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Transcript
Bacterial physiology The biochemical reactions that together enable bacteria to live, grow, and reproduce. Strictly speaking, metabolism describes the total chemical reactions that take place in a cell, while physiology describes the role of metabolic reactions in the life processes of a bacterium. Cell Metabolism Cell metabolism is the total energy released and consumed by a cell. Metabolism describes all of the chemical reaction that are happening in the cell. Some reactions, called anabolic reactions, create’ needed products. Other reactions, called catabolic reactions, break down products. Your body is, performing both anabolic. and catabolic reactions at the same time and around the clock, twenty four hours a day, to keep your body alive and functioning. Even while you ‘sleep, your cells are busy metabolizing. • Catabolism: The energy releasing process in which a chemical or food is used (broken down) by degradation or decomposition, into smaller pieces. • Anabolism: Anabolism is just the opposite of catabolism. In this portion of metabolism, the cell consumes energy to produce larger molecules via smaller ones. ATP is the currency of the cell. Physical and chemical growth determinate Main Requirements Many food-poisoning bacteria have to multiply to high numbers in food before they are likely to cause illness. The four main requirements for bacterial growth are nutrition, moisture, warmth and time. Nutritional Requirements 1-Basic nutritional requirements for growth : a-Carbon - building blocks of cell components b-Nitrogen - production of proteins, nucleic acids c-Hydrogen - occur in organic compounds d-Oxygen - involved in the production of energy e-Minerals, Trace Elements - required in small amount. 2-Special metabolites ( growth factors ) a-Substances required for growth that the cell cannot produce using the basic requirements already listed ( Ex. : vitamins, amino acids, carbohydrates, blood factors ) b- Organisms may be described as being fastidious Two types organisms based on source of nutrients : 1-Autotrophs - utilize inorganic compounds ( C - CO2, carbonates; N - NH4, N2, NO3 ) 2-Heterotrophs - utilize organic compounds ( C - CHO, lipids; N - proteins ) a- Saprophytes - nonliving organic material b- Parasites - viable (living) organic material Moisture Most foods naturally contain sufficient moisture to provide bacteria with the water they need in order to grow. Where moisture has been deliberately removed (e.g. in dehydrated foods such as milk powder, soup mixes, etc.), then bacteria will not grow whilst the food remains dry, but once water is added then bacterial growth may occur once more. Warmth / Temperature Bacteria have varying requirements in terms of the range of temperatures in which they will grow. Those which grow at low temperatures (usually below 20°C) are called psychrophiles and at high temperatures (above 45°C) are thermophiles. Most pathogens are known as mesophiles. They will grow at temperatures between 5°C and 63°C, commonly referred to as the growth or 'danger' zone and have an optimum temperature for growth of about 37°C. Time In ideal conditions (i.e. in moist foods at 37°C) bacteria will grow and multiply by dividing into two every 20 minutes. Other Factors Affecting Growth pH Level The acidity or alkalinity of foods will affect bacterial growth. Most bacteria like neutral conditions (pH value of 7) . Concentration of H+, OH- ions 1 ----------------------------------- 7 ------------------------------14 Acid Neutral Basic . Divided into four groups according to pH range : . *Neutrophiles - 5 to 8 *Alkalinophiles - 8 to 12 *Optimum pH 7.0 - 7.2 *Acidophiles - 0 to 5 Oxygen Pathogens vary in their oxygen requirements. Those which require oxygen are called aerobes, e.g. Bacillus cereus. Those which do not need oxygen are called anaerobes, e.g. Clostridium perfringens .Those which will grow or survive with or without oxygen are known as facultative anaerobes and include Salmonella species and Staphylococcus aureus. Microaerophilic - require the presence of small amounts of oxygen (2% 10%) . Osmotic pressure a- Exerted by solutes in water b- Increase o.p. outside cell - water leaves cell ( very high o.p. - dehydrates cell ) c- Decreased o.p. outside cell - water enters cell ( very low o.p. - lysis of cell ) d- Halophiles - require the presence of 3% NaCl ( extreme halophiles - 20 to 30% NaCl ) LIGHT ( RADIATION ) 1-Very small