Microorganisms and Types of Microbes or Germs

What are microorganisms?

Microorganisms are living creatures that are too small to observe with the naked eye. A microscope is typically required to detect these tiny creatures, which are smaller than 0.04mm. In informal language, microorganisms are also referred to as microbes, bugs, or germs. There are many different species of microbes, and each species possesses unique characteristics.

The study of microorganisms falls under the purview of the field of microbiology. The study of microorganisms has become a very important area due the significant impact that microbes have on our lives. Many microorganisms live inside and on the human body without causing any disease. These constitute the normal flora of the human body. The presence of this normal microbial flora is beneficial for the human body since it restricts the growth of pathogenic bacteria.

However, overgrowth of some of the microbes that constitute the normal flora can cause diseases. This usually occurs when the body’s immune system is weakened. The pathogenic potential of a microorganism is determined by its virulence, access to internal tissues, and the microbial load (number of microbes).

Types of Microorganisms

Microorganisms can be classified in many different ways. The following are some of the common ways in which microbes are usually categorized.

Classification based on kingdoms

There have been many attempts to classify all life forms on earth into different kingdoms. At various points of time, different kingdom classifications have been proposed and adopted. However, no single classification has managed to classify all life forms neatly according to specific sets of characteristics. The latest classification scheme (as of 2015) involves grouping all life forms into 7 major kingdoms: bacteria, archaea, protozoa, chromista, plantae, fungi, and animalia.

Microorganisms occupy positions in multiple kingdoms. The major microbial groups include bacteria, fungi, parasites, and viruses. The viruses are obligate intracellular parasites that are not able to replicate outside a host system. They do not have the metabolic machinery to live independently. This makes their classification into any of the existing kingdoms problematic.

Classification based on cellularity

Microorganisms may also be classified as unicellular, multicellular, and acellular types. Unicellular microbes exist as single cells. Most of the bacterial species fall under this category. Yeasts and certain parasites are also unicellular microorganisms. Multi-cellular microbes have a body that is composed of many cells. Many fungi and parasites belong to this category. Viruses are acellular microorganisms that do not have a cellular structure.

Classification based on pathogenic potential

Some microorganisms can cause infections and diseases in humans. These kinds of microbes are commonly referred to as pathogens. On the other hand, microorganisms that do not cause any disease are known as non-pathogenic microbes. However, some non-pathogenic microbes may be opportunistic pathogens, which may cause disease under certain conditions (such as a weakened immune system).

Main Categories of Microorganisms

Bacteria, fungi, parasites, and viruses are the major microbes of human interest due to their impact on human health. Many of these microbes are responsible for serious human diseases, including diseases with fatal outcomes.


Bacteria is one of the main kingdoms of life. This class of life contains unicellular organisms that are only a few microns in size. Bacteria are prokaryotic cells. They lack a well-defined nucleus. However, bacterial cells contain all the other essentials of life, such as DNA, cytoplasmic structures for carrying out metabolism and reproduction, inner cell membrane, outer cell wall, and capsule. Some bacteria contain whip-like tails known as flagella, which allows them to move. Unlike us, bacteria reproduce asexually by splitting one cell into two daughter cells.

There are many species of bacteria, each with its own unique set of characteristics. For example, some of the bacteria are pathogenic, whereas others are non-pathogenic. Depending on the results of gram staining procedure, bacteria can be classified as either gram-positive bacteria (such as Streptococcus) or gram-negative bacteria (such as Klebsiella). The gram staining property depends on the composition of the cell wall. Bacteria can also be classified as aerobic or anaerobic.

Aerobic bacteria (such as Mycobacterium tuberculosis) require oxygen to survive and grow, whereas anaerobic bacteria (such as Clostridium difficile) can grow in the absence of oxygen. Some bacteria (such as Pseudomonas aeruginosa) can survive in both the presence and absence of oxygen. Such bacteria are referred to as facultative anaerobes. Bacterial species may also be differentiated based on the shape of the bacteria. For example, spherical bacteria are known as cocci (such as Staphylococcus aureus), elongated bacteria are known as bacilli (such as Lactobacillus acidophilus), and spiral bacteria (such as Treponema pallidum) are known as spirochetes.

Bacteria can live in a variety of different conditions. Some of these conditions are extremely harsh, such as boiling water, freezing temperatures, starvation conditions, and desiccation. To survive in such harsh environments, some bacteria have the ability to form endospores, which are tough external capsules that protect the dormant bacteria lying inside. It is possible for bacterial spores to survive many years (even millions of years).

Bacterial infections can be treated by antibacterial drugs such as antibiotics. Some of the common pathogenic bacteria are Staphylococcus aureus, Streptococcus pyogenes, Salmonella, Neisseria gonorrhoeae, Helicobacter pylori, and Mycoplasma. Examples of non-pathogenic bacteria include Lactobacillus acidophilus and Staphylococcus epidermidis. These bacteria are part of the normal flora of the gut and the skin.

Read more on staph skin infections.

Some non-infectious bacteria that reside in the gut (such as Escherichia coli and Enterobacter) can cause infections when an opportunity arises. Such bacteria are opportunistic pathogens. Bacteria can cause both localized and systemic infections. Laboratory tests help in the identification of the causative bacteria. Vaccinations can help in preventing the spread of many bacterial infections (such as tuberculosis, typhoid, cholera, diphtheria, tetanus, and pertussis).


Viruses are acellular microorganisms that need a live host to reproduce. These are the simplest forms of life that contain a genetic material (DNA or RNA) enclosed within a proteinaceous capsule. Viruses typically cause systemic infections in the human body. Examples of pathogenic viruses include Human Immunodeficiency Virus (commonly abbreviated as HIV), Rhinovirus, Herpes simplex virus, Influenza virus, and Ebola virus.

Most of the viruses cannot survive outside the host for long. Transmission of viral infections occurs through contaminated air, food, water, and physical contact with infected things or persons. Viruses are too small to be seen even with a traditional light microscope (which can detect other microorganisms). Therefore, viruses are identified indirectly by detecting the presence of antibodies that the body may have generated against the virus.

Read more on STD symptoms in men.

Some viral infections can be treated through anti-viral medications. Other viral infections are self-limiting and resolve on their own. For many viral infections, the treatment is symptomatic. It is also possible to prevent some viral infections (such as rabies) through vaccination.


Fungi are eukaryotic microbes that can exist in both unicellular and multicellular forms. Examples of unicellular fungi include yeasts (budding and fission types). Mushrooms are examples of multicellular fungi. Examples of pathogenic fungi include candida, dermatophytes, and Aspergillus.


Human parasites include various protozoa and worms. Parasites that infect the skin are referred to as ectoparasites, whereas parasites that infect the internal tissues are known as endoparasites. Many parasitic infections in humans tend to occur in the human gut. The eggs and cysts of the parasites can be detected in the feces of an infected individual.

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