Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Chemoautotrophic and chemolithotrophic bacteria

115

•

Many plants, including edible ones, produce discreet

amounts of some toxic compound that plays a role in plant

protection against some natural predator. Some of these natu-

ral compounds may also be genotoxic for humans and ani-

mals, when that plant is consumed frequently and in great

amounts. For instance, most edible mushrooms contain a fam-

ily of chemical mutagenes known as hydrazines; but once

mushrooms are cooked, most hydrazines evaporate or are

degraded into less toxic compounds.

Among the most aggressive man-made chemical muta-

genes are:

• asbestos


• DDT


• insecticides and herbicides containing arsenic


• industrial products containing benzene


• formaldehyde


• diesel and gasoline exhaust


• polychlorinated biphenyl (PCB)
  Exposure to some of these compounds may occur in the
  work place, others can be present in the polluted air of great
  cities and industrial districts. For instance, insecticide and her-
  bicide sprayers on farms, tanners, and oil refinery workers are
  frequently exposed to arsenic and may suffer mutations that
  lead to lung or skin cancers. Insulation and demolition work-
  ers are prone to contaminationwith asbestos and may eventu-
  ally develop lung cancer. Painters, dye users, furniture
  finishers, and rubber workers are often exposed to benzene,
  which can induce mutations in stem cells that generate white
  blood cells, thus causing myelogenous leukemia. People
  working in the manufacture of wood products, paper, textiles
  and metallurgy, as well as hospital and laboratory workers, are
  frequently in contact with formaldehyde and can thus suffer
  mutations leading to nose and nasopharynx tumors. Cigarette
  and cigar smoke contains a class of chemical mutagenes,
  known as PAH (polycyclic aromatic hydrocarbons), that leads
  to mutation in lung cells. PAH is also present in gas and diesel
  combustion fumes.
  Except for the cases of accidental high exposure and
  contamination, most chemical mutagenes or their metabolites
  (i.e., cell-transformed by-products) have a progressive and
  gradual accumulation in DNA, throughout years of exposi-
  tion. Some individuals are more susceptible to the effects of
  cumulative contamination than others. Such individual
  degrees of susceptibility are due to discreet genetic varia-
  tions, known as polymorphism, meaning several forms or
  versions of a given group of genes. Depending on the poly-
  morphic version of Cytochrome P450 genes, an individual
  may metabolize some mutagenes faster than others.
  Polymorphism in another group of genes, NAT (N-acetyl-
  transferase), is also implied in different individual suscepti-
  bilities to chemical exposure and mutagenesis.

See alsoImmunogenetics; Mutants, enhanced tolerance or

sensitivity to temperature and pH ranges; Mutations and muta-

genesis

CHEMOAUTOTROPHIC AND

CHEMOLITHOTROPHIC BACTERIAChemoautotrophic and chemolithotrophic bacteria

Autotrophic bacteriaobtain the carbon that they need to sus-

tain survival and growth from carbon dioxide (CO 2 ). To

process this carbon source, the bacteria require energy.

Chemoautotrophic bacteria and chemolithotrophic bacteria

obtain their energy from the oxidation of inorganic (non-car-

bon) compounds. That is, they derive their energy from the

energy already stored in chemical compounds. By oxidizing

the compounds, the energy stored in chemical bonds can be

utilized in cellular processes. Examples of inorganic com-

pounds that are used by these types of bacteria are sulfur,

ammonium ion (NH4+), and ferrous iron (Fe2+).

The designation autotroph means “self nourishing.”

Indeed, both chemoautotrophs and chemolithotrophs are able

to grow on medium that is free of carbon. The designation

lithotrophic means “rock eating,” further attesting to the abil-

ity of these bacteria to grow in seemingly inhospitable envi-

ronments.

Most bacteria are chemotrophic. If the energy source

consists of large chemicals that are complex in structure, as is

the case when the chemicals are derived from once-living

organisms, then it is the chemoautotrophic bacteria that utilize

the source. If the molecules are small, as with the elements

listed above, they can be utilized by chemolithotrophs.

Only bacteria are chemolithotrophs. Chemoautotrophs

include bacteria, fungi, animals, and protozoa.

There are several common groups of chemoautotrophic

bacteria. The first group is the colorless sulfur bacteria. These

bacteria are distinct from the sulfur bacteria that utilize sun-

light. The latter contain the compound chlorophyll, and so

appear colored. Colorless sulfur bacteria oxidize hydrogen

sulfide (H 2 S) by accepting an electron from the compound.

The acceptance of an electron by an oxygen atom creates

water and sulfur. The energy from this reaction is then used to

reduce carbon dioxide to create carbohydrates. An example of

a colorless sulfur bacteria is the genus Thiothrix.

Another type of chemoautotroph is the “iron” bacteria.

These bacteria are most commonly encountered as the rusty

coloured and slimy layer that builds up on the inside of toilet

tanks. In a series of chemical reactions that is similar to those

of the sulfur bacteria, iron bacteria oxidize iron compounds

and use the energy gained from this reaction to drive the for-

mation of carbohydrates. Examples of iron bacteria are

Thiobacillus ferrooxidans and Thiobacillus thiooxidans.

These bacteria are common in the runoff from coal mines. The

water is very acidic and contains ferrous iron. Chemoauto-

trophs thrive in such an environment.

A third type of chemoautotrophic bacteria includes the

nitrifying bacteria. These chemoautotrophs oxidize ammonia

(NH 3 ) to nitrate (NO 3 - ). Plants can use the nitrate as a nutrient

source. These nitrifying bacteria are important in the operation

of the global nitrogen cycle. Examples of chemoautotrophic

nitrifying bacteria include Nitrosomonas and Nitrobacter.

The evolutionof bacteria to exist as chemoautotrophs or

chemolithotrophs has allowed them to occupy niches that

womi_C 5/6/03 2:04 PM Page 115