Microbiology and Immunology

Biogeochemical cycles WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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the sugary network. This mass represents the biofilm. The

sugar constituent is known as glycocalyx, exopolysaccharide,

or slime.

As the biofilm thickens and multiple layers of bacteria

build up, the behavior of the bacteria becomes even more

complex. Studies using instruments such as the confocal

microscopecombined with specific fluorescent probes of var-

ious bacterial structures and functional activities have demon-

strated that the bacteria located deeper in the biofilm cease

production of the slime and adopt an almost dormant state. In

contrast, bacteria at the biofilm’s periphery are faster-growing

and still produce large quantities of the slime. These activities

are coordinated. The bacteria can communicate with one

another by virtue of released chemical compounds. This so-

called quorum sensingenables a biofilm to grow and sense

when bacteria should be released so as to colonize more dis-

tant surfaces.

The technique of confocal microscopy allows biofilms

to be examined without disrupting them. Prior to the use of the

technique, biofilms were regarded as being a homogeneous

distribution of bacteria. Now it is known that this view is

incorrect. In fact, bacteria are clustered together in “micro-

colonies” inside the biofilm, with surrounding regions of bac-

teria-free slime or even channels of water snaking through the

entire structure. The visual effect is of clouds of bacteria ris-

ing up through the biofilm. The water channels allow nutrients

and waste to pass in and out of the biofilm, while the bacteria

still remain protected within the slime coat.

Bacterial biofilms have become important clinically

because of the marked resistance to antimicrobial agents that

the biofilm bacteria display, relative to both their planktonic

counterparts and from bacteria released from the confines of

the biofilm. Antibioticsthat swiftly kill the naked bacteria do

not arm the biofilm bacteria, and may even promote the devel-

opment of antibiotic resistance. Contributors to this resistance

are likely the bacteria and the cocooning slime network.

Antibiotic resistant biofilms occur on artificial heart

valves, urinary catheters, gallstones, and in the lungs of those

afflicted with cystic fibrosis, as only a few examples. In the

example of cystic fibrosis, the biofilm also acts to shield the

Pseudomonas aeruginosa bacteria from the antibacterial

responses of the host’s immune system. The immune response

may remain in place for a long time, which irritates and dam-

ages the lung tissue. This damage and the resulting loss of

function can be lethal.

See alsoAnti-adhesion methods; Antibiotic resistance, tests

for; Bacterial adaptation

BBiogeochemical cyclesIOGEOCHEMICAL CYCLES

The term biogeochemical cycle refers to any set of changes

that occur as a particular element passes back and forth

between the living and non-living worlds. For example, car-

bon occurs sometimes in the form of an atmospheric gas (car-

bon dioxide), sometimes in rocks and minerals (limestone and

marble), and sometimes as the key element of which all living

organisms are made. Over time, chemical changes occur that

convert one form of carbon to another form. At various points

in the carbon cycle, the element occurs in living organisms

and at other points it occurs in the Earth’s atmosphere, litho-

sphere, or hydrosphere.

The universe contains about ninety different naturally

occurring elements. Six elements, carbon, hydrogen, oxygen,

nitrogen, sulfur, and phosphorus, make up over 95% of the

mass of all living organisms on Earth. Because the total

amount of each element is essentially constant, some cycling

process must take place. When an organism dies, for example,

the elements of which it is composed continue to move

through a cycle, returning to the Earth, to the air, to the ocean,

or to another organism.

All biogeochemical cycles are complex. A variety of

pathways are available by which an element can move

among hydrosphere, lithosphere, atmosphere, and biosphere.

For instance, nitrogen can move from the lithosphere to the

atmosphere by the direct decomposition of dead organisms

or by the reduction of nitrates and nitrites in the soil. Most

changes in the nitrogen cycle occur as the result of bacterial

action on one compound or another. Other cycles do not

require the intervention of bacteria. In the sulfur cycle, for

example, sulfur dioxide in the atmosphere can react directly

with compounds in the earth to make new sulfur compounds

that become part of the lithosphere. Those compounds can

then be transferred directly to the biosphere by plants grow-

ing in the earth.

Most cycles involve the transport of an element

through all four parts of the planet—hydrosphere, atmo-

sphere, lithosphere, and biosphere. The phosphorous cycle is an

exception since phosphorus is essentially absent from the atmos-

phere. It does move from biosphere to the lithosphere (when

organisms die and decay) to the hydrosphere (when phospho-

rous-containing compounds dissolve in water) and back to the

biosphere (when plants incorporate phosphorus from water).

Hydrogen and oxygen tend to move together through

the planet in the hydrologic cycle. Precipitation carries water

from the atmosphere to the hydrosphere and lithosphere. It

then becomes part of living organisms (the biosphere) before

being returned to the atmosphere through respiration, transpi-

ration, and evaporation.

All biogeochemical cycles are affected by human activ-

ities. As fossil fuels are burned, for example, the transfer of

carbon from a very old reserve (decayed plants and animals

buried in the earth) to a new one (the atmosphere, as carbon

dioxide) is accelerated. The long-term impact of this form of

human activity on the global environment, as well as that of

other forms, is not yet known. Some scientists assert, however,

that those affects can be profound, resulting in significant cli-

mate changes far into the future.

See alsoBiodegradable substances; Carbon cycle in microor-

ganisms; Composting, microbiological aspects; Economic

uses and benefits of microorganisms; Evolution and evolu-

tionary mechanisms; Evolutionary origin of bacteria and

viruses; Nitrogen cycle in microorganisms; Oxygen cycle in

microorganisms

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