Let us make an in-depth study of the influence of environmental factors on the growth of microorganisms. The below given article will help you to learn about the following things: 1. Chemical Factors 2. Physical Factors and 3. Biological Factors.
A. Chemical Factors:
(4) Surface active substances;
(5) Phenols; creosols and derivatives;
(7) Salts of heavy metals;
(8) Oxidising agents;
B. Physical Factors:
(5) High pressure;
(6) Osmotic pressure.
C. Biological Factors:
A. Chemical Factors:
1. Nutrition of Microorganisms:
(a) Metabolism means the digestion and utilisation of food to (i) synthesize the proteins, fats, carbohydrates and other substances of which living cells are made up and to (ii) furnish the energy necessary for living and reproduction. The protoplasm (cell substance) of all living cells is, therefore, identical. All living cells utilise and metabolize all foods by means of enzymic mechanisms. There are slight differences among different species of bacteria.
Food of Microorganisms:
Since Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P) and Sulphur (S) with smaller amounts of Magnesium (Mg), Iron (Fe), Copper (Cu), Sodium (Na), Potassium (K) and other elements are the main components of protoplasm and other structures of all living cells; the nutrients of all cells must contain all these elements.
In addition, the growth factors, such as nicotinic acid (niacin), thiamine, riboflavine are utilised by bacteria in much the same manner as those cells of human body do. The composition and metabolism of human cells are similar to those of many microorganisms.
Organic compounds. Proteins, carbohydrates, fats.
Inorganic compounds. Sodium chloride, water, carbon dioxide, sulphur, iron, hydrogen.
Carbon Source for Growth:
Like plants some non-parasitic bacteria are able to utilise carbon- dioxide (CO2) as the main source of carbon and are called autotrophs (Grautos—self, trophe—nutrition) or lithographs. Energy is obtained in these organisms by the oxidation of inorganic compounds (Chemosynthetic autotrophs) or from the sunlight (Photosynthetic autotrophs).
The majority of bacteria require organic nutrients, such as carbohydrates, amino acids, peptides or lipids to serve as the source of carbon and energy. These organisms are called heterotrophs (Gr. heteros — another, trophe — nutrition) or organotrophs and they obtain their energy by breakdown of the organic carbon source.
They differ in the utilisation of organic compound. Thus some species of the genus Pseudomonas can utilise any one over a hundred organic compounds (sugars, acids, alcohols, etc.) as the sole source of carbon and energy. On the other hand, many bacteria are much more specific in their requirements.
Nitrogen Source for Growth:
Bacteria differ widely in their ability to synthesize the main nitrogenous structural units – amino acids and nucleotides. Some are able to grow on ammonium salts as the sole source of nitrogen, while others require a variety of amino acids and nucleotides preformed in the medium.
2. Influence of Oxygen and Redox Potential:
Majority of bacteria are able to grow either aerobically, i.e., in presence of air and free oxygen or anaerobically, in the absence of oxygen and they are called as facultative anaerobes (e.g., Salmonella typhi, Staphylococcus aureus, and Streptococcus pyogenes). Certain other bacteria will grow only in the presence of air or free oxygen and are described as obligate aerobes (e.g., Tubercle bacilli, Bacillus cereus, common saprophytes of the soil).
Still others will grow only in absence of free oxygen and are usually killed in its presence, they are known as strict anaerobes (e.g., Clostridium tetani). In the latter case, the ultimate determining factor is the state of oxidation of the environment, this being described in terms of oxidation- reduction or “redox” potential.
A sufficiently low redox potential for the growth of strict anaerobes is usually provided by placing the culture media in an atmosphere of hydrogen with the complete exclusion of oxygen (e.g., Mcintosh and Fieldes anaerobic jar).
It has been suggested that in the presence of oxygen, a strict anaerobe is liable to produce toxic peroxides which it cannot destroy owing to lack of catalase, an enzyme present in most aerobes and facultative anaerobes. Finally, there is a group of organisms which grow best in the presence of a trace of free oxygen and often prefer an increased concentration of carbon dioxide (CO2), these are called microaerophilic.
3. Influence of Hydrogen Ion Concentration:
Hydrogen ion concentration. Hydrogen atoms, when dissolved in water, immediately assume a positive electrical charge, they are called hydrogen ions and the acidity of any solution is due to them. Acids, such as hydrochloric or acetic acid, possess the property of acidity because they give off hydrogen ions (i.e., they ionize) in aqueous solutions.
