Bacteria and Virus PDF

Bacteria and Virus  Bacteria Bacteria are unicellular organisms. Because they have no nucleus, the cells are described as prokaryotic. The three major basic shapes of bacteria are bacillus, coccus, and spiral. Most bacteria have a peptidoglycan cell wall; they divide by binary fission, and they ma...

Bacteria and Virus  Bacteria Bacteria are unicellular organisms. Because they have no nucleus, the cells are described as prokaryotic. The three major basic shapes of bacteria are bacillus, coccus, and spiral. Most bacteria have a peptidoglycan cell wall; they divide by binary fission, and they may possess flagella. Bacteria can use a wide range of chemical substances for their nutrition. Archaea consist of prokaryotic cells; they lack peptidoglycan in their cell walls. Archaea include methanogens, extreme halophiles, and extreme thermophiles. Fungi (mushrooms, molds, and yeasts) have eukaryotic cells (cells with a true nucleus). Most fungi are multicellular. Fungi obtain nutrients by absorbing organic material from their environment. Protozoa are unicellular eukaryotes. Protozoa obtain nourishment by absorption or ingestion through specialized structures. Algae are unicellular or multicellular eukaryotes that obtain nourishment by photosynthesis. Algae produce oxygen and carbohydrates that are used by other organisms. Viruses are noncellular entities that are parasites of cells. Viruses consist of a nucleic acid core (DNA or RNA) surrounded by a protein coat. An envelope may surround the coat. The principal groups of multicellular animal parasites are flatworms and roundworms, collectively called helminths.. The microscopic stages in the life cycle of helminths are identified by traditional microbiological procedures Prokaryotic Cells: Shapes Average size: 0.2 –1.0 µm  2 – 8 µm Most bacteria are monomorphic A few are pleomorphic The Requirements for Growth Physical requirements Temperature pH Osmotic pressure Chemical requirements Carbon Nitrogen, sulfur, and phosphorous Trace elements Oxygen Organic growth factor Physical Requirements Temperature Minimum growth temperature Optimum growth temperature Maximum growth temperature Cardinal temperatures Depend on environmental factors such as pH and available nutrients a) Minimum temperature - below which cells are inactive reduced membrane fluidity – perhaps affects nutrient transport or proton gradient formation b) Optimum temperature highest rate of growth and reproduction, always nearer maximum temperature c) Maximum temperature - above which growth is not possible Growth stops because of inactivation of one or more key proteins, damages transport carriers or other proteins, or thermal disruption of membrane  Cardinal temperatures vary for different organisms Medium composition can have a slight affect Temperature optima usually vary from 0°C to 75°C Pyrolobus fumarii (archaeon) - maximum temperature = 113°C Growth temperature range for a particular organisms usually spans 30 to 40°C Distinguish five groups of microbes based on temperature optima Psychrotrophs Psychrophiles are cold-loving bacteria having an optimal temperature for growth at about 15°C or lower, a maximal temperature for growth at about 20°C and a minimal temperature for growth at 0°C or lower. Psychrotrophs are cold-tolerant bacteria that have the ability to grow at low temperatures but have optimal and maximal growth temperatures above 15°C and 20°C, respectively. Cause food spoilage Psychrophiles Grow well at 0°C and have an optimum temperature ≤ 15°C and a maximum temperature around 20°C heat sensitive and unable to survive temperate climates Adaptations to Psychrophily - Enzymes, transport systems and protein synthetic apparatus work well at low temperatures enzymes with low temperature optima - greater amounts of α-helix and lesser amounts of β sheet secondary structure o greater amounts of polar amino acids and lesser amounts of hydrophobic amino acids membranes contain higher amounts of unsaturated fatty acids - some psychrophiles have membranes higher in polyunsaturated fatty acids ii) Psychrotolerant (psychrotrophs, facultative psychrophiles) grow at 0°C but have optima of 20 - 30°C iii) Mesophiles Optimum temperature between 25 and 40°C Minimum temperature between 15 and 20°C Maximum temperature ≤ 45°C Most common type of microbe e.g., E. coli Optimum temperature < 39°C Maximum temperature < 48°C Minimum