PPT 2

• The growth of microbes is encouraged; in other words, scientists want them to grow.
• Factors That Affect Microbial Growth

  • Availability of Nutrients
  • Osmotic Pressure and Salinity
  • Temperature
  • Moisture
  • Barometric Pressure
  • Gaseous Atmosphere
• Availability of Nutrients


  To survive in a particular environment, appropriate nutrients must be available. Many nutrients are energy sources; organisms will obtain energy from these chemicals by breaking chemical bonds.
• Moisture


  There are certain microbial stages (e.g., bacterial endospores, protozoan cysts), however, that can survive the complete drying process (desiccation). The organisms contained within the spores and cysts are in a dormant or resting state; if they are placed in a moist, nutrient-rich environment, they will grow and reproduce normally.
• Temperature


  Every microorganism also has a minimum growth temperature, below which it ceases to grow, and a maximum growth temperature, above which it dies.

  
• Osmotic Pressure and Salinity


  The pressure that is exerted on a cell membrane by solutions both inside and outside the cell.

  
• pH


  Most microorganisms prefer a neutral or slightly alkaline growth medium (pH 7.0–7.4). Acidophilic microbes (acidophiles), such as those that can live in the human stomach and in pickled foods, prefer a pH of 2 to 5. Alkaliphiles prefer an alkaline environment (pH >8.5).
• Barometric Pressure


  Most bacteria are not affected by minor changes. Others, known as piezophiles, thrive deep in the ocean and in oil wells, where the atmospheric pressure is very high.
• Gaseous Atmosphere


  To grow a particular microorganism in the laboratory, it is necessary to provide the atmosphere that it requires.
• In microbiology research laboratories, scientists must culture microbes so that they can learn more about them, harvest antibiotics and other microbial products, test new antimicrobial agents, and produce vaccines.
• Microbes must also be cultured in genetic engineering laboratories and in the laboratories of certain food and beverage companies, as well as other industries.
• The earliest successful attempts to culture microorganisms in a laboratory setting were made by Ferdinand Cohn (1872), Joseph Schroeter (1875), and Oscar Brefeld (1875).
• Robert Koch described his culture techniques in 1881. Initially, Koch used slices of boiled potatoes on which to culture bacteria, but he later developed both liquid and solid forms of artificial media.
• Gelatin was initially used as a solidifying agent in Koch's culture media, but in 1882, Fanny Hesse, the wife of Dr. Walther Hesse— one of Koch's assistants—suggested the use of agar.
• Frau Hesse (as she is most commonly called) had been using agar in her kitchen for many years as a solidifying agent in fruit and vegetable jellies.
• Another of Koch's assistants, Richard Julius Petri, invented glass Petri dishes in 1887 for use as containers for solid culture media and bacterial cultures.
• The Petri dishes in use today are virtually unchanged from the original design, except that most of today's laboratories use plastic, presterilized, disposable Petri dishes.
• In 1878, Joseph Lister became the first person to obtain a pure culture of a bacterium (Streptococcus lactis) in a liquid medium.
• As a result of their ability to obtain pure cultures of bacteria in their laboratories, Louis Pasteur and Robert Koch made significant contributions to the germ theory of disease.
• Bacterial growth


  Refers to an increase in the number of organisms rather than an increase in their size. Refers to the proliferation or multiplication of bacteria. A bacterial colony is a mound or pile of bacteria containing millions of cells. The time taken for one cell to become two cells by binary fission is called the generation time.
• In the laboratory, under ideal growth conditions, E. coli, V. cholerae, Staphylococcus spp., and Streptococcus spp. all have a generation time of about 20 minutes, Pseudomonas and Clostridium spp. may divide every 10 minutes, and Mycobacterium tuberculosis may divide only every 18 to 24 hours.
• Types of culture media

  • Artificial media or synthetic media
  • Chemically defined medium
  • Complex medium
  • Enriched medium
  • Selective medium
  • Differential medium
• Chemically defined medium


  A medium in which all the ingredients are known; this is because the medium was prepared in the laboratory by adding a certain number of grams of each of the components (e.g., carbohydrates, amino acids, salts).
• Complex medium


  Complex media contain ground-up or digested extracts from animal organs (e.g., hearts, livers, brains), fish, yeasts, and plants, which provide the necessary nutrients, vitamins, and minerals.
• Enriched medium


  A broth or solid medium containing a rich supply of special nutrients that promotes the growth of fastidious organisms.
• Selective medium


  A medium that has added inhibitors that discourage the growth of certain organisms without inhibiting growth of the organism being sought.
• Differential medium


  A medium that allows one to readily differentiate among the various types of organisms that are growing on the medium.
• Inoculation of culture media


  Inoculation of a liquid medium involves adding a portion of the specimen to the medium. Inoculation of a solid or plated medium involves the use of a sterile inoculating loop to apply a portion of the specimen to the surface of the medium; a process commonly referred to as streaking.
• Aseptic technique


  Practiced to prevent: (a) microbiology professionals from becoming infected, (b) contamination of their work environment, and (c) contamination of clinical specimens, cultures, and subcultures.
• Types of aseptic techniques

  • Medical asepsis
  • Surgical asepsis
• Sepsis


  Refers to the presence of pathogens in blood or tissues.
• Asepsis


  Means the absence of pathogens.
• Antisepsis


  The prevention of infection.
• Sterile technique


  Practiced when it is necessary to exclude all microorganisms from a particular area, so that the area will be sterile.
• Types of incubators used in a clinical microbiology laboratory

  • CO2 (carbon dioxide) incubator
  • Non-CO2 incubator
  • Anaerobic incubator
• Bacterial population counts


  The microbiologist may: (a) determine the total number of bacterial cells in the liquid (the total number would include both viable and dead cells) or (b) determine the number of viable (living) cells.
• Spectrophotometer


  An instrument used to determine the total number of cells.
• Viable plate count


  Used to determine the number of viable bacteria in a liquid sample, such as milk, water, ground food diluted in water, or a broth culture.

  
• Phases of a bacterial population growth curve

  • Lag phase
  • Log phase
  • Stationary phase
  • Death phase
• Fungal culture in the laboratory is usually carried out on agar plates, shake flasks, and bench top fermenters starting with an inoculum that typically features fungal spores.