Scientists use a Gram stain to classify them. Gram-positive bacteria have a thick peptidoglycan layer that retains purple dye, while Gram-negative bacteria have a thinner layer and an outer membrane, appearing pink.
Bacteria are microscopic, single-celled organisms that exist in virtually every environment on Earth, from deep-sea vents to the human digestive tract. They are , meaning they lack a nucleus and membrane-bound organelles. While often associated with disease, the vast majority of bacteria are either harmless or essential to life. 1. Structure and Classification bacterium
Bacteria play a critical role in maintaining the planet's balance: Scientists use a Gram stain to classify them