Microbial Growth and Temperature

• Microbial growth is about the number of cells, not their size.
• Microbes increase in number and form colonies.
• Bacteria form biofilms to survive in nutrient-poor environments.
• Microbial growth requirements into physical and chemical categories.

Physical Requirements

• Physical aspects include temperature, pH, and osmotic pressure.
• Microorganisms grow best at human-favored temperatures, 98.6 F (37 C).
• Some bacteria thrive in temperature extremes.

Temperature Classifications

• Microorganisms are classified by preferred temperature range.
• Psychrophiles are cold-loving microbes.
• Mesophiles are moderate-temperature-loving microbes.
• Thermophiles are heat-loving microbes.
• Bacteria grow in a limited temperature range.
• Minimum growth temperature is the lowest temperature for growth.
• Optimum growth temperature is the temperature where a species grows best.
• Maximum growth temperature is the highest temperature for growth.
• Refrigeration preserves food by slowing microbial reproduction.
• Psychrophiles grow at 0°C with an optimal growth temperature of 15°C and cannot grow in warm rooms at 25°C.
• Mesophiles' optimum growth temperature is 25 to 40°C and are common as spoilage and disease organisms.
• Thermophiles are capable of growth at high temperatures.
• Many thermophiles' optimal growth temperature is 50 to 60°С, like hot tap water.
• Many thermophiles cannot grow at temperatures below 45°C.

pH

• pH measures the acidity or alkalinity of a solution.
• pH 7 is neutral; below 6 is acidic; above 8 is alkaline.
• Bacteria grow best near neutrality, pH 6.5-7.5.
• Few bacteria grow below pH 4.

Osmotic Pressure

• Microorganisms need water to obtain nutrients, with a composition of 80-90% water.
• High osmotic pressures remove water from cells, causing plasmolysis (cytoplasm shrinkage).
• Extreme halophiles need high salt concentrations to grow.

Chemical Requirements

• Chemical requirements include sources of carbon, nitrogen, sulfur, phosphorus, oxygen, trace elements, and organic growth factors.

Carbon

• Carbon is an important requirement for microbial growth, as it is the structural backbone of living matter.
• Chemoheterotrophs get carbon from organic materials.
• Chemoautotrophs and photoautotrophs derive carbon from carbon dioxide.

Nitrogen, Sulfur, and Phosphorus

• Cellular material synthesis requires nitrogen, sulfur, and phosphorus.
• Protein synthesis needs nitrogen and some sulfur.
• DNA and RNA synthesis require nitrogen and phosphorus; phosphorus is used to synthesize nucleic acids and phospholipids of cell membranes.
• Potassium, magnesium, and calcium act as enzyme cofactors.

Trace Elements

• Microbes need small amounts of iron, copper, molybdenum, and zinc as trace elements.
• They are added to laboratory media but are assumed to be in tap water and media components.

Oxygen

• Aerobes use molecular oxygen to extract more energy from nutrients.
• Anaerobes do not use oxygen.
• Obligate aerobes need oxygen to live.
• Facultative anaerobes can use oxygen or grow without it.
• Anaerobes cannot use molecular oxygen and are harmed by its presence.

Organic Growth Factors

• Organic growth factors are essential compounds an organism cannot synthesize and must get from the environment.
• Vitamins are an organic growth factor which humans and some bacteria cannot synthesize.
• Some bacteria lack enzymes for vitamin synthesis, so those vitamins are organic growth factors.

Biofilms

• Microorganisms live in communities called biofilms, not as isolated colonies.
• Biofilms are thin, slimy layers of bacteria on a surface.
• Quorum sensing is cell-to-cell chemical communication that allows bacteria to coordinate activity in communities.
• Biofilms allow bacteria to share nutrients and protection from desiccation, antibiotics, and the immune system.
• A biofilm forms when a planktonic bacterium attaches to a surface.
• Cell density in bacterial cells alters gene expression through quorum sensing.
• Quorum sensing allows bacteria to communicate and coordinate behavior.
• Bacteria using quorum sensing produce and secrete inducer.
• Inducer diffuses into the medium, prompting other cells to produce it; inducer concentration increases with cell numbers, attracting more cells and initiating inducer synthesis.

Culture Media

• Culture medium is a nutrient material for microorganism growth in a lab.
• Inoculum is microbes that are introduced into a culture medium to initiate growth.
• A culture is the microbes that grow and multiply in or on a culture medium.
• Culture medium requirements: The right nutrients, sufficient moisture, and properly adjusted pH, the proper oxygen level and must be sterile.
• The growing culture should be incubated at a proper temperature.
• Agar has valuable properties for microbiology.
• Few microbes degrade agar, so it remains solid.
• Agar liquefies at 100° C (212 F) and remains liquid until about 40° C (104 F).
• In lab use, agar is held in water baths at about 50° C (122 F) and does not injure bacteria.
• Solidified agar can be incubated at temperatures close to 100° C without reliquifying, useful for thermophilic bacteria.