If a solution contains a large number of these ions, it is very acidic and it is said to have a high hydrogen ion concentration. If it is only slightly acidic or alkaline, it is said to have a low hydrogen ion concentration. pH Hydrogen ion concentration is usually expressed by numbers used in the symbol of pH, which stands for hydrogen ion concentration. pH is an essential factor in bacterial metabolism and growth. The majority of commensal and pathogenic bacteria grow best at a neutral or very slightly alkaline reaction (pH 7.2 to 7.6).
Some bacteria flourish in the presence of considerable degree of acidity and are termed acidophilic, e.g., lactobacillus. Others are very sensitive to acid, but tolerant to alkali, e.g., Vibrio cholerae. Most of the bacteria are rapidly killed by strong acid or alkali solutions, e.g., 5 per cent hydrochloric acid or sodium hydroxide but the mycobacteria (e.g., tubercle bacilli) are resistant to them.
According to their effect on bacteria, bactericidal chemical substances can be divided into surface active substances, phenols and their derivatives, salts of heavy metals, oxidising agents and the formaldehyde group.
4. Surface Active Substances:
Surface active substances change the electrical charges. Bacterial cells lose their negative charge and acquire positive charge which impairs the normal function of the cytoplasmic membrane. Bactericidal substances with surface active action include fatty acids and soaps which harm only the cell wall and do not penetrate into the cell.
5. Phenol, Creosol and Related Derivatives:
Phenol, creosol and related derivatives first of all injure the cell wall and then cell proteins. Some substances of this group inhibit the function of coenzyme (diphosphopiridine nucleotide) which participates in the dehydrogenation of glucose and lactic acid.
Dyes are able to inhibit the growth of bacteria. The basis of this action is the marked affinity for the phosphoric acid groups of nucleoproteins. Dyes with bactericidal properties include brilliant green, acriflavine etc.
7. Salts of Heavy Metals:
(lead, copper, zinc, silver, and mercury) cause coagulation of the cell proteins. Silver, gold, copper, zinc, tin, lead etc. have an oligodynamic action (bactericidal activity).Thus, for example, silver-plated objects, silverware in contact with water may render the metal bactericidal to many species of bacteria.
The mechanism is that the positively charged metallic ions are adsorbed on to the negatively charged bacterial surface and alter the permeability of the cytoplasmic membrane. There by, the nutrition and reproduction are, possibly, disturbed. Under the influence of the salts of heavy metals, viruses may become irreversibly inactivated as they are very sensitive to these salts.
8. Oxidising Agents:
Chlorine is commonly used in decontaminating water, because chlorine acts on dehydrogenase, hydrolase, amylase, proteinase of bacteria; chloride of lime and chloroform are used as disinfectants. In medicine; iodine is used successfully as antimicrobial substance in the form of tincture iodine which oxidizes proteins of bacterial cytoplasm and causes their denaturation. Potassium permanganate, hydrogen peroxideare other oxidising agents.
Many species of viruses are resistant to heat, chloroform, ethyl and methyl alcohol and volatile oils. All viruses are resistant to 50 per cent glycerine, except Rinderpest virus, sodium hydroxide, chloride of lime and chlorine can destroy viruses as they are oxidising agents.
Formalin (40 per cent formaldehyde) has bactericidal effect, because it possibly unites with the amino groups of proteins and brings out denaturation of proteins. Formaldehyde destroys both the vegetative forms and the spores of bacteria. It is also used to decontaminate diphtheria and tetanus toxins and at the same time it detoxifies the toxins transforming them into antitoxins (toxoids). Phage and tobacco mosaic virus inactivated by formalin can sometimes renew their infectivity.
Antiseptic is of great importance in medical practice. The science of antiseptics played a great role in the development of surgery, brought a decrease in the number of post-operative complications like gas gangrene.
B. Physical Factors:
Since the bacterial cell consists of water, moisture is essential for its growth. Drying in air is injurious to many microorganisms; it may prevent the growth of microorganisms and may not kill them. One method adopted by microbiologists to keep certain microorganisms alive for long periods (20: years or more) is to dry them and store them in a refrigerated vacuum.
