Microbial Growth

Physical Requirements

Temperature adaptations: “optimum” temperature

Psychrophiles – cold-loving (15o )

Psychrotrophs – (20-30o) e.g., bacteria that spoil refrigerated food

Mesophiles –moderate temperature-loving (25-40o ), e.g, most pathogens & food spoilers; thermoduric microbes may survive short exposure to high temperature conditions

Thermophiles – heat-loving (50-60o )

Extreme thermophiles (hyperthermophiles) (80o or higher) e.g., many archaea organisms

Optimum pH:

Most bacteria prefer neither acidic nor basic (ph=6.5-7.5) but a few species are acidophiles, i.e, tolerate acidic conditions.

Optimum osmotic pressure:

Most bacteria cannot tolerate “hypertonic” medium (e.g., very salty, very sweet); they tend to shrink (plasmolyze)

Some bacteria with weak cell wall may burst in “hypotonic” medium (e.g., distilled water)

 But some species are halophiles, i.e., either prefer high salt concentrations or tolerate them better

Macronutrients

• Elements Carbon (from CO2 or organics), Nitrogen, Sulfur and Phosphorus are required in large amounts from organic compounds.
• Oxygen (from O2 ) is required by obligate aerobic and facultatively anaerobic organisms for cell respiration; note that obligate anerobic organisms may be harmed by the presence of oxygen.

Micronutrients (or trace elements)

• Iron, copper, zinc, molybdenum are required in minute amounts and mainly come from tap water.

Organic growth factors

• Organics that the organisms are unable to synthesize themselves which therefore has to directly come from the environment, e.g., vitamins

Biofilms (also called slime, hydrogel)

• a microbial community, specifically bacteria, that usually forms as a slimy layer on a surface; microbes communicate & coordinate with each other to better protect themselves, improve efficiency in resource use
• may have the potential to destroy materials that form the surface, cause infection if growing on living surface

Growing bacteria

Culture medium

• nutrient material prepared in the lab for microbial growth, e.g., (solid) agar plate, agar deep, agar slant; (liquid) broth

Culture – microbes that grow in or on a culture medium

colony- clone (genetically identical) of bacterial cells growing on a solid medium, visible to the naked eye

Types of culture media

Based on chemical composition

• Chemically defined media (synthetic) – exact composition is known; for fastidious organisms or microbial assay
• Complex media (non-synthetic) – ingredients not precisely defined, such as meat or yeast extracts e.g., nutrient broth (NB) when liquid, nutrient agar (NA) when solid
• Anaerobic growth media – for growth of anerobes; oxygen depleted from media

Based on specific functions

• Selective media – suppress growth of unwanted microbes
• Differential media – contain indicators which differentiate among microbes
• Enrichment media – enhance growth of microbes of interest

Microbial population growth characteristics

Note: A population refers to a group of organisms of the same species living in the same place.

Bacterial growth generally refers to population growth in bacterial cultures due to increase in cell number (population size) NOT increase in cell size.

Describing bacterial growth

• One way to describe how fast a population is growing is by calculating its generation time: average time it takes for a new generation to show up, i.e., for bacteria, time required for cells to divide
• g., most bacteria’s generation time is from 1 to 3 hours
• Calculating population growth rate: population size (# of cells) at any given time

Four Phases of Microbial Growth

Lag phase – slow increase

Log phase – rapid (logarithmic) increase

Stationary phase – no change in population size

Death (decline) phase – population decreases