Comparing Fresh and Spoiled Milk Under a Microscope

Musical biology lab techSci-Inspi, who previously made a compilation of the microscopic world around him, used a microscope to examine a sample from a jug of whole homogenized milk fresh from a supermarket. He also examined another sample of milk long after its expiration date, per a viewer's request. He then compared the two in order to see the differing bacteria.

I ...tested for bacteria at each stage with a petri dish to quantify bacterial colonies. I believe the most common bacteria after the milk was incubated to be lactobacillus because of the following criteria: gram positive rods, growth on complex medium (milk, Tryptic soy agar), non-pathogenic since the milk was pasteurized, Indole negative, Citrate negative. This is the best guess I can make with the medium and tests available to me in my lab.

He also notes that some bacteria remain even after pasteurization.

Pasteurization kills most but not all microbes. The ones that survived grow in the milk.