group photosynthetic bacteria (cyanobacteria) - require UV light 2- Nonphotosynthetic bacteria (eubacteria) - UV light is lethal (causes mutations) Competition Where there are a number of different bacteria present in food, they compete for the same nutrients. Pathogens are often not as competitive as spoilage bacteria and unless present in high numbers, will usually die. Bacterial growth Bacterial growth is the division of one bacterium into two daughter cells in a process called binary fission. Providing no mutational event occurs the resulting daughter cells are genetically identical to the original cell. Bacterial Reproduction 1- Occurs by binary fission (simple division) . Process *Slight enlargement in cell size due to : 1-Increase in metabolic activities 2-Production of energy and cell parts 3-DNA replicates (duplicated) 4-Cell wall and membrane grow inward separating DNA 5-Divides contents of cell and DNA molecules 6-Two daughter cells formed GENERATION TIME Time required for one cell to produce two new cells. A-Varies with type organism and environmental conditions. B-Average 15 - 20 min. (varies - 10 minutes to 24 hrs.) MEASUREMENT OF BACTERIAL GROWTH 1-Optical density 2-Plate count 3-Direct microscopic count Phases of growth In autecological studies, bacterial growth in batch culture can be modeled with four different phases: lag phase (A), exponential or log phase (B), stationary phase (C), and death phase (D). Growth is shown as L = log(numbers) where numbers is the number of colony forming units per ml, versus T (time.) 1. During lag phase, bacteria adapt themselves to growth conditions. It is the period where the individual bacteria are maturing and not yet able to divide. During the lag phase of the bacterial growth cycle, synthesis of enzymes and other molecules occurs, no increase in number of cells. 2. Exponential phase (sometimes called the log phase or the logarithmic phase) is a period characterized by cell doubling. The number of new bacteria appearing per unit time is proportional to the present population. The slope of this line is the specific growth rate of the organism, which is a measure of the number of divisions per cell per unit time. 3. The "stationary phase" is due to a growth-limiting factor; this is mostly depletion of a nutrient, and/or the formation of inhibitory products such as organic acids. Rate of growth influenced by environmental factors. Stationary Phase - rate of reproduction = rate of death. Due to exhaustion of nutrients, accumulation of wastes. 4- At death phase, bacteria run out of nutrients and die. rate of death rate of reproduction.Some species die quickly, others survive longer. Sporulation The sporulation process begins when nutritional conditions become unfavorable, near depletion of the nitrogen or carbon source (or both) Many environmental bacteria are able to produce stable dormant, or resting, forms as a branch of their life cycle to enhance their survival under adverse conditions. Such dormant forms are called endospores, cysts, or heterocysts (primarily seen in cyanobacteria), depending on the method of spore formation, which differs between groups of bacteria. Sporulation involves the production of many new structures, enzymes, and metabolites along with the disappearance of many vegetative cell components. These changes represent a true process of differentiation: A series of genes whose products determine the formation and final composition of the spore are activated. Germination The germination process occurs in three stages: activation, initiation, and outgrowth: Activation Most endospores cannot germinate immediately after they have formed. But they can germinate after they have rested for several days or are first activated, in a nutritionally rich medium, by one or another agent that damages the spore coat. Among the agents that can overcome spore dormancy are heat, abrasion, acidity, and compounds containing free sulfhydryl groups. Initiation Once activated, a spore will initiate germination if the environmental conditions are favorable. Different species have evolved receptors that recognize different effectors as signaling a rich medium: Thus, initiation is triggered by L-alanine in one species and by adenosine in another. Outgrowth Degradation of the cortex and outer layers results in the emergence of a new vegetative cell consisting of the spore protoplast with its surrounding wall. Outgrowth requires a supply of all nutrients essential for cell growth.