Unit 1.1 Introduction to Microbiology
Outline
Learning Objectives
After reading the following, you should be able to:
- Be able to define the terms microbiology, microorganism and pathogen.
- Be able to give examples of the non-pathogenic roles that microorganism play.
- Be able to name the different types of microorganisms discussed in this course.
From boiling thermal hot springs to deep beneath the Antarctic ice, microorganisms can be found almost everywhere on earth in great quantities. Microorganisms or microbes, as they are also called) are small organisms. Most are so small that they cannot be seen without a microscope and the study of these organisms is known as Microbiology.
Most microorganisms are harmless to humans and, in fact, many are helpful. They play fundamental roles in ecosystems everywhere on earth, forming the backbone of many food webs. People use them to make biofuels, medicines, and even foods. Without microbes, there would be no bread, cheese, or beer. Our bodies are filled with microbes, and our skin alone is home to trillions of them. Some of them we can’t live without; others are pathogens that cause diseases that can make us sick or even kill us.
Although much more is known today about microbial life than ever before, the vast majority of this invisible world remains unexplored. Microbiologists continue to identify new ways that microbes benefit and threaten humans. In this course we will be discussing several different categories of microorganism. These can be separated in to two broad categories: Prokaryotic microorganisms and Eukaryotic microorganisms, which will be discussed in greater detail in Unit 2.
Prokaryotic Microorganisms
Bacteria are single celled microbes that are found in nearly every habitat on earth, including within and on humans. Bacteria are prokaryotic because their genetic material (DNA) is not housed within a true nucleus. Most bacteria are harmless or helpful, but some are pathogens, causing disease in humans and other animals.
Most bacteria have cell walls that contain peptidoglycan. They have a wide range of metabolisms and can grow in a variety of environments, using different combinations of nutrients. Some bacteria are photosynthetic, producing oxygen. Other types of bacteria are non-photosynthetic, obtaining their energy from organic or inorganic compounds in their environment. Environmentally, they play a major role in decomposition, nitrogen fixation and are the foundation of many food webs. In our bodies, non-pathogenic bacteria are involved in a number of processes, such as the production of vitamin K and making feces brown.
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, which are too hostile for other types of organism to survive in (Figure 1.2). Some archaea live in the human body, but none have been shown to be human pathogens.
Eukaryotic Microorganisms
Algae (singular: alga) are plant-like organisms that can be either unicellular or multicellular (Figure 1.3). Their cells are usually 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. They produce approximately 50% of the atmospheric oxygen and serve as a major environmental carbon sink. Because other organisms can use their waste products for energy, algae are important parts of many ecosystems. 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) 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 1.4).
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 1000 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 and some even cause diseases (Figure 1.5).
Other fungi of interest to microbiologists are multicellular organisms called molds. Molds are made up of long filaments that form visible colonies (Figure 1.6). Molds are found in many different environments, from soil to rotting food to dank bathroom corners. Molds play a critical role in the decomposition of dead plants and animals. Some molds can cause allergies, and others produce disease-causing metabolites called mycotoxins. Molds 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.
Multicellular Animal Parasites
Multicellular parasitic worms called helminths are not technically microorganisms, as most are large enough to see without a microscope. However, these worms fall within the field of microbiology because diseases caused by helminths involve microscopic eggs and larvae. As with all animals, they lack cells walls. While there are many other worms that do not cause disease, also those that fall within this category are pathogens. Parasitic helminthes are some of the most widely distributed infectious organisms, affecting ~2 billion people worldwide.
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. Viruses can infect all types of host cells, from human cells to the cells of other microorganisms. In humans, viruses are responsible for numerous diseases, from the common cold to deadly Ebola (Figure 1.8).