Microbial Ecology and Environmental Microbiology

The term "microbial ecology"is now used in a general way to describe the presence and contributions of microorganisms, through their activities, to the places where they are found. Students of microbiology should be aware that much of the information on microbial presence and contributions to soils, waters, and associations with plants, now described by this term, would have been considered as "environmental microbiology" in the past. Thomas D. Brock, the discoverer of Thermus aquaticus, which is known the world over as the source of Taq polymerase for the polymerase chain reaction (PCR), has given a definition of microbial ecology that may be useful: "Microbial ecology is the study of the behavior and activities of microorganisms in their natural environments." The important operator in this sentence is their environment instead of the environment. To emphasize this point, Brock has noted that "microbes are small; their environments also are small." In these small environments or "microenvironments," other kinds of microorganisms (and macroorganisms) often also are present, a critical point that was emphasized by Sergei Winogradsky in 1947.

Environmental microbiology, in comparison, relates primarily to all-over microbial processes that occur in a soil, water, or food, as examples. It is not concerned with the particular "microenvironment" where the microorganisms actually are functioning, but with the broader-scale effects of microbial presence and activities. One can study these microbially mediated processes and their possible global impacts at the scale of "environmental microbiology" without knowing about the specific microenvironment (and the organisms functioning there) where these processes actually take place. However, it is critical to be aware that microbes function in their localized environments and affect ecosystems at greater scales, including causing global-level effects. In the last decades the term "microbial ecology" largely has lost its original meaning, and recently the statement has been made that "microbial ecology has become a 'catch-all' term." As you read various textbooks and scientific papers, possible differences between "microbial ecology" and "environmental microbiology" should be kept in mind.

These physical associations can be intermittent and cyclic or permanent. Examples of intermittent and cyclic associations of microorganisms with plants and marine animals are shown in table 28.1. Important human diseases, including listeriosis, malaria, leptospirosis, legionellosis, and vaginosis also involve such intermittent and cyclic symbioses. These diseases will be discussed in chapters 39 and 40. Interesting permanent relationships also occur between bacteria and animals, as shown in table 28.2. Hosts include squid, leeches, aphids, nematodes, and mollusks. In each of these cases, an important characteristic of the host animal is conferred by the permanent bacterial symbiont.

Although it is possible to observe microorganisms in these varied physical associations with other organisms, the fact that there is some type of physical contact provides no information on the types of interactions that might be occurring. These interactions can be positive (mutualism, protocooperation, and

Table 28.1 Intermittent and Cyclical Symbioses of Microorganisms with Plants and Marine Animals

Symbiosis

Host

Cyclical Symbiont

Plant-bacterial

Marine animals

Gunnera

(tropical angiosperm) Azolla (rice paddy fern) Phaseolus (bean) Ardisia (angiosperm) Coral coelenterates

Luminous fish Squid

Nostoc (cyanobacterium)

Anabaena (cyanobacterium) Rhizobium (N2 fixer) Protobacterium

Symbiodinium (dinomastigote)

Vibrio, Photobacterium Photobacterium fischerei

Adapted from L. Margulis and M. J. Chapman. 1998. Endosymbioses: Cyclical and permanent in evolution. Trends in Microbiology 6(9):342-46, tables 1, 2, and 3.

Table 28.2 Examples of Permanent Bacterial-Animal Symbioses and the Characteristics Contributed by the Bacterium to the Symbiosis

Animal Host

Symbiont

Symbiont Contribution

Sepiolid squid (Euprymna scolopes) Medicinal leech (Hirudo medicinalis) Aphid (Schizaphis graminum) Nematode worm (Heterorhabditis spp.) Shipworm mollusk (Lyrodus pedicellatus)

Luminous bacterium

Enteric bacterium (Aeromonas veronii)

Bacterium (Buchnera aphidicola)

Luminous bacterium (Photorhabdus luminescens)

Gill cell bacterium

Luminescence (Vibrio fisheri)

Blood digestion

Amino acid synthesis

Predation and antibiotic synthesis

Cellulose digestion and nitrogen fixation

Source: From E. G. Ruby, 1999. Ecology of a benign "infection": Colonization of the squid luminous organ by Vibrio fischeri. In Microbial ecology and infectious disease, E. Rosenberg, editor, American Society for Microbiology, Washington, D.C., 217-31, table 1.

Prescott-Harley-Klein: I VIII. Ecology and I 28. Microorganism I I © The McGraw-Hill

Microbiology, Fifth Edition Symbiosis Interactions and Microbial Companies, 2002

Ecology

598 Chapter 28 Microorganism Interactions and Microbial Ecology

Figure 28.1 Microbial Interactions. Basic characteristics of positive (+) and negative (—) interactions that can occur between different organisms.

Interaction quality

Interaction type

Interaction example

Positive

Mutualism

Obligatory

Obligatory

Negative

Predation

Prey

Predator f

Predator

Prey

Parasitism

Parasite

f

\

°

Host

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