BACTERIOLOGY (STM 311) THIRD LECTURE (B) (THE YOUNG LECTURER NOTE) (PRINCIPLE)

BACTERIOLOGY THIRD LECTURE (B)

(THE YOUNG LECTURER NOTE)

PRINCIPLE

          Generally prokaryotes multiply by the process of cell division called binary fusion. After a cell has reached its maximum size, all its cell parts are doubled and the cell divides into four, those two divide to give rise to four cells, the four gives eight and so on exponentially. Thus microbial growth is defined as an increase in the number of microbial cell population.

          The time taken for a cell or population to double in size or number is the generation time or doubling time. This varies depending on the species of the organism and prevailing conditions. E.coli can double in about 20 minutes while Mycobacterium tuberculosis takes 12 to 24 hours to double.

          If the doubling time of an organism is known and the number of cells initially present is also known, the number of cells present at any given time can be calculated based on a simple equation.

          N_t = N_o X 2ⁿ

          Where;

          N_t = is the number of cells at the required time.

          N_o = initial number of cells i.e at inoculation

          N = number of divisions undergone during the given time.

          n = number of division undergone during the given time.

Example: if 20 cells are inoculated into a nutrient medium with generation time of 20 minutes, first we have to determine the number of divisions that will occur in the given time. Because the organism divides every 20 minutes, i.e. 3 times per hour, in 4 hours, knowing the N_o = 22 and n = 12, the number of cells in the colony will be;

           

20 X 2¹² = N_t = 8,192.

Using  Nt = No X 2n

ENVIRONMENTAL FACTORS AFFECTING MICROBIAL GROWTH

          Microbes are ubiquitous; they are adapted differently to survive in different environment most extreme mophiles belong to the Archae. The environmental factor that influence microbial growth are either physical temperature, PH and osmotic pressure or chemical C, N, O, S, phosphorus, trace element and organic factor.

1.   Temperature: Each prokaryotic species has a unique cell wall defined upper and lower temperature range or boundary within which it can thrive. Usually the temperature difference within the upper and the lower boundaries is about 25­­­­­­­^oC. Within this range is the optimum temperature is usually close to the upper limits this is a kind of general rule because the rate of enzymatic activity in the cell doubles with approximately every 10^oC rise in temperature at a critical temperature the enzymes become denatured leading to death of the cell. Based on optimum temperature requirement, microbes are divided into 5 groups.

i.             Psychrophiles: Have optimum temperature below -20^oC and 15^oC. These are mostly found in the poles and glacier fed lakes e.g. Pseudomonas Syrinage, chryseobacterium greeulandensis, psycrobacter cryhalolentis.

ii.           Psychrotrophs: With optimum temperature between 20^oC and 30^oC e.g. Listeria monocytogenes.

iii.         Mesophiles: These have optimum between 25^oC and 45^oC e.g. E.coli, most inhabit the human body.

iv.          Thermophiles: Optimum of 50^oC and 70^oC e.g. Thermus aquaticus, Bacillus stearothermophiles, they are commonly found in hot springs.

v.            Hyperthermophiles: With optimum of 85^oC and 121^oC. They usually don’t grow in temperature less than 55^oC. They are mostly Archae e.g. Methanopyrus Kandleri. Bacteria examples include Pyrolobus fumarii, Pyrococcus abyssi.

2.   PH: Each bacteria specie has a tolerable PH range like in temperature within this range lies optimum PH. Despite the external environmental PH, cells try to maintain a stable internal or cytoplasmic PH of their own. Most bacteria have PH optimum near neutral. They are called Neutrophils e.g. Helicobacter Pylori. This organism produces the enzyme urease which breaks down stomach acidic urea into ammonia and carbon dioxide the ammonia neutralizes the stomach acid in its immediate environment where it causes cancer.

