Chapter 6 Microbial Growth The Requirements for Growth

Chapter 6 Microbial Growth

The Requirements for Growth: Physical Requirements • Temperature • Minimum growth temperature • Optimum growth temperature • Maximum growth temperature

Temperature Celcius to fahrenheit conversion Figure 6. 1

• Psychrophiles: cold-loving microbes • Psychrotrophs (“moderate psychrophiles”): cold tolerant; cause food spoilage • Mesophiles: moderate-temperature-loving microbes • Thermophiles: hot-loving microbes

Figure 6. 2

The Requirements for Growth: Physical Requirements • p. H • Most bacteria grow between p. H 6. 5 and 7. 5 • Molds and yeasts grow between p. H 5 and 6 • Acidophiles • Alkalophiles • neutralophiles

The Requirements for Growth: Physical Requirements • Water - (osmotic pressure) • Hypertonic environments can cause plasmolysis • Extreme or obligate halophiles require high [salt] • Facultative halophiles tolerate high [salt] • Hypotonic environment may cause cells to burst

Figure 6. 4

The Requirements for Growth: Chemical Requirements • Carbon • Form structural and functional organic molecules; energy source • Heterotrophs • Autotrophs

The Requirements for Growth: Chemical Requirements • Nitrogen • Used to make amino acids (proteins), nucleic acids • Bacteria acquire nitrogen from various sources: • Most by decomposing proteins • Some bacteria use NH 4+ (ammonium), NO 3 (nitrate), or N 2 (nitrogen gas)

The Requirements for Growth: Chemical Requirements • Sulfur • Is used to make some amino acids, thiamine, biotin • Bacteria acquire sulfur from various sources: • Most bacteria decompose proteins • Some bacteria use SO 42 (sulfate) or H 2 S • Phosphorus • Is used to make DNA, RNA, ATP, and membranes

The Requirements for Growth: Chemical Requirements • Trace Elements • Inorganic elements required in small amounts • Usually as enzyme cofactors

The Requirements for Growth: Chemical Requirements • Organic Growth Factors • Organic compounds that the microbe cannot synthesize • Obtained from the environment • Vitamins, amino acids, other compounds

The Requirements for Growth: Chemical Requirements • Oxygen (O 2) obligate aerobes Facultative anaerobes Obligate anaerobes Aerotolerant anaerobes Microaerophiles

Culture Media • Medium: Nutrients prepared for microbial growth • Sterile: No living microbes • Inoculation: Introduction of microbes into medium • Inoculum: the microbes used to inoculate a medium • Culture: Microbes growing in/on a medium

Agar • Complex polysaccharide • Used as solidifying agent for culture media in Petri plates, slants, and deeps • Generally not metabolized by microbes • Liquefies at 100°C • Solidifies ~40°C

Culture Media • Chemically Defined Media: Exact chemical composition is known • Complex Media: Extracts and digests of yeasts, meat, or plants • i. e. Nutrient broth • i. e. Nutrient agar • Extract: water soluble preparation of broken down cells • Digest: water soluble preparation of partially broken down proteins

Culture Media Table 6. 2 & 6. 4

Anaerobic Culture Methods • Reducing media • Contains thioglycolate that combines with O 2 • http: //www. mc. maricopa. edu/~johnson/labtools/Dbiochem/oxy. html • Oxyrase – commercial product that reduces O 2 to H 2 O (removes the oxygen)

Anaerobic Culture Methods • Anaerobic jar • Chemical reactions remove oxygen Figure 6. 5

Capnophiles require high CO 2 • Candle jar • CO 2 -packet Figure 6. 7

Selective Media • Suppresses unwanted microbes and allows desired microbes. • How? i. e. vary the [salt] or p. H • (Enrichment Media: encourages growth of microbes present in small numbers and/or are fastidious) • Fastidious: require specific nutrients • How? Use special ingredient

Differential Media • Make it easy to distinguish colonies of different microbe types. • How? • i. e. hemolysis on blood agar • i. e. mannitol fermentation • i. e. p. H indicator (color)

Pure Culture • A pure culture contains only one species or strain • A colony is a population of cells arising from a single cell or spore or from a few attached cells • A colony = colony-forming unit (CFU)

Preserving Bacterial Cultures • Deep-freezing: -50°C to -95°C • Lyophilization (freeze-drying): Frozen (-54° to -72°C) and dehydrated in a vacuum; stored at room temp. • Bacteria can be preserved for years by these methods

Reproduction in Prokaryotes • Binary fission

• Budding • Microbial growth = increase in number of cells, not cell size

Generation Time • The time period for one cell to divide; therefore, for the population to double • Generation time varies from microbe to microbe • E. coli = 20 – 30 minutes • Mycobacterium tuberculosis = 15 – 20 hours