The controlled growth of microorganisms, known as bacterial culturing, is a foundational technique across medical, industrial, and research laboratories. This process requires providing a managed environment and nutrient source to promote the multiplication of specific bacterial strains. Growing pure and robust cultures is necessary for diagnosing infectious diseases, developing antibiotics, and manufacturing biotechnology products. Successfully cultivating bacteria relies on specialized equipment that manages temperature, sterility, and atmospheric conditions.
The Core Apparatus: Incubators
The most fundamental piece of equipment for growing bacterial cultures is the incubator, which acts as an environmental chamber providing stable conditions for proliferation. Microorganisms have defined optimal growth temperatures, and the incubator maintains this setting with high precision over extended periods. For many common laboratory bacteria, the incubator is typically set to \(37^\circ\text{C}\).
Heating elements warm the air, while internal fans circulate the heat to ensure temperature uniformity across all culture vessels. The duration of incubation is strictly controlled, relating directly to the organism’s generation time and experimental requirements. For organisms that thrive in cooler environments, known as psychrophiles, a cooled incubator maintains temperatures below ambient, sometimes as low as \(4^\circ\text{C}\).
Achieving Asepsis: Sterilization Equipment
Before bacteria are introduced, growth media, glassware, and reusable tools must be rendered completely sterile to prevent contamination. The autoclave is the standard apparatus, utilizing high-pressure saturated steam to kill all forms of microorganisms, including highly resistant bacterial spores. Operating like a heavy-duty pressure cooker, it typically reaches \(121^\circ\text{C}\) and 15 psi, maintained for 15 to 30 minutes. The intense heat and moisture denatures microbial proteins, ensuring sterilization of liquid media and heat-tolerant glassware.
For items that cannot tolerate moisture, such as certain metal instruments or powders, a dry heat sterilizer (specialized oven) is necessary. This method relies on oxidation of the microbial cellular components, requiring higher temperatures and longer exposure times than steam. Dry heat sterilization is commonly performed at \(160^\circ\text{C}\) to \(170^\circ\text{C}\) for two hours or more, eliminating viable contaminants without introducing water vapor.
Tools for Safe Transfer and Inoculation
Inoculation, the process of moving bacteria into sterile media, requires specialized tools and careful aseptic technique to maintain culture purity. The inoculation loop and needle are the most common instruments for this transfer, consisting of a handle with a small wire loop or straight wire. These tools are sterilized immediately before and after use by heating the wire portion until it glows red, often using a Bunsen burner or an electric micro-incinerator.
A Bunsen burner also creates a localized zone of minimal air disturbance through convection, which helps minimize the risk of airborne contaminants. For more complex or sensitive procedures, a laminar flow hood, or biological safety cabinet, provides a larger, enclosed workspace. This cabinet maintains a constant, filtered stream of sterile air across the work surface, controlling the environment for manipulation and preventing cross-contamination.
Specialized Growth Environments
While a standard incubator is suitable for many bacteria, certain organisms or experimental designs require specialized modifications to the growth environment. The shaking incubator combines precise temperature control with continuous orbital agitation, primarily for growing bacteria in liquid broth cultures. The mechanical shaking (typically \(150\) to \(250\) RPM) serves two important functions for aerobic organisms.
First, agitation increases the surface area exposed to air, enhancing the rate at which dissolved oxygen is transferred into the liquid medium. Second, movement ensures nutrient molecules are uniformly distributed throughout the culture, preventing localized depletion. For bacteria sensitive to oxygen (obligate anaerobes), an anaerobic chamber or incubator is used. This sealed unit replaces atmospheric air with a non-oxygen gas mixture, maintaining oxygen levels below \(0.1\) percent, allowing these unique organisms to thrive.