Because “protists” are so diverse, the term “protist” is no longer used. Instead, the categories of microbes are bacteria, archaea, fungi, algae, protozoa, slime molds, and viruses (because they are not cells, viruses aren’t technically living). There so many uses (such as vaccines, biofuel, and yogurt) that microbes give us, but they’re also harmful too.
Various Types of Microbes
Bacteria are unicellular, meaning that they are only one cell. Most bacteria are prokaryotic (unlike our cells, they don’t have a nucleus. They have what’s called a “nucleoid” – where the unbound chromosome sits). This bacteria have a cell wall, which covers the plasma membrane of the cell. Most bacteria have this special molecule for their cell walls called peptidoglycan (big word that means “protein sugar”) that can help protect them from danger. They can live anywhere and depending on what kind of bacteria, are pathogenic or symbiotic (or helpful to us humans). These guys are microscopic. A number of them are heterotrophic (eat other organisms) or autotrophic (make their sugars by photosynthesis).
Archaea are, as far as we know, the oldest microbes. Depending on which kind of archaea, they can survive harsh treatments like high salt environments (called “halophiles”) or extreme hot temperatures (called “thermophiles”), etc. They are typically unicellular (usually prokaryotic) and have cell walls. Because they don’t have peptidoglycan, their cell walls are different from bacteria. Archaea have distinct lipids (a fancy word for a collection of fat and wax molecules) in their walls.They are microscopic. Some eat other organisms or make their own food via photosynthesis.
Fungi are eukaryotes, meaning that have the same kind of cells that we, humans, have. However, these cells are different from bacterial, archaeal, and human cells. Fungi have a chitin (strong nitrogen-formed outer layer) cell wall*. Fungi, depending on which type, are multicellular or unicellular. Multicellular fungi are the type of fungi that grow on the forest floor. Unicellular fungi is the kind that’s used to make beer or bread. They’re either microscopic or macroscopic (macroscopic just means that we can see without looking into a microscope). Every single known fungi is heterotrophic (eats other living tissue).
Algae have eukaryotic cells. They are both macroscopic and microscopic; unicellular and multicellular. They have a cell wall, but unlike bacteria and fungi, their cell wall has pectin (gelatinous polysaccharide** outer layer) or a cellulose (insoluble polysaccharide outer layer). These guys are autotrophic. They’re very important to the environment (and our survival!). They make a lot of of the oxygen we breathe!
Protozoa are unicellular eukaryotic organisms that have no cell wall and are typically microscopic and heterotrophic. Most protozoa can move and can survive in different places including the intestines of some animals!
Slime Molds are eukaryotic microbes that act like protozoa in the beginning of their life by engulfing their food and then act like fungi later in life by growing on their food.
Viruses aren’t technically living and attack a cell to use its nutrients or its resources. They are very small! Viruses insert their DNA or RNA (depending on the virus) into the cell’s DNA. Viruses have no cell wall and are acellular (meaning it’s not a cell). Bacteriophages are viruses that insert their DNA into bacteria. Bacteriophages or just phages are used in science to help cure diseases!
There are other acellular pathogens such as prions and viroids. Prions are proteins that causes other proteins to change shape and become dysfunctional. Viroids are RNA molecules that cause plant disease.
*Just so you know humans don’t have cell walls.
**A ridiculously big word that means “many sugars.”
Willey, Joanne M. Sherwood, Linda M. Woolverton, Christopher J (2014). Prescott’s Microbiology. (9th Ed.). New York, NY: McGraw Hill