Gram Positive Bacteria

GRAM-POSITIVE BACTERIA are characterised by having as part of their cell wall structure peptidoglycan as well as polysaccharides and/or teichoic acids. The peptidoglycans which are sometimes also called murein are heteropolymers of glycan strands, which are cross-linked through short peptides.

THE BASIS OF THE MUREIN are chains of alternating residues of N-acetylglucosamine and N-acetyl muramic acid which are Beta -1,4-linked. The muramic acid is a unique substance associated with bacterial cell walls. These chains are cross-linked by short polypetide chains consisting of both L- and D-aminoacids. While in Gram-negative bacteria the peptidoglycan is simple in structure and comparatively uniform throughout most genera, in Gram-positive bacteria there is a very big variation in structure and composition. In general the peptidoglycan is multilayered. There have also been recorded some minor variations in composition in some groups. Thus, in Mycobacterium and Nocardia the N-acetyl moiety of the muramic acid is replaced by the oxydised form N-glycolyl. The amino acid composition of the both the cross-linking as well the stem polypeptides can vary extensively with different groups. These differences form the basis for the taxonomy of these organisms.

THE LIPOPOLYSACCHARIDES of the Gram-positive bacteria have not been extensively studied apart from those of the streptococci, where they are the basis of their serological subdivision. They have also been used in investigating the serological relationships between streptococci, enterococci and lactobacilli. The specific polysaccharides associated with the acid-fast bacteria of the Corynebacterium-Mycobacterium-Nocardia group have also been extensively studied. They are arabinogalactans and arabinomannans linked to mycolic acids. The presence in the cell wall of these genera of arabinose, galactose and meso-diaminopimelic acid is characteristic of these genera.

THE TEICHOIC ACIDS , which are polyols consisting predominantly of glycerol, ribitol and mannitol, are covalently linked to the peptidoglycan through phosphodiester bonds and can be substituted by sugars, aminosugars or D-alanine residues. Some rare teichoic acids lack polyols. Teichoic acids are found in some actinomycetes, bacilli, lactobacilli, listeria and staphylococci.

IN ONLY A LIMITED NUMBER of Gram-positive genera including the Corynebacterium-Mycobacterium-Nocardia group are found the mycolic acids. They are 3-hydroxy acids of high molecular weight with a long alkyl branch in position 2.

Introduction of Bacteria

Bacteria are among the oldest living organisms on Earth, and are very small. Because the bacteria structure is so minute, it can only be seen through a microscope. Bacteria is commonly found in the ground, water and in other living organisms. While some types of bacteria can cause diseases and become harmful to the environment, animals and humans, others offer benefits that we likely could not live without.

Some types of bacteria can attack plants, causing diseases like leaf spot and fireblight.

In human hosts, certain types of bacteria can cause tetanus, pneumonia, syphilis, tuberculosis and other illnesses. As long as the host is not infected with antibiotic resistant bacteria, they can be treated with antibiotics, which kill bacteria or at least hamper their growth. Antiseptics, sterilization and disinfectants can help prevent contamination and risk of infection from bacteria.

The term “friendly bacteria” is used to describe the types of bacteria that offer some benefit. Not only does bacteria help produce the food we eat and keeps the soil fertile, it also helps us digest our food. Bacteria in our digestive system help to convert milk protein into lactic acid and inhibit the growth of potentially harmful bacteria.

About Angiosperm

The flowering plants (angiosperms), also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Together with gymnosperms, they are the only extant groups of seed-producing plants, but they can be distinguished from the gymnosperms by a series of synapomorphies (derived characteristics). These characteristics include flowers, endosperm within the seeds, and the production of fruits that contain the seeds.

The ancestors of flowering plants diverged from gymnosperms around 245–202 million years ago, and the first flowering plants known to exist are from 140 million years ago. They diversified enormously during the Lower Cretaceous and became widespread around 100 million years ago, but replaced conifers as the dominant trees only around 60-100[citation needed] million years ago.


Flowers
The flowers, which are the reproductive organs of flowering plants, are the most remarkable feature distinguishing them from other seed plants. Flowers aid angiosperms by enabling a wider range of adaptability and broadening the ecological niches open to them. This has allowed flowering plants to largely dominate terrestrial ecosystems.

