Bacteriology lab

Question 1

What are some common diagnostic techniques

Answer 1

Culture (sterile and non sterile sites)
Serology
Molecular techniques
Antimicrobial susceptibility testing

Question 2

Give examples of sterile and non-sterile sites

Answer 2

Blood, CSF

Skin

Question 3

Describe the process of culturing for bacteria

Answer 3

Watch cell growth
Mostly grown for 5 days at 37 degrees
Disc changes colour due to bacteria reproducing + producing CO2 → alter pH

Question 4

Describe the use of serology for bacterial diagnosis

Answer 4

1st infection: IgG peaks later than IgM

2nd infection: IgG rises rapidly first + higher than IgM

Question 5

Describe the use of molecular techniques

Answer 5

PCR chlamydia in genital specimens

Rapid PCR for MRSA

Question 6

Describe specimen collection

Answer 6

In the acute phase of illness and before staring antimicrobials
Collection from proper site, avoiding contamination by normal flora
Prompt transport to lab since micro-organisms multiply in transit
Adequate quantity and appropriate number of specimens
Acute sera and Convalescent sera (paired), for rising antibody titres

Question 7

Describe urine testing

Answer 7

Bedside - naked eye (clear, clouds, haemorrhagic)
Dipstick - ketones may indicate infection, nitrates suffuse bacteriuria
Microscopy - WBC (pyuria -> infection)
Culture on MacConkey agar
Antibiotic sensitivity testing of bacterial growth

Question 8

Describe faeces testing

Answer 8

Naked eye - consistency, blood stain, colour, worm presence
Microscopy - ova, cysts, parasites
Culture on inhibitory media e.g. deoxycholatecitrate agar (DCA) selenite
Toxin detection (clostridium difficile)
Special stains e.g. cryptospridia

Question 9

What may the cause of diarrhoea be

Answer 9

Bacterial, viral, or parasitic

Bacteria: salmonella, shigella, campylobacter, E Coli 0157, C. difficile, cholera

Question 10

Describe the grams stain

Answer 10

Separates into gram positive and negative
Gram +ve - thick wall - purple
Gram -ve - thin wall + peptidoglycan wall - pink

Question 11

Describe the test for staphylococci

Answer 11

Microscope with gram stain
Purple (gram positive)
Appears like clumps of grapes
Can be separated by using a coagulase test

Question 12

Describe staphylococcus aureus

Answer 12

Includses MRSA

severe infections e.g. skin/soft tissue, endocarditis. osteomyelitis

Question 13

Compare coagulase -ve and +ve staphylococci

Answer 13

-ve = skin commensals of low pathogenic potential. Probably contaminants from taking blood

+ve = appears in chains - S. aureus (might be MRSA)

Question 14

Which investigations are available for stool samples

Answer 14

Bacteria
Culture on agar plates (salmonella, campylobacter routinely)
Clostridium difficile (toxin detection or PCR for toxin gene)

Parasite - concentrate and use special stains

Question 15

What is the MIC

Answer 15

Minimum Inhibitory Concentration

Minimum concentration of antibiotic needed to inhibit bacterial growth

Question 16

What its peptidoglycan made up of

Answer 16

NAM=N-acetylglucosamine

NAG=N-acetylmuramic acid

Question 17

What tissue do gram positive and negative bacteria generally infect

Answer 17

Gram positive = skin and soft tissue

Gram negative = abdomen and urinary tract

Question 18

What is chocolate agar and what is it used for

Answer 18

Cooked blood - certain bacteria can’t lyse RBCs

Cooking releases nutrients in blood agar to allow certain bacteria to grow e.g. Haemophilus influenzae

Question 19

What is MacConkey and neomycin agar

Answer 19

MacConkey Agar = designed to grow Gram negative organisms

Neomycin agar = growth of anaerobic microorganisms

Question 20

What is important when sending for agar plates

Answer 20

Usually incubate for 24 hours on plate
Where possible, try + send cultures BEFORE giving patients ABs
However, give antibiotics ASAP in meningitis or meningococcal septicaemia

Question 21

What is coagulase

Answer 21

Coagulase converts fibrinogen to fibrin
Coagulase is virulence factor which helps S. aureus to cause infection
It doesn’t tend cause infection unless opportunistic circumstances e.g. central lines

Question 22

What infections can coagulase cause

Answer 22

Can infect prosthetic material causing line, pacemaker infections

Question 23

How are groups of streptococci divided

Answer 23

Divided depending on blood agar

Question 24

How is alpha haemolysis detected and give example of bacteria that may cause it

Answer 24

Incomplete haemolysis that turns the agar green

streptococci e.g. S. pneumoniae = pneumonia + meningitis

Question 25

How is beta haemolysis detected

Answer 25

Complete haemolysis - clears agar

Question 26

Give an example of a group A beta-haemolytic streptococci and what it may cause

Answer 26

S. pyogenes

Causes skin and soft tissue infections e.g. necrotising fasciitis
Causes rheumatic and scarlet fever

Question 27

Give an example of a group B beta-haemolytic streptococci and what it may cause

Answer 27

S. agalactiae

Causes sepsis in young children (neonatal septicaemia)
Infections in diabetics

Question 28

Can streptococci infections be treated with antibiotics

Answer 28

There is no penicillin resistance in these bacteria

Some (e.g. group A) are aggressive but treatable with antibiotics

Question 29

Describe the use of breakpoints with MIC

Answer 29

Set breakpoints correlate MIC when using the antibiotic

MIC below breakpoint - good correlation with clinical success if use that antibiotic

MIC ABOVE breakpoint - reported as resistant