Microbial Ecology (PDF)

INTRODUCTORY MICROBIAL ECOLOGY Microorganisms exist in communities, ranging from 1010-1017 individuals, representing at least 107 different taxa. These huge microbial populations exist and survive together, because they can interact with each other and with their immediate environment. Micr...

INTRODUCTORY MICROBIAL ECOLOGY Microorganisms exist in communities, ranging from 1010-1017 individuals, representing at least 107 different taxa. These huge microbial populations exist and survive together, because they can interact with each other and with their immediate environment. Microbial ecology describes the presence, behaviour, activities and contributions of microorganisms in their natural environments. ME also focuses on ways microorganisms deal with or adapt to environmental factors such as heat, cold, gases, acid, radiation, osmotic and hydrostatic pressure, and even other microbes.  That is: ❑ Temperature adaptations ❑ Gas requirements ❑ Effects of pH ❑ Osmotic pressure ❑ Miscellaneous environmental factors ❑ Ecological interactions among microorganisms  MICROBIAL INTERACTIONS ▪ Microorganisms can associate physically with other organisms in a variety of ways: one organism can be located on the surface of another, as an ectosymbiont or, one organism can be located within another organism as an endosymbiont. ▪ The simplest microbial interactions involve two members, a symbiont and its host. A number of interesting organsims host more than one symbiont, a term known as consortium is used to describe this physical relationship. ▪ Some associations are between similar or dissimilar types of microbes; others involve multicellular organisms such as animals or plants. ▪ Interactions can have beneficial, harmful, or no particular effects on the organisms involved; they can be obligatory or non-obligatory to the members; and they can often involve nutritional interactions.  Microbial associations can be symbiotic or non-symbiotic o In symbiotic organisms live in close nutritional relationship; required by one or both members. o Non-symbiotic, organisms are free-living; relationships not required for survival. Symbiotic relationships: Non-symbiotic ✔ Mutualism Amensalism ✔ Cooperation Competition ✔ Commensalism ✔ Predation ✔ Parasitism  MUTUALISM Mutualism (from the Latin word mutuus, meaning borrowed or reciprocal) defines the relationship in which some reciprocal benefit accrues to both partners; i.e. both members benefit. This is an obligatory relationship in which the mutualist and the host are dependent on each other. In many cases the individual organisms will not survive when separated. Mutualistic associations are common between insects and microorganisms. The required vitamins and amino acids are provided by bacterial symbionts in exchange for a secure habitat and ample nutrients in the insects (hosts). The aphid is an excellent example of this mutualistic relationship. Cells of this insect harbour the γ-proteobacterium, Buchnera aphidicola, and a mature insect contains literally millions of these bacteria in its body. B. aphidicola provides its host with 10 essential amino acids, and if the insects is treated with antibiotics, it dies. The protozoan-termite relationship is another classic example of mutualism in which flagellated protists live in the gut of wood roaches and termites. The protists engulf wood particles ingested by their host, digest the cellulose, and metabolize it to acetate and other products. Termites then use the acetate released by the protists as their true carbon source. Because the host is almost always incapable of synthesizing cellulases, needed to hydrolyse cellulose, it therefore depends on the mutualistic protists for its existence.  COOPERATION Cooperation and commensalism are two positive but not obligatory types of symbiosis found widely in the microbial world. They involve syntrophic relationships. Syntrophism (from the Greek word syn, meaning together, and trophe, nourishment) is an association in which the growth of one organism either depends on or is improved by growth factors, nutrients, or substrates provided by another organism. Sometimes both organisms benefit. The non-obligatory form of cooperation differentiates it from mutualism. In cooperation, cooperating organisms can be separated from one another and remain viable, whereas mutualistic organisms cannot survive when separated. Two examples of a cooperative relationship include the association between Desulfovibrio and Chromatium, in which the carbon and sulphur cycles are linked, and the interactions of a nitrogen-fixing microorganisms with a cellulolytic organism such as Cellulomonas. In the latter example, the cellulose-degrading microorganism liberates glucose from the cellulose, which can be used by nitrogen-fixing microbes.  COMMENSALISM Commensalism (Latin com, meaning together, and mensa, table) is a relationship in which one symbiont, the commensal, benefits while the other (sometimes called the host) is neither harmed nor benefitted. The spatial proximity of the two partners permits the commensal to feed on substances captured or ingested by the host, and the commensal often obtains shelter by living either on or in the host. The commensal is not directly dependent on the host metabolically, so when it is separated from the host experimentally, it can survive without the addition of factors of host origin. Commensalistic relationships between microorganisms include situations in which the waste product of one microorganism is a substrate for another species. One good example is nitrification- the oxidation of ammonium ion to nitrate. This occurs in two steps: first, microorganisms such as Nitrosomonas oxidize ammonium to nitrite, and, second, nitrite is oxidized to nitrate by Nitrobacter and similar bacteria. Nitrobacter benefits from its association with Nitrosomonas because it uses nitrite to obtain energy for growth. Commensalistic associations also occur when one microbial group modifies the environment to make it better suited for another organism. For example, the synthesis of acidic waste products during fermentation stimulates the proliferation of more acid-tolerant microorganisms, which may be only a minor part of the microbial community at neutral pH.  PREDATION As is the case with larger organisms, predation among microbes involves a predator species that attacks and usually kills its prey. Examples of predators are Bdellovibrio, Vampirococcus and Daptobacter. Bdellovibrio is a motile organism, that swims about looking for susceptible Gram-negative bacterial prey. Upon sensing such a cell, Bdellovibrio swims faster until it collides with the prey cell. It then bores a hole through the outer membrane of its prey and enters the periplasmic space. As it grows, it forms a long filament that eventually septates to produce progeny bacteria. Lysis of the prey cell releases new Bdellovibrio. Bdellovibrio will not attack mammalian cells, and Gram-negative prey bacteria have never been observed to acquire resistance to Bdellovibrio attack.. This has raised interest in the use of Bdellovibrio as a ‘probiotic’ to treat infected wounds. Predation can provide a protective, high-nutrient environment for particular prey. Protists ingest the Gram-positive bacterium Legionella and protect this important pathogen from chlorine, which is often used to control Legionella in cooling towers and air-conditioning units. Fig. 1. Bdellovibrio, a periplasmic predator that penetrates the cell wall of its prey and grows outside the plasma membrane  PARASITISM Parasitism is a relationship between two organisms in which one benefits from the other and the host is usually harmed. It can involve nutrient acquisition, physical maintenance in or on the host, or both. In parasitism, there is always some coexistence between host and parasite. This is because a host that dies immediately after parasite invasion may prevent the microbe from reproducing to sufficient numbers to ensure colonization of a new host. Depending on the host-parasite equilibrium, if the balance favours the host (perhaps by a strong immune defense or antimicrobial therapy), the parasite loses its habitat and may be unable to survive. On the other hand, if the equilibrium is shifted to favour the parasite, the host becomes ill, and eventually die, depending on the host-parasite relationship. A controlled parasite-host relationship can be maintained for long periods of time. For example, lichens, the association between specific ascomycetes (a fungus) and certain genera of either green algae or cyanobacteria. The fungal partner is termed the mycobiont and the algal or cyanobacterial partner, the phycobiont. The fungus obtains nutrients from its partner by projections of fungal hyphae called haustoria, which penetrate the phycobiont cell wall. It also uses O2 produced by the phycobiont in carrying out respiration. In turn, the fungus protects the phycobiont from high light intensities, provides water and minerals, and creates a firm substratum within which the phycobiont can grow protected from environmental stress. In the past the lichen symbiosis was considered to be a mutualistic interaction.