Stamens with two pairs of pollen sacs
Stamens are much lighter than the corresponding organs of gymnosperms and have contributed to the diversification of angiosperms through time with adaptations to specialized pollination syndromes, such as particular pollinators. Stamens have also become modified through time to prevent self-fertilization, which has permitted further diversification, allowing angiosperms eventually to fill more niches.

Reduced male parts, three cells
The male gametophyte in angiosperms is significantly reduced in size compared to those of gymnosperm seed plants. The smaller pollen decreases the time from pollination — the pollen grain reaching the female plant — to fertilization of the ovary; in gymnosperms fertilization can occur up to a year after pollination, while in angiosperms the fertilization begins very soon after pollination. The shorter time leads to angiosperm plants setting seeds sooner and faster than gymnosperms, which is a distinct evolutionary advantage.

Closed carpel enclosing the ovules (carpel or carpels and accessory parts may become the fruit)
The closed carpel of angiosperms also allows adaptations to specialized pollination syndromes and controls. This helps to prevent self-fertilization, thereby maintaining increased diversity. Once the ovary is fertilized, the carpel and some surrounding tissues develop into a fruit. This fruit often serves as an attractant to seed-dispersing animals. The resulting cooperative relationship presents another advantage to angiosperms in the process of dispersal.

Reduced female gametophyte, seven cells with eight nuclei
The reduced female gametophyte, like the reduced male gametophyte, may be an adaptation allowing for more rapid seed set, eventually leading to such flowering plant adaptations as annual herbaceous life cycles, allowing the flowering plants to fill even more niches.

Endosperm
Endosperm formation generally begins after fertilization and before the first division of the zygote. Endosperm is a highly nutritive tissue that can provide food for the developing embryo, the cotyledons, and sometimes for the seedling when it first appears.

These distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans. The major exception to the dominance of terrestrial ecosystems by flowering plants is the coniferous forest.

Bryophyta

Introduction

Diversity of mosses has been classified in approximately 10,000 species, 700 genera, and about 110-120 families. This places the mosses as the third most diverse group of land plants, only after the angiosperms and ferns. Mosses are small plants requiring stereoscopes and compound microscopes for routine examination. The conspicuous green leafy shoots are the gametophytes, haploid organisms, on which the diploid embryo develops into a mature sporophyte (Figure 1). The sporophyte is chlorophyllose and photosynthetic only in early stages of development, and it is mostly dependent on the gametophyte. Moss colonies are a very important element in many ecosystems, from the tundra to the tropical rain forest, reducing soil erosion, capturing water and nutrients, providing shelter for microfauna, and nurseries for seedlings in succession or regeneration processes.

As a lineage, mosses are a historically crucial group in the understanding of the transition to life on land. The green leafy shoots (gametophytes) retain some features of the green algal ancestors (chlorophylls a and b, starch, sperm with two forward undulipodia), but the needle-like shoots that produce the spores (sporophytes) display key innovations for the life outside water, such as stomates, a simple strand of conductive cells [in an unbranched sporophyte], and airborne spores produced in a single apical capsule (sporangium). This is the simplest structural level among all land plants. The next organizational level is found in two fossil groups: Horneophythopsida and Aglaophyton (Rhynia) major, where the sporophyte is branched and produces several sporangia. The sporophyte shows the most complex structural organization in the tracheophytes.
Characteristics

There is no controversy that the mosses are monophyletic. Synapomorphies for the mosses are: i) leaves in the gametophyte; these green laminar organs are attached densely along the shoot. ii) multicellular rhizoids; these branched filaments composed of a series of multiple cells develop from the surface of the gametophyte axis at the point of contact with the substrate. iii) columnella; this is a cylinder of sterile cells located in the center of the capsule. (Mishler & Churchill, 1984). Other possible synapomorphies are features of male gamete [spermatozoid] ultrastructure (Mishler, Lewis & et al 1994).
Click on an image to view larger version & data in a new window
Click on an image to view larger version & data in a new window
Life cycle of a dioicous moss

