Microbiology and Immunology

Mesophilic bacteria WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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specific setup of DNA bases upstream of the initiation site in

order for transcriptionto occur. If the number of bases was

increased or reduced from what was required, the genewould

not be transcribed. Meselson also found that there were par-

ticular DNA sequences that could be recombined or moved

around within the entire chromosome of DNA. These move-

able segments are termed transposons. Transposons, when

inserted into particular sites within the sequence, can either

turn on or turn off expression of the gene that is near it, caus-

ing mutationswithin the fly. These studies contributed to the

identity of particular regulatory and structural features of the

fruit fly as well as to the overall understanding of the proper-

ties of DNA.

Throughout his career as a scientist, Meselson has writ-

ten over 50 papers published in major scientific journals and

received many honors and awards for his contributions to the

field of molecular biology. In 1963, Meselson received the

National Academy of Science Prize for Molecular Biology,

followed by the Eli Lilly Award for Microbiology and

Immunologyin 1964. He was awarded the Lehman Award in

1975 and the Presidential award in 1983, both from the New

York Academy of Sciences. In 1990, Meselson received the

Science Freedom and Responsibility Award from the

American Association for the Advancement of Science.

Meselson has also delved into political issues, particularly on

government proposals for worldwide chemical and biological

weapon disarmament.

See alsoMicrobial genetics; Transposition

MMesophilic bacteriaESOPHILIC BACTERIA

Mesophiles are microorganisms such as some species of

Bacteria, Fungi, and even some Archaeathat are best active at

median temperatures. For instance, bacterial species involved

in biodegradation (i.e., digestion and decomposition of organic

matter), which are more active in temperatures ranging from

approximately 70° - 90°F (approx. 15°–40°C), are termed

mesophilic bacteria. They take part in the web of micro-organic

activity that form the humus layer in forests and other fertile

soils, by decomposing both vegetable and animal matter.

At the beginning of the decomposition process, another

group of bacteria, psychrophylic bacteria, start the process

because they are active in lower temperatures up to 55°F

(from below zero up to 20°C), and generate heat in the

process. When the temperature inside the decomposing layer

reaches 50–100°F, it attracts mesophilic bacteria to continue

the biodegradation. The peak of reproductive and activity of

mesophilic bacteria is reached between 86–99°F (30–37°C),

and further increases the temperature in the soil environment.

Between 104–170°F (40–85°C, or even higher), another group

of bacteria (thermophyllic bacteria) takes up the process that

will eventually result in organic soil, or humus. Several

species of fungi also take part in each decomposing step.

Mesophilic bacteria are also involved in food contami-

nationand degradation, such as in bread, grains, dairies, and

meats. Examples of common mesophilic bacteria are Listeria

monocytogenes, Pesudomonas maltophilia, Thiobacillus nov-

ellus, Staphylococcus aureus, Streptococcus pyrogenes,

Streptococcus pneumoniae, Escherichia coli, and Clostridium

kluyveri.Bacterial infections in humans are mostly caused by

mesophilic bacteria that find their optimum growth tempera-

ture around 37°C (98.6°F), the normal human body tempera-

ture. Beneficial bacteria found in human intestinal flora are

also mesophiles, such as dietary Lactobacillus acidophilus.

See alsoArchaeobacteria; Bacteria and bacterial infection;

Biodegradable substances; Composting, microbiological

aspects; Extremophiles

MMetabolismETABOLISM

Metabolism is the sum total of chemical changes that occur in

living organisms and which are fundamental to life. All

prokaryotic and eukaryotic cells are metabolically active. The

sole exception is viruses, but even viruses require a metaboli-

cally active host for their replication.

Metabolism involves the use of compounds. Nutrients

from the environment are used in two ways by microorgan-

isms. They can be the building blocks of various components

of the microorganism (assimilation or anabolism). Or, nutri-

ents can be degraded to yield energy (dissimilation or catabo-

lism). Some so-called amphibolic biochemical pathways can

serve both purposes. The continual processes of breakdown

and re-synthesis are in a balance that is referred to as turnover.

Metabolism is an open system. That is, there are constant

inputs and outputs. A chain of metabolic reactions is said to be

in a steady state when the concentration of all intermediates

remains constant, despite the net flow of material through the

system. That means the concentration of intermediates

remains constant, while a product is formed at the expense of

the substrate.

Primary metabolism comprises those metabolic

processes that are basically similar in all living cells and are

necessary for cellular maintenance and survival. They include

the fundamental processes of growth (e.g., the synthesis of

biopolymers and the macromolecular structures of cells and

organelles), energy production (glycolysis and the tricar-

boxylic acid cycle) and the turnover of cell constituents.

Secondary metabolism refers to the production of substances,

such as bile pigments from porphyrins in humans, which only

occur in certain eukaryotic tissues and are distinct from the

primary metabolic pathways.

Metabolic control processes that occur inside cells

include regulation of geneexpression and metabolic feedback

or feed-forward processes. The triggers of differential gene

expression may be chemical, physical (e.g., bacterial cell den-

sity), or environmental (e.g., light). Differential gene expres-

sion is responsible for the regulation, at the molecular level, of

differentiation and development, as well as the maintenance of

numerous cellular “house-keeping” reactions, which are

essential for the day-to-day functioning of a microorganism.

In many metabolic pathways, the metabolites (substances pro-

duced or consumed by metabolism) themselves can act

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