Streak plating is a microbiology laboratory method that has two major disadvantages. Firstly, users will not be able to grow obligate anaerobes using this method. Secondly, only organisms that were viable in the original sample are able to be grown.
Streak plating is a method that allows microorganisms to be grown from a sample on an agar plate. The agar provides the nutrients needed by the organism(s) being grown. Obligate anaerobes require a complete lack of oxygen to thrive, and the streak plate method is aerobic. When a sample is streaked onto the surface of the agar in the Petri dish, it is exposed to atmospheric oxygen. Assuming the sample is aerobic, then the next limitation that must be considered is that streak plating will only grow bacteria (known as colony forming units or CFUs) and microorganisms that are alive.
For example, if your interest is in knowing the amount of individual microorganisms present in a known volume of the sample of lake water, streak plating will limit the results to those that are still alive and able to thrive on the agar chosen as a growth medium. Therefore, direct counting via a slide, using a microscope, would be best.