Bryophyta

Introduction

Diversity of mosses has been classified in approximately 10,000 species, 700 genera, and about 110-120 families. This places the mosses as the third most diverse group of land plants, only after the angiosperms and ferns. Mosses are small plants requiring stereoscopes and compound microscopes for routine examination. The conspicuous green leafy shoots are the gametophytes, haploid organisms, on which the diploid embryo develops into a mature sporophyte (Figure 1). The sporophyte is chlorophyllose and photosynthetic only in early stages of development, and it is mostly dependent on the gametophyte. Moss colonies are a very important element in many ecosystems, from the tundra to the tropical rain forest, reducing soil erosion, capturing water and nutrients, providing shelter for microfauna, and nurseries for seedlings in succession or regeneration processes.

As a lineage, mosses are a historically crucial group in the understanding of the transition to life on land. The green leafy shoots (gametophytes) retain some features of the green algal ancestors (chlorophylls a and b, starch, sperm with two forward undulipodia), but the needle-like shoots that produce the spores (sporophytes) display key innovations for the life outside water, such as stomates, a simple strand of conductive cells [in an unbranched sporophyte], and airborne spores produced in a single apical capsule (sporangium). This is the simplest structural level among all land plants. The next organizational level is found in two fossil groups: Horneophythopsida and Aglaophyton (Rhynia) major, where the sporophyte is branched and produces several sporangia. The sporophyte shows the most complex structural organization in the tracheophytes.
Characteristics

There is no controversy that the mosses are monophyletic. Synapomorphies for the mosses are: i) leaves in the gametophyte; these green laminar organs are attached densely along the shoot. ii) multicellular rhizoids; these branched filaments composed of a series of multiple cells develop from the surface of the gametophyte axis at the point of contact with the substrate. iii) columnella; this is a cylinder of sterile cells located in the center of the capsule. (Mishler & Churchill, 1984). Other possible synapomorphies are features of male gamete [spermatozoid] ultrastructure (Mishler, Lewis & et al 1994).
Click on an image to view larger version & data in a new window
Click on an image to view larger version & data in a new window
Life cycle of a dioicous moss

New discovery at Mercury

July 3, 2008: Mercury's magnetic field is "alive." Volcanic vents ring the planet's giant Caloris Basin. And Mercury has shrunk in on itself more than previously suspected.

These are just a few of the new discoveries by NASA's MESSENGER spacecraft, which flew past Mercury on January 14, 2008. The results are described in a series of 11 papers published in a special July 4th issue of Science magazine.

Six of the papers in Science report studies of the planet's surface--its colors, mineralogy, and the shape of its terrain. For instance, the color enhanced image below reveals evidence of volcanic vents along the margins of Caloris basin, one of the Solar System's largest and youngest impact basins:



Above: A color image of the Caloris basin and adjacent regions. Orange hues just inside the Caloris basin rim mark the locations of features thought to be volcanic. Courtesy of Science/AAAS [Larger image] [more]

"By combining Mariner 10 and MESSENGER data, the science team was able to reconstruct a comprehensive geologic history of the entire Caloris basin interior," says James Head of Brown University, lead author of one of the Science reports. "The basin was formed from an impact by an asteroid or comet during a period of heavy bombardment in the first billion years of Solar System history. As with the lunar maria, a period of volcanic activity followed, producing lava flows that filled the basin interior. This volcanism is responsible for the comparatively light, red material of the interior plains intermingled with [newer] impact crater deposits."

Finding volcanic vents around Caloris resolves an old debate among planetary scientists: Are smooth plains on Mercury, such as the interior of Caloris basin, caused by erupting lava or some other process? Lava has won the day.

Right: Near the rim of Caloris basin, this broad, smooth dome or shield-like feature is interpreted to be a volcano. The bright halo surrounding the kidney-shaped depression is probably an explosive volcanic eruption deposit. Courtesy of Science/AAAS [Larger image] [diagram]

One of the most exciting results announced in Science involves Mercury's magnetic field. Until Mariner 10 discovered Mercury's magnetic field in the 1970s, Earth was the only other terrestrial planet known to have a global magnetic field. Earth's magnetism is generated by the planet's churning hot, liquid-iron core via a mechanism called a magnetic dynamo. Researchers have been puzzled by Mercury's field because its iron core was supposed to have cooled long ago and stopped generating magnetism. Some researchers have thought that the field may have been a relic of the past, frozen in the outer crust.

