Beneficial Bacteria

Magnificent Microbes

The term bacteria is a plural form of the Latin bacterium, meaning “staff” or “rod.” Bacteria are prokaryotes and among the most abundant organisms on earth. The vast majority play a positive role in nature. Bacteria digest sewage into simple chemicals; extract nitrogen from the air and make it available to plants for protein production; break down the remains of everything that dies, recycling the carbon and other elements; and produce foods for human consumption and products for industrial technology. Many biologists believe that life on earth as we know it would be impossible without bacteria. They are the Master’s magnificent microbes.

Bacteria have adapted to more different living conditions than any other group of organisms. They inhabit the air, soil, and water, and exist in enormous numbers on the surfaces of virtually all plants and animals. They can be isolated from Arctic ice, thermal hot springs, the fringes of space, and the tissues of animals. Some can withstand the searing acid in volcanic ash, the crushing pressures of ocean trenches, and the powerful activity of digestive enzymes. Other bacteria survive in oxygen-free environments, boiling water, and extremely dry locations. In short, they are ubiquitous (everywhere). The Creator must love them because they are the closest things to omnipresence on earth. Bacteria have so completely invaded the earth that the mass of bacterial cells is estimated to outweigh the mass of all plants and animals combined. They are Providence’s prokaryotic wonders.

This chapter explains the importance of bacteria to our everyday lives. It also includes a discussion of their designed structures, size, classification, environmental functions, and their role in disease. Though most bacteria are beneficial, some are harmful. Certain species multiply within the human body, where they digest tissues or produce toxins resulting in disease. Other bacteria infect plant crops and animal herds. Disease-causing bacteria (i.e., pathogenic bacteria) are a global threat to all life forms, and in the 21st century, they have already been used as agents of bioterror. Bacteria have threatened human existence over the millennia through tuberculosis, bubonic plague, typhus, pneumonia, streptococcal infections, diphtheria, E. coli, and anthrax.

Prokaryota

The prokaryotic cells of bacteria are generally much smaller and simpler in structure than eukaryotic cells (Table 2.2). Their definitive feature is the absence of a nuclear envelope separating the genetic material from the cytoplasm. Today, bacteria still outnumber other organisms, thriving in almost every conceivable habitat, but because of their tiny size, they were the last major group of organisms discovered. Some of the larger bacteria were first observed and described by Anton van Leeuwenhoek in 1676. In the 1860s and 1870s, Louis Pasteur, Joseph Lister, and Robert Koch discovered the role of bacteria in causing food spoilage and many diseases. It is also interesting to note that Leeuwenhoek, Pasteur, and Lister were all creation scientists who sought God’s thoughts (Ps. 139:17) in biology and in medicine. They sought not only to classify their newly found bacteria, but also to find useful applications to benefit mankind. Bacteria, like other living things, are classified by their morphology, molecular composition, staining characteristics, metabolic way of life, and growth characteristics. The table below (Table 2.2) gives the basic characteristics of prokaryotes.

In the second edition of Bergey’s Manual, prokaryotes are grouped into two domains, Archaea and Bacteria. Each domain is further divided into “clades,” each clade into phyla, each phylum into classes, and so on. Bacteria are divided into five clades. Clades refer to distinct lines of phylogeny from a cladogram. This concept is based upon assumptions of common descent and neo-Darwinian evolution. Many creationists would refer to these unrelated, large, inclusive categories as polybaramins. Perhaps the largest collections of typical bacteria are the Eubacteria and the Cyanobacteria. Both possess “true” prokaryotic cells. Cyanobacteria have cells like bacteria, but form colonies and filaments like algae, with all being capable of photosynthesis. Hence, they were traditionally referred to as blue-green algae. Clearly they are a distinct kind, defying neat, clear classification according to modern taxonomic methods. Although some creationists still place them in the alga category, we are placing them in this chapter because most biologists today classify them with bacteria. We will focus our discussion of Cyanobacteria on their role in the nitrogen cycle and their overall importance to ecology. Archaea have unique attributes and are also difficult to classify. Archaea cells have many attributes similar to bacteria, but have some characteristics similar to Eukarya.

Domain Bacteria

Most of us have been conditioned to think of bacteria as invisible, potentially harmful little creatures. Actually, relatively few species of bacteria cause disease in humans, animals, plants, or any other organisms. Indeed, once you have completed a biology course, you will realize that without bacteria, the majority of life as we know it would be impossible. Evolutionists assert that all eukaryotic organisms probably evolved from Bacteria or Archaea. They believe that Archaea were some of the earliest forms of life on earth.

