Microbial Growth: Physical Requirements

Microbial Growth

• Microbial growth refers to the number of cells doubling, not individual cell size increasing.
• Petri dish culture plate contains a blood agar growth medium inoculated with Serratia marcescens bacilli.

The Requirement for Growth

Physical Requirements

• Temperature, pH, and osmotic pressure are crucial for microbial growth.
• Psychrophiles are cold-loving microbes.
• Mesophiles are moderate-temperature-loving microbes.
• Thermophiles are heat-loving microbes.
• Minimum growth temperature is the lowest temperature a species can grow.
• Optimum growth temperature is the temperature at which a species grows best.
• Maximum temperature is the highest temperature a species can grow.
• Refrigeration is a common method for preserving household supplies by reducing microbial reproduction rates.
• Most bacteria prefer a narrow, near-neutral pH range between 6.5 and 7.5.
• Acidophiles are tolerant of acidity.
• Chemoautotrophic bacteria can survive in pH 1 in coal mine drainage and oxidizes sulfur to form sulfuric acid.
• Molds and yeasts can grow over a greater pH range than bacteria, optimally at pH 5-6.

Chemical Requirements

• Carbon, nitrogen, sulfur, phosphorus, potassium, magnesium, and calcium are essential.
• Trace elements and growth factors are necessary cofactors for enzymes and metabolic processes.
• Carbon is the structural backbone of organic compounds.
• Chemoheterotrophs get most of their carbon from organic materials like proteins, carbohydrates, and lipids.
• Photoautotrophs and chemoheterotrophs derive their carbon from carbon dioxide.
• Nitrogen is needed for protein synthesis, RNA, and DNA.
• Cyanobacteria use nitrogen fixation.
• Phosphorus synthesizes ATP and forms amino acids, nucleic acids, and cell membrane phospholipids.
• Sulfur synthesizes sulfur-containing amino acids and vitamins, such as thiamine and biotin.
• Obligate aerobes require oxygen for aerobic growth.
• Facultative anaerobes can grow aerobically and anaerobically, but grow better with oxygen.
• Obligate anaerobes grow only in the absence of oxygen, and their growth ceases if oxygen is present.
• Aerotolerant anaerobes grow only anaerobically, but their growth continues even if oxygen is present.
• Microaerophiles need oxygen in low concentrations.
• Catalase and superoxide dismutase (SOD) allow organisms to neutralize toxic forms of oxygen.

Culture Preparation

• A culture medium is a nutrient material prepared for the growth of microorganisms.
• Inoculum is microbes introduced into a culture medium for growth initiation.
• Culture refers to microbes that grow and multiply on a culture medium.

Criteria for Culture Medium

• Must be sterile, and incubated at proper temperature.
• Must have right nutrients, sufficient moisture
• Must contain properly adjusted pH, a suitable level of oxygen
• Must be no oxygen

Defined vs. Undefined Media

• Known quantities and provides defined carbon and nitrogen sources, along with trace elements and vitamins.
• Glucose or glycerol are used as carbon sources in defined media.
• Ammonium salts or nitrates are used as inorganic nitrogen sources.
• Has complex ingredients like yeast extract with a mixture of many, many chemical species in unknown proportions.
• Sometimes chosen based on price, and some microorganisms have never been cultured in defined media.

Types of Culture Media

• Nutrient broth is a liquid medium used for cultivating various organisms from clinical specimens.
• Solid media contain agar and are used to grow microorganisms in their full physical form.
• Agar is a solidifying agent that liquefies at 100°C and remains liquid until the temperature drops to 40°C, few microbes can degrade agar.
• Semi-solid media contain 0.5% of agar, has a soft-custard consistency, and is used to study microaerophilic organisms or determine motility of bacteria.

Chemically Defined Medium

• Exact chemical composition is known.
• Ideal for chemoheterotrophs needing specific organic growth factors.

Complex Media

• Containing nutrients from yeasts, meat, plants, or protein digests.
• Source of carbon, nitrogen, and sulfur.

Reducing Media

• Contains substances (thioglycollic acid or sodium thioglycolate) that absorb oxygen or slow its penetration.
• Suitable for anaerobic bacteria cultivation.

Special Culture Technique

• Mycobacterium leprae is usually grown in armadillos due to their relatively low body temperature.
• Candle jars are used to grow capnophiles needing increased carbon dioxide.

Selective Media

• Suppresses unwanted bacteria growth while encouraging desired microbe growth.
• Examples: Bismuth Sulphite Agar and Saboraud Dextrose Agar (pH 6).

Differential Media

• Distinguishes desired organism colonies from others growing on the same plate.
• Red blood cells contained.
• Examples: Blood Agar, Eosin Methylene Blue (EMB) agar, and MacConkey (MCK) agar.

Enrichment Media

• Increases small bacterial numbers to detectable levels, often used for fecal or soil samples.
• The media selectively provides conditions for the desired organisms, inhibiting other bacteria.
• Example: Selenite F Broth.
• Chocolate Agar (enriched) is used for isolating fastidious microorganisms like Neisseria and Haemophilus.
• Chocolate agar is a version of blood agar.

Explore microbial growth, focusing on physical requirements such as temperature, pH, and osmotic pressure. Learn about psychrophiles, mesophiles, and thermophiles and how they thrive in different conditions. Discover the significance of minimum, optimum, and maximum growth temperatures for microbial reproduction.