General bacteriology Flashcards

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1
Q

Purposes of laboratory diagnosis of bacterial infection

A
  1. Identification
  2. treatment
  3. surveillance purpose
  4. for outbreak investigation
  5. to start PEP (Post exposure prophylaxis)
  6. to initiate appropriate infection control measures
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2
Q

The different types of laboratory diagnosis of bacterial infections

A
  1. Specimen collection
  2. Direct detection (microscope, antigen detection, molecular diagnosis)
  3. culture
  4. identification (bacterial identification, automated identification)
  5. antimicrobial susceptibility testing
  6. serology
  7. molecular methods
  8. typing methods
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3
Q

Louis Pasteur and Robert Koch in culture media development

A

Louis Pasteur developed liquid media/ broth
Rapid growth
Robert Koch introduced solid media using gelatin (15%)
Visible colonies

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4
Q

Disadvantages of gelatin as solidifying medium

A
  1. Proteolysed by some bacteria
  2. Solid only at temperatures less than 24°C
    15% gelatin was used

Other solidifying agents are serum and egg

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5
Q

Agar agar properties

A
Extracted from sea weeds and red algae
Also called Chinese grass
Polysaccharide containing: 
70% agarose 
30% agaropectin
Inorganic phosphate and calcium
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6
Q

Agar agar and its concentration

A
  1. 2 - 2 % for solid media (2%)
  2. 2-0.5 % soft agar medium for checking motility of bacteria, Craigie’s tube (u-tube)

5-6% firm agar medium for inhibition for swarming

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7
Q

Methods to inhibit swarming of bacteria

A
  1. Firm agar (5-6%)
  2. CLED (cysteine lactose electrolyte deficient) or MacConkey agar
  3. Add alcohol, boric acid, bile salts, chloral hydrate, sulfonamides, sodium azide, surface acting agents
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8
Q

Gram positive bacteria that shows swarming

A
Clostridium tetani
Bacillus cereus (‘serious’)
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9
Q

Gram negative bacteria that shows swarming

A

Proteus vulgaris
Proteus mirabilis
Vibrio alginolyticus
Vibrio parahemolyticus

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10
Q

Simple or basal culture media

A

Contains only sources of C and N for the nutritionally non-fastidious bacteria

  1. Peptone water
  2. Meat extract (+peptone water)
  3. Agar (+ meat extract +. ) or nutrient agar
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11
Q

Enriched culture media

A
  1. 5% sheep blood agar
  2. Chocolate agar
    Egg mediums (for mycobacterium) like:
  3. LJ medium
  4. Dorset egg medium
    Serum containing medium like
  5. Loeffler’s serum slope for C. diphtheriae
  6. PPLO broth/agar
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12
Q

Blood agar production

A

5 mL sterile sheep blood and 95 mL of autoclave nutrient agar is cooled to 50-55•C (70-75•C in case of chocolate agar)

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13
Q

Selective culture media

A
  1. Change pH
  2. Adding antibiotics
  3. Chemical agents
  4. Dye
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14
Q

pH of culture media and bacteria

A

For pathogenic bacteria it is usually 7.2-7.4
For vibrio it is 8.2-8.4
Example TCBS medium

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15
Q

Culture media for Neisseria

A
Thayer Martin medium contains:
vancomycin
Colistin
Nystatin 
while modified Thayer Martin medium contains trimethoprim in addition to the other antibiotics
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16
Q

PPLO medium contains

A

Penicillin

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17
Q

MacConkey medium

A

Contains bile salts
1. Makes it selective for gram negative bacteria
2. Differentiate between lactose fermenting (pink/magenta) and non lactose fermenting (pale) gram -ve bacteria
Indicator: neutral red

18
Q

DCA medium

A

Contains sodium deoxycholate citrate for salmonella and shigella

19
Q

Potassium tellurite and culture medium

A

All corynebacterium are resistant to 0.3-0.4% concentration of Potassium tellurite
Example tinsdale medium

