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Microbial Growth. Requirements for Growth. Temperature pH Osmotic pressure Oxygen Nutrients. Temperature. Psychrophiles – cold loving Mesophiles – moderate Thermophiles – heat loving Most important human pathogens are mesophiles Optimum growth temperature is 37 °C.
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Requirements for Growth • Temperature • pH • Osmotic pressure • Oxygen • Nutrients
Temperature • Psychrophiles – cold loving • Mesophiles – moderate • Thermophiles – heat loving • Most important human pathogens are mesophiles • Optimum growth temperature is 37°C
pH and Growth • Most bacteria and especially the important human pathogens grow best at a pH near neutral, 6.5 – 7.5 • Neutrophiles • Acidophiles – bacteria that can survive and grow at low pH • Alkalinophiles – bacteria that can survive and grow at high pH
Osmotic Pressure • Bacteria are 80-90% water • If the cells are placed in a high salt or sugar solution the cells lose water and cannot grow • PLASMOLYSIS • Few bacteria survive high salt • HALOPHILES
How about some nice salt pork? • Because high salt or sugar prevents bacterial growth such foods as salted meats, honey, jellies, etc will usually not spoil if kept at room temperature • Salt preserved meats were used for long ocean trips • Molds can grow however in high sugar, so be careful! • Always refrigerate when in doubt
Chemical Requirements • Carbon source – needed to make all organic compounds • Nitrogen, sulfur, phosphorus – synthesis of proteins, nucleic acids, and ATP • Trace elements – iron, zinc, copper
Oxygen Requirements • Strict aerobe – requires oxygen • Facultative anaerobe – grows in the presence of oxygen but can without oxygen • Obligate anaerobes – cannot use oxygen for growth • Aerotolerant anaerobes – do not use oxygen for growth but can tolerate it
What’s the problem with oxygen? • During normal respiration bacteria produce oxygen free radicals O2- which combine with 2H+ to produce hydrogen peroxide H2O2 which is toxic • The enzyme that makes this reaction happen is SUPER OXIDE DISMUTASE (SOD) • Another enzyme CATALASE breaks down the H2O2 into H2O and O2 • Obligate anaerobes usually have neither SOD or catalase and are killed by oxygen and toxic by-products
Growth of Bacterial Cultures • Bacteria reproduce by BINARY FISSION • Time required for a cell to divide is the generation time • Generation time is highly variable • Generally 1-3 hours but can be as short as 30 min
Stages of Growth Curve • Lag – little increase in number, cells are “getting ready” to grow • Log – rapid, exponential growth, cells most metabolically active, most rapid generation time • Stationary – growth rate slows, equilibrium • Death – decline, cell death exceeds cells formed
Measuring Bacterial Populations • Plate counts • Most common way to measure bacterial number • Make a series of dilutions, plate selected dilutions, count colonies • Multiply number of colonies by dilution factor to get COLONY FORMING UNITS
On what do you grow bacteria? • Culture media • Culture media can either be solid or liquid and contains all the nutrients the microbe needs to grow • Adding bacteria to sterile media is called INOCULATION • The growth is called a CULTURE • Visible growth on solid media is called a colony
What is agar? • Agar is the solid agent on which bacteria grow • Agar is made from seaweed • Bacteria cannot break down the agar so it is the perfect solid media on which to observe bacterial growth
Chemically defined media Exact chemical composition is known Complex media Exact chemical composition if media is not known Most common type of lab media Types of media
Selective media Special media that inhibits growth of unwanted bacteria and encourages the growth of desired bacteria Differential media Media that allows you to distinguish different types of bacteria growing on a plate usually by color of colonies or growth patterns Many media are BOTH selective and differential Selective and Differential Media