MESSENGER data suggest otherwise: Mercury's field appears to be generated by an active dynamo in the planet's core. It is not a relic.

"MESSENGER's measurements indicate that, like Earth, Mercury's magnetic field is mostly dipolar, which means it has a north and south magnetic poles," says lead author Brian Anderson of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. "The fact that it is dipolar, and that we did not find the signature shorter-wavelength anomalies that would signify patches of magnetized crust, supports the view that we’re seeing a modern dynamo. We are eager for the October flyby and the year in orbit to see if this is the case elsewhere on the planet and confirm that the field comes from the core."

Right: A crater deformed by a lobate scarp. Click to view more examples. Courtesy of Science/AAAS

Mercury's core makes up 60% of its mass, which is at least twice as large as any other planet. Cooling of this outsized core has led to a remarkable contraction of the planet, revealing itself in the form of cliff-like "wrinkles" called lobate scarps (pictured right). MESSENGER Principal Investigator Sean Solomon, at the Carnegie Institution of Washington, explains:

"The dominant tectonic landforms on Mercury are lobate scarps, huge cliffs that mark the tops of crustal faults that formed during the contraction of the surrounding area. They tell us how important the cooling core has been to the evolution of the surface. After the end of the period of heavy bombardment, cooling of the planet's core not only fuels the magnetic dynamo, but also led to contraction of the entire planet. And the data from the flyby indicate that the total contraction is a least one third greater than we previously thought."

The flyby also made the first-ever observations of charged particles in Mercury's unique exosphere. The exosphere is an ultrathin atmosphere where the molecules are so far apart they are more likely to collide with the surface than with each other. Material in the exosphere comes mainly from the surface of Mercury itself, knocked aloft by solar radiation, solar wind bombardment and meteoroid vaporization:



"MESSENGER was able to observe Mercury's exosphere in three areas—the dayside, the day/night line, or terminator, and its 25,000 mile-long (40,000 km) sodium tail," says author Bill McClintock of the University of Colorado. "Atoms of hydrogen, helium, sodium, potassium, and calcium have been seen in the exosphere, and many other elements almost certainly exist there. These atoms are accelerated away from Mercury by solar-radiation pressure and form a long tail of atoms flowing away from the Sun. But their abundances differ depending on whether it's day or night, effects from the magnetic field and solar wind, and possibly the latitude."

"Mercury's exosphere is remarkably active," he marvels.

Another significant scientific surprise involves Mercury's magnetosphere--the bubble of magnetism surrounding the planet. Thomas Zurbuchen of the University of Michigan explains: "Mercury's magnetosphere is full of many [kinds of charged particles], both atomic and molecular. What is in some sense a 'Mercury plasma nebula' is far richer in complexity and makeup than the Io plasma torus in the Jupiter system." The composition of the nebula doesn't match that of the solar wind, leading researchers to conclude "that this material came from the planet's surface. This observation means that this flyby got the first-ever look at surface composition."

Right: Data from MESSENGERS FIPS sensor reveal the composition of Mercury's plasma nebula. Courtesy of Science/AAAS [Larger image] [more]

"When you look at the planet in the sky, it looks like a simple point of light," remarked MESSENGER Project Scientist Ralph McNutt, of APL. "But when you experience Mercury close-up through all of MESSENGER's 'senses' seeing it at different wavelengths, feeling its magnetic properties, and touching its surface features and energetic particles, you perceive a complex system and not just a ball of rock and metal."

"It's remarkable that this rich lode of data came from two days of imaging, just 30 minutes of sampling the planet's magnetosphere and exosphere, and less than ten minutes carrying out altimetry and collecting other data near the time of its closest approach," adds Solomon. "MESSENGER's flyby was a huge success."

And it was just the beginning. Two more flybys are scheduled for Oct. 2008 and Sept. 2009. Then, MESSENGER will actually go into orbit around Mercury in 2011. Exciting times lie ahead. Stay tuned to Science@NASA for updates.

