What are the limitations of direct count?

Direct count is one of many techniques used to measure bacterial growth. In this method, individual bacterial cells are directly observed and counted using a Petroff-Hausser counting chamber. 

Limitations of direct microscopic count

• Unable to differentiate viable and nonviable cells so both living and dead cells are counted
• Is not sensitive to populations of less than 1 million cells
• The particles in the samples and the cells are mixed together 
• Precision is difficult to accomplish 
• Some cells are likely missed because of the difficulty in being able to see smaller cells under the microscope
• Higher susceptibility to eye fatigue
• When using a sample that is not stained, a phase contrast microscope is needed