MEASUREMENT OF MICROBIAL GROWTH – INDIRECT METHOD

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 bacterial growth in the graph. 

Spectrophotometer 

Advantages

The benefits of measuring Turbidity to estimate population growth include 

a) Ease of use 

b) Speed.

Disadvantage 

a) About 10 million to 100 million cells per milliliter are needed to make a suspension turbid enough to be read on a Spectrophotometer. Therefore, turbidity is not a useful measure of contamination of liquids by relatively small numbers of bacteria.

b) If the bacteria form either a pellicle (a film of cells at the surface) or a sediment (an accumulation of cells at the bottom), their number will be underestimated. 

c) Spectrophotometry does not distinguish between living and dead cells.

2) Metabolic activity

• Another indirect way to estimate bacterial numbers is to measure a population’s Metabolic activity. 
• This method assumes that the amount of a certain metabolic product, such as acid or CO2, is in direct proportion to the number of bacteria present. 
• An example of a practical application of a Metabolic test is the Microbiological assay in which Acid production is used to determine amounts of Vitamins. 

3) Dry weight

• The abundance of some microorganisms, particularly Filamentous microorganisms is difficult to measure by Direct methods. In plate counts of Actinobacteria and Fungal molds, it is mostly the number of Asexual spores that is counted instead. This is not a good measure of growth.
• In Dry weight estimation method, the organisms are filtered from their culture medium, dried, and weighed.
• The Dry weight method is suitable for broth cultures, but growth cannot be followed over time because the organisms are killed during the process.
• For bacteria also the same basic procedure is followed.