The majority of heterotrophic microorganisms are chemoorganotrophs that derive both carbon and energy from organic compounds. Processing these organic molecules by respiration or fermentation releases energy in the form of ATP. We will explore these topics in greater depth in chapter 8. An example of chemoheterotrophy is aerobic respiration, the principal energy-yielding pathway in animals, most protozoa and fungi, and aerobic bacteria. It can be simply represented by the equation:
Note that this reaction is complementary to photosynthesis. Here, glucose and oxygen are reactants and carbon dioxide is given off. Indeed, the earth’s balance of both energy and metabolic gases greatly depends on this relationship. Chemoheterotrophic microbes are all similar in requiring an organic carbon source, but they differ in how they obtain it. This category includes a wide variety of organisms that feed on other organisms: animals, protozoa, fungi, and many types of bacteria. Examples of varied nutritional patterns include grazers, scavengers, and predators, but the dominant chemoheterotrophic microorganisms fit the description of saprobes or symbionts. Saprobes are free-living microbes that feed primarily on organic detritis released by dead organisms, and symbionts derive their organic nutrients from the bodies of living organisms. These categories are not absolute, in that some saprobes can adapt to living organisms, and some symbionts can obtain nutrients from a nonliving source. Because many of these nutritional patterns are also ecological in nature, they will be given fuller coverage in section 7.5. In this section, we introduce saprobes and parasites as they relate to nutrition.
Saprobic Microorganisms
The primary niche of saprobes is as decomposers of plant litter, animal matter, and dead microbes. If not for the work of decomposers, the earth would gradually fill up with organic material, and the nutrients it contains would not be recycled. Most saprobes, notably bacteria and fungi, have a rigid cell wall and cannot engulf large particles of food. To compensate, they release enzymes to the extracellular environment and digest the food particles into smaller molecules that can be transported into the cell (figure 1).
Fig1. Extracellular digestion in a saprobe with a cell wall (bacterium or fungus). (a) A walled cell is inflexible and cannot engulf large pieces of organic debris. (b) In response to a usable food substrate, the cell synthesizes enzymes that are transported across the wall into the extracellular environment. (c) The enzymes hydrolyze the bonds in the debris molecules. (d) Digestion produces molecules small enough to be transported into the cytoplasm.
Many saprobes exist strictly on dead organic matter in environmental reservoirs such as soil and water, and are unable to adapt to the body of a live host. This group includes free-living protozoa, most fungi, and a variety of bacteria. Apparently there are fewer of these microbes than was once thought, and many supposedly nonpathogenic saprobes can adapt to and invade a susceptible host. When a saprobe does infect a host, it is considered a facultative parasite. Because such an infection usually occurs when the host is compromised, the microbe is said to be an opportunistic pathogen. A notable example is Pseudomonas aeruginosa—its natural habitat is soil and water, yet it is a frequent cause of infections in hospital patients and compromised individuals, especially burn patients and those suffering from cystic fibrosis.
Parasitic Microorganisms
Most human infections are caused by heterotrophic microorganisms, but those having the greatest adverse impact on human health are parasites. By definition, a parasite is a microbe that invades the body of a host, uses it as a habitat and source of nutrients, and, in the process, harms the host to some degree. Such parasitic microbes that grow inside sterile tissues and cause damage, and even death, are also termed pathogens. Parasites range from viruses to helminth worms, and they can live on the body (ectoparasites), in the organs and tissues (endoparasites), or even within cells (intracellular parasites, the most extreme type). The more successful parasites generally have no fatal effects and may eventually evolve to a less harmful relationship with their host.
Obligate parasites (for example, the leprosy bacillus and the syphilis spirochete) are so dependent on their hosts that they are un able to grow outside of the host. Less strict parasites, such as the gonococcus and pneumococcus, can be cultured artificially if provided with the correct nutrients and environmental conditions in the laboratory.
