What are the methods used to measure bacterial growth?

There are several methods that can be used to measure bacterial growth: 

• Direct microscopic count: This method involves directly observing and counting individual bacterial cells under a microscope using a Petroff-Hausser counting chamber, which contains an etched grid. Bacterial growth is estimated by determining the average amount of bacteria present within specified squares of the grid. The direct microscope count is the simplest and most accurate method for measuring bacterial growth. 
• Quantification via electronic means: In this technique, a bacterial cell solution is suspended within electronic particle counters. The bacteria are counted as they pass through relatively small orifices. 
• Bacterial counting via filtration: Filtration methods are usually employed when counting bacteria in environments where microorganism count is very low, such as in naturally occurring streams or ponds. In this technique, a thin filter membrane with pore sizes smaller than bacteria is used to pass a threshold volume of water, enabling bacterial counting. 
• Estimation of bacterial growth based on turbidity: In certain experiments, turbidity is the only practical way to monitor bacterial growth. As bacteria multiply and grow in a liquid medium, the medium starts to become increasingly more turbid or cloudy, which is an indication of growth. 
• The plate counting technique: There are two plate counting techniques – spread plate and pour plate technique. Both techniques calculate the proliferation of molecules under specific conditions and are used extensively for estimating bacterial activity in various substances such as water, milk, and food items.  
• Nitrogen content determination: Since proteins constitute the primary cellular material, nitrogen, a constituent of proteins, serves as a measure of bacterial nitrogen content, either by microbial species or cell yield.
• Cell dry weight determination:  This technique quantitatively measures bacterial growth by assessing cell mass. It makes up for the drawbacks of more traditional methods such plate count, which does not account for increase in filamentous mass.