Micrographia: Specimens: Bacteria (Monera).

Bacteria from a stagnant pond surface film.

The Bacteria.

Bacteria are dealt with in these pages under two headings: the classical bacteria as understood by earlier biologists, and the cyanobacteria, formerly known as the blue-green algae or cyanophyta, which are now classified amongst the Bacteria.

Classical Bacteria.
Those described as bacteria before the inclusion of the cyanobacteria.

Cyanobacteria.
Described in older textbooks as blue-green algae or cyanophyta.

The modern kingdom Bacteria therefore incudes all prokaryotes -- all of the creatures whose cells have no nucleus, and which in older systems were classified as Monera.

The Bacteria: an Introduction.

The present kingdom Bacteria represents those organisms most fundamentally important to the existence and nature of life on Earth. There are a number of ways in which this is apparent, but probably the most crucial is the role played by bacteria in the supply of nitrogen to other living creatures.

The element nitrogen is an essential constituent of all protein (and DNA), and protein is essential to the structure, metabolism and growth of all living things. (Nitrogen is about 16% of the weight of protein).
The Bacteria are the only creatures which are able to convert the nitrogen gas of the atmosphere into chemical forms (ammonia, nitrites and nitrates) which can be utilized by other organisms. Their ability to "fix" nitrogen constitutes a major contribution to the environment in which all other creatures have developed, and upon which those creatures are now, as in the distant past, totally dependent.
The ability of the bacteria to process nitrogen is at least as important to the present state of life on Earth as the ability of plants to produce oxygen.

It has been estimated that the weight of bacteria in the topsoil of fertile pasture is in the order of a hundred times greater than the weight of herbivores the pasture can sustain. The ability of the bacteria to fix nitrogen determines the rate at which the grass can grow (given sunlight), and this in turn determines the number of animals which can be sustainably grazed on the pasture. It is the availability of assimilable nitrogen which is the limiting factor to growth (and therefore an important evolutionary pressure) in many locations within the biosphere. (It is believed that lightning strikes, and to a lesser extent, human contributions such as the exhaust gases from the internal combustion engine, may provide around 10% of the nitrogen incorporated into living creatures, but the rest is due to the activities of bacteria).

Another clear example of dependence upon bacteria is seen in the gut organisms which facilitate much of the food digestion in animals from termites to elephants. Deprived of their gut flora, none of these animals could survive. In our own case, it has been estimated that we humans carry a larger number of bacterial cells than we possess human cells.
Animal existence at this level is very clearly a partnership. We are each a microcommunity, made up of cells which are themselves the descendants of microcommunities (also involving bacteria) established very early in the history of life on Earth.

The metabolic activities of ancient bacteria are also believed to to be responsible for many of our largest mineral deposits, particularly those of iron, copper, zinc, lead, manganese, gold, and sulphur.

The recent discovery of highly specialized bacteria living around volcanic vents on the deep ocean floor, and living in the spaces between the grains of sedimentary rocks thousands of metres below the Earth's surface, has greatly expanded our awareness of the extent to which our planet, in terms of both numbers and biomass, is inhabited largely by bacteria.
We have also had to rethink our ideas on the transience and longevity of life-forms, for the bacteria trapped in these ancient sediments are at least as old as the rocks themselves -- the nutrients transported by the slow percolation of groundwater have demonstrably sustained vast numbers of individual bacteria for hundreds of millions of years.

A certain amount of bacteria-hysteria has been encouraged by the advertisers of such products as kitchen disinfectants and lavatory cleaners. In terms of public perception, they have done for bacteria what the movie "Jaws" did for sharks, and "King Kong" did for large primates. It is hoped that these notes will go some way towards restoring a more rational perspective on these essential and ubiquitous creatures.

Bacteria.

	In stagnant and polluted waters, bacteria of various kinds often form a film which appears as a scum on the pond surface. It is in effect a two-dimensional colony, held together by secretions from the bacteria themselves, increasing in area as the the bacteria multiply. When a coverglass is lowered onto such a water sample, the film adheres to the glass and is preserved exactly as it was when on the water surface. In this picture, a colony of smaller rod bacteria centred around a clump of debris in the lower left of the picture has been overgrown by a film of larger rod bacteria which fill the frame. Phase contrast, x1000.
	Another bacterial film in the process of drying out as the water evaporates from the specimen. The individual bacterial cells are thrown into extra relief by the meniscus of water which still surrounds them. Phase contrast, x1000.
	A dense growth of purple sulphur bacteria from a polluted (and evil-smelling) small stagnant pool. These are relatively large rod bacteria and they catch the light from the darkfield condenser like tiny glass tubes, giving flashes of reflected light as they tumble and turn. The visual impact of an entire field of actively motile cells glittering as they form and reform living bridges from one dense clump to another can only be described as spectacular. Darkfield, x100.
	Same specimen of purple sulphur bacteria as above, at higher magnification. Darkfield, x400.
	Curved filaments of the sulphur bacterium Beggiatoa. The highly refractile granules within the filaments are elemental sulphur, a byproduct of the bacterium's metabolism which utilizes hydrogen sulphide (H₂S, rotten egg gas) as a source of hydrogen. This bacterium thrives in organically polluted waters, and is especially common near sewage outfalls where it is known, somewhat inaccurately, as sewage fungus. Darkfield, x800.
Movie: 229KB. Takes a minute to load.	Here is a short movie sequence of the filamentous bacterium Beggiatoa in brightfield illumination. The filaments are in constant longitudinal movement which causes the filaments to loop and tangle, curve and straighten, and they can impose an erratic movement upon even quite large particles which come into contact with them. The particles of debris adhering to some of the filaments give an indication of the speed with which the filaments move. Brightfield, x400.

Bacterial Forms. A Footnote.

The Bacteria cannot be identified using the microscope alone, but they have been traditionally divided into three groups on the basis of their microscopical appearance -- the cocci (spherical), the bacilli (rod-shaped), and the spiral forms.
The spiral forms and some varieties of bacilli are particularly interesting as many of them have a flagellum at one or both ends, and this flagellum is attatched to the bacterium by the only known example in natural world of a ball-and-socket joint. In this short video sequence (471KB, 100 seconds to load) spiral bacteria called vibrios are seen attatched to the coverglass by their flagellum, and the rotation which normally propels them through the water at high speed is causing them to spin rapidly.