            Acidophiles grow at optimum PH below 5.5 e.g. Thiobacillus ferroxidans. Pirocrophilus oshimae an Archae has an optimum PH less than 1. Alkolophiles grow optimally at PH above 8.5 e.g. Bacillus alkalophilus grow best at PH 10.5

3.   Osmotic pressure: this generally is a measure of the tendency of a solution to take up more pure solvent. All microbes require water however of present, the water might not be available for microbial use owing to dissolved substances such as salts and sugars e.g. Nacl. If the organism’s environment has more salt or solute concentration than the cell cytoplasm, water is drawn out of the cell due to osmosis dehydrating the cell. This is called plasmolysis. The cytoplasm is forced to shrink from the cell wall.

    Some bacteria can tolerate high salt concentration e.g. staphylococcus epidemidis. They are said to be osmotolerant. Those that require high salt concentration (3% Nacl) to grow are called halophiles. Extreme halophiles are mostly Archae withstanding up to 9% Nacl or more.

4.   Oxygen Requirment: The percentage availability of O₂ differentiates various microbial habitats. Some bacteria require O_2; others don’t, while some die in its presence. The oxygen requirements of cells can be determined by growing them in Shake tubes prokaryotes are classified based on O₂ requirement into.

i.             Obligate aerobes: These cannot survive in the absence of O₂. They require O₂ for aerobic respiration e.g. Micrococcus Luteus. They are commensal that inhabit human skin and mucosa.

ii.           Obligate anaerobes: These don’t multiply in the presence of O₂. They might even die in the presence of trace O₂ owing to toxic O₂ derivatives (super oxides and catalases that degrade the O₂ derivatives). E.g. of obligate Anaerobes Cibotulium and bacteriodes that inhabit human large intestines. Obligate Anaerobes transform energy by fermentation.

iii.         Facultative Anaerobes: These aerobically produce more ATP in the presence of O₂ –and switch to fermentation or anaerobic respiration in the absence of O₂ absence. Example is Escherichia coli.

iv.          Microaerophiles: These require small amount of O₂ about 2 - 10% high concentrations inhibits their growth and may lead to death e.g. Spirillum volutans common in aquatic habitats and Helicobacter pylori.

v.            Aerotolerant Anaerobes: These are called obligate fermenters. They do not use aerobic nor anaerobic respiration to transform energy. They are indifferent to O₂. They can grow in the presence of O₂ but do not use it. Example is Streptococcus pyogenes that causes strep throat.

CHEMICAL FACTORS INFLUENCING MICROBIAL GROWTH

          Cells synthesize all its components from chemical elements. They have remarkable ability to use diverse sources of elements e.g. they are the only organism capable of using atmospheric N₂ and a N­₂ source.

          Elements that make up cell constituents are called major elements; these include carbon, hydrogen, oxygen, sulfur, nitrogen, phosphorus, potassium, magnesium, calcium and ion. They are essential components of proteins, carbohydrates, lipids and nucleic acids.

          Prokaryotes are distinguished based on their source of carbon. Those that are organic carbon are called heterotrophs while those that use inorganic carbon are Autotrophs. They play a role in the cycling of carbon dioxide converting inorganic CO₂ to organic form, this process is called carbon fixation. Other chemical elements are provided for organisms as organic salts. Ammonium sulphate supplies both Nitrogen and Sulfur. Some prokaryotes can convert inorganic nitrogen gas to ammonia; this process is called Nitrogen fixation. Elements required in small amount are trace elements. E.g. zinc, cobalt, Molybdenum and Manganese. Trace element form parts of enzymes or may be required for enzyme formation.

GROWTH FACTOR

          These are low molecular weight compounds necessary to be provided to any particular bacterium growth. Some bacteria cannot synthesize some of their cell constituents such as amino acids, vitamins, purines and pyrimidines from the major elements, Microbes have a wide range of growth facto requirements, this reflects differences in their biosynthetic capability e.g. E.coli does not require any growth factor, it grows in a medium containing only glucose and about 6 different inorganic salts. In contrast Neisseria species require at least no additional ingredients including 7 vitamins and all the 20 amino acids. Bacteria that require many growth factor are called Fastidious e.g. Neisseria.