Nature of impact on the microbial cell
The external environment exerts a constant influence on the vital activity of microorganisms. Depending on the intensity, concentration, and nature of the factor, the response of a microbial cell develops according to three main scenarios:
- Bactericidal effect: the factor causes irreversible damage leading to complete cell death.
- Bacteriostatic effect: viability is preserved, but microorganisms completely lose their ability to reproduce and grow.
- Mutagenic effect: the factor affects the genetic apparatus, provoking persistent changes in the hereditary properties of the bacterium.
Temperature factor and bacterial classification
Temperature is a crucial regulator of microbial activity. Regarding temperature range, bacteria are divided into three large groups.
1. Psychrophiles (cold-loving) They grow within the range of −10 to 40 °C (optimum 15–40 °C). In nature, these are inhabitants of sewage, fresh and marine waters, as well as soil (iron bacteria, luminous bacteria, pseudomonads). In everyday life, they often cause food spoilage in refrigerators. Among them are dangerous pathogens with pronounced psychrotolerance. For example, the pseudotuberculosis pathogen successfully multiplies at 4 °C, while the plague agent (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 precisely when the temperature drops.
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 strong heating with an optimum from 40 to 90 °C. Extreme species are able to survive in hydrothermal vents (at the ocean floor) at a pressure of 265 atmospheres and a temperature 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 its contamination with 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 resistant and require autoclaving at 120 °C.
Humidity and desiccation resistance
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, when embedded in mucus (sputum), remains viable for up to 90 days. Slime-producing and capsular bacteria also exhibit increased resistance.
- Spore forms: possess absolute resistance. Bacillus anthracis spores are capable of persisting 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 products can be stored for years without losing their original properties.
Radiation and chemicals
Radiation Short-wave radiation exerts a strong destructive effect on microflora. Ionizing radiation is used for "cold" sterilization of items that cannot be heated: disposable plastics, dressings, nutrient media, and certain drugs. An exception is made by specific radioresistant microorganisms (e.g., Micrococcus radiodurans, which was isolated from a nuclear reactor circuit). Ultraviolet (UV) radiation is also widely used in practice. Germicidal lamps generating UV in the 200–400 nm range 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).
Chemical substances Chemicals can act as a source of nutrition, be indifferent to the cell, or exert a pronounced suppressive effect (bacteriostatic and bactericidal). In medicine, aggressive chemicals are used as disinfectants and antiseptics. They possess a wide spectrum of activity, including virucidal and fungicidal action. The main groups of such chemical agents are strong oxidizing agents and halogen-containing compounds (chlorine-, bromine-, and iodine-based preparations).
Briefly: answers to topical questions
What is the mechanism of the bactericidal action of ultraviolet radiation on a microbial cell?
The mechanism of the destructive action of ultraviolet radiation consists in the destruction of the microorganism's DNA structure. Radiation acts predominantly on pyrimidine bases, causing the formation of covalent bonds between adjacent residues in a single strand.
As a result, the following are formed:
- Thymine dimers — cross-linking of two thymine residues;
- Cytosine dimers — cross-linking of cytosine residues;
- Thymine-cytosine dimers — cross-linking of thymine and cytosine.
These damages disrupt the DNA structure and prevent normal replication, ultimately leading to the death of the microbial cell.
Which groups of chemical substances are used in medicine as disinfectants and antiseptics?
Various groups of chemical substances, classified by their chemical structure, are used in medicine as disinfectants and antiseptics.
The main groups include:
- Halogen-containing compounds — chlorine, iodine, and bromine preparations;
- Oxidizing agents — peroxide and oxygen-containing compounds;
- Surfactants — detergents, quaternary ammonium compounds;
- Alcohols — ethyl alcohol;
- Aldehydes — formaldehyde, glutaraldehyde;
- Acids and alkalis — boric, salicylic, acetic acids, ammonia;
- Heavy metal compounds — salts of mercury, silver, zinc, copper;
- Phenolic compounds — lysol, hexachlorophene;
- Guanidine derivatives — biguanides;
- Dyes — brilliant green, methylene blue;
- Synthetic derivatives — nitrofurans, quinolones.
What is the mechanism of the bactericidal action of halogen-containing disinfectants?
The mechanism of the bactericidal action of halogen-containing disinfectants is based primarily on oxidative damage to the microbial cell. In accordance with the classification by mechanism of action, halogens implement an oxidative mechanism that causes the destruction of microbial proteins and nucleic acids.
In addition, some halogen-containing compounds have additional mechanisms of damage. For example, triclosan causes disruption of microbial cellular respiration.
To memorize the types of action of factors: CID — kills (bacteri*cid*al, sounds like suicide), STAT — stops (bacterio*stat*ic, like statics), MUT — changes genes (*mut*agenic).
Class and exam questions
How do low temperatures affect bacteria?
Microorganisms mostly tolerate the cold well. At low temperatures, they enter a state of suspended animation, which is widely used in microbiology for the long-term preservation of cultures.
Can temperature alter the dangerous properties of a pathogen?
Yes, the temperature regime strongly affects the phenotype. For example, the plague pathogen exhibits significantly greater virulence at 25 °C than at the standard 37 °C.
What is the essence of lyophilization?
This is a laboratory preservation method in which frozen biological material is gently dried under vacuum conditions. It allows live vaccines and cultures to be stored for years without altering their original properties.
What else to study on this topic
- Mechanisms of action of halogen-containing antiseptics on the cell wall
- Specific spore resistance: chemical composition and structure
- Radioresistant bacteria using Micrococcus radiodurans as an example
- The problem of food spoilage by psychrophiles during cold storage
- The effect of temperature on the synthesis of virulence factors in pathogens
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