Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Nosocomial infections

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Some species of plants live in an intimate and mutually

beneficial symbiosis with microbes that have the capability of

fixing dinitrogen. The plants benefit from the symbiosis by

having access to a dependable source of fixed nitrogen, while

the microorganisms benefit from energy and habitat provided

by the plant. The best known symbioses involve many species

in the legume family (Fabaceae) and strains of a bacterium

known as Rhizobium japonicum. Some plants in other families

also have dinitrogen-fixing symbioses, for example, red alder

(Alnus rubra) and certain member of Actinomycetes. Bacteria

from the genera Frankiaand Azospirillumare also able to

establish symbiotic relationships with non-leguminous plants.

Many species of lichens, which consist of a symbiotic rela-

tionship between a fungus and a blue-green bacterium, can

also fix dinitrogen.

Ammonification is a term for the process by which the

organically bound nitrogen of microbial, plant, and animal

biomass is recycled after their death. Ammonification is car-

ried out by a diverse array of microorganisms that perform

ecological decay services, and its product is ammonia or

ammonium ion. Ammonium is a suitable source of nutrition

for many species of plants, especially those living in acidic

soils. However, most plants cannot utilize ammonium effec-

tively, and they require nitrate as their essential source of

nitrogen nutrition.

Nitrate is synthesized from ammonium by an impor-

tant bacterial process known as nitrification. The first step in

nitrification is the oxidation of ammonium to nitrite (NO 2 -),

a function carried out by bacteria in the genus Nitrosomonas.

Once formed, the nitrite is rapidly oxidized further to nitrate,

by bacteria in the genus Nitrobacter. The bacteria responsi-

ble for nitrification are very sensitive to acidity, so this

process does not occur at significant rates in acidic soil

or water.

Denitrification is another bacterial process, carried out

by a relatively wide range of species. In denitrification,

nitrate is reduced to either nitrous oxide or dinitrogen, which

is then emitted to the atmosphere. One of the best studies

bacterial examples is Pseudomonas stutzeri. This bacterial

species has almost 50 genes that are known to have a direct

role in denitrification. The process of denitrification occurs

under conditions where oxygen is not present, and its rate is

largest when concentrations of nitrate are large. Conse-

quently, fertilized agricultural fields that are wet or flooded

can have quite large rates of denitrification. In some

respects, denitrification can be considered to be an opposite

process to dinitrogen fixation. In fact, the global rates of

dinitrogen fixation and denitrification are in an approximate

balance, meaning that the total quantity of fixed nitrogen in

Earth’s ecosystems is neither increasing nor decreasing sub-

stantially over time.

See alsoBiogeochemical cycles; Economic uses and benefits

of microorganisms

NON-CULTURABLE BACTERIA•seeVIABLE

BUT NON-CULTURABLE BACTERIA

NON-SELECTIVE MEDIA•seeGROWTH AND

GROWTH MEDIA

NON-SPECIFIC IMMUNITY•seeIMMUNITY,

ACTIVE, PASSIVE, AND DELAYED

NNosocomial infectionsOSOCOMIAL INFECTIONS

A nosocomial infection is an infection that is acquired in a

hospital. More precisely, the Centers for Disease Controlin

Atlanta, Georgia, defines a nosocomial infection as a localized

infection or one that is widely spread throughout the body that

results from an adverse reaction to an infectious microorgan-

ism or toxin that was not present at the time of admission to

the hospital.

The term nosocomial infection derives from the nosos,

which is the Greek word for disease.

Nosocomial infections have been a part of hospital

care as long as there have been hospitals. The connection

between the high death rate of hospitalized patients and the

exposure of patients to infectious microorganismswas first

made in the mid-nineteenth century. Hungarian physician Ignaz

Semmelweis(1818–1865) noted the high rate of death from

puerperal fever in women who delivered babies at the Vienna

General Hospital. Moreover, the high death rate was confined to

a ward at which medical residents were present. Another ward,

staffed only by midwives who did not interact with other areas

of the hospital, had a much lower death rate. When the residents

were made to wash their hands in a disinfectant solution prior to

entering the ward, the death rate declined dramatically.

At about the same time, the British surgeon Joseph

Lister (1827–1912) also recognized the importance of

hygienic conditions in the operating theatre. His use of pheno-

lic solutions as sprays over surgical wounds helped lessen the

spread of microorganisms resident in the hospital to the

patient. Lister also required surgeons to wear rubber gloves

and freshly laundered operating gowns for surgery. He recog-

nized that infections could be transferred from the surgeon to

the patient. Lister’s actions spurred a series of steps over the

next century, which has culminated in today’s observance of

sterile or near-sterile conditions in the operating theatre.

Despite these improvements in hospital hygienic prac-

tices, the chance of acquiring a nosocomial infection still

approaches about 10%. Certain hospital situations are even

riskier. For example, the chance of acquiring a urinary tract

infection increases by 10% for each day a patient is equipped

with a urinary catheter. The catheter provides a ready route for

the movement of bacteriafrom the outside environment to the

urinary tract.

The most common microbiological cause of nosocomial

infection is bacteria. The microbes often include both Gram-

negative and Gram-positive bacteria. Of the Gram-negative

bacteria, Escherichia coli, Proteus mirabilis,and other mem-

bers of the family known as Enterobacteriacaea are predomi-

nant. These bacteria are residents of the intestinal tract. They

are spread via fecal contaminationof people, instruments or

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