Microbial Ecology

The study of microorganisms too often treats each species as a pure strain. However, in the environment this is rarely the case. Many of the important behaviors depend on the interactions among microbial populations and between them and other organisms. Here we describe some of these interactions.

Cooperation is the positive interaction among organisms within a single population. Bacteria cooperate in numerous ways. Bacteria form flocs, biofilms, and other kinds of aggregates for the purpose of degrading solid substrates, whether lignin, cellulose, or rocks. The aggregates can benefit more efficiently from the investment of extracelllular products than a single individual can. In laboratory-batch microorganism growth experiments, a lag phase of growth is often missing when a large initial concentration is used. It may be that at a high initial density, the cells share growth factors with each other. A certain minimum number of microorganisms, called the infective dose, is required to transmit a disease to a host. The bacteria in this case cooperate to overwhelm the host's defenses.

Individuals also compete within a population. They certainly use the same resources. In the case of photoautotrophic microbes, there is competition for light.

Microorganisms exhibit most of the types of two-population interactions shown in Table 14.5. Neutralism (0/0) is uncommon among microorganisms if there is any overlap in their niches. Commensalism (+/0) is common. One microbial population may solubi-lize rock minerals, and other microbes benefit from the nutrients. One microbe may manufacture growth factors that others require. Biotin is a growth factor that is very important in marine habitats. Methanogenesis may result from commensal relationships. Desulfovi-brio produces acetate and hydrogen by fermentation, which is then used by Methanobac-terium to reduce CO2 to methane. Several of the microbially mediated pathways in biogeochemical cycling involve one population using the products of another (e.g., Nitro-somonas and Nitrobacter).

Cometabolism is, by some definitions, commensal. Mycobacterium vaccae can come-tabolize cyclohexane if it has propane available for its own benefit. The cyclohexane is converted to cyclohexanone, which can be used by other bacteria. The Mycobacterium does not assimilate the cyclohexanone itself.

One population may utilize a substance that is toxic to another population. An example is the oxidation of H2S. Some marine bacteria produce organics that chelate heavy metals, reducing their bioavailability.

Synergism (+/+) refers to a loose two-way benefit, in which both populations can exist separately, but benefit when together. Sometimes two populations together can produce metabolic products that neither can produce by themselves. For example, neither Sterpto-coccus faecalis nor Escherichia coli can convert arginine to putrescine, but together, they can. Cyclohexane can be degraded by Nocardia and Pseudomonas together, but not separately. The reason is that Nocardia needs biotin and other growth factors produced by Pseudomonas to do the job. Together, Arthrobacter and Streptomyces can degrade and grow on the organophosphate pesticide diazinon, but not separately. Pseudomonas stutzeri combines with Pseudomonas aeruginosa to mineralize parathion. One form of synergism is called syntrophy or cross-feeding. In this situation one species provides a nutritional requirement for another, and vice-versa. For example, Enterococcus faecalis requires folic acid for growth, and Lactobacillus arabinosus requires phenylalanine. Together, they can grow in a medium that contains neither, because E. faecalis produces pheynylalanine and L arabinosus produces folic acid.

Mutualism, or symbiosis (+/+), is an obligatory relationship between two organisms that enables them to occupy a habitat that they otherwise could not. Lichens, an association between a fungus and a photoautotroph, are the best known example of symbiosis. Some protozoans form mutualistic associations with algae. For example, Paramecium can contain many cells of the alga Chlorella within its protoplasm. Under conditions of stress, the protozoan may digest its algae. Paramecium aurelia can also harbor a bacterium of the Rickettsia group. These paramecia have an advantage over a different strain from the same species that does not contain the bacteria. Apparently, the Rickettsia manufacture a toxin used to inhibit neighbors.