Even delicate, non-sporing organisms may survive drying for many years if they are desiccated at a high vacuum (0.01 mm. Hg. or less) in sealed glass ampoule which is stored at room temperature in the dark. This is the basis of lypholisation or freeze drying process of preserving bacterial cultures or vaccines and viral vaccines in the laboratory or also during long-distance transport. Quick freezing of bacterial and viral suspension at a very low temperature provokes conditions at which crystals do not form and subsequent disruption of the microorganisms does not occur.
Certain fragile microorganisms, Neisseria gonorrhoeae (gonococci) causing gonorrhea, Treponema pallidum causing syphilis, Entamoeba histolytica causing amoebic dysentery,die almost at once when subjected to drying, while tubercle bacilli, Staphylococcus aureus and small pox virus may survive for several months in sputum, pus, crust, respectively.
Low temperatures halt putrefying and fermentative processes. The process of metabolism is inhibited, as a result bacteria die and the cells are destroyed under the influence of the formation of ice crystals during freezing. Vibrio cholerae loses its viability at a temperature of -32°C. Some species of bacteria, remain viable at a temperature of liquid air -190°C and of liquid hydrogen (-253°C).
Diphtheria bacilli are able to withstand freezing for 3 months and enteric fever bacteria (Salmonella typhi) are able to live long in ice. Bacillus spores withstand a temperature of -253°C for 3 days. Many microorganisms remain viable at low temperature and viruses are especially resistant to low temperatures. Thus, for example, Japanese encephalitis virus in 10% brain suspension does not lose its pathogenicity at -70°C for a year; the causative agents of Influenza and Trachoma at -70°C for 6 months and Coxsachie virus at -40°C for 1.5 years.
Only certain species of pathogenic bacteria are very sensitive to low temperature (e.g., meningococci, gonococci). During short periods of cooling, these species die quite rapidly. This is taken into account in laboratory diagnosis and the specimens for the presence of meningococci or gonococci should be sent to the laboratory protected from cold. Some microorganisms can grow best at a low temperature (4° to 10°C) and are said to be psychrophilic, whereas those bacteria growing best at higher temperature (from 20°C to 37°C) are called mesophils.
Most of the pathogenic non-sporing bacteria are killed at 58°C to 60°C for 30-60 minutes. Bacillus spores can withstand boiling (100°C) from a few minutes to 3 hours, but under the effect of dry heat at 160°C- 170°C only 60 – 90 minutes. Heating at 121 °C (15 lbs per square inch in gauge pressure) kills the bacteria and their spores within 20 – 30 minutes.
Some microorganisms will grow only at high temperatures ranging from 45°C to 75°C and are called thermophilic, e.g., Bacillus stearothermophilus. The bactericidal action of high temperature is due to the inhibition of the catalase, oxidase, dehydrogenase activity, protein denaturation and an interruption of the osmotic pressure.
High temperature causes rapid destruction of viruses, but some of the viruses (viruses of serum hepatitis, poliomyelitis) are resistant to environmental factors. They remain viable long in water, in the feces of patients or carriers and are resistant to heat at 60°C and to small concentration of chlorine in water.
Dessication is accompanied by dehydration of the cytoplasm and denaturation of bacterial proteins. Gonococci, meningococci, treponema, leptospira and phages are sensitive to dessication. Vibrio cholerae persist for 2 days on exposure to dessication; dysentery bacteria for 7; plague bacilli for 8, diphtheria bacilli for 30; bacilli for enteric fever for 70; staphylococci and tubercle bacilli for 90 days. The dry sputum of tubercular patients remains infectious for 10 months.
Dessication in a vacuum at a low temperature does not kill bacteria, rickettsiae, viruses. This method of preserving cultures is employed in the manufacture of stable long storage of live vaccines against tuberculosis, plague, brucellosis, small pox, influenza.
Direct sunlight has the greatest bactericidal action. Different kinds of light have bactericidal or sterilizing effects. They are ultraviolet rays (electromagnetic rays with a wave length of 200- 300 mµ; X-rays (electromagnetic rays with a wave length of 0.005 -1 mµ.); gamma rays (short wave X- rays); beta particles or cathode rays (high speed electrons); alpha particles (high speed helium nuclei) and neutrons.
Rays of short waves have a high bactericidal effect and are used for the disinfection of wards, infectious material, for the conservation of products, the preparation of vaccine, treating operation and maternity wards. Viruses are very quickly inactivated under the influence of ultraviolet rays with a wave length of 200-300 mµ.
