General bacteriology Flashcards
Purposes of laboratory diagnosis of bacterial infection
- Identification
- treatment
- surveillance purpose
- for outbreak investigation
- to start PEP (Post exposure prophylaxis)
- to initiate appropriate infection control measures
The different types of laboratory diagnosis of bacterial infections
- Specimen collection
- Direct detection (microscope, antigen detection, molecular diagnosis)
- culture
- identification (bacterial identification, automated identification)
- antimicrobial susceptibility testing
- serology
- molecular methods
- typing methods
Louis Pasteur and Robert Koch in culture media development
Louis Pasteur developed liquid media/ broth
Rapid growth
Robert Koch introduced solid media using gelatin (15%)
Visible colonies
Disadvantages of gelatin as solidifying medium
- Proteolysed by some bacteria
- Solid only at temperatures less than 24°C
15% gelatin was used
Other solidifying agents are serum and egg
Agar agar properties
Extracted from sea weeds and red algae Also called Chinese grass Polysaccharide containing: 70% agarose 30% agaropectin Inorganic phosphate and calcium
Agar agar and its concentration
- 2 - 2 % for solid media (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
Methods to inhibit swarming of bacteria
- Firm agar (5-6%)
- CLED (cysteine lactose electrolyte deficient) or MacConkey agar
- Add alcohol, boric acid, bile salts, chloral hydrate, sulfonamides, sodium azide, surface acting agents
Gram positive bacteria that shows swarming
Clostridium tetani Bacillus cereus (‘serious’)
Gram negative bacteria that shows swarming
Proteus vulgaris
Proteus mirabilis
Vibrio alginolyticus
Vibrio parahemolyticus
Simple or basal culture media
Contains only sources of C and N for the nutritionally non-fastidious bacteria
- Peptone water
- Meat extract (+peptone water)
- Agar (+ meat extract +. ) or nutrient agar
Enriched culture media
- 5% sheep blood agar
- Chocolate agar
Egg mediums (for mycobacterium) like: - LJ medium
- Dorset egg medium
Serum containing medium like - Loeffler’s serum slope for C. diphtheriae
- PPLO broth/agar
Blood agar production
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)
Selective culture media
- Change pH
- Adding antibiotics
- Chemical agents
- Dye
pH of culture media and bacteria
For pathogenic bacteria it is usually 7.2-7.4
For vibrio it is 8.2-8.4
Example TCBS medium
Culture media for Neisseria
Thayer Martin medium contains: vancomycin Colistin Nystatin while modified Thayer Martin medium contains trimethoprim in addition to the other antibiotics
PPLO medium contains
Penicillin