Microbiology and Immunology

Microbiology, clinical WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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drogram appears as a tree oriented on a horizontal axis. The

dendrogram becomes increasingly specialized—that is, the

similarity coefficient increases—as the dendrogram moves

from the left to the right. The right hand side consists of the

branches of the trees. Each branch contains a group of

microorganisms.

The dendrogram depiction of relationships can also be

used for another type of microbial taxonomy. In this second

type of taxonomy, the criterion used is the shared evolutionary

heritage. This heritage can be determined at the genetic level.

This is termed molecular taxonomy.

Molecular microbial taxonomy relies upon the genera-

tion and inheritance of genetic mutationsthat is the replace-

ment of a nucleotide building block of a geneby another

nucleotide. Sometimes the mutation confers no advantage to

the microorganism and so is not maintained in subsequent

generations. Sometimes the mutation has an adverse effect,

and so is actively suppressed or changed. But sometimes the

mutation is advantageous for the microorganism. Such a muta-

tion will be maintained in succeeding generations.

Because mutations occur randomly, the divergence of

two initially genetically similar microorganisms will occur

slowly over evolutionary time (millions of years). By sequenc-

ing a target region of genetic material, the relatedness or dis-

similarity of microorganisms can be determined. When

enough microorganisms have been sequenced, relationships

can be established and a dendrogram constructed.

For a meaningful genetic categorization, the target of

the comparative sequencing must be carefully chosen.

Molecular microbial taxonomy of bacteria relies on the

sequence of ribonucleic acid(RNA), dubbed 16S RNA, that is

present in a subunit of prokaryotic ribosomes. Ribosomes are

complexes that are involved in the manufacture of proteins

using messenger RNA as the blueprint. Given the vital func-

tion of the 16S RNA, any mutation tends to have a meaning-

ful, often deleterious, effect on the functioning of the RNA.

Hence, the evolution(or change) in the 16S RNA has been

very slow, making it a good molecule with which to compar

microorganisms that are billions of years old.

Molecular microbial taxonomy has been possible

because of the development of the technique of the poly-

merase chain reaction. In this technique a small amount of

genetic material can be amplified to detectable quantities

The use of the chain reaction has produced a so-called

bacterial phylogenetic tree. The structure of the tree is even

now evolving. But the current view has the tree consisting of

three main branches. One branch consists of the bacteria.

There are some 11 distinct groups within the bacterial branch.

Three examples are the green non-sulfur bacteria, Gram- pos-

itive bacteria, and cyanobacteria.

The second branch of the evolutionary tree consists of

the Archae, which are thought to have been very ancient bac-

teria that diverged from both bacteria and eukaryotic organ-

isms billions of years ago. Evidence to date places the Archae

a bit closer on the tree to bacteria than to the final branch (the

Eucarya). There are three main groups in the archae:

halophiles (salt-loving), methanogens, and the extreme ther-

mophiles (heat loving).

Finally, the third branch consists of the Eucarya, or the

eukaryotic organisms. Eucarya includes organisms as diverse

as fungi, plants, slime moldsand animals (including humans).

See also Bacterial kingdoms; Genetic identification of

microorganisms

MMicrobiology, clinicalICROBIOLOGY, CLINICAL

Clinical microbiology is concerned with infectious microor-

ganisms. Various bacteria, algae and fungiare capable of

causing disease.

Disease causing microorganisms have been present for

millennia. Examples include anthrax, smallpox, bacterial

tuberculosis, plague, diphtheria, typhoid fever, bacterial diar-

rhea, and pneumonia. While modern technological advances,

such as mass vaccination, have reduced the threat of some of

these diseases, others remain a problem. Some illnesses are re-

emerging, due to acquisition of resistance to many antibiotics.

Finally, other diseases, such as the often lethal hemorrhagic

fever caused by the Ebola virus, have only been recognized

within the past few decades.

Many bacterial diseases have only been recognized

since the 1970s. These include Legionnaires’ disease,

Campylobacter infection of poultry, toxic shock syndrome,

hemolytic uremic syndrome, Lyme disease, peptic ulcer dis-

ease, human ehrlichiosis, and a new strain of cholera. Clinical

microbiology research and techniques were vital in identifying

the cause of these maladies, and in seeking treatments and

ultimately a cure for each malady.

Clinical microbiology involves both the detection and

identification of disease-causing microorganisms, and

research to find more effective means of treating the infection

or preventing infections from occurring. The symptoms of the

ailment, and the shape, Gram stainreaction (in the case of bac-

teria), and biochemical reactions of an unknown organism are

used to diagnose the cause of an infection. Knowledge of the

identity of the microbe suggests means of treatment, such as

the application of antibiotics. Many clinical microbiologists

are also researchers. In many cases, the molecular basis of an

organism’s disease-causing capability is not clear. Unraveling

the reasons why a disease is produced can help find ways to

prevent the disease.

There are several groups or categories of bacteria that

are of medical importance. They are grouped into five cate-

gories based on their shape and reaction to the Gram stain.

These criteria apply to the light microscope, as typically a first

step in the identification of bacteria in an infection is the light

microscope examination of material obtained from the infec-

tion or from a culture. The groups are Gram-positive bacilli

(rod-shaped bacteria), Gram negative bacilli, Gram positive

cocci (round bacteria), Gram negative cocci, and bacteria that

react atypically to the Gram stain.

A group of spiral shaped bacteria called spirochetesare

responsible for leptospirosis in dogs, and syphilisand Lyme

disease in humans. These bacteria are easily identified under

the light microscope because of their wavy shape and

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