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Enumeration of bacteria 293

Читайте также:
  1. BACTERIAL GROWTH 291
  2. CHAPTER 10 BACTERIAL REPRODUCTION AND GROWTH OF MICROORGANISMS
  3. CHAPTER 10 BACTERIAL REPRODUCTION AND GROWTH OF MICROORGANISMS
  4. Deep Sea Thermal Vent Bacteria
  5. Enriching for Specific Bacteria
  6. ENUMERATION OF BACTERIA 295



 


 


FIG. 10-7 The spread plate technique for isolating and enumerating microorganisms.

It is assumed that each colony arises from an indi­vidual bacterial cell. By counting the number of colonies that develop, the colony forming units (CFUs), and by taking into account the dilution fac­tors, the concentration of bacteria in the original sam­ple can be determined. Preferably two or three plates are counted to determine numbers of bacteria in a sample. Each plate counted should have 30 to 300


FIG. 10-8 The pour plate technique for isolating and enu­merating microorganisms.

colonies. If bacterial numbers in a sample are low, it is sometimes necessary to filter the suspension to concentrate the bacterial cells by collecting the cells on a membrane filter. The typical membrane filter for collecting bacterial cells is made of nitrocellulose or cellulose acetate and has a pore size of 0.2 to 0.45 |лт, which is small enough to trap most bacterial cells. The membrane filter with the trapped bacteria is then placed onto a suitable medium so that bacterial re­production can occur, and the colonies that develop on the filter are counted. Countable plates are those having between 30 and 300 colonies. Less than 30 colonies is not acceptable for statistical reasons and more than 300 colonies on a plate are likely to pro­duce colonies too close to distinguish as individual CFUs. Such samples are noted as TNTC (too numer­ous to count).

A major limitation of the viable plate count proce­dure is that it is selective. There is no single combina­tion of incubation conditions and medium composi­tion that permits the growth of all bacterial types. The nature of the growth medium and the incubation con­ditions determine which bacteria can grow and thus be counted. Viable counting measures only cells that are capable of growth on the given plating medium under the set of incubation conditions that are used. Sometimes cells are viable but nonculturable unless rigorous steps are taken to acclimate the microorgan­isms to laboratory culture conditions. The viable plate



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Читайте в этой же книге: Авиаперелет оплачивается отдельно (в зависимости от выбранной авиакомпании). | Binary Fission | CHAPTER 10 BACTERIAL REPRODUCTION AND GROWTH OF MICROORGANISMS | Generation time or doubling time is the unit of me sure of bacterial growth; it is the time it takes for the size of a bacterial population to double. | ENUMERATION OF BACTERIA 295 | Enriching for Specific Bacteria | Deep Sea Thermal Vent Bacteria | FACTORS INFLUENCING BACTERIAL GROWTH 301 | Growing Cultures of Aerobic and Anaerobic Bacteria | FACTORS INFLUENCING BACTERIAL GROWTH 303 |
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