Question 12

Explain differences between gram positive and negative bacteria.

How does this affect gram staining?

Answer 12

Gram positive: Have a single cell membrane sorrounded by a peptidoglycan layer.

Gram negative: Have a cytoplasmic membrane and a lipopolysaccharacide membrane with a peptidoglycan layer between them

Gram positive appear purple due to the thick peptidoglycan wall which retains the dye.

Gram negative appears pink due to thick cell wall (double membrane with pep membrane in-between), causing dye to be incapable of staining so appears pink.

Question 13

What is the most common type of bacteria?

How do they appear under a microscope?

Answer 13

Gram positive cocci such as:

Staphylococci

Appear purple after staining (gram positive), round in shape and clumped together like grapes.

Question 14

Give an example of a test which can be used to determine the type of staphyloccoci in a sample.

Answer 14

Coagulase test: (Determines ability to coagulate)

+ indicates s.aureus

• indicates common skin microbes

Question 15

What is staphylococcus aureus and what conditions can it cause?

Answer 15

Coagulase postive staphylococci which are highly pathogenic and can lead to severe infections such as:

Skin infections

Endocarditis

Osteomyelitits

Question 16

What kind of investigations may be performed?

Answer 16

culture e.g. agar or a broth

Serology

Molecular techniques

Question 17

What can coagulase negative staphylococci cause?

Answer 17

It is less pathogenic in comparison to coagulase positive staphylococci (aka staph A).

They tend to live on the skin and have low pathogenic potential.

However if they breach the skin (e.g by infecting prosthetic material) they can cause infections such as:

Line infections

Pacemaker infections

Endocarditis

Question 18

How do streptococci look under the microscope?

Answer 18

Form chains and appear purple in gram stain (gram positive)

Question 19

What are the types of streptococci on blood agar?

What type of haemolysis can be displayed and state some common examples of streptococci in these groups and their clinical manifestations?

Answer 19

Alpha haemolytic streptococci: due to incomplete haemolysis it turns green.

e,g streptococcus pneumonia

Beta haemolytic streptococci: Due to complete haemolysis, clears the agar

Have two different groups:

Group A s.pyogenes (Cause pharyngitis, cellulitis, erysipelas)

Group B s.agalactiae (Causes neonatal sepsis and neonatal meningitis)

Question 20

How do bacilli look under the microscope?

Answer 20

Appear pink as theyre gram negative and rod shaped

Question 21

What are the possible causes of diarrhoea?

Answer 21

1)Bacteria:

E.g: Salmonella, Shigella, Campylobacter [Common in food poisoning], E. Coli, C. difficile, cholera.

2)Parasites:

E.g. Amoeba, Giardia, Cryptosporidium.

3) Viruses.
* N.B Must inform if patient has travel history so they can test for specifics such as cholera as it isnt normally tested*

Question 22

When is c.difficile normally tested for?

Answer 22

Tested if someone has diarrhoea after being on AB usage.

AB can kill commensal bacteria and cause c.difficile to thrive in uncompetitive niche.

Question 23

When are viruses normally detected for if someone is presenting with diarrhoea?

Answer 23

If someone comes in from nursing home or school and other individuals have been experiencing the same problems.

Question 24

Explain the culturing proccess for bacteria that cause diarrhoea

Answer 24

Salmonella: XLD agar: 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 cholerae: TCBS agar: cholera makes the agar turn green.

Question 25

How can bacteria involved in diarrhoea be investigated?

Answer 25

Agar culture: salmonella, shigella and campylobacter looked for routinely as they are found in faeces

PCR to detect toxic gene of c.difficile. This bacteria is difficult to culture

Question 26

How are parasites involved in diarrhoea investigated?

Answer 26

Special stains

Concentrations

Question 27

What is the positive predictive value dependent on?

What is the significance of this?

Answer 27

Pre-test probability of the sample being positive

The more likely a patient is to have the disease, the more likely a positive test is a true positive

Therefore there is no need to test for everything

Question 28

What is the MIC?

Answer 28

Minimum Inhibitory Concentration

The lowest amount of AB needs to inhibit the growth of bacteria in vitro

Question 29

How is MIC used in evaluating the success of antibiotics?

Answer 29

The breakpoint of AB is set correcting the MIC with the clinical success of a AB

The breakpoint is a chosen conc of AB which can be used to determine whether a bacteria is susceptible or resistant tto a certain AB.

A bacterium with a MIC below the breakpoint means the AB has a change of being successfully used

A MIC that goes above the breakpoint indicates that the bacterium is resistant

Question 30

How is MIC tested?

Answer 30

1) Gradient MIC strip:

Gradient antibiotic strips on agar:

Has decreasing conc of AB on strip

Look for crossover of bacteria on the strip (When it touches the strip) to determine MIC.

2) Disc diffusion:

Where a set conc of AB on discs are incubated for 24hrs

A zone size is compared to breakpoints on an AB table

Siameter is measured

Question 31

What are the limitaions of microbiological investiagtions

Answer 31

slow - some e.g. TB can take 6 weeks to grow MIC tests difficult to interpret

Question 32

What are the limitations of PCR?

Answer 32

-need to know what looking for to design primers -potentially hundreds of primers needed as many genes confer resistance -PCR less sensitive than culture