Short answers, pulled from the story.
Bacteria are mostly free-living microorganisms, often a single biological cell only a few micrometres long, that make up a large domain of prokaryotes. They lack a nucleus, carry a circular chromosome, and rarely hold membrane-bound organelles.
Bacteria were among the first life forms on Earth, appearing about 4 billion years ago. For roughly 3 billion years afterward, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.
Humans carry approximately 10 to the 13th through 10 to the 14th power of bacteria, with most in the gut and many on the skin. Most are harmless or beneficial, especially the gut species, which help synthesise vitamins and inhibit harmful bacteria.
Pathogenic bacteria cause infectious diseases including cholera, syphilis, anthrax, leprosy, tuberculosis, tetanus and bubonic plague. The most common fatal bacterial diseases are respiratory infections, and antibiotics used to treat them have driven a growing problem of antibiotic resistance.
Antonie van Leeuwenhoek first observed bacteria in 1676 using a single-lens microscope of his own design. He called all the microorganisms he saw animalcules, and reported his findings in letters to the Royal Society of London.
Gram-positive bacteria have a thick cell wall with many layers of peptidoglycan and teichoic acids, while Gram-negative bacteria have a thin peptidoglycan layer wrapped in a second lipid membrane. The Gram stain, developed in 1884 by Hans Christian Gram, distinguishes them by colour, purple for Gram-positive and pink for Gram-negative.