Microbial Growth refers to increase in number of cells not in size

cell division of bacteria binary fission-a cell duplicates its components and divides into two cells budding-occurs in yeast and a few bacteria when a small new cell develops from the surface of an existing cell and separates from the parent cell

binary fission unchecked E. coli - generation time of 20 min. 20 generations (about 7 hrs.) 1 million cells 30 generations ( about 10 hrs.) 1 billion cells 72 generations ( about 24 hrs.) 1 x ,000,000,000,000,000,000,000 cells

phases of growth standard bacterial growth curve

lag phase Bacteria are first introduced into an environment or media Bacteria are “checking out” their surroundings cells are very active metabolically # of cells changes very little 1 hour to several days

log phase Rapid cell growth (exponential growth) population doubles every generation microbes are sensitive to adverse conditions antibiotics anti-microbial agents the generational time for most bacteria is less than 1 hour

log growth is limited by decreasing nutrients, oxygen and living space increasing organisms, metabolic wastes limiting factor for log growth is the rate at which ATP can be produced

stationary phase-when cells produced at the same rate as old cells die because of environmental stress death (decline) phase-the number of live cells decreases at a logarithmic rate because of less supportive medium

growth in colonies cells divide exponentially to form a small colony (all descendants of the original cell) the colony grows rapidly at its edges cells nearer the center grow more slowly or begin to die

factors affecting bacterial growth Physical pH, temperature, oxygen concentration, moisture, hydrostatic pressure, osmotic pressure, and radiation Nutritional (biochemical) availability of carbon, nitrogen, sulfur, phosphorus, trace elements

Physical-pH acidity or alkalinity of a medium Most bacteria grow between pH pH 7.5 many foods, such as sauerkraut, pickles, and cheeses are preserved from spoilage by acids produced during fermentation

temperature most species of bacteria can grow over a 30*C temp range according to their growth temperature range, bacteria can be classified as: psychrophiles mesophiles thermophiles

oxygen aerobes-require oxygen to grow anaerobes-do not require oxygen Aerobic and anaerobic bacteria can be identified by growing them in a liquid culture: 1: Obligate aerobic bacteria gather at the top of the test tube in order to absorb maximal amount of oxygen. 2: Obligate anaerobic bacteria gather at the bottom to avoid oxygen. 3: Facultative bacteria gather mostly at the top, since aerobic respiration is the most beneficial one; but as lack of oxygen does not hurt them, they can be found all along the test tube. 4: Microaerophiles gather at the upper part of the test tube but not at the top. They require oxygen but at a low concentration. 5: Aerotolerant bacteria are not affected at all by oxygen, and they are evenly spread along the test tube.

moisture all actively metabolizing cells generally require a water environment single-celled organisms are exposed directly to their environment most cells can only live a few hours without moisture

osmotic pressure membranes of all microorganisms are selectively permeable water moves by osmosis into and out of the cell most bacteria can tolerate a wide range of concentrations of dissolved substances yet, if concentrations outside the cells is too high, water loss can inhibit growth or kill cells

the use of salt as a preservative in curing ham and bacon and in making pickles is based on the fact that high concentrations of dissolved substances exert sufficient osmotic pressure to kill or inhibit microbial growth the use of sugar as a preservative in making jellies and jams is based on the same principle

nitrogen all organisms, including microorganisms, need nitrogen to synthesize enzymes, other proteins, and nucleic acids some microorganisms obtain nitrogen from inorganic sources many disease-causing organisms obtain amino acids for making proteins and other nitrogenous molecules from the cells of humans

Limiting factors in the environment Lack of food, water or nutrients space accumulation of metabolic wastes lack of oxygen changes in pH temperature