Preparación medios de cultivo - Microbiología

Preparation of Culture Media

Introduction to Culture Media

• The video introduces the concept of culture media, which serves as a substrate or nutrient solution for growing and multiplying microorganisms in a laboratory setting. The goal is to isolate different bacterial species for identification and further studies.

Types of Culture Media

Liquid and Solid Media

• Liquid media contain nutrients with added substances to stabilize pH levels; examples include nutrient broth and peptone broth.

• Solid media are created by adding agar, a polysaccharide derived from marine algae, to liquid media, facilitating the isolation of bacterial colonies.

Synthetic vs. Natural Media

• Synthetic (chemically defined) media consist of known chemical products used for metabolic studies, such as nutrient broth and methylene blue agar.

• Natural (chemically undefined) media are prepared from natural substances (animal or plant origin), like milk or carrot.

Simple vs. Enriched Media

• Simple media provide basic nutritional requirements for general bacterial growth; examples include nutrient agar.

• Enriched media have additional elements (e.g., blood, serum, glucose) that support the growth of nutritionally demanding bacteria.

Selective and Differential Media

Selective Media

• Selective media contain harmful chemicals for certain bacteria or altered physical conditions to inhibit unwanted growth; examples include violet crystal agar and bile lactose broth.

Differential Media

• Differential media allow specific bacteria to grow while providing observable characteristics that differentiate them from others; examples include methylene blue agar differentiating E. coli from Enterobacter aerogenes based on colony color.

Practical Procedure for Preparing Culture Media

Storage and Labeling

• Culture media are stored in labeled plastic containers detailing composition, expiration date, hazard pictograms, and preparation formula crucial for calculating desired volumes.

Calculating Required Amounts

• For preparing 100 mL of nutrient broth requiring 13 grams per liter: using rule of three gives 1.3 grams needed.

• For sulfide-indole-motility medium needing 36.23 grams per liter: calculation yields 3.6 grams required for 100 mL.

Agar Preparation

• Nutrient agar requires 23 grams per liter; thus, preparing 300 mL needs approximately 6.9 grams.

• Methylene blue agar requires about 37.46 grams per liter; hence preparing 300 mL necessitates around 11.2 grams.

Final Steps in Preparation

• For HGC agar needing 40 grams per liter: preparation of 300 mL results in approximately 12 grams being dissolved in distilled water.

Preparation and Sterilization of Culture Media

Steps for Preparing Culture Media

• The process begins with heating 100 milliliters of the medium while continuously stirring with a glass rod until it reaches the first boil, after which it should be removed from heat. This procedure is repeated three times to ensure proper mixing.

• After preparation, the media intended for Petri dishes are covered with aluminum foil and prepared for sterilization in an autoclave. This step is crucial to prevent contamination before use.

Autoclaving Process

• Tubes containing the medium must not be completely sealed; lids should remain loose. They are placed in a heat-resistant plastic container for sterilization, ensuring that water levels touch the metal rack inside the autoclave both before and after sterilization.

• Once connected to power, the autoclave is turned on, adjusting the thermostat knob to reach specific pressure and temperature settings (15 psi and 121°C). The media are then placed inside an internal aluminum container within the autoclave. Proper sealing is essential to maintain pressure during sterilization.

Monitoring Sterilization Conditions

• It’s important to check that all seals on the autoclave lid are secure; otherwise, air may escape, preventing effective sterilization. The air control valve must also be closed properly before starting the process. As pressure builds up, it should stabilize at 15 psi for 20 minutes while maintaining 121°C temperature conditions.

• After completing this duration, turn down the thermostat to zero and switch off the autoclave; wait until pressure drops back down to 5 psi before opening any valves or removing items from inside. This ensures safety during handling post-sterilization.

Post-Sterilization Handling

• Once safe to open, remove items carefully by releasing locks in pairs opposite each other and take them out into a laminar flow cabinet for further processing. This minimizes exposure to contaminants after sterilization has been completed successfully.

• For tubes that were previously prepared, they need complete sealing before being placed in gravel for refrigeration storage until needed again later on in experiments or procedures involving culture media preparation.

Final Steps Before Use

• Petri dishes are wrapped with kraft paper prior to their placement into an oven set at 20°C for two hours of additional sterilization time—this step ensures thorough disinfection of all surfaces involved in culturing processes later on when used with microbial samples or cell cultures as required by protocols established beforehand.

• After removal from the oven once cooled down sufficiently enough not cause burns upon contact with skin surfaces (as indicated), they can then be transferred into a laminar flow cabinet where sterile conditions will allow safe handling without risk contamination occurring during subsequent steps leading up serving media directly onto plates themselves!

Serving Culture Media