Lecture 5 microbial growth Flashcards
Bacterial growth times
- Asexual process known as binary fission; division is geometrical
- Generation time (optimal conditions) ex.
B. stearothermophilus: 11 min.
E. coli: 20 min.
S. aureus: 28 min.
L. acidophilus: 60 to 80 min.
M. tuberculosis: 360 min
T. pallidum: 1980 min.
Fungal growth
- moulds hyphae -> mycelium
- extension at tip (apical growth)
- up to 40 microns/min.
- provides penetrating power (fresh nutrients)
Temp adaptations
Psychtrophile: no harm to humans
- Opt. temp < 15 C (cannot grow > 20 C)
- found in snow fields, polar ice, deep ocean
Facultative Psychtrophile: harmful to humans (contaminants in deli meats in fridge)
- grow slowly in cold conditions
- Optimal temp > 20 C
- Ex. S. aureus, L. monocytogenes
Mesophile: very harmful
- Opt. temp. @ 20 to 40 C (capable @ 10 to 50 C)
- group containing human pathogens
- Ex. E. coli
Thermophile
- Opt. temp >45 C (capable 45 to 85 C)
- incapable of growth at usual body temp
Microbes divided into 4 groups based on oxygen requirements
- Obligate aerobes
- Microaerophiles: grow in presence of O2 but tolerate only 4% b/c lack catalase
- Obligate Anaerobes: brief exposure will kill; No SOD or catalase
- Facultative Anaerobes: can switch b/w 2 types of O2 metabolism (presence or absence); grow best under aerobic conditions, Ex.
Notes:
- Bacteria: all 4 groups
- Fungi/moulds: normally aerobic
- Yeasts & Enterobacteriacea: facultative anaerobes
Water activity microbial categories
- Xerotolerant: lower Aw
- fungi & yeasts grow @ Aw=0.60
- cold cuts, grated cheese - Halophiles
- high [solute], low Aw
Chemical factor microbial categories (partial)
Hydrogen/electron
- lithotrophs: reduced inorganic molec
- organotrophs: organic molec
Note: most pathogens are chemorganotrophic heterotrophs
See Lecture 5 slide 34-35 for rest
Reduced medium
- addition of reducing agent Ex. thioglycolate, cystine or ascorbate
- removes O2 so anaerobes can grow
Differential media
- more than 1 type of organism can grow but separation b/w genera & sp. (visible changes in media)
ex. MacConkey agar: selective medium designed to selectively grow Gram-negative bacteria and differentiate them for lactose fermentation
- lactose present as sole nutrient source
- so E. coli will use the lactose and produce acid to produce red
- S. Typhi does not use lactose
- P. aeruginosa produces grey/blue pigment on Mac cultures.
ex2. Eosin-methylene blue (EMB): E. coli & enterobacter aerogenes
Note: media can be selective & differential or enriched and differential at the same time
Fungal cultivation
- Primary (isolation): Sabouraud’s Agar
- peptone (protein hydrolysate)
- pH 5.6 (inhibits faster growing bacteria)
- antibiotics: penicillin, streptomycin, tetracycline (inhibit bacteria) - Secondary (culture & species id): Corn meal agar
- incubated @ 25C, several days
- characteristic sex structures develop
- ID based on morphology (microscopy)
Enriched media & hemolysis
- basal growth support media + nutritive supplements added Ex. Blood agar
- Beta-hemolysis (clearing): complete hemolysis due to molecular mimicry; Ex. Strep. pyogenes
- Alpha-hemolysis (greening): Ex. Strep. viridans, Strep. pneumoniae
- Gamma-hemolysis (no change): Ex. Enterococcus faecalis