DNA varient may heavy boozing in team sport

Here’s some not-so-sobering news for party people, barhoppers and clubgoers. Individuals who inherit a particular gene variant that tweaks the brain’s reward system are especially likely to drink a lot of alcohol in the company of heavy-boozing peers.

That’s the preliminary indication of a new study directed by psychology graduate student Helle Larsen of Radboud University Nijmegen in the Netherlands. Adults carrying at least one copy of a long version of the dopamine D4 receptor gene, dubbed DRD4, imbibed substantially more alcohol around a heavy-drinking peer than did others who lacked that gene variant, Larsen’s group reports in a paper published online July 7 in Psychological Science.

“Carriers of the long gene may be more attuned to, and influenced by, another person’s heavy drinking than noncarriers are,” Larsen says.

Her study provides the first evidence that a gene influences human alcohol use in social situations.

Scientists have yet to decipher the precise brain effects of DRD4’s long form. Larsen hypothesizes that in the presence of heavy drinkers, the gene variant may increase dopamine activity in brain areas that amplify alcohol’s appeal as a rewarding social activity.

“If this gene-environment interaction stands, and I don’t see why it shouldn’t, there is every reason to expect the effect would extend to drugs besides alcohol, as well to many motivated pursuits,” remarks biopsychologist Kent Berridge of the University of Michigan in Ann Arbor, who was not involved with the new study.

Sociologist Michael Shanahan of the University of North Carolina in Chapel Hill lauds the new study for ruling out the possibility that carriers of the key gene simply like to drink a lot of booze and tend to do so with other heavy drinkers. Instead, alcohol use jumped among volunteers with a long DRD4 gene who happened to see a stranger imbibe heavily for a brief time.

Larsen and her colleagues asked 60 women and 53 men to evaluate advertisements for an alcohol-abuse prevention campaign. Each volunteer entered a room that had been furnished as a typical Dutch pub, accompanied by a person of the same sex who the volunteer thought was another participant but who was actually working with the researchers.

In between two 10-minute evaluation sessions, volunteers and the researchers’ confederates were given a break. An experimenter asked them to sit at a bar stocked with peanuts, beer, wine, soda and mineral water and to drink whatever they wanted.

As instructed, confederates took the initiative and drank either two sodas, one alcoholic drink and then one soda; or three alcoholic drinks for women and four alcoholic drinks for men over a 30-minute period.

DNA analyses of saliva identified 31 volunteers as carriers of the long DRD4 gene, which contains an amino acid sequence that repeats seven times.

When confederates stuck to sodas or drank one alcoholic beverage, long-gene carriers and noncarriers alike limited themselves to an average of less than half a glass of wine or half a bottle of beer.

When confederates quaffed multiple alcoholic drinks, carriers of the gene variant consumed an average of almost two wine or beer servings, versus almost one serving for noncarriers.

These results held for men and women, all of whom said they drink socially, regardless of how much alcohol they reported drinking weekly.

Deceptive research techniques can backfire if volunteers see through them and don’t admit it to researchers (SN: 6/20/98, p. 394). But when interviewed after testing, none of the participants guessed the study’s real aim or the confederate’s agenda.

Other researchers need to confirm these findings, Larsen says. Some attempts to replicate findings from other studies of gene-environment interactions have yielded mixed results, including follow-up work on a study by researchers from Duke University in Durham, N.C., that found that another gene variant promotes depression in people who experience stress (SN: 7/18/09, p. 10).

DNA varient may heavy boozing in team sport

Here’s some not-so-sobering news for party people, barhoppers and clubgoers. Individuals who inherit a particular gene variant that tweaks the brain’s reward system are especially likely to drink a lot of alcohol in the company of heavy-boozing peers.

That’s the preliminary indication of a new study directed by psychology graduate student Helle Larsen of Radboud University Nijmegen in the Netherlands. Adults carrying at least one copy of a long version of the dopamine D4 receptor gene, dubbed DRD4, imbibed substantially more alcohol around a heavy-drinking peer than did others who lacked that gene variant, Larsen’s group reports in a paper published online July 7 in Psychological Science.

