Microbiology is the study of microorganisms, which are living things too small to be seen with the naked eye. They include bacteria, viruses, fungi, protozoa, and archaea. Anton van Leeuwenhoek was the first person to observe living microorganisms, which he called “animalcules,” using a simple microscope he made himself. Louis Pasteur is credited with developing the germ theory of disease, which says that many diseases are caused by microorganisms. Robert Koch later proved this with Koch’s postulates. These state that the microbe must be found in every case of the disease, it must be isolated and grown in pure culture, it must cause the same disease when put into a healthy host, and it must be isolated again from that host.
Bacteria are prokaryotic organisms, which means they have no true nucleus. They are classified by shape. Rod-shaped bacteria are called bacilli, round ones are cocci, and spiral ones are spirilla. Bacteria are often identified by Gram staining, a technique that separates them based on the thickness of the peptidoglycan layer in their cell wall. Gram-positive bacteria have a thick peptidoglycan layer and appear purple or violet. Gram-negative bacteria have a thin layer and appear pink or red. Useful bacteria such as Lactobacillus are used to make yogurt and cheese.
Carl Woese proposed the three-domain classification of life: Bacteria, Archaea, and Eukarya. It was based mainly on comparing ribosomal RNA (rRNA) sequences. Archaea differ from bacteria because their cell walls do not contain peptidoglycan. Methanogens, which produce methane and are poisoned by oxygen, belong to the domain Archaea. Halophiles are microorganisms that thrive in environments with a very high concentration of salt.
Viruses are much smaller than bacteria and differ from them because they are not made of cells and can only reproduce inside a living host cell. The protein coat that surrounds the genetic material of a virus is called the capsid. A virus that specifically infects bacteria is called a bacteriophage. Fungi, such as yeasts and molds, obtain nutrients by absorption: they release enzymes to digest food outside their bodies and then absorb the digested nutrients.
Alexander Fleming discovered penicillin. Antibiotics are effective against bacteria, but not against viruses. Antibiotic resistance is a growing problem. Some bacteria resist penicillin by producing the enzyme beta-lactamase, which breaks the beta-lactam ring of the antibiotic. An efflux pump contributes to resistance by actively pumping the antibiotic out of the bacterial cell before it can work. Resistance genes spread rapidly between bacteria mainly through horizontal gene transfer, especially conjugation, in which genes are passed on plasmids.
Many bacteria form a biofilm, which is a community of microorganisms attached to a surface and embedded in a protective slimy matrix. Biofilms are harder to treat with antibiotics.
Pasteurization is the process of using heat to kill harmful microorganisms in food, especially in milk, without changing its quality much.