Saran's Learn Microbiology

INDIRECT MEASUREMENT OF MICROBIAL GROWTH  1) Turbidity Estimating turbidity is a practical way of monitoring bacterial growth.  As bacteria multiply in a Liquid medium, the medium becomes Turbid, or Cloudy with cells.  The instrument used to measure Turbidity is a Spectrophotometer or Colorimeter. In the Spectrophotometer, a beam of light is transmitted through a bacterial suspension to a light sensitive Detector. As bacterial numbers increase, less light will reach the Detector. This change of light will register on the instrument’s scale as the Percentage transmission. Also printed on the instrument’s scale is a logarithmic expression called the Absorbance (sometimes called Optical density, or OD, which is calculated as Abs = 2 − log of % transmittance).  The Spectrophotometer meter has two scales. The bottom scale displays Absorbance and the top scale, Percent transmission. Absorbance increases as percent transmission decreases. The Absorbance is used to plot...

DIRECT MEASUREMENT OF MICROBIAL GROWTH   1) Direct Microscopic Count  A specially designed slide called a Petroff-Hausser Cell Counter is also used in Direct Microscopic Count. Petroff-Hausser Cell Counter. In the Direct Microscopic Count method, a measured volume of a bacterial suspension is placed within a defined area on a Microscope slide. Because of time considerations, this method is often used to count the number of bacteria in milk.  A 0.01 ml sample is spread over a marked square centimeter of slide, stain is added so that the bacteria can be seen, and the sample is viewed under the oil immersion objective lens. The area of the viewing field of this objective can be determined. Once the number of bacteria has been counted in several different fields, the average number of bacteria per viewing field can be calculated. From these data, the number of bacteria in the square centimeter over which the sample was spread can also be calculated. Petroff-Hausser C...