What are microorganisms?
Technically a microorganism or microbe is an organism that is microscopic. The study of microorganisms is called microbiology. Microorganisms can be bacteria, fungi, archaea, or protists. The term microorganisms do not include viruses and prions, which are generally classified as non-living.
There is currently a great deal of discussion about the organization and classification of life, particularly in the study of microorganisms. The basic distinction divides living organisms into two groups:
- Prokaryotes (cells without internal membrane-bound organelles – the monera, including most microorganisms).
- Eukaryotes (cells containing membrane-bound organelles – protists, fungi, plants, and animals).
Before the advent of the microscope, living organisms were conveniently divided into two kingdoms: plant or animal. This division was unsatisfactory, however – what about fungi?
Today kingdom taxonomists have defined systems based on five or six kingdoms (Archaea – the sixth, Monera, Protoctista, Fungi, Plantae, and Animalia), neither of which include viruses (or prions). Viruses are considered to be on the line between living and non-living.
It has been argued extensively whether viruses are living organisms. Most virologists consider them non-living, as they do not meet all the criteria of the generally accepted definition of life.
For instance, most viruses do not respond to changes in the environment, which is a definitive trait for living organisms. In addition, viruses can replicate themselves only by infecting a host cell. They therefore cannot reproduce on their own.
Prions are a recently discovered infectious agent. They are proteins that are folded abnormally (folding is a property of proteins enabling them to take on a shape that is critical for their function), and which can convert normally folded proteins to abnormally folded ones.
The advent of genome analysis has both simplified and complicated the question. DNA sequence analysis led Dr. Carl Woese in the 70s to propose a three-domain division: bacteria, archaea, and Eukaryota.
The grounds for this division can be seen in the tree of life he produced from the sequence data. This tree of life clearly shows that microorganisms account by far for the majority of life on earth. Scientists estimate that 99% of the microorganisms on earth have not yet been identified!
Types of microorganisms
The major groups of microorganisms namely bacteria, archaea, fungi (yeasts and molds), algae, protozoa, viruses, and Multicellular Animal Parasites are summarized below. Links to the more detailed articles on each of the major groups are provided.
Bacteria are unicellular organisms. The cells are described as prokaryotic because they lack a nucleus. They exist in four major shapes:
- Bacillus (rod shape)
- Coccus (spherical shape)
- Spirilla (spiral shape)
- Vibrio (curved shape)
Most bacteria have a peptidoglycan cell wall; they divide by binary fission, and they may possess flagella for motility. The difference in their cell wall structure is a major feature used in classifying these organisms.
According to the way their cell wall structure stains, bacteria can be classified as either Gram-positive or Gram-negative when using the Gram staining.
Bacteria can be further divided based on their response to gaseous oxygen into the following groups:
- Aerobic (living in the presence of oxygen)
- Anaerobic (living without oxygen)
- Facultative anaerobes (can live in both environments)
According to the way they obtain energy, bacteria are classified as heterotrophs or autotrophs. Autotrophs make their own food by using the energy of sunlight or chemical reactions, in which case they are called chemoautotrophs. Heterotrophs obtain their energy by consuming other organisms. Bacteria that use decaying life forms as a source of energy are called saprophytes.
Archaea or Archaebacteria differ from true bacteria in their cell wall structure and lack peptidoglycans. They are prokaryotic cells with avidity to extreme environmental conditions.
Based on their habitat, all Archaeans can be divided into the following groups:
- Methanogens (methane-producing organisms)
- Halophiles (archaeans that live in salty environments)
- Thermophiles (archaeans that live at extremely hot temperatures)
- Psychrophiles (cold-temperature Archaeans)
Archaeans use different energy sources like hydrogen gas, carbon dioxide, and sulfur. Some of them use sunlight to make energy, but not the same way plants do.
They absorb sunlight using their membrane pigment, bacteriorhodopsin. This reacts with light, leading to the formation of the energy molecule adenosine triphosphate (ATP).
Fungi (mushrooms, molds, and yeasts) are eukaryotic cells (with a true nucleus). Most fungi are multicellular and their cell wall is composed of chitin.
They obtain nutrients by absorbing organic material from their environment (decomposers), through symbiotic relationships with plants (symbionts), or harmful relationships with a host (parasites).
They form characteristic filamentous tubes called hyphae that help absorb the material. The collection of hyphae is called mycelium. Fungi reproduce by releasing spores.
Protozoa are unicellular aerobic eukaryotes. They have a nucleus, complex organelles, and obtain nourishment by absorption or ingestion through specialized structures. They make up the largest group of organisms in the world in terms of numbers, biomass, and diversity.
Their cell walls are made up of cellulose. Protozoa have been traditionally divided based on their mode of locomotion: flagellates produce their own food and use their whip-like structure to propel forward, ciliates have tiny hair that beat to produce movement, amoeboids have false feet or pseudopodia used for feeding and locomotion, and sporozoans are non-motile. They also have different means of nutrition, which groups them as autotrophs or heterotrophs.
Algae also called cyanobacteria or blue-green algae, are unicellular or multicellular eukaryotes that obtain nourishment by photosynthesis. They live in water, damp soil, and rocks and produce oxygen and carbohydrates used by other organisms. It is believed that cyanobacteria are the origins of green land plants.
Viruses are noncellular entities that consist of a nucleic acid core (DNA or RNA) surrounded by a protein coat. Although viruses are classified as microorganisms, they are not considered living organisms. Viruses cannot reproduce outside a host cell and cannot metabolize on their own. Viruses often infest prokaryotic and eukaryotic cells causing diseases.
Multicellular Animal Parasites
A group of eukaryotic organisms consists of flatworms and roundworms, which are collectively referred to as helminths. Although they are not microorganisms by definition, since they are large enough to be easily seen with the naked eye, they live a part of their life cycle in microscopic form. Since the parasitic helminths are of clinical importance, they are often discussed along with the other groups of microbes.