20
Q

All staphylococcus are resistant to

A

7-10% salt

Example: salt milk agar

21
Q

Potassium tellurite and culture medium

A

All corynebacterium are resistant to 0.3-0.4% concentration of Potassium tellurite
Example tinsdale medium

22
Q

Eosin methylene blue agar

A

Selective media for gram negative bacteria

23
Q

LJ culture medium

A

Selective media for mycobacterium

Malachite green dye

24
Q

Examples of enrichment media

A
  1. Alkaline peptone water for vibrio

2. Selenite F broth for salmonella and shigella

25
Q

Differential medium

A

Based in colony morphology and colour
Examples:
1. Blood agar
2. MacConkey medium

26
Q

Blood agar

A
Differentiate between haemolysis types
1. Alpha haemolysis:
 Green or partial haemolysis
2. Beta haemolysis:
 Clear yellow with complete haemolysis
3. Gamma haemolysis:
 No haemolytic 
 you can see the colour of colonies and the media
27
Q

TCBS medium

A

Differentiates sucrose fermenting vibrio from non sucrose fermenting vibrio
Bromothymol blue gives yellow colour to sucrose fermenters

28
Q

Mannitol salt agar

A

Mannitol fermenting staphylococcus (yellow) and non mannitol fermenting staphylococcus (pale) are differentiated

29
Q

CLED medium

A

Cysteine lactose electrolyte deficient medium

Lactose fermenting urinary pathogens (yellow) from non lactose fermenting primary pathogens (pale)

30
Q

Hugh Leifson oxidative fermenting medium

A

Differentiates the utilisation of glucose by bacteria
1. Oxidative utilisation
2. Fermentative utilisation (both oxidative and not)
3. No utilisation
pH indicator bromothymol blue and glucose are found
For anaerobic condition petroleum gel is used
Indicator: bromothymol blue

31
Q

Indicator media examples

A
  1. MacConkey media: neutral red
  2. TCBS medium: bromothymol blue
  3. Hugh Leifson medium: bromothymol blue
32
Q

Transport media

A

Maintain the original count
Do not allow commensals to overgrow
1. VR medium: vibrio
2. Pike’s medium: S. pyogenes in throat swabs
3. Thioglycolate broth for anaerobic bacteria
4. Cary Blair medium: universal stool transport medium

33
Q

Standard inoculum of test bacterium

A

Special suspension of test bacterium with a specified turbidity (in a units called Macfarland or McF)
Temperature of incubation: 36- 37°C
Time of result interpretation: 16-18 hours

34
Q

Methods of sensitivity testing classification

A
1. Dilution (reference method:
 Agar dilution
 Broth dilution
   Microbroth dilution
   Macrobroth dilution
2. Disc diffusion
3. E test
35
Q

Broth dilution method

A

Sensitivity testing
Nutrient broth medium used
Standard inoculum:
Overnight broth culture in peptone water

36
Q

Interpretation of dilution method

A

Resistant:
MIC> breakpoint value

Sensitive:
MIC value < breakpoint value

37
Q

Disadvantages of broth dilution method

A
  1. Cumbersome and laborious

2. Fastidious organisms cannot be tested

38
Q

Agar dilution method

Media, advantages and disadvantages

A

Cation adjusted Mueller Hinton agar
For both fastidious (add blood) and non fastidious media
For anaerobic bacteria, Wilkren Chalgren agar
Still cumbersome procedure but MIC can be calculated

39
Q

Kirby Bauer disc diffusion method

A

Mueller Hinton agar
6 discs impregnated with standardised concentration of antibiotics
Very easy to do
Cannot exactly determine MIC

40
Q

Stoke’s method

A

Control strain and test strain are incubated in the same petridish
Easy to do
Does not determine MIC

41
Q

Epsilometer test out E test

A

Combination of dilution and disc diffusion method
Easy and quantitative test
Determine MIC