Microbial Growth: Temperature, pH & Oxygen

Microbial Growth Requirements

• Microbes need both physical and chemical conditions to grow.
• Physical factors include: temperature, pH, and osmotic pressure.
• Chemical factors include: carbon, nitrogen, sulfur, phosphorus, trace elements, oxygen, and organic growth factors

Temperature Preferences of Microbes

• Psychrophiles thrive in cold environments like ocean depths and polar regions.
• Mesophiles, the most common type, favor moderate temperatures, living in animals and causing disease, and are easy to incubate.
• Thermophiles prefer heat, like thermal water.
• Psychotrophs tolerate cold.
• Hyperthermophiles thrive in extreme heat, with optimum growth above 80°C.

pH Preferences of Microbes

• Most bacteria prefer a neutral pH range of 6.5 to 7.5.
• Molds and yeasts thrive in more acidic conditions, between pH 5 and 6

Oxygen Requirements of Bacteria

• Obligate aerobes need oxygen.
• Facultative anaerobes grow using fermentation or anaerobic respiration when oxygen is absent.
• Anaerobes cannot use oxygen and are harmed by it.
• Aerotolerant anaerobes mainly grow without oxygen but can tolerate it.
• Microaerophiles require oxygen concentrations lower than in air.

Biofilms

• Biofilms are microbial communities.
• They create slime or hydrogels that stick to different surfaces.
• Biofilms enable microbes to share nutrients.
• They shelter bacteria from harmful environmental factors.

Agar Usage

• Agar is a solidifying agent for culture media in petri plates, slants and deeps.
• Agar liquefies at 100°C.
• Agar solidifies at approximately 40°C.

Anaerobic Growth Method

• Reducing media is used for anaerobic growth.
• It cultivates anaerobic bacteria.
• Contains chemicals that combine with O2 to deplete it.
• Heated to drive off any remaining O2.

Selective vs. Differential Media

• Selective media suppress unwanted microbes and encourage desired ones, containing inhibitors.
• Differential media allows the observer to distinguish colonies of different microbes on the same plate.

Colonies

• A colony is a population of cells arising from a single cell, spore, or group of attached cells.
• It’s also known as a colony-forming unit

Bacterial Cell Division

• Bacteria divide through binary fission

Direct Measurement of Microorganisms

• Methods for this include: plate count, filtration, most probable number (MPN) method, and direct microscopic count.

Turbidity Measurement

• Turbidity is the measurement of cloudiness.

Sepsis vs. Asepsis

• Sepsis refers to bacterial contamination
• Asepsis refers to the absence of significant contamination.

Sterilization vs. Disinfection

• Sterilization removes and destroys all microbial life.
• Disinfection destroys harmful microorganisms.
• Sanitation lowers microbial counts on eating utensils to safe levels.

Actions of Microbial Control Agents

• Alteration of membrane permeability: This damages the plasma membrane.
• Damage to proteins: This affects enzyme function and cause denatured protein.
• Damage to nucleic acids: This prevents cells from replicating.

Physical Microbial Growth Methods

• Heat, moist heat sterilization, pasteurization, sterilization, dry heat sterilization.
• Also includes: filtration, low temperature, high pressure, desiccation, osmotic pressure, radiation

Sterilization with Autoclave

• Autoclaves use steam under pressure achieving 121°C at psi for 15 minutes.
• This process effectively kills all organisms including endospores.
• Steam must directly contact the items requiring sterilization.

Pasteurization

• Pasteurization uses mild heat to reduce spoilage organisms without harming the product.

Plasmolysis

• Plasmolysis is when osmotic pressure uses high concentrations of salts and sugars to create a hypertonic environment.

Effects of Radiation on Microorganisms

• Ionizing radiation damages DNA by causing lethal mutations.
• Nonionizing radiation damages DNA by creating thymine dimers
• Microwaves kill by heat, but are not especially antimicrobial.

Heavy Metals for Microbial Control

• Heavy metals used include: silver, mercury, copper and zinc.
• Silver nitrate prevents ophthalmia neonatorum.
• Mercuric chloride prevents mildew in paint.
• Copper sulfate is an algicide.
• Zinc chloride is found in mouthwash.

Alcohol's Effect on Microbes

• Alcohols denature proteins and dissolve lipids.
• They kill bacteria but does not affect endospores.
• 100% ethanol can still allow bacteria to grow because as it acts, it evaporates.

Plasma

• Plasma is the fourth state of matter.
• Plasma is used to destroy microbes by free radicals.

Learn about the physical and chemical conditions required for microbial growth, including temperature, pH, osmotic pressure, carbon, nitrogen, oxygen and other factors. Understand temperature preferences of microbes such as psychrophiles, mesophiles, thermophiles, and hyperthermophiles. Discover pH preferences for bacteria, molds, and yeasts, as well as oxygen requirements for different types of bacteria.