Prokaryotae WORLD OF MICROBIOLOGY AND IMMUNOLOGY450•
PProbioticsROBIOTICS
Probiotics is a term that refers to the consumption of certain
microorganismsin an effort to improve overall health and the
functioning of the body’s microflora.
The use of microorganisms as a health aid is not new.
People have asserted the health fortifying attributes of yogurt
and fermented milk for thousands of years. However, the cause
of the beneficial effect was unknown. A century ago, the
Russian microbiologist Élie Metchnikoff(1845–1916) began the
scientific assessment of the probiotic role of microorganisms.
Based on Metchnikoff’s work and that of others, it
appeared well established (but not clinically proven) by the
1920s that Lactobacillus acidophilusacted to relieve the con-
flicting conditions of constipation and diarrhea. Capsules con-
taining living bacteriawere popular items in drug stores of the
day. However, with the advent of antibioticsas a cure for
many ailments, the public interest in probiotics waned. The
emphasis shifted to the treatment of infections, as opposed to
the prevention of infections.
In the 1990s the interest in probiotics surged. A number
of studies established the clinical significance of Lactobacillus
and Bifidobacteriumin improving the efficiency of lactose
absorption, in the treatment of diarrhea in children, and in
combating recurrent vaginal yeastinfections.
Probiotic bacteria exert their effect by colonizing sur-
faces, such as found in the intestinal tract or the vagina.
Compounds can also be produced by the adherent bacteria that
are inhibitory to other types of bacteria. The net effect of these
processes is the competitive exclusion of potentially harmful
bacteria by the beneficial probiotic bacteria.
The exclusion process can extend to other infectious
agents as well. For example, colonization of the intestinal tract
with LactobacillusGG has been shown to significantly reduce
the length of diarrheal illness caused by rotavirus. The
rotavirus had no place to adhere and were washed out of the
intestinal tract.
Probiotics also have shown potential in relieving skin
disorders that are the result of an allergic reaction to a food.
The colonization of the intestinal wall appears to restore the
ability of nutrients to cross from the intestinal canal to the
bloodstream. This ability to absorb food nutrients is disrupted
in those with some food allergies.
The molecular basis for the competitive exclusion
behavior of bacteria such as Lactobacillus and
Bifidobacteriumis the subject of continuing study. The identi-
fication of the precise molecular agents that are responsible
for surface blocking will expand the use of probiotics.See alsoLactobacillus; Microbial flora of the stomach and
gastrointestinal tractPProkaryotaeROKARYOTAE
Prokaryote is a kingdom, or division, in the classification
scheme devised for all life on Earth. This kingdom, which is
also designated as Monera, includes all bacteriaand blue-green algae(which are also called Cyanobacteria). There are
four other kingdoms in the classification system. The classifi-
cation is based on the structure of a subunit of the ribosome.
This criterion was selected because the structure of the subunit
seems to have been maintained with little change throughout
the millions of years that life has existed on Earth.
Besides the kingdom Prokaryotae, there are the Protista
(eukaryotic organisms’ organisms that have a nucleus
enclosed in a well-defined membrane), Fungi, Animalia
(eukaryotesorganized into complex organisms), and Plantae.
The use of kingdoms in the classification of organisms
arose with the work of Carolus Linneus who, in the mid-
1700s, devised the system that is still used today. The Linnean
system of classification has kingdoms as the highest level,
with six other subdivisions down to the species level. Bacteria
are divided into various genera. A group of bacteria derived
from a single cell is called a strain. Closely related strains con-
stitute a bacterial species. For example, the complete classifi-
cation of the bacterium Escherichia coliis as follows:- Kingdom: Prokaryotae (Monera)
- Division (also called Phylum): Gracilicutes
- Class: Scotobacteria
- Order: Enterobacteriales
- Family: Enterobacteriaceae
- Genus: Escherichia
- Species: Escherichia coli
The Prokaryotae are further divided into two subking-
doms. These are called the Eubacteriobonta (which contains
the so-called Eubacteria) and the Archaebacteriobonta (which
contains the so-called Archaebacteria). This split arose from
the research of Carl Woese. He showed that the so-called 16 S
ribosomal subunit of bacteria divide bacteria into two groups;
the Eubacteria and the Archaeobcteria.
Archaebacteria are a very diverse group of bacteria and
have several features that set them apart from the other
Prokaryotae. Their cell walls lack a structure called the pepti-
doglycan, which is a rigid and stress-bearing network neces-
sary for the survival of other bacteria. Archaebacteria live in
extreme environments such as deep-sea vents, hot springs, and
very salty water. Finally, some metabolic processes of
Archaebacteria are different from other bacteria.
The feature that most distinguishes the bacteria and
blue-green algal members of the Prokaryote from the mem-
bers of the other kingdoms is the lack of membrane-bound
structure around the genetic material. The genetic material,
deoxyribonucleic acid(DNA), is dispersed through the inside
of the microorganism, a region that is typically referred to as
the cytoplasm. In contrast, eukaryotic organisms have their
genetic material compartmentalized inside a specialized
membrane.
A second distinctive feature of the Prokaryotae concerns
their method of reproduction. Most bacteria reproduce by
growing and then splitting in two. This is called binary fission.
Eukaryotic organisms have a more complex process that
involves the replication of their differently organized genetic
material and the subsequent migration of the material to spe-
cific regions of the cell.
womi_P 5/7/03 11:10 AM Page 450