Some methanogenic cultures, once thought to be pure, have been found to consist of mutualistic associations. Methanobacterium omelianskii is associated with another strain, called "S." Methane formation involves an electron transfer reaction that actually starts in the S organism and ends in the Methanobacterium. It is thought that some other metha-nogenic mechanism may involve interactions among three organisms. Methanogens also participate in a mutualistic relationship with eubacteria. The methanogens require simple substrates such as acetate, CO2, formate, and methanol. These are waste products for the eubacterial degradation of more complex organics. Thus eubacter benefits by having their wastes removed, and the archaean methanogens benefit by having substrates provided.

Bacteria can also enter into symbiotic relationships with viruses. Corynebacterium diphtheriae harbors a virus that enables it to produce a toxin and to infect host organisms, causing the disease diptheria. Without the virus, it cannot do either.

Competition (—/—) is also uncommon because of the principle of competitive exclusion. Unless competition is weak, two species cannot occupy the same niche. However, if each species has some distinguishing feature, they can compete in overlapping portions of their niches. The classic example of these behaviors is laboratory work by Gause (Figure 15.30). This shows two species of Paramecium that although they do not attack each other or secrete toxins, compete to the degree that one species is eliminated after 16 days. In a separate experiment, P. caudatum and P. bursaria were found to be able to coexist because they tended to occupy different areas of the laboratory flask.

Nutrient limitations can also produce coexistence. When the diatoms Asterionella formosa and Cyclotella meneghiniana were cultured together, the former would be limited by silica and the latter by phosphate.

Some organisms are relatively efficient at low substrate concentrations but are outcom-peted at high nutrient levels. This causes the microbial populations in raw sewage to shift

P. aurelia

P. aurelia


P. caudatum

P. caudatum


Figure 15.30 Competition between P. aurelia and P. caudatum. Solid diamonds show growth of each paramecium species in the absence of the other. Open circles show growth in mixed culture. The curves are for equation (14.28) fitted to the data by eye. For P.aurelia, r — 1.5 day-1, K — 100 volume units, and b — 1.5. For P.caudatum, r = 1.0 day-1, K — 60, and b — 0.8. (Original data from Gause, 1934.)


Figure 15.30 Competition between P. aurelia and P. caudatum. Solid diamonds show growth of each paramecium species in the absence of the other. Open circles show growth in mixed culture. The curves are for equation (14.28) fitted to the data by eye. For P.aurelia, r — 1.5 day-1, K — 100 volume units, and b — 1.5. For P.caudatum, r = 1.0 day-1, K — 60, and b — 0.8. (Original data from Gause, 1934.)

to the typical populations found in rivers and streams as substrate is consumed. This also explains why some organisms that cause filamentous bulking in the activated sludge process, such as Sphaerotilus natans, are favored by low-loading conditions. Figure 15.31 shows how this could occur by comparing the growth rate, rg, as a function of substrate concentration, S, for two hypothetical microorganisms. Species A has a higher maximum growth rate, mm; species B has a lower Monod coefficient, KS. As a result, species B has the higher growth rate at low substrate concentration (to the left of the dashed line).

Among bacteria, amensalism (antagonism) (0/-), usually comes in the form of allelopathy, the production of chemical inhibitors. In some cases it takes the form of acid production leading to pH changes, many produce toxic short-chain fatty acids, and some produce complex inhibitors that we exploit as antibiotics. Inhibitors allow the first population to gain a foothold in an environment to exclude newcomers.

Thiobacillus oxidans oxidizes sulfur, producing sulfuric acid, which lowers the pH of mine drainage to between 1 and 2, inhibiting most other microbes. Microorganisms on the skin produce fatty acids that are believed to prevent colonization by yeasts and other microbes. The fatty acids produced during methanogenesis are not just intermediates in the reaction but inhibit microbes that otherwise would disrupt the electron transfer between Methanobacterium and S.

Antibiotics are substances that kill or inhibit at very low concentration. Their role in natural habitats is unclear. They do not seem to accumulate to effective levels under natural conditions.

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