Bacterial growth factors primarily include temperature, pH, salt concentration, light source, nutritional and gaseous requirement etc. Microorganisms live in diverse natural habitats such as atmosphere, hydrosphere and lithosphere.
The environmental and nutritional factors may favour or restrict the growth of microorganisms. Some microbes survive in extreme conditions or the changing environment, while few can not thrive. The bacterial growth factors decides the increase in cell size and mass of the bacterial cell.
Content: Bacterial Growth Factors
It is one of the most crucial factors which decides the multiplication rate of microorganisms. A temperature can categorize into three kinds, namely minimal, optimal and maximal.
- Minimal temperature: Below which no growth occurs
- Optimal temperature: At which fastest growth occurs
- Maximal temperature: Above which no growth occurs
Cardinal temperatures for microbial growth: Microorganisms belong to the different groups according to the optimal temperature for division and metabolism.
Psychrophiles: The microorganisms belong to this group also refer as “Cold loving” and these can grow at a temperature of 0 degrees Celsius. It includes two subgroups:
- True psychrophiles: The true psychrophilic microorganisms achieve their optimum growth at 15 degrees Celsius or below, but are sensitive to the temperature above 20 degrees Celsius. It is ubiquitous in the North Pole and ocean depth.
- Psychrotrophs: The psychrotrophs achieve their optimum growth between 20-30 degrees Celsius. These are widely distributed in nature compared to the psychrophiles.
Mesophiles: The microorganisms belong to this group also refer as “Middle loving” where these grow best at a normal temperature ranging between 25-40 degrees Celsius. The optimum temperature for the mesophilic bacteria is 37 degrees Celsius. It includes commensals, saprophytes, plant parasites etc.
Thermophiles: The microorganisms belong to this group also refer as “Heat-loving” where these show optimum growth at a temperature between 50-60 degrees Celsius. Many thermophilic organisms cannot grow below 45 degrees Celsius. The bacteria belong to this group can adapt themselves to live in a harsh environment like hot springs, sunlit soil, compost piles etc. Some thermophilic bacteria also possess heat resistance endospores.
There is another category refer as extreme thermophiles or hyperthermophiles, which grow optimum at a temperature of 80 degrees Celsius or above. Thermophilic bacteria like archaebacteria, Thermus aquaticus are abundantly found in volcanic and ocean vents.
Bacteria are sensitive to various pH range. The bacterial species grow at a pH range above or below the preferred value; they will not be able to survive. But, bacteria at optimum pH shows their best growth in nearly neutral pH, i.e. 6.5-7.5.
The acidic pH usually restricts the microbial growth and frequently uses in the food preservation methods. The basic pH also inhibits microbial growth. To stabilize the pH, buffers are generally used. Therefore, depending upon the pH range, the organisms can be grouped into three categories:
Acidophiles: The organisms belong to this group also refer as “Acid loving” which grow at very low pH ranging between 0.1-5.4. Bacillus sp, Micrococcus sp, sulphobolus sp. etc. comes in a category of acidophilic organisms.
Neutrophiles: The organisms belong to this group grow at a normal pH ranging between 5.4-8.5. It includes most human commensals like members belong to the family Enterobacteriaceae.
Alkalophiles: The organisms belong to this group also refer as “Alkali loving” which grow at alkaline pH ranging between 7-12. Vibrio cholera, Alkaligenes sp, Agrobacterium sp etc. comes in a category of alkalophilic organisms.
Microorganisms can adjust to changes in the environment pH by maintaining an internal pH that is near neutrality. Some bacteria also synthesize shock proteins in response to pH. The pH of the medium also determines which pathway of metabolism will operate.
The bacterial species that use molecular oxygen (O2) produce more energy from nutrients than anaerobes because oxygen functions as “Terminal electron acceptor” for an electron transport chain, during aerobic respiration. On the basis of oxygen concentration, the microorganisms can be categorized into:
Obligate aerobes: These include microorganisms like Pseudomonas sp, common nosocomial pathogen etc. require oxygen to survive.
Facultative anaerobes: These include E. coli, staphylococcus sp etc. makes the use of oxygen, but can also grow without its presence.
Obligate anaerobes: These include Clostridium sp, Pasteurianum sp etc. cannot use oxygen and are harmed by the presence of toxic forms of oxygen.
Aerotolerant anaerobes: These include Lactobacillus sp, Enterococcus faecalis etc. can use oxygen, but can tolerate its presence.
Microaerophiles: These include Campylobacter sp etc. require oxygen, but at low concentration and sensitive to toxic forms of oxygen.
Basis of different oxygen sensitivities
Oxygen can easily reduce toxic products like singlet oxygen and hydrogen peroxide. Singlet oxygen is an extremely reactive form that is found in the most phagocytic cells. Hydrogen peroxide ion is also toxic, which can be broken down by the two enzymes, namely catalase and peroxidase.
Superoxide free radical is an extremely toxic and reactive form of oxygen. The organisms which are growing at the temperature include aerobes, facultative anaerobes and aerotolerant but not by anaerobes or microaerophiles.
The group of bacteria, which utilizes a higher amount of CO2 for their growth, refers to capnophilic bacteria (H. influenzae, Brucella abortus, etc.). For their optimum growth, they require the presence of 5–10% CO2 and 15% O2. In candle jar, 3% CO2 can be achieved. Capnophiles found in the normal flora of some ruminants.
It is another factor which is also affecting the bacterial growth, and those bacteria which makes the use of light source can be classified as:
Phototrophs: It can define as a group of bacteria, which derives energy by capturing photons mainly from the sunlight. Phototrophs can be either classified into autotrophs (fix carbon) or heterotrophs (utilizes carbon). Examples: Rhodobacter capsulatus, Chromatium, Chlorobium etc.
Chemotrophs: It can define as the group of bacteria, which derives energy by oxidizing electrons primarily from the chemical sources. Chemotrophs can be either grouped into organic (chemoorganotrophs) or inorganic (chemolithotrophs). These are found on ocean floors where the sunlight cannot reach.
Microbes require minerals or nutrients for their growth, which can be obtained from the surrounding water. Osmotic pressure and salt concentration of the solution can influence bacterial growth. The bacterial cell wall gives a mechanical strength that allows the bacteria to withstand alternations in the osmotic pressure.
Osmophilic bacteria requires high osmotic pressure. When the bacterial cell is subjected to the hypertonic solution, it may cause osmotic removal of water, resulting in plasmolysis or osmotic shrinkage of the protoplasm.
In contrast, when the bacterial cell is subjected to the distilled water from the from the high concentration, it may cause excessive imbibition of water resulting in plasmoptysis or cell bursting.