Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Eubacteria

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Outside of the intestinal tract, E. colidies quickly. This trait

has been exploited, and E. coliis a popular indicator of drink-

ing water quality, as its presence indicates the recent contami-

nationof the water with feces.

One of the most harmful types of E. coli is a strain

called O157:H7. Researchers surmise that O157:H7 arose

when an innocuous E. colibacterium was infected by a virus

carrying the genes coding for a powerful toxin called Shiga-

like toxin. The toxin can destroy cells in the intestinal tract

and, if they enter the bloodstream, can impair or destroy the

kidneys and the liver. The intestinal damage causes a lot of

bleeding. In children and elderly people, this hemorrhaging

can be lethal. In other people, damage to the kidney and liver

can be permanent or even lethal. In the summer of 2000, more

than 2,000 people in Walkerton, Ontario, Canada were sick-

ened and seven people died from drinking water which had

been contaminated with O157:H7.

Strain O157:H7 was first linked to human disease in

1983, when it was shown to have been the cause of two out-

breaks of an unusual and severe gastrointestinal ailment in the

Unites States. Since then, the number of documented human

illnesses and deaths caused by O157:H7 has increased steadily

worldwide. Disease caused by E. coliis preventable, by proper

hand washing after bowel movements, avoidance of unpas-

teurized milk or apple cider, washing of raw foods before con-

sumption and thorough cooking of ground meat.

Modern genetics techniques have been successful in

obtaining the sequence of the genetic material of E. coli.

Frederick Blattner and his colleagues published the genome

sequence of strain K–12 in 1997. The genome was discovered

to have approximately 4300 protein coding regions making up

about 88 per cent of the bacterial chromosome. The most

numerous types of proteins were transport and binding pro-

teins—those necessary for the intake of nutrients. A fairly large

portion of the genome is reserved for metabolism—the process-

ing of the acquired nutrients into useable chemicals. In 2000,

Nicole Perna and her colleagues published the genome

sequence of O157:H7. The O157:H7 genome shows similarity

to tat of k12, reflecting a common ancestry. But, in contrast to

K12, much of the genome of O157:H7 codes for unique pro-

teins, over 1,300, some of which may be involved in disease

causing traits. Many of these genes appear to have been

acquired from other microorganisms, in a process called lateral

transfer. Thus, strain O157:H7 appears to be designed to

undergo rapid genetic change. This distinction is important;

indicating that strategies to combat problems caused by one

strain of E. colimight not be universally successful. Knowledge

of the genetic organization of these strains will enable more

selective strategies to be developed to combat E.coliinfections.

See alsoFood safety; Microbial flora of the stomach and gas-

trointestinal tract; Microbial genetics; Waste water treatment;

Water purification; Water quality

ESCHERICHIA COLI(E. COLI) INFECTION

• seeE. COLIO157:H7 INFECTION

ESCHERICHIA COLI, ENTEROHEMOR-

RHAGIC• seeESCHERICHIA COLI(E. COLI)

EEubacteriaUBACTERIA

The Eubacteria are the largest and most diverse taxonomic

group of bacteria. Some scientists regard the Eubacteria group

as an artificial assemblage, merely a group of convenience

rather than a natural grouping. Other scientists regard eubac-

teria as comprising their own kingdom. Another recent classi-

fication holds Eubacteria and Archaebacteria as domains or

major groupings, classified above the kingdom level. The

Eubacteria are all easily stained, rod-shaped or spherical bac-

teria. They are generally unicellular, but a small number of

multicellular forms do occur. They can be motile or non-

motile and the motile forms are frequently characterized by

the presence of numerous flagellae. Many of the ecologically

important bacteria responsible for the fixation of nitrogen,

such as Azotobacterand Rhizobium,are found in this group.

The cell walls of all of these species are relatively

thick and unchanging, thus shape is generally constant

within groups found in the Eubacteria. Thick cell walls are

an evolutionary adaptation that allows survival in extreme

situations where thinner walled bacteria would dry out.

Some of the bacteria are gram positive while others are gram

negative. One commonality that can be found within the

group is that they all reproduce by transverse binary fission,

although not all bacteria that reproduce in this manner are

members of this group.

Eubacteria are often classified according to the manner

in which they gain energy. Photoautotrophic Eubacteria man-

ufacture their own energy through photosynthesis.

Cyanobacteria, often called blue-green algae, are common

photoautotrophic Eubacteria that are found in ponds and wet-

lands. Although not true algae, Cyanobacteria grow in chain-

like colonies and contain chloroplasts as do aquatic algae.

Cyanobacteria fossils are among the oldest-known fossils on

Earth, some more than 3.5 billion years old.

Heterotrphic Eubacteria depend upon organic molecules

to provide a source of energy. Heterotrophic Eubacteria are

among the most abundant and diverse bacteria on Earth, and

include bacteria that live as parasites, decomposers of organic

material (saprophytes), as well as many pathogens (disease-

causing bacteria). Chemoautotrophic Eubacteria bacteria

obtain their own energy by the oxidation of inorganic mole-

cules. Chemoautotrophic bacteriaare responsible for releasing

the sulfur resulting in a sulfur taste of freshwater near many

beaches (such as in Florida), and for supplying nitrogen in a

form able to be used by plants.

See also Autotrophic bacteria; Heterotrophic bacteria;

Nitrogen cycle in microorganisms; Oxidation-reduction

reaction; Photosynthetic microorganisms

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