What Are Microorganisms?

Microorganisms (Latin micro = small) are living beings so small (< 40 µm or 0.04 mm) that they are not visible by the naked eye. Microorganisms related to human health include certain bacteria, viruses, fungi, and parasites.

Synonyms: microbes, germs, bugs.

Types of Microorganisms

Microorganisms can be, according to their characteristics, divided into several groups:

  • bacteria, viruses, certain fungi, and parasites
  • pathogenic (capable of causing disease), non-pathogenic, and opportunistic (causing disease when they have an opportunity, like in people with low immune system)
  • acellular (without cell, like viruses), unicellular (bacteria, yeasts, and certain parasites), or multi-cellular (molds)

1. Bacteria

Bacteria are unicellular organisms, about a few microns in size (1 micron (µm) = 1/1,000 of a millimeter), consisting of DNA, cytoplasm, structures needed for metabolism and reproduction, cell membrane, cell wall, and capsule (Picture 1). Certain bacteria use flagella, tail-like appendages, to propel themselves.800px-Prokaryote_cell_diagram_svg

Bacteria can be divided into several groups::

  • Spheres or cocci (like Staphylococcus aureus), rods or bacilli (like Lactobacillus acidophilus), spirals or spirochetes (like Treponema pallidum); bacterial shape can help in their recognition under the microscope
  • Aerobic bacteria, like Mycobacterium tuberculosis, need oxygen to thrive, while anaerobic, like Clostridium difficile, do not. Facultative anaerobic bacteria, like Pseudomonas aureginosa, can live in aerobic and anaerobic environments.
  • Gram-positive (G+) bacteria, like Streptococcus, and Gram-negative (G-) bacteria, like Klebsiella
  • Pathogenic and non-pathogenic bacteria

Certain bacteria can form endospores, a kind of encapsulated bunkers within a bacteria that enable vital parts of bacteria to survive in harsh conditions, like freezing or boiling water, desiccation, lack of nutrients, etc. Some bacteria can survive weeks and some millions of years in this form.

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In the human body, bacteria usually cause localized infections, like pneumonia or skin infections. Read more about symptoms, diagnosis, and treatment of bacterial infections. Bacterial infections can be diagnosed by growing a bacterial culture from a sample of infected body fluid (e.g. urine, blood), stool, discharge (e.g. sputum), or tissue (e.g. a mucosal layer of the stomach). Most of bacterial infections can be successfully treated by anti-bacterial drugs – antibiotics

Examples of bacteria pathogenic for a human are:

  • Staphylococcus aureus, causing skin infections, pneumonia, infection of the heart valves, etc.
  • Streptococcus pyogenes, causing “strep throat”, cellulitis, etc.
  • Neisseria gonorrhea, causing gonorrhea
  • Salmonella, causing diarrhea in food poisoning
  • Helicobacter pylori, causing chronic gastritis
  • Mycoplasma, causing atypical pneumonia
  • List of bacteria, pathogenic for human

Examples of non-pathogenic bacteria:

  • Staphylococcus epidermidis, a part of normal skin flora
  • Lactobacillus acidophilus, a part of normal intestinal flora

Examples of opportunistic bacteria:

  • Certain intestinal bacteria, like Escherichia coli and Enterobacter, live in the human intestine without causing any symptoms, but in a person with a lowered immune system, they may overgrow and cause a bowel infection.

2. Viruses

Viruses are simple microorganisms, containing only DNA or RNA molecules and capsules. They can not survive outside the host for long periods, so they are mainly transmitted by blood-to-blood or stool-to-mouth route. In the human body, they have to invade the cells to multiply (Picture 4).

Viruses usually cause systemic infections, affecting the whole body. Examples of viruses, pathogenic for a human:

  • Rhinovirus, causing common cold
  • Influenzavirus, causing flu, bird flu, swine flu
  • Herpes simplex virus causing herpes labialis (cold sore) or herpes genitalis
  • HIV, causing AIDS
  • Ebolavirus, causing hemorrhagic fever
  • List of viruses pathogenic for a human
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Viruses can be diagnosed by finding specific antibodies in the sample of blood (serologic tests). Vaccination against several virus infections is possible; only few viral infections can be treated by anti-viral medications, though. Read more about viral infections.

3. Fungi

Fungi are widely present in the environment and also on the human skin, gut, and vagina. Read more about fungi.

4. Parasites

Read about human intestinal parasites.

Beneficial Microorganisms

Microorganisms, like certain bacteria and yeasts, living on the human skin or in the nose, mouth, throat, small and large intestine, and vagina, are part of the normal human flora; they prevent the overgrowth of harmful microorganisms. Some of these microbes, when overgrown, may become pathogenic, though.

Harmful or Pathogenic Microorganisms

Pathogenic means capable of causing disease. An actual harmful effect of a microbe to the body depends on:

  • microbial virulence – a relative ability of a microbe to cause disease; for example, a certain, highly virulent subtype of influenza virus may cause the bird flu, which is deadly in a high percent, while “usual” influenza virus is not.
  • invasion through the body’s barriers; staph bacteria might not cause any harm to a person with intact skin but can cause a severe infection of a skin wound.
  • amount of microbes; eating a few bites of food contaminated with staph bacteria may go unnoticed, while eating the whole portion of the same food may cause severe food poisoning.
  • body’s defense (immune) system; patients with a weak immune system, like those receiving corticosteroids, often get oral thrush (candida infection of the mouth), while otherwise healthy people do not.