Microorganisms differ from each other not only in size, but also in structure, habitat, metabolism, and many other characteristics. While they are typically thought of as being unicellular, there are also many multicellular organisms that are too small to be seen without a microscope. Some microorganisms, such as viruses, are acellular (not composed of cells).

Size of Microorganisms

Most microorganisms are unicellular and small enough that they require artificial magnification to be seen. However, there are some unicellular microbes that are visible to the naked eye, and some multicellular organisms that are microscopic. An object must measure about 100 micrometres (µm) to be visible without a microscope, but most microorganisms are many times smaller than that. For some perspective, consider that a typical animal cell measures roughly 10 µm across but is still microscopic. Bacterial cells are typically about 1 µm, and viruses can be 10 times smaller than bacteria (Figure 7.5) with units of length used in microbiology shown in Table 7.1.

Table 7.1. Units of length commonly used in microbiology

Microorganisms are found in each of the three domains of life: Archaea, Bacteria, and Eukarya. Microorganisms within the domains Bacteria and Archaea are all prokaryotes (their cells lack a nucleus), whereas microorganisms in the domain Eukarya are eukaryotes (their cells have a nucleus). Some microorganisms, such as viruses, do not fall within any of the three domains of life (Figure 7.6).

Bacteria

Bacteria (singular: bacterium) are found in nearly every habitat on earth, including within and on humans. Most bacteria are harmless or helpful, but some are pathogens, causing disease in humans and other animals. Bacteria are prokaryotic because their genetic material (DNA) is not confined within a true nucleus. They have a wide range of metabolic capabilities and can grow in a variety of environments, using different combinations of nutrients. Some bacteria are photosynthetic, such as oxygenic cyanobacteria and anoxygenic green sulphur and green non-sulphur bacteria; these bacteria use energy derived from sunlight and fix carbon dioxide for growth. Other types of bacteria are non-photosynthetic, obtaining their energy from organic or inorganic compounds in their environment.

Bacteria are often described in terms of their general shape (Figure 7.7). Common shapes include spherical (coccus), rod-shaped (bacillus), or curved (spirillum, spirochete or vibrio).

Archaea

Archaea are also unicellular prokaryotic organisms. Archaea and bacteria have different evolutionary histories, as well as significant differences in genetics, metabolic pathways and the composition of their cell walls and membranes. Unlike most bacteria, archaeal cell walls do not contain peptidoglycan, but their cell walls are often composed of a similar substance called pseudopeptidoglycan. Like bacteria, archaea are found in nearly every habitat on earth, even extreme environments that are very cold, very hot, very basic, or very acidic. Some archaea live in the human body, but none have been shown to be human pathogens as yet.

Protists

Protists are an informal grouping of eukaryotes that are not plants, animals, or fungi. Some algae are protists and others are bacteria; all protozoa are examples of protists.

Algae (singular: alga) are mostly made up of protists that can be either unicellular or multicellular and vary widely in size, appearance and habitat. Algal protists are surrounded by cell walls made of cellulose, a type of carbohydrate. Algae are photosynthetic organisms that extract energy from the sun and release oxygen and carbohydrates into their environment. Many consumer products contain ingredients derived from algae, such as carrageenan or alginic acid, which are found in some brands of ice cream, salad dressing, beverages, lipstick, and toothpaste. A derivative of algae also plays a prominent role in the microbiology laboratory. Agar, a gel derived from algae, can be mixed with various nutrients and used to grow microorganisms in a Petri dish. Algae are also being developed as a possible source for biofuels.

Protozoa (singular: protozoan) are protists that make up the backbone of many food webs by providing nutrients for other organisms. Protozoa are very diverse. Some protozoa move with help from hair-like structures called cilia or whip-like structures called flagella. Others extend part of their cell membrane and cytoplasm to propel themselves forward. These cytoplasmic extensions are called pseudopods (“false feet”). Some protozoa are photosynthetic; others feed on organic material. Some are free-living, whereas others are parasitic, only able to survive by extracting nutrients from a host organism. Most protozoa are harmless, but some are pathogens that can cause disease in animals or humans (Figure 7.8).

Fungi

Fungi (singular: fungus) are also eukaryotes. Some multicellular fungi, such as mushrooms, resemble plants, but they are actually quite different. Fungi are not photosynthetic and their cell walls are usually made out of chitin rather than cellulose.

Unicellular fungi — yeasts — are included within the study of microbiology. There are more than 1,000 known species. Yeasts are found in many different environments, from the deep sea to the human navel. Some yeasts have beneficial uses, such as causing bread to rise and beverages to ferment; but yeasts can also cause food to spoil. Some even cause diseases, such as vaginal yeast infections and oral thrush (Figure 7.9).

Other fungi of interest to microbiologists are multicellular organisms called moulds (filamentous fungi). Moulds are made up of long filaments that form visible colonies (Figure 7.10). Moulds are found in many different environments, from soil to rotting food to dank bathroom corners. Moulds play a critical role in the decomposition of dead plants and animals. Some moulds can cause allergies and others produce disease-causing metabolites called mycotoxins. Moulds have been used to make pharmaceuticals, including penicillin, which is one of the most commonly prescribed antibiotics and cyclosporine, used to prevent organ rejection following a transplant.

Helminths

Multicellular parasitic worms called helminths are not technically microorganisms, as most are large enough to see without a microscope (eg the beef tapeworm, Figure 7.11). However, these worms fall within the field of microbiology because diseases caused by helminths involve microscopic eggs and larvae. One example of a helminth is the guinea worm, or Dracunculus medinensis, which causes dizziness, vomiting, diarrhoea and painful ulcers on the legs and feet when the worm works its way out of the skin (Figure 7.11). Infection typically occurs after a person drinks water containing water fleas infected by guinea-worm larvae. In the mid-1980s, there were an estimated 3.5 million cases of guinea-worm disease, but the disease has been largely eradicated.

Viruses

Viruses are acellular microorganisms, which means they are not composed of cells. Essentially, a virus consists of proteins and genetic material—either DNA or RNA, but never both—that are inert outside of a host organism. However, by incorporating themselves into a host cell, viruses are able to co-opt the host’s cellular mechanisms to multiply and infect other hosts.