Metabolism and Ways of Life

We have already mentioned that bacteria are the most numerous organisms on earth. A handful of soil may contain billions of them. They occur in habitats as remote as the sea floor, icebergs, and hot springs, or as close as your mouth. They are before us even as we speak. This far-flung distribution is possible because many bacteria have metabolic abilities not found in any eukaryote. Some carry on fermentation, others respiration. Aerobes require O₂ for respiration; obligate anaerobes are killed by O₂; facultative anaerobes can live in the presence or absence of O₂. Some anaerobic bacteria use nitrate (NO₃^-), sulfate (SO₄^-), or iron (Fe³⁺) instead of O₂ in cellular respiration. Most bacteria that undergo aerobic respiration yield more energy than those that undergo anaerobic respiration or fermentation.

Because of their rigid cell walls, bacteria cannot take in large food particles. Rather, they absorb small molecules that filter through their walls. Many bacteria can make any organic molecule they need by starting with only one kind of organic molecule, such as glucose or fatty acids. Others require a more complex diet. The “picky eaters” are referred to as fastidious. When conditions are unfavorable for growth, some bacteria form thick-walled, resting spores resistant to heat and drying. Most bacterial spores are not a means of reproduction, because no increase in cell number occurs. Rather, spore formation permits cells to survive adverse conditions and disperse to new locations, where a new cell may develop from the spore.

Bacteria undoubtedly owe their biological success to their varied metabolic abilities, coupled with small size, rapid reproductive rate, and ability to form resistant spores. These features permit bacteria to live in many habitats that are here today and gone tomorrow. A raindrop on a leaf evaporates in a few hours, but by then a bacterium has divided several times, and its descendants have formed spores that can blow away to new habitats.

Bacterial Anatomy

Individual bacterial bodies have one of three basic shapes, but the shapes have many variations and arrangements (left). The cells of some bacteria are spherical; some are cylindrical or rod-shaped; and others have the form of a somewhat rigid, curved rod or of a short spiral. The spherical forms are called cocci (singular coccus); the cylindrical forms bacilli (singular bacillus); and the spiral-shaped forms spirilla (singular spirillum). Each of the many kinds of true bacteria is placed in one of these three morphological groups.

Bacilli

In Latin, bacillus means “a little stick or rod,” and the bacilli have the shape of a little rod or cylinder. Individual kinds of bacilli show almost infinite variations on this basic shape. Some are so short and thick that they are nearly indistinguishable from cocci; others are long and slender. Their ends may be square-cut, rounded, or tapered to a blunt or fine point. Most of the bacilli are straight, rigid rods, but some are slightly curved and less rigid. Many species are motile, moving through liquids by means of delicate, whip-like appendages called flagella. Others lack flagella and are non-motile. Like cocci, bacilli are widely distributed, and numerous species are known. One common example of a bacillus is Escherichia coli, the common intestinal bacterium.

Cocci

The word coccus comes from a Greek word meaning “berry,” and all cocci have a spherical form like a tiny berry. Many are not perfectly round, but are flattened on one side or more or less elongated. There are many species of cocci, and they are widely distributed. One common example of a coccus is Streptococcus pyogenes, the cause of strep throat.

Spirilla

In this group are bacteria having a helical shape like a corkscrew. Some spirilla are short, comma-shaped organisms with less than one complete spiral turn; these are classified in the genus Vibrio (e.g., Vibrio cholera). Other spirilla are longer, more delicately coiled threads; these are placed in the genus Spirillum. The body of a spirillum is rigid; its motility is due to flagella. Spirilla must be distinguished from spirochetes, which are delicate, flexible, filamentous forms, having the general shape of a rather long, more or less tightly coiled wire spring. They are motile, but do not have special locomotive appendages (i.e., flagella). One common example of a spirochete is Treponema palladium, the cause of syphilis.

Size of Bacteria

Illustration of a typical bacterial cell, sectioned to reveal its internal structures.

As we have already learned, bacteria are so minute that they cannot be measured accurately by any familiar scale. A special unit of measurement, the micron (μ), is 1/1,000 of a millimeter, or about 1/25,000 of an inch. Most bacteria are smaller than eukaryotic cells but larger than the nucleus (Table 2.3). Two notable exceptions are Epulopiscium sp. and Thiomargarita namibiensis. These are quite exceptional and atypical. The first giant bacterium is over 0.6 mm long—600 μ compared to 2 μ for E. coli dwarfing Paramecium. Found in the gut of surgeonfish near Australian reefs, the bacterium is the second largest. After the discovery of Epulopiscium in 1991, another “Goliath” bacterium was found in 1999. Thiomargarita namibiensis, meaning “sulfur pear of Namibia,” was found in sediments off the coast of Africa. This bacterium is 750 μm, or a bit larger than the size of a period. Normally the fact that nutrients must enter the cytoplasm by simple diffusion limits the size of prokaryotic cells. In addition, they do not reproduce through binary fission. These observations present problems to evolutionary biologists; they cannot explain how such prokaryotic cells could evolve, and the giant prokaryotes cast doubt on current phylogenetic trees illustrating divergence of prokaryotes from eukaryotes. Table 2.3 gives examples of actual sizes of some of the best-known prokaryotes.