temperature ≤ 8°C iv) Thermophiles Optimum temperature between 50 and 60°C Minimum temperature around 45°C Maximum temperature ≤ 45°C Only prokaryotes grow above 60°C The most thermophilic organisms are Archaea Nonphototrophic organisms are able to grow at higher temperatures than phototrophic forms v) Hyperthermophiles Optimum temperature > 80°C Extreme thermophiles are usually Archaea The highest growth temperatures for an archaeon is 113°C (Pyrolobus fumarii)  Adaptations to Thermophily i) Enzymes and other proteins are heat stable ii) Macromolecules function optimally at high temperatures iii) Membrane is heat stable Membrane lipids are more branched, rich in saturated fatty acids and of higher molecular weight In some cases they have lipid monolayers (diglycerol tetraethers) iv) DNA is stabilized by special histone – like proteins  Applications of Thermophily High temperature enzymes e.g., feed pelleting process PCR – Taq DNA polymerase from Thermus aquaticus pH Most bacteria grow between pH 6.5 and 7.5 Molds and yeasts grow between pH 5 and 6 Acidophiles grow in acidic environments  pH is a measure of the hydrogen ion activity of a solution and is defined as the negative logarithm of the hydrogen ion concentration (expressed in terms of molarity). pH log [H] log(1/[H]) The pH scale extends from pH 0.0 (1.0 M H) to pH 14.0 (1.0 1014 M H), and each pH unit represents a tenfold change in hydrogen ion concentration. Osmotic Pressure Hypertonic environments, or an increase in salt or sugar, cause plasmolysis Extreme or obligate halophiles require high osmotic pressure Facultative halophiles tolerate high osmotic pressure Plasmolysis Oxygen Oxygen Concentration An organism able to grow in the presence of atmospheric O2 is an aerobe, whereas one that can grow in its absence is an anaerobe. Almost all multicellular organisms are completely dependent on atmospheric O2 for growth—that is, they are obligate aerobes. Oxygen serves as the terminal electron acceptor for the electron-transport chain in aerobic respiration. In addition, aerobic eucaryotes employ O2 in the synthesis of sterols and unsaturated fatty acids. Facultative anaerobes do not require O2 for growth but grow better in its presence. In the presence of oxygen they use aerobic respiration. Aerotolerant anaerobes such as Enterococcus faecalis simply ignore O2 and grow equally well whether it is present or not. In contrast, strict or obligate anaerobes (e.g., Bacteroides, Fusobacterium, Clostridium pasteurianum, Methanococcus, Neocallimastix) do not tolerate O2 at all and die in its presence. Aerotolerant and strict anaerobes cannot generate energy through aerobic respiration and must employ fermentation or anaerobic respiration for this purpose. Finally, there are aerobes such as Campylobacter, called microaerophiles, that are damaged by the normal atmospheric level of O2 (20%) and require O2 levels below the range of 2 to 10% for growth. The nature of bacterial O2 responses can be readily determined by growing the bacteria in culture tubes filled with a solid culture medium or a special medium like thioglycollate broth, which contains a reducing agent to lower O2 levels  Although obligate anaerobes are killed by O2, they may be recovered from habitats that appear to be toxic. In such cases they associate with facultative anaerobes that use up the available O2 and thus make the growth of strict anaerobes possible. For example, the strict anaerobe Bacteroides gingivalis lives in the mouth where it grows in the anoxic crevices around the teeth. Chemical Requirements Carbon Structural organic molecules, energy source Chemoheterotrophs use organic carbon sources Autotrophs use CO2  Nitrogen In amino acids and proteins Most bacteria decompose proteins Some bacteria use NH4+ or NO3– A few bacteria use N2 in nitrogen fixation  Sulfur In amino acids, thiamine, and biotin Most bacteria decompose proteins Some bacteria use SO42– or H2S Phosphorus In DNA, RNA, ATP, and membranes PO43– is a source of phosphorus  Trace elements Inorganic elements required in small amounts Usually as enzyme cofactors Bacterial growth curve and its significance When a broth culture is inoculated with a small bacterial inoculum, the population size of the bacteria increases showing a classical pattern. When plotted on a graph, a distinct curve is obtained referred to as the bacterial growth curve. Method of Obtaining Bacterial Growth Curve