Bacteriology Lecture (6) PDF

LECTURE (6) 3. Oxygen Relationships of Microorganisms: Microorganisms can be grouped into categories based on their requirement or intolerance of O₂. Aerobes grow in the presence of air that contains molecular oxygen. Obligate aerobes require O₂ for growth and carry out aerobic respiration...

LECTURE (6) 3. Oxygen Relationships of Microorganisms: Microorganisms can be grouped into categories based on their requirement or intolerance of O₂. Aerobes grow in the presence of air that contains molecular oxygen. Obligate aerobes require O₂ for growth and carry out aerobic respiration. Other microorganisms, called microaerophiles, grow only at reduced concentrations molecular oxygen. Such organisms require O₂ for growth but only at concentrations (~5%) reduced from that of atmospheric levels (20%). Generally, microaerophilic organisms will not grow in air.  Facultative anaerobes can grow in the presence or absence of air. Many facultative anaerobes such as E.coli switch between aerobic respirations and fermentation, depending on the availability of molecular oxygen. They usually carry out fermentation metabolism in the absence of O₂ and aerobic respiration in the presence of O₂. This group of facultative anaerobes also includes strictly fermentative bacteria, such as streptococci, that are insensitive to oxygen (oxyduric) and hence can grow in the presence of O₂.  Other bacteria are anaerobes and grow only in the absence of air. Obligate anaerobes carry out fermentative metabolism. Various bacteria (for example, the sulfate –reducer Desulfovibrio), archaea (for example, methanogenic archaea). Strict anaerobes are sensitive to oxygen and even a brief exposure to O₂ will kill such organisms. Clostridium species can be classified as obligate, strict anaerobes.  4. Effects of acidity and pH The acidity or alkalinity of an environment can greatly affect microbial growth. Most organisms grow best between pH 6 and 8, but some organisms have evolved to grow best at low or high pH. The internal pH of a cell must stay relatively close to neutral even though the external pH is highly acidic or basic. – Acidophiles : organisms that grow best at low pH (Helicobacter pylori, Thiobacillus thiooxidans ) – Alkaliphiles : organisms that grow best at high pH (Vibrio cholera) – Most of pathogenic bacteria are neutrophiles.  The pH of a solution describes the hydrogen ion concentration [H⁺]. Bacterial growth rates are greatly influenced by pH values and are based largely on the nature of proteins. Enzymes are normally inactive at very high and low pH values. Also, bacteria are less tolerant of higher temperatures at low pH value than they are at neutral pH values. In culture media and industrial fermenters, pH values are controlled to achieve optimal growth rates. This is normally accomplished by buffering the solution.  Buffers are used to maintain the pH value within a certain range, thus dampening changes pH and permitting continued bacterial growth. At neutral pH values, a phosphate buffer may be used. At alkaline pH values borate buffers are often employed. Citrate buffers often are used for maintaining acidic conditions. Bacteria vary in their pH tolerance ranges and most grow well within a range of 6 to 9 pH.  Most bacteria can therefore be considered neutralophiles because they tend to thrive under neutral conditions. Although most bacteria are unable to grow at low pH values, there are exceptions, and certain bacteria tolerate pH values as low as 0.8. The pH of the medium will determine which pathways of carbohydrate metabolism are dominant. The fate of free amino acids in the cell is also decided by pH. At an acid reaction, they are decorboxylated to the corresponding amines, whereas at an alkaline reaction they are deaminated to an acid.  5. Effects of pressure The growth of all cells is affected by the external and internal pressures. These forces include osmotic pressure and hydrostatic pressure. Hydrostatic pressure results from the weight of a column of water on cells such as those found in the deepest parts of the ocean. Osmotic pressure results from water diffusing across the cell membrane in response to solute concentrations. Solute concentration affects the availability of water and also the osmotic pressure. This is associated often with the salt concentration (salinity) surrounding the cell.  Osmotic pressure and salinity The cell wall structures of bacteria make them relatively resistant to changes in osmotic pressure, however, extreme osmotic pressures can result in the death of bacteria. In hypertonic solutions. Bacteria may shrink and become desiccated. In hypotonic solutions the cell may burst. Organisms that can grow in solutions with high solute concentrations are called osmotolerant. These organisms can withstand high osmotic pressures and also grow at low water activities. Some microorganisms are osmophiles, requiring a high solute concentration for growth. Salinity has an important effect on osmotic pressure. Some bacteria have specific responses to concentrations of salt (NaCl). Some bacteria and archaea, known as halophiles, require NaCl for growth.  6. Effects of light Photosynthetic bacteria require light to carry out their photoautotrophic generation of ATP. These bacteria function optimally at specific light intensities. They utilize specific light wavelengths. Certain photosynthetic bacteria move through their environment in response to light, called phototaxis. Some of these phototactic bacteria have mechanisms that regulate flagellar rotation in response to changing intensities of light.  Bacteria may also respond to specific wavelengths of light. Visible light, as well as ultraviolet light, can cause structural damage to proteins and DNA. Many bacterial cells that are exposed to bright light in their environment protect themselves from harmful radiation damage by synthesizing carotenoids and other pigments. These pigments absorb light of certain wavelengths before the light can cause damage and kill the cell.  7. Effects of radiation For the photosynthetic green and purple bacteria, light essential for growth. The effect of light on other bacteria is mostly toxic. Its toxicity depends almost entirely on the UV part of the spectrum, which is absorbed by the nucleic acis and proteins. The UV light prevents the replications of DNA and this results in some cases in a lethal mutation.  8. Effects of chemicals Although chemical substances are known to be used as nutrients that are essential for bacterial growth, some of them may prevent growth of the bacteria, some of them may prevent growth of the bacteria, acting as a bacteriostatic agents, and some others can act as a bactericidal causing death of bacteria. Chemical substances that affect microorganisms may destroy the structural organization of the cell, as for example alcohols, which act as denaturing agent of proteins.  Phenol and cresols which interfere with the function of the semi-permeable cytoplasmic membrane of the cell. Lysozyme is an example of those enzymes that dissolved the cell wall (affect mainly Gram +ve bacteria) Some other chemicals serve as inactivating agents for some cellular enzymes or co-enzymes and prevent, or, at least, interfere, in this way, with the synthesis of essential macromolecules as proteins and nucleic acids.  9. Effects of antibiotics Penicillin specially inhibits the transpeptidase, which affects the cross linking of the short peptide chains in the peptidoglycan biosynthesis do not occur in eukaryotic cells. Thus the action of penicillin is strictly antibacterial. Oxamyeine (cycloseine), bacitracin and vancomycin are cell wall antibiotics, through their inhibition of correct cell wall biosynthesis, result in structurally weak murein. This causes lysis and death of bacterial cells. Actinomycin interferes with RNA synthesis, sunce this antibiotic bind specifically to the DNA of the cell to form actinomycin-DNA complex, which cannot be transcribed by RNA polymerase.