Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Chronic bacterial disease

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The linear chromosomes of bacteria are similar to those

of eukaryotes such as yeast in that they have specialized

regions of DNA at the end of each double strand of DNA.

These regions are known as telomeres, and serve as bound-

aries to bracket the coding stretches of DNA. Telomeres also

retard the double strands of DNA from uncoiling by essen-

tially pinning the ends of each strand together with the com-

plimentary strand.

There are two types of telomeres in bacteria. One type

is called a hairpin telomere. As its name implies, the telomers

bends around from the end of one DNA strand to the end of

the complimentary strand. The other type of telomere is

known as an invertron telomere. This type acts to allow an

overlap between the ends of the complimentary DNA strands.

Replication of a linear bacterial chromosome proceeds

from one end, much like the operation of a zipper. As replica-

tion moves down the double helix, two tails of the daughter

double helices form behind the point of replication.

Research on bacterial chromosome structure and func-

tion has tended to focus on Escherichia colias the model

microorganism. This bacterium is an excellent system for such

studies. However, as the diversity of bacterial life has become

more apparent in beginning in the 1970s, the limitations of

extrapolating the findings from the Escherichia colichromo-

some to bacteria in general has also more apparent. Very little

is known, for example, of the chromosome structure of the

Archae, the primitive life forms that share features with

prokaryotes and eukaryotes, and of those bacteria that can live

in environments previously thought to be completely inhos-

pitable for bacterial growth.

See alsoGenetic identification of microorganisms; Genetic

regulation of prokaryotic cells; Microbial genetics; Viral

genetics; Yeast genetics

CChronic bacterial diseaseHRONIC BACTERIAL DISEASE

Chronic bacterial infections persist for prolonged periods of

time (e.g., months, years) in the host. This lengthy persist-

ence is due to a number of factors including masking of the

bacteriafrom the immune system, invasion of host cells, and

the establishment of an infection that is resistance to anti-

bacterial agents.

Over the past three decades, a number of chromic bac-

terial infections have been shown to be associated with the

development of the adherent, exopolysaccharide-encased

populations that are termed biofilms. The constituents of the

exopolysaccharide are poorly immunogenic. This means that

the immune system does not readily recognize the

exopolysaccharide as foreign material that must be cleared

from the body. Within the blanket of polysaccharide the bac-

teria, which would otherwise be swiftly detected by the

immune system, are protected from immune recognition. As

a result, the infection that is established can persist for a

long time.

An example of a chronic, biofilm-related bacterial infec-

tionis prostatitis. Prostatitis is an inflammationof the prostate

gland that is common in men over 30 years of age. Symptoms

of this disease can include intense pain, urinary complications,

and sexual malfunction including infertility. Chronic bacterial

prostatitis is generally associated with repeated urinary tract

infections. The chronic infection is typically caused by

biofilms of Escherichia coli.

A second biofilm-related chronic bacterial infection is

the Pseudomonas aeruginosalung infection that develops

early in life in some people who are afflicted with cystic fibro-

sis. Cystic fibrosis is due to a genetic defect that restricts the

movement of salt and water in and out of cells in the lung. The

resulting build-up of mucus predisposes the lungs to bacterial

infection. The resulting Pseudomonas aeruginosainfection

becomes virtually impossible to clear, due the antibiotic resis-

tanceof the bacteria within the biofilm. Furthermore, the

body’s response to the chronic infection includes inflamma-

tion. Over time, the inflammatory response is causes breathing

difficulty that can be so pronounced as to be fatal.

Another chronic bacterial infection that affects the

lungs is tuberculosis. This disease causes more deaths than

any other infectious disease. Nearly two billion people are

infected with the agent of tuberculosis, the bacterium

Mycobacterium tuberculosis. As with other chronic infec-

tions, the symptoms can be mild. But, for those with a weak-

ened immune system the disease can become more severe.

Each year some three million people die of this active form of

the tuberculosis infection.

Tuberculosis has re-emerged as a health problem in the

United States, particularly among the poor. The develop-

ment of drug resistance by the bacteria is a factor in this re-

emergence.

Beginning in the mid 1970s, there has been an increas-

ing recognition that maladies that were previously thought to

be due to genetic or environmental factors in fact have their

basis in chronic bacterial infections. A key discovery that

prompted this shift in thinking concerning the origin of certain

diseases was the demonstration by Barry Marshallthat a bac-

terium called Helicobacter pyloriis the major cause of stom-

ach ulcers. Furthermore, there is now firm evidence of an

association with chronic Helicobacter pyloristomach and

intestinal infections and the development of certain types of

intestinal cancers.

At about the same time the bacterium called Borrelia

burgdorferiwas established to be the cause of a debilitating

disease known as Lyme disease. The spirochaete is able to

establish a chronic infection in a host. The infection and the

host’s response to the infection, causes arthritis and long-last-

ing lethargy.

As a final recent example, Joseph Penningerhas shown

that the bacterium Chlamydia trachomatisis the agent that

causes a common form of heart disease. The bacterium chron-

ically infects a host and produces a protein that is very similar

in three-dimensional structure to a protein that composed a

heart valve. The host’s immune response to the bacterial pro-

tein results in the deterioration of the heart protein, leading to

heart damage.

Evidence is accumulating that implicates chronic bacte-

rial infection with other human ailments including schizo-

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