Bacterial Ecology

Bacterial ecology is quite a large subject and here I have only presented a brief outline of the more central aspects not covered in other parts of this chapter.

Bacterial Ecology Menu
Energy Ecological Importance Morphological Variations
Eating Reproduction Surviving the Bad Times

Energy

All organisms need energy. We get our energy from the food we eat, in other words from organic matter. All other animals are the same. Plants and some of the Protista get their energy from the sun. Various species of bacteria do both these things. When an organism gets its energy from the sun we call it a 'Phototroph' from the Greek works 'photo' for light and 'troph' meaning to feed. Literally this means 'light eating'. Organisms which do not get their energy from the sun get it from the energy already stored in chemicals. These organisms are called 'chemotrophs' meaning chemical eating. Most bacteria, and Protists as well as all animals and fungi are chemotrophs. If the food chemicals are large, complicated molecules of organic origin (ie they were once organisms), then we call the bacteria and all the other animals that eat them 'Chemoorganotrophs', but if the molecules are small and inorganic (ie iron, sulphur, hydrogen sulphide, etc) we call the bacteria which eat them 'Chemolithotrophs'. Only bacteria are chemolithotrophs, whereas animals, fungi and many protozoa are all chemoorganotrophs. Chemolithotrophic bacteria are unique in our world, and because they require special adaptations to gain their energy they are quite different to the more common chemoorganotrophic bacteria.

 

Some Long Words to Remember
Phototrophic = Getting energy from the sun
Chemoorganotrophic = Getting energy from organic molecules
Chemolithotrophic = Getting energy from inorganic molecules
Autotrophic = Able to live with CO2 as the only carbon source

Ecologically Important

Bacteria are important in soil formation, not only through the breakdown of organic matter in already easily recognisable soils, but also in the conversion of rock to soil. Any organic matter, however small a particle, is attractive to bacteria. The bacteria which live by degrading organic compounds are called Chemooganotrophs. The carbon dioxide produced during respiration by chemoorganotrophic bacteria is converted to carbonic acid which is an important agent in the break down of rocks.

Bacteria also live in aquatic environments - ponds, streams, lakes, rivers, seas and oceans. In many aquatic environments cyanobacteria, sometimes called Blue-Green Algae because of their colour, are the most important primary producers. They contain chlorophyll and trap energy from the sun in the form of light. In the depths of the sea, miles below the surface where there is no light bacteria can still be the primary producers. Only here they derive their energy from oxidising or reducing naturally occurring sulphur compounds. In certain places in the ocean are upwellings of warm, minerally enriched water called Hydrothermal Vents. They occur where the earth's crust is spreading. Around these vents live strange communities of organisms including giant Pogonophoran worms, giant clams and various other invertebrates. As no light ever reaches these areas all the energy for life comes from the heat and mineral content of the water. The first step in this process are the bacteria, either free living or living as partners (symbiotically) with the worms and the clams.

Some Come With Extra Bits

As mentioned earlier many bacteria are regularly shaped (cocci, rods etc.), but this is not true of all bacteria. Some have unusual appendages or attachments like long stalks or tubes and these are called appendaged bacteria. Sometimes these appendages are used in reproduction, sometimes they help the bacterium stick to a surface so it can stay where it wants to be, and sometimes we do not know yet what they are for.

These various shapes are general indications only - as with all things in nature there is really a vast continuum of shapes varying between these various central forms, some bacteria are half way between a rod and a coccus, or only slightly curved.

Some bacteria have other appendages which are much smaller than those mentioned above. These come in two sorts neither of which are very well understood from an ecological point of view though they both look like fine hairs sticking out from the bacterium in scanning electron microscopes images. One sort are called fimbriae (singula = fimbria) which are much shorter than flagella and often much more numerous. Scientists think that they may help the bacteria to cling to surfaces. The second sort are called pili (singular = pilus), they are slightly larger than fimbriae and only occur in ones or twos on any given cell. They are believed to be involved in conjugation, an aspect of Bacterial Reproduction.

Finally, many bacteria secrete a layer around themselves which is made up of polysaccharides with a few proteins. The resulting layer is quite variable between species, being thick or thin and/or rigid (solid) or flexible (runny). If it is rigid it is called a capsule and if it is flexible it is called a slime layer. These layers help bacterial cells bind to surfaces, resist predators and retain moisture.

Surviving the Bad Times

Some bacteria, notably species of soil bacteria such as Bacillus and Clostridium, can form a dormant, highly resistant form called an endospore. An endospore is formed inside the normal cell and contains copies of all the cell's DNA. Endospores are inactive and are used by bacteria to survive times when the environmental conditions do not allow for normal living. In this way they are a bit like seeds, except that one bacterium can only become one endospore.

