Factors Affecting Bacterial Growth PDF

Factors Affecting Bacterial Growth RAKESH SHARDA Department of Veterinary Microbiology NDVSU College of Veterinary Science & A.H., MHOW  Bacterial requirements for growth PHYSICAL OR ENVIRONMENTAL FACTORS oxygen (presence or absence) Temperat...

Factors Affecting Bacterial Growth RAKESH SHARDA Department of Veterinary Microbiology NDVSU College of Veterinary Science & A.H., MHOW  Bacterial requirements for growth PHYSICAL OR ENVIRONMENTAL FACTORS oxygen (presence or absence) Temperature pH osmotic pressure moisture radiation  Bacterial requirements for growth CHEMICAL OR NUTRITIONAL FACTORS carbon nitrogen phosphorous sulphur metal ions trace elements vitamins growth factors  PHYSICAL OR ENVIRONMENTAL FACTORS  OXYGEN OXYGEN REQUIREMENTS OBLIGATE OBLIGATE FACULTATIVE MICXRO AEROTOLERANT AEROBES ANAEROBES ANAEROBES AEROPHILES  Obligate aerobes are organisms that grow only in the presence of oxygen. They obtain their energy through aerobic respiration. Obligate anaerobes are organisms that grow only in the absence of oxygen and, in fact, are often inhibited or killed by its presence. They obtain their energy through anaerobic respiration or fermentation. Facultative anaerobes are organisms that grow with or without oxygen, but generally better with oxygen. They obtain their energy through aerobic respiration if oxygen is present, but use fermentation or anaerobic respiration if it is absent. Most pathogenic bacteria are facultative anaerobes. Aerotolerant anaerobes do not use oxygen to transform energy but can tolerate and grow in its presence. They obtain energy only by fermentation and are also known as obligate fermenters. Microaerophiles are organisms that require a low concentration of oxygen (2% to 10%) for growth, but higher concentrations are inhibitory. They obtain their energy through aerobic respiration..  OXYGEN REACTIVITY Oxygen is a very reactive molecule and is a potent cellular poison unless a cell has a mechanisms to enzymatically inactivate it. Various reactive oxygen radicals (ROS) are invariably generated by cells in the presence of O2. These ROS are H2O2 (peroxide), superoxide (O2-), singlet oxygen (O-), peroxide anions and hydroxyl radicals (OH-).  OXYGEN DETOXIFICATION All organisms which can live in the presence of O2 (whether or not they utilize it in their metabolism) contain enzyme superoxide dismutase, which destroys superoxide (O2-) ion. Nearly all organisms also contain the enzyme catalase, which decomposes H2O2. Some organisms decompose H2O2 by means of peroxidase enzymes which derive electrons from NADH2 to reduce peroxide to H2O. The action of superoxide dismutase, catalase and peroxidase enzymes on toxic oxygen radicals  Distribution of superoxide dismutase, catalase and peroxidase in prokaryotes with different O2 tolerances. Group Superoxide Catalase Peroxidase dismutase Obligate + + - aerobes and most facultative anaerobes (e.g. Enterics) Most + - + aerotolerant anaerobes (e.g. Streptococci) Obligate - - - anaerobes (e.g. Clostridia)  Terms used to describe O2 Relations of Microorganisms Group Aerobic Anaerobic O2 Effect Obligate aerobe Growth No growth Required (utilized for aerobic respiration) Obligate No growth Growth Toxic anaerobe Facultative Growth Growth Preferentially anaerobe utilized when (Facultative available aerobe) Microaerophile Growth if level No growth Required but at not too high levels below 0.2 atm Aerotolerant Growth Growth Not required anaerobe and not utilized  Capnophiles ▪ A small amount of CO2 is required by all organisms for growth, which is provided by metabolism. ▪ Some organisms requires relatively high concentrations of carbon dioxide (1-5%) for their optimum growth, particularly for primary isolation; these are known as capnophiles, e.g., Brucella abortus  TEMPERATURE TEMPERATURE REQUIREMENT HYPER PSYCHROPHILES MESOPHILES THERMOPHILES THERMOPHILES Bacteria have a minimum, optimum, and maximum temperature for growth and can be divided into following groups based on their optimum growth temperature (temperature at which an organism grows best): Psychrophiles are cold-loving bacteria. Their optimum growth temperature is between -5°and 15°C with maximum up to 20°C. They are usually found in the Arctic and Antarctic regions and in streams fed by glaciers. Mesophiles are bacteria that grow best at moderate temperatures. Their optimum growth temperature is between 25° and 40°C with maximum below 45°C. Most bacteria are mesophilic and include common soil bacteria and PATHOGENS. Thermophiles are heat-loving bacteria. Their optimum growth temperature is between 45° and 70°C and are commonly found in hot springs and in compost heaps, e.g. Bacillus stearothermophilus  Hyperthermophiles - grow at very high temperatures. Their optimum growth temperature is between 70° and 110°C. They are usually members of the Archae and are found growing near hydrothermal vents at great depths in the ocean, e.g. Sulfolobus acidocaldarium ▪ Obligate psychrophiles – are unable to grow above 20ºC. ▪ Facultative psychrophiles - are able to grow above 20ºC, but not in mesophilic range. ▪ Psychrotroph - A variant of a psychrophile which can grow at temperature in the range of mesophiles ▪ Thermoduric - Mesophilic organisms that can survive brief exposures to relatively high temperatures are termed thermoduric.  TEMPERATURE TOLERANCE Psychrophilic bacteria are adapted to their cool environment due to high concentration of unsaturated fatty acids in their plasma membranes. Thermophilic bacteria adapted to their hot environment due to high concentration of highly saturated fatty acids in their plasma membranes. Often thermophiles have a high G + C content in their DNA The membranes of hyperthermophiles are composed of repeating subunits of the C5 compound, phytane, which help them to live in superheated environments.  