These waves are absorbed by the nucleic acid of viruses. Longer waves are weaker and do not render viruses harmless. Viruses in comparison to bacteria are less resistant to X-rays and gamma rays. Beta-rays are more markedly viricidal.
5. High pressure and mechanical injury to microbes. Bacteria withstand easily atmospheric pressure. Some bacteria, yeasts and moulds withstand a pressure of 3,000 atmosphere pressure. The movement of liquid media has a harmful effect on microbes.
The movement of water in rivers and streams, undulations in stagnant water are important factors in self-purification of reservoirs from microbes. Ultrasonic oscillation (waves with a frequency of about 20,000 hertz per second) has a bactericidal activity and it is used for the preparation of vaccines and disinfection of various objects.
6. Osmotic Pressure:
Like other living cells, bacteria have a semipermeable cytoplasmic membrane which is subjected to osmotic pressure. Bacteria are very tolerant of changes in the osmotic pressure of their environment and can grow in media with widely varying contents of salts, sugar and other such solutes because of the thickness and mechanical strength of their walls. For most species, the upper limit of sodium chloride concentration permitting growth lies between 5 and 15 per cent, though holophilic (or osmophilic) species occur which can grow at higher concentration up to saturation.
Sudden exposure of bacteria to solution of high concentration (e.g., 2 – 25 per cent sodium chloride) may cause plasmolysis (i.e., temporary shrinkage of the protoplast and its retraction from cell wall due to osmotic withdrawal of water, this occurs much more rapidly in Gram-negative than in Gram-positive bacteria). Sudden transfer from a concentrated to a weak solution, or to distilled water, may cause plasmolysis i.e., swelling and bursting of cell through excessive imbibition of water.
C. Biological Factors:
1. Symbiosis is an intimate mutually beneficial relation of organisms of different species. They develop together better than separately. Sometimes, the adaptation of two organisms becomes so close that they lose their ability to exist separately (symbiosis of the fungus and blue green algae).
2. Metabiosis is one type of relationship in which an organism continues the process caused by another organism, liberating it from all its life activities and thus creating conditions for its own further development (nitrifying bacteria).
3. Satellism, a symbiont known as the favourable microbe, accelerates the growth of the other (some yeasts producing amino acids, vitamins etc. enhance the growth of microbes which are very specific regarding their nutrient requirements).
4. In Synergism, there is an increase in the physiological function of microbial association (Borrelia and fusobacterium).
5. In antagonism there is struggle for oxygen, nutrients and a habitat (antibiotics).
Application to Nursing:
The nurse will use the methods of the control of the microorganisms by the knowledge of the relation of microorganisms to their environmental factors. For example, high temperatures affect the living cell structures causing death; pathogenic bacteria are capable to produce infection or toxin according to the presence or absence of oxygen.
The inhibition or destruction of microorganisms depends upon the removal of food, water and oxygen. In nursing practice, the microbial growth can be controlled by altering the temperature and subjecting the microorganisms to disinfectants.
In nursing situations (operating rooms, nurseries, communicable disease units, isolation wards, intensive care units) microorganisms can be destroyed by physical environmental factors (X-rays, gamma rays etc.). Introduction of antiseptics in surgical practice prevented the access of microbes into the wounds. Aseptics are attained by disinfection of the air and equipment’s of the operating room, by sterilisation of surgical instruments and materials and by disinfecting the hands of surgeon and the skin of the operating field.
Modern methods of aseptics have been well perfected, by which almost all operations are accompanied by primary healing of wounds without suppuration and the post-operative septicemia has been completely eliminated. The nurse would be able to adopt the suitable method to control the growth of microorganisms according to their relationship with the environment. The growth of microorganisms can be slowed down or completely stopped by altering the environmental growth factors.
The high temperature can affect the structure of the living bacterial cells, so that they may be completely destroyed. Anaerobic bacteria can grow very well in absence of oxygen and elaborate the toxin which is responsible for the pathogenic effect. In the presence of oxygen, the same bacteria cannot produce such toxin and, hence, they are not pathogenic.
It may not be always possible to inhibit or destroy microorganisms by simple removal of food, oxygen, water. In nursing practice, the microbial growth can be controlled primarily by subjecting the microorganisms to the sterilisation and disinfection. Ultraviolet rays can be sometimes used to kill the microorganisms in nursing situations (operation rooms, nurseries, communicable diseases).