Endospores can survive a very, very long time. In 1981 some spores stored in 1947 were successfully germinated showing that they can survive for at least 34 years. More recently, some scientists have claimed to have found viable (capable of germinating) endospores from the bottom of a lake in Minnesota which are 7000 years old. Even more amazing, in 1995 a group of scientists claimed to have grown bacteria from endospores taken from the gut of a bee trapped in amber 25-30 million years ago. Finally in the year 2000 scientists grew some bacteria from endospores 250 million years old. This remain to be checked by other scientists, but if it is true then it means that endospores might one day be found and revived from bacteria which lived with the dinosaurs, the trilobites or even earlier.

 

How Bacteria Eat

Like all living organisms bacteria need to eat in order to live, grow and reproduce. However, bacteria are far too small to have a mouth. Instead they have special channels in their cell walls and cell membranes which allow, or even assist some molecules to cross. Once the molecules are inside the cell they can be broken down into their componant parts before being rebuilt into the macromoloecules the bacteria needs in order to buils and repare itself, or generate energy. Unfortunately for the bacteria the surrounding environment is not always full of free-floating molecules of the correct sort. Instead, the molecules may be all bound together in tissues such as a dead leaf or you or me, or even our half digested dinner in our intestines. To solve this problem bacteria have evolved the habit of leaking enzymes out into the environment around around them. These enzymes then do what ever it is they do, attack specific tissues and molecules (proteases attack proteins, cellulases attack cellulose etc) and break them up into smaller units. Eventually molecules of a size that the bacterium can take into itself are madse that the cell can then absorb them through the channels mentioned above.

Reproduction in Bacteria

Bacteria live strange lives. In optimal conditions they can reach maturity in 20 minutes, they can then reproduce and one becomes two. twenty minutes later two becomes four. In another twenty minutes they become eight. After the first hour there are 8. After two hours there are 64. If the first bacterium fell into a perfect soup at the stroke of midnight there would be 2 097 152 of them by the time you woke up at 7.00 am. By the time you had morning coffee or recess at 10.40 am there would be 4.4 billion (that is 4.4 British Billions or 4.4 million million = 4 294 967 296 bacteria). The numbers just keep doubling every twenty minutes and they get horrendously large and my old bacteriology teacher assures me that in 48 hours they total weight of all the bacterial offspring of just this one bacterium would weigh 400 times the weight of the planet earth. If this seems unbelievable experiment with the maths, the power of doubling is incredible. In 45 hours the bacteria would only weigh 78% of the earths weight but a mere three hours later they would out weigh it 400 times. Fortunately for us, bacteria never actually end up in a perfect soup. In real life a cell normally takes between 1 and 24 hours to reach maturity. In extreme conditions where there is very little food, or low temperatures it can take much longer still, which is good for us. Also, many things eat bacteria or cause them to die, which is why we aren't swimming in a soup of bacteria. Still it is worth remembering how quickly one bacteria can become many when it finds something it likes to eat.

The simplest form of bacterial reproduction is called binary fission. Basically, this is where a bacterium grows to about twice the size of the smallest bacterium and splits in two. There is a little more to it than that, first the DNA in the cell makes a copy of itself. The two copies separate in the cell and the cell grows two new cell membranes and two new cell walls through its middle, effectively cutting the cell in half, to make two cells. This is asexual (the 'a' in front meaning without) reproduction because both the daughter cells have exactly the same DNA as the original cell and only one cell is involved. With this sort of reproduction you can start a population with just one bacterium.

Sexual reproduction, like that used by most animals and plants involves at lease two individuals and normally these are called males and females. Bacteria are not quite the same as the higher animals but they do transfer DNA from one individual to another and they can have a sort of sex.

Some methods of DNA transfer between cells seem almost accidental. When one bacterial cell dies and its cell wall is ruptured the contents are released into the environment. This includes the DNA which may be complete or broken into bits. Other nearby bacterial cells can absorb this DNA and add it to their own and in this way they gain extra DNA, if the dead cells DNA codes for a property they do not have then they have gained this property. Viruses which attack bacterial cells can also be the agents of the transfer DNA from one cell to another.

Bacterial DNA is normally a single circular double strand. Many bacteria can also contain smaller sections of DNA in the form of Plasmids. Plasmids are still circles of DNA and they can that replicate themselves within a bacterial cell. They often give the host cell extra abilities such as resistance to antibiotics or the ability to make a useful enzymes. Some plasmids give the bacterial cells that possess them a sort of masculinity. They cause the cell to grow special sex pili which reach out from the cell and attach to other cells that they contact. They then pull the two cells together. Once their cell walls are touching these plasmids reproduce copies themselves into the other cell. In doing so they give the other cell the ability to generate sex pili effectively making the second cell male as well. Sometimes this plasmid DNA becomes can become joined up with, or incorporated into the normal bacterial cell DNA and then the whole cell DNA is replicated into the other cell. This is called conjugation and it is as close to what we think of as sex as bacteria get. Plasmids can also move to new bacterial cells when a cell dies the same as ordinary cell DNA can as mentioned above.

 

 

 

 

Book Reviews


Microbiology, Principles and Explorations 5th Ed., by Jacquelyn G. Black 5th (2002)
Liaisons with Life, by Tom Wakeford (2001)

 

 

 

 

 

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