Saturday, October 27, 2007

Don't Overlook the GOOD Bacteria...

We wouldn’t want students to think that ALL bacteria are bad – in truth, most of the thousands of different species of bacteria in our bodies are harmless or even beneficial. Bacteria living within our bodies contribute to such important functions as maturation of the human immune system, the synthesis of vitamins and the digestion of foodstuffs. It may not be a good idea to try to wipe them all out, as an article in Newsweek this week points out. (“Caution: Killing Germs May be Hazardous to Your Health”, Newsweek Oct. 29, 2007, pp. 44-48). It’s an easy read, well worth your students' time, and it might serve as a counter-balance to the constant barrage of news about truly bad bacteria such as MRSA (see post on Oct. 26).

Friday, October 26, 2007

The Rise of a Drug-Resistant "Superbug"

A drug-resistant strain of “superbug” called MRSA (methicillin-resistant Staphylococcus aureus) has been in the news recently. MRSA is transmitted by skin-to-skin contact or by sharing infected items such as towels or sports equipment. It generally enters the skin through cuts and abrasions, causing boils and painful abscesses in the skin. However, if it enters the bloodstream it can attack internal organs and kill the patient.

MRSA was a rare hospital-based infection until the 1990s. Then it began to show up in prisons, locker rooms and gyms, and poor urban communities. It is now the most common cause of soft tissue and skin infections seen in hospital emergency rooms (“Methicillin-resistant S. aureus infections among patients in the emergency department”, N. Engl. J. Med. 355:666-674, 2006). It is often misdiagnosed and is very difficult to treat; more than half of all patients who come to emergency rooms with MRSA infections are treated with antibiotics that don’t kill it.

Last week, scientists at the Centers for Disease Control and Prevention reported that MRSA may be even more prevalent and more dangerous than previously thought. They estimate that MRSA may contribute to more deaths per year in this country than AIDS (“Invasive Methicillin-Resistant Staphylococcus aureus Infections in the United States”, JAMA 298:1763-1771, Oct. 17, 2007).

Also last week, the news media reported the death of Ashton Bonds, a 17-year-old high-school senior, due to a MRSA infection that had spread to his liver, kidneys, and lungs. As a result of his death, all 21 public schools in Bedford County, Virginia were shut down briefly for cleaning and disinfection.

Epidemiologists worry that the rise of MRSA is the natural outcome of the over-use of antibiotics, which only encourages bacteria to evolve toward antibiotic resistance (for more on the subject of antibiotic resistance, see p. 18 of Human Biology, 5th ed.) This bug will be hard to kill. Keep an eye on it.

Wednesday, October 24, 2007

Global Warming and Loss of Species

Many scientists believe that the Earth is losing animal and plant species at a more rapid rate than in past millennia, in part because of the effects of humans on complex ecosystems. For example, human activities such as building cities and highways, clearing and tilling land for food crops, and diverting water resources for our own use all affect the habitats of specific species. But now scientists report that humans may also increase the rate of species extinctions through our effect on carbon dioxide emissions and global warming.

In an article published online in Proceedings of the Royal Society. B, Biological Sciences (doi:10.1098/rspb.2007.1302), British ecologists report that four out of five periods of mass extinctions of species over the last 520 million years coincided with periods of warmer world climate. And according to climate scientists, Earth is now on track to reach similarly warm temperatures within 100 years unless global emissions of carbon dioxide are reduced soon. Fully 20-30% of all species assessed so far “are likely to be at risk of extinction” if temperatures rise just 3-4 degrees Fahrenheit, according to the highly respected and authoritative Intergovernmental Panel on Climate Change (IPCC).

A big issue of the century will be what, if anything, to do about it.

Thursday, October 11, 2007

A Function for the Appendix?

It has long been thought that the small blind-ended pouch near the end of the small intestine called the appendix is vestigial, i.e. it no longer serves any useful function in humans. But now scientists at Duke University hypothesize that the appendix may indeed still have a function. In a September 7 article online in the Journal of Theoretical Biology, the researchers speculate that the appendix serves as a reservoir (or safe haven) for “good” bacteria in the event that a severe bout of diarrhea flushes out most of the bacteria in the small and large intestines. After the diarrhea has passed, according to the scientists, the bacteria in the appendix could emerge to repopulate the intestines.

It’s an interesting thought, but this is speculation, not yet proved. The Journal of Theoretical Biology is a vehicle for ideas, not hard scientific data. My question would be if the appendix does serve a useful function, why don’t most animals have one? (Apparently only humans, rabbits, and two species of marsupials have an appendix.) An admittedly equally speculative explanation might be that while the appendix does harbor bacteria that help repopulate the gut after a bout of diarrhea in humans, that function is not so significant that it confers significant survival value any more. In other words, the appendix ultimately still may be considered vestigial.

The concept of survival value of a given structure or function is an important one for students to understand when discussing evolution, of course.

Monday, October 8, 2007

Breast Cancer Around the World

The cover story in this week’s issue of Time magazine (Oct. 15) is about the worldwide incidence of breast cancer. The article discusses how the economic situation in different countries affects the likelihood of screening for breast cancer and the probability that the cancer will be adequately treated once it is detected. The reasons for a failure to detect and treat breast cancers are different in different countries, but they include a shortage of doctors, diagnostic equipment, and x-ray film; the high cost of treatment relative to income; and the inability to reach a medical facility because of the lack of transportation.

Aside from the usual genetic influences, students might be interested to learn that cultural differences also influence the diagnosis and treatment of breast cancer. The article points out that in some countries, women are afraid to admit to having breast cancer out of fear that their husbands will leave them or that their daughters will be shunned by prospective marriage partners. Diet, too, has a significant effect, for reasons pointed out in the article. And finally, there is just plain ignorance about the disease – in one instance, a woman who had an obviously-visible tumor admitted that she had not been diagnosed earlier because it didn’t hurt. Clearly, the fight against breast cancer will need to be fought differently from country to country.

Monday, October 1, 2007

DNA Evidence Exonerates the Innocent

More than 300 persons convicted of rape or murder between 1989 and 2003 were later exonerated by new DNA or other evidence that proved their innocence, according to a study by a University of Michigan Law School professor (Samuel R. Gross, et. al. Exonerations in the United States 1989-2003. Journal of Criminal Law & Criminology 95:523-560, 2005.) In most cases their wrongful convictions were at least partially due to false identification by eyewitnesses. How many persons were erroneously convicted or even put to death in the past? We may never know.

The certain knowledge (with the help of DNA evidence) that mistakes were made in the past is fueling widespread reform of our criminal justice system, and thousands of innocent convicts may have their second day in court as a result. Its also prompting legal changes, including requirements in many states that confessions be recorded (to reduce the number of false confessions), and that informants’ testimony be corroborated before it can be used in a courtroom. States are also tightening up their procedures for how witnesses identify suspects in lineups and in photos.

On page 468 of Human Biology 5th ed. I describe the scientific basis for how a DNA sample is used to identify a particular individual. It's nice to see that its being used to protect the innocent, as well as convict the guilty.