“Carriers of the long gene may be more attuned to, and influenced by, another person’s heavy drinking than noncarriers are,” Larsen says.

Her study provides the first evidence that a gene influences human alcohol use in social situations.

Scientists have yet to decipher the precise brain effects of DRD4’s long form. Larsen hypothesizes that in the presence of heavy drinkers, the gene variant may increase dopamine activity in brain areas that amplify alcohol’s appeal as a rewarding social activity.

“If this gene-environment interaction stands, and I don’t see why it shouldn’t, there is every reason to expect the effect would extend to drugs besides alcohol, as well to many motivated pursuits,” remarks biopsychologist Kent Berridge of the University of Michigan in Ann Arbor, who was not involved with the new study.

Sociologist Michael Shanahan of the University of North Carolina in Chapel Hill lauds the new study for ruling out the possibility that carriers of the key gene simply like to drink a lot of booze and tend to do so with other heavy drinkers. Instead, alcohol use jumped among volunteers with a long DRD4 gene who happened to see a stranger imbibe heavily for a brief time.

Larsen and her colleagues asked 60 women and 53 men to evaluate advertisements for an alcohol-abuse prevention campaign. Each volunteer entered a room that had been furnished as a typical Dutch pub, accompanied by a person of the same sex who the volunteer thought was another participant but who was actually working with the researchers.

In between two 10-minute evaluation sessions, volunteers and the researchers’ confederates were given a break. An experimenter asked them to sit at a bar stocked with peanuts, beer, wine, soda and mineral water and to drink whatever they wanted.

As instructed, confederates took the initiative and drank either two sodas, one alcoholic drink and then one soda; or three alcoholic drinks for women and four alcoholic drinks for men over a 30-minute period.

DNA analyses of saliva identified 31 volunteers as carriers of the long DRD4 gene, which contains an amino acid sequence that repeats seven times.

When confederates stuck to sodas or drank one alcoholic beverage, long-gene carriers and noncarriers alike limited themselves to an average of less than half a glass of wine or half a bottle of beer.

When confederates quaffed multiple alcoholic drinks, carriers of the gene variant consumed an average of almost two wine or beer servings, versus almost one serving for noncarriers.

These results held for men and women, all of whom said they drink socially, regardless of how much alcohol they reported drinking weekly.

Deceptive research techniques can backfire if volunteers see through them and don’t admit it to researchers (SN: 6/20/98, p. 394). But when interviewed after testing, none of the participants guessed the study’s real aim or the confederate’s agenda.

Other researchers need to confirm these findings, Larsen says. Some attempts to replicate findings from other studies of gene-environment interactions have yielded mixed results, including follow-up work on a study by researchers from Duke University in Durham, N.C., that found that another gene variant promotes depression in people who experience stress (SN: 7/18/09, p. 10).

DNA varient may heavy boozing in team sport

Here’s some not-so-sobering news for party people, barhoppers and clubgoers. Individuals who inherit a particular gene variant that tweaks the brain’s reward system are especially likely to drink a lot of alcohol in the company of heavy-boozing peers.

That’s the preliminary indication of a new study directed by psychology graduate student Helle Larsen of Radboud University Nijmegen in the Netherlands. Adults carrying at least one copy of a long version of the dopamine D4 receptor gene, dubbed DRD4, imbibed substantially more alcohol around a heavy-drinking peer than did others who lacked that gene variant, Larsen’s group reports in a paper published online July 7 in Psychological Science.

“Carriers of the long gene may be more attuned to, and influenced by, another person’s heavy drinking than noncarriers are,” Larsen says.

Her study provides the first evidence that a gene influences human alcohol use in social situations.

Scientists have yet to decipher the precise brain effects of DRD4’s long form. Larsen hypothesizes that in the presence of heavy drinkers, the gene variant may increase dopamine activity in brain areas that amplify alcohol’s appeal as a rewarding social activity.

“If this gene-environment interaction stands, and I don’t see why it shouldn’t, there is every reason to expect the effect would extend to drugs besides alcohol, as well to many motivated pursuits,” remarks biopsychologist Kent Berridge of the University of Michigan in Ann Arbor, who was not involved with the new study.

