Temperature Requirements for Cultivation
One of the primary parameters for classifying bacteria is their temperature optimum for growth. Depending on the temperature most favorable for metabolism and division, microorganisms are divided into three main groups:
- Mesophiles. The optimal temperature for their cultivation ranges between $20–40\ ^\circ\text{C}$. This is the most significant group in medical microbiology because the temperature optimum coincides with the human body temperature. The vast majority of pathogenic bacteria are mesophiles.
- Thermophiles. Heat-loving microorganisms that actively multiply at high temperatures—from $40$ to $60\ ^\circ\text{C}$. Typical representatives of thermophiles include actinomycetes and certain spore-forming bacilli.
- Psychrophiles. Cold-loving bacteria with a temperature optimum ranging from $0$ to $20\ ^\circ\text{C}$.
Methods for Creating Anaerobic Conditions
To grow oxygen-sensitive bacteria, special gas conditions must be established. There are two main approaches to creating anaerobiosis:
- Chemical method. Based on the use of special substances that absorb oxygen from the environment (e.g., alkaline pyrogallol). Hermetic containers equipped with systems such as "gas packs" are also widely used in practice. The main purpose of chemical methods is the cultivation of aerotolerant bacteria capable of surviving in the presence of residual oxygen.
- Mechanical (physical) method. Involves the complete physical removal of air from the working volume, followed by replacement with an inert gas. This process requires specialized equipment, such as anaerobic jars or hermetic anaerobic workstations.
Incubation Parameters: Time and Light
In addition to temperature and gas composition, time and light regimes are critical factors.
The time factor directly depends on the division rate (generation time) of a specific microorganism species. For most bacteria, the standard cultivation period ranges from 18 to 48 hours. However, there are slow-growing species whose diagnosis takes significantly longer:
- The causative agent of pertussis requires incubation for 5 days.
- Mycobacterium tuberculosis forms visible growth only after 3–4 weeks.
The light regime is vital for phototrophic microorganisms. In medical microbiology, light has important diagnostic significance when working with opportunistic mycobacteria: illumination directly affects their ability to form pigment, which is a key criterion for intraspecific identification.
Specific Methods and Industrial Cultivation
A major challenge in microbiology is the cultivation of obligate intracellular parasites. This group includes members of the genera Rickettsia, Ehrlichia, Coxiella, and Chlamydia. Since they cannot multiply on cell-free artificial media, living systems are used for their growth:
- Cell cultures;
- Susceptible animal or arthropod hosts;
- Chicken embryos.
Note: Chicken embryos are suitable for cultivating most intracellular agents, as well as bacteria with a high degree of heterotrophy (e.g., Borrelia and Legionella). An exception is Ehrlichia, which do not grow in chicken embryos.
Industrial cultivation has completely different scales and goals. It aims to obtain massive microbial biomass, which is then used to produce antibiotics, vaccines, probiotic preparations, and diagnostic kits. This process is carried out in specialized apparatuses called fermenters and requires rigorous automated control of all bacterial population growth parameters.