Nature of Impact on the Microbial Cell
The external environment continuously influences microbial activity. Depending on the strength, concentration, and nature of the factor, the response of the microbial cell follows three main scenarios:
- Bactericidal action: The factor causes irreversible damage leading to complete cell death.
- Bacteriostatic action: Viability is preserved, but microorganisms completely lose the ability to reproduce and grow.
- Mutagenic action: The factor affects the genetic apparatus, provoking stable changes in the hereditary properties of the bacterium.
Temperature Factor and Bacterial Classification
Temperature is a crucial regulator of microbial activity. Based on temperature range, bacteria are divided into three major groups.
1. Psychrophiles (Cold-loving) Grow in the range of −10 to 40 °C (optimum 15–40 °C). In nature, they inhabit sewage, fresh and marine waters, and soil (iron bacteria, luminous bacteria, pseudomonads). They frequently cause food spoilage in refrigerators. Among them are dangerous pathogens with pronounced psychrotolerance. For example, the pseudotuberculosis pathogen successfully reproduces at 4 °C, while the plague pathogen (Yersinia pestis) has a growth range from 0 to 40 °C (optimum around 25 °C). Interestingly, changing the cultivation temperature can alter the phenotype of microorganisms:
- Serratia marcescens synthesizes more red pigment (prodigiosin) at 20–25 °C than at 37 °C.
- The plague pathogen becomes more virulent at 25 °C.
- The synthesis of protective capsular polysaccharides in certain bacteria is activated specifically upon temperature reduction.
2. Mesophiles The most significant group for medical microbiology, as it includes the bulk of pathogenic and opportunistic bacteria. Their temperature boundaries are 10–47 °C, and the optimal growth temperature coincides with human body temperature (around 37 °C).
3. Thermophiles Adapted to high heat with an optimum from 40 to 90 °C. Extreme species can survive in hydrothermal vents (at the ocean floor) under a pressure of 265 atmospheres and temperatures of 250–300 °C. They inhabit hot springs, as well as self-heating hay, grain, and manure. The detection of a high concentration of thermophiles in soil serves as an indicator of contamination by compost or manure.
In practice, high temperatures are used for sterilization. Vegetative forms die at 60 °C in 20–30 minutes. Spores are significantly more resilient and require autoclaving at 120 °C.
Humidity and Resistance to Desiccation
The presence of water is critical for bacteria: dehydration disrupts basic physiological functions. The degree of sensitivity to drying varies among different species.
- Highly sensitive: Pathogens of meningitis, gonorrhea, dysentery, typhoid fever, and cholera die very quickly upon drying.
- Protected forms: Survival increases if the bacterium is sheltered by external barriers. Mycobacterium tuberculosis, residing in mucus (sputum), remains viable for up to 90 days. Slime-forming and encapsulated bacteria also exhibit increased resistance.
- Spore forms: Possess absolute resistance. Spores of the anthrax pathogen can persist in dried soil for centuries.
To preserve microorganisms and immunobiological preparations in laboratories, lyophilization is actively used. The method consists of drying pre-frozen material under vacuum conditions. As a result, biological preparations can be stored for years without losing their original properties.
Radiation and Chemicals
Radiation Short-wave radiation has a strong destructive effect on microflora. Ionizing radiation is used for "cold" sterilization of items that cannot be heated: single-use plastics, dressings, culture media, and certain drugs. An exception is specific radioresistant microorganisms (e.g., Micrococcus radiodurans, which was isolated from a nuclear reactor cooling circuit). Ultraviolet (UV) radiation is also widely applied in practice. Germicidal lamps generating UV in the range of 200–400 nm are massively used for air and surface disinfection in maternity hospitals, clinics, and laboratories (natural solar UV light reaching Earth has a wavelength of 290 nm).
Chemicals Chemicals can act as a nutrient source, be indifferent to the cell, or exert a pronounced inhibitory effect (bacteriostatic and bactericidal). In medicine, aggressive chemicals are used as disinfectants and antiseptics. They possess a broad spectrum of activity, including virucidal and fungicidal action. The main groups of such chemical agents are strong oxidizers and halogen-containing compounds (chlorine-, bromine-, and iodine-based preparations).