Sociologist Michael Shanahan of the University of North Carolina in Chapel Hill lauds the new study for ruling out the possibility that carriers of the key gene simply like to drink a lot of booze and tend to do so with other heavy drinkers. Instead, alcohol use jumped among volunteers with a long DRD4 gene who happened to see a stranger imbibe heavily for a brief time.

Larsen and her colleagues asked 60 women and 53 men to evaluate advertisements for an alcohol-abuse prevention campaign. Each volunteer entered a room that had been furnished as a typical Dutch pub, accompanied by a person of the same sex who the volunteer thought was another participant but who was actually working with the researchers.

In between two 10-minute evaluation sessions, volunteers and the researchers’ confederates were given a break. An experimenter asked them to sit at a bar stocked with peanuts, beer, wine, soda and mineral water and to drink whatever they wanted.

As instructed, confederates took the initiative and drank either two sodas, one alcoholic drink and then one soda; or three alcoholic drinks for women and four alcoholic drinks for men over a 30-minute period.

DNA analyses of saliva identified 31 volunteers as carriers of the long DRD4 gene, which contains an amino acid sequence that repeats seven times.

When confederates stuck to sodas or drank one alcoholic beverage, long-gene carriers and noncarriers alike limited themselves to an average of less than half a glass of wine or half a bottle of beer.

When confederates quaffed multiple alcoholic drinks, carriers of the gene variant consumed an average of almost two wine or beer servings, versus almost one serving for noncarriers.

These results held for men and women, all of whom said they drink socially, regardless of how much alcohol they reported drinking weekly.

Deceptive research techniques can backfire if volunteers see through them and don’t admit it to researchers (SN: 6/20/98, p. 394). But when interviewed after testing, none of the participants guessed the study’s real aim or the confederate’s agenda.

Other researchers need to confirm these findings, Larsen says. Some attempts to replicate findings from other studies of gene-environment interactions have yielded mixed results, including follow-up work on a study by researchers from Duke University in Durham, N.C., that found that another gene variant promotes depression in people who experience stress (SN: 7/18/09, p. 10).

Morphology of bacteria

A more or less typical bacterium, shown here, is comparatively much simpler than a typical eukaryotic cell. View the transmission electron micrograph of a typical bacterium, E. coli, below and compare it with the diagram above.

Bacteria lack the membrane-bound nuclei of eukaryotes; their DNA forms a tangle known as a nucleoid, but there is no membrane around the nucleoid, and the DNA is not bound to proteins as it is in eukaryotes. Whereas eukaryote DNA is organized into linear pieces, the chromosomes, bacterial DNA forms loops. Bacteria contain plasmids, or small loops of DNA, that can be transmitted from one cell to another, either in the course of sex (yes, bacteria have sex) or by viruses. This ability to trade genes with all comers makes bacteria amazingly adaptible; beneficial genes, like those for antibiotic resistance, may be spread very rapidly through bacterial populations. It also makes bacteria favorites of molecular biologists and genetic engineers; new genes can be inserted into bacteria with ease.
Bacteria do not contain membrane-bound organelles such as mitochondria or chloroplasts, as eukaryotes do. However, photosynthetic bacteria, such as cyanobacteria, may be filled with tightly packed folds of their outer membrane. The effect of these membranes is to increase the potential surface area on which photosynthesis can take place.
The cell membrane is surrounded by a cell wall in all bacteria except one group, the Mollicutes, which includes pathogens such as the mycoplasmas. The composition of the cell wall varies among species and is an important character for identifying and classifying bacteria. In this diagram, the bacterium has a fairly thick cell wall made of peptidoglycan (carbohydrate polymers cross-linked by proteins); such bacteria retain a purple color when stained with a dye known as crystal violet, and are known as Gram-positive (after the Danish bacteriologist who developed this staining procedure). Other bacteria have double cell walls, with a thin inner wall of peptidoglycan and an outer wall of carbohydrates, proteins, and lipids. Such bacteria do not stain purple with crystal violet and are known as Gram-negative.