Terms used to describe microorganisms in relation to temperature requirements for growth Group Minimum Optimum Maximum Comments Psychrophile Below 0 10-15 Below 20 Grow best at relatively low T Psychrotroph 0 15-20 Above 25 Able to grow at low T but prefer moderate T Mesophile 10-15 25-40 Below 45 Most bacteria especially those living in association with warm-blooded animals Thermoduric 10-15 30-40 45-60 Mesophiles that can survive brief exposures to relatively high temperatures Thermophile 45 50-70 Above Shows wide 100 ºC variation in (boiling) optimum and maximum T Hyperthermophile 70 80-110 Above occurring in hot 121 springs or deep sea vents in mid- ocean ridges  pH pH ACIDOPHILES NEUTROPHILES ALKALIPHILES Microorganisms can be placed in one of the following groups based on their optimum pH requirements: Acidophiles - Organisms whose optimum pH is moderately to highly acidic, usually 5.5. Several genera of Archaea, including Sulfolobus and Thermoplasma, are obligate acidophiles. Among eukaryotes, many fungi are acidophiles. Neutrophiles - Organisms whose optimum pH is 7± 1.5 with a range of 5 to 8, e.g. Staphylococcus aureus (7.0-7.5), E. coli (6.0-7.0) Alkaliphiles - Organisms whose optimum pH is usually above 8.0, e.g. Nitrobacter  WATER AVAILABILITY Water is the solvent in which the molecules of life are dissolved, and the availability of water is, therefore, a critical factor that affects the growth of all cells. The availability of water for a cell depends upon its presence in the atmosphere (relative humidity) or its presence in solution (water activity). Microorganisms live over a range of Aw from 1.0 to 0.7. The concept of lowering water activity in order to prevent bacterial growth is the basis for preservation of foods by drying (in sunlight or by evaporation) or by addition of high concentrations of salt or sugar. Xerophiles - Organisms which live in dry environments (made dry by lack of water).  OSMOTIC PRESSURE Most bacteria require an isotonic environment or a hypotonic environment for optimum growth. Osmophiles - organisms that are able to live in hypertonic environments high in salt or sugar. Osmotolerant - organisms that can tolerate salt concentration up to approximately 10%. Depending on the requirement of salt concentrations microorganisms are classified as follows: ▪ Non-halophiles require less than 1% salt (E. coli or Pseudomonas) ▪ mild halophiles require 1-6% salt ▪ moderate halophiles require 6-15% salt (S. aureus) ▪ Extreme halophiles require 15-30% NaCl for growth (archaea such as Halococcus). ▪ Halotolerant - bacteria that are able to grow at moderate salt concentrations, even though they grow best in the absence of NaCl. CHEMICAL OR NUTRITIONAL FACTORS  NUTRITIONAL REQUIREMENTS All living organisms are grouped into different nutritional groups according to their energy source, carbon source and hydrogen / electrons source. Energy source Phototrophs use light as the primary source of energy Chemotrophs use chemical compounds as the primary source of energy Carbon source Autotrophs require only carbon dioxide as a carbon source. An autotroph can synthesize organic molecules from inorganic nutrients. Heterotrophs require organic forms of carbon. A heterotroph cannot synthesize organic molecules from inorganic nutrients. Hydrogen/Electron source Lithotroph use inorganic substrates such as ammonium, nitrite, hydrogen sulfide or elementary hydrogen as a source of hydrogen/ electron donor Organotroph use reduced organic compounds such as glucose and fatty acids as a source of hydrogen/ electron donor  Major nutritional types of prokaryotes Nutritional Type Energy Carbon Examples Source Source Photoautotrophs Light CO2 Cyanobacteria, some Purple and Green Bacteria Photoheterotrophs Light Organic Some Purple and compounds Green Bacteria Chemoautotrophs Inorganic CO2 A few Bacteria and or Lithotrophs compounds, many Archaea (Lithoautotrophs) e.g. H2, NH3, NO2, H2S Chemoheterotrophs Organic Organic Most Bacteria, or Heterotrophs compounds compounds some Archaea NUTRITIONAL FACTORS ❑ carbon ❑ nitrogen ❑ phosphorous ❑ sulphur ❑ metal ions ❑ trace elements ❑ vitamins ❑ growth factors  Major elements, their sources and functions in bacterial cells Element % of Source Function dry weight Carbon 50 organic Main constituent of compounds or cellular material CO2 Oxygen 20 H2O, organic Constituent of cell compounds, CO2, material and cell water; and O2 O2 is electron acceptor in aerobic respiration Nitrogen 14 NH3, NO3, organic Constituent of amino compounds, N2 acids, nucleic acids nucleotides, and coenzymes Hydrogen 8 H2O, organic Main constituent of organic compounds, H2 compounds and cell water Phosphorus 3 inorganic Constituent of nucleic acids, phosphates (PO4) nucleotides, phospholipids, LPS, teichoic acids Sulfur 1 SO4, H2S, So, Constituent of cysteine, organic sulfur methionine, glutathione, several compounds coenzymes Potassium 1 Potassium salts Main cellular inorganic cation, cofactor for enzymes Magnesium 0.5 Magnesium salts Inorganic cellular cation, cofactor for enzymes Calcium 0.5 Calcium salts Inorganic cellular cation, cofactor for certain enzymes, a component of endospores. Iron 0.2 Iron salts Component of cytochromes and certain nonheme iron-proteins, cofactor for some enzymes Growth factors Growth factors are organic compounds such as amino acids, purines, pyrimidines, and vitamins that a cell must have for growth but cannot synthesize itself. Organisms having complex nutritional requirements and needing many growth factors are said to be fastidious.