Salt: The First Antibiotic - Antibiotics in Action

Salt: The First Antibiotic

Salt being traded in Mali.

Every chemistry student knows salt is sodium chloride, NaCl. No one knows when people began using salt, because salt was used long before people started writing things down. Even so, salt is mentioned in the Chinese publication Peng-Tzao-Kan-Mu, written about 2700 BC. This work contains the earliest known treatise on pharmacology. In it is found a large section devoted to the details of more than forty kinds of salt.

Since salt is a biological necessity of human life, the trade in salt was very important in ancient times. Sometimes salt was even used as money. Marco Polo of Venice, Italy (1254?-1324?), traveler to East Asia, took note of the well organized Chinese salt trade in which salt was shipped up the Yangtze River to the interior of China from coastal cities. Among other techniques and sources, the Chinese developed equipment to pump brine from wells drilled in salt beds. In the Medieval era, the business of salt extraction, transport, and trade formed an important base for political power as well as personal wealth. In West Africa on the southern edge of the Sahara desert, empires were built on the salt trade. In some trading cities in this region (like the famous Timbuktu) salt could be traded ounce-for-ounce for gold. European salt mining centers gave names to towns and cities still in existence today. The names of cities in Austria and aouthern Germany like Salzburg, Hallein, Hallstatt, and Bad Reichenhall all contain either of two German words, salz or hall, both meaning "salt." During the United States Civil War, part of the Union's military strategy included the capture and/or destruction of sources of salt for the Confederate army.

Today, there are remnants of the once global salt trade network in African countries like Mali and Niger. Salt is obtained either from shallow mines or isolated from soil. The latter involves mixing salt-containing soil with water in large pans. Salt is a polar and ionic compound, so it dissolves in the water. But the soil, made up mostly of nonploar compounds, does not dissolve. When the soil settles out, the salty water is scooped out of the large pans into smaller pans. The water evaporates, leaving the salt behind. The salt crystals are collected, then transported across the desert by camel. Obviously this labor intensive and harsh occupation is dying out as the next generation finds alternative occupations away from these isolated villages.

Our language still contains words and phrases that have their origins in the early history of salt usage. For example, Roman soliders received salt as part of their wages. This salt was called salarium argentum, meaning “salt money” in Latin. The phrase eventually became the English word “salary.” A far-flung trade in ancient Greece involving the exchange of salt for slaves gave rise to the expression “not worth his salt.”

In addition to being necessary for human health, salt early on became an important food preservative. Although the mechanism by which salt keeps perishable foods (particularly meat) from decaying was not understood until the early 1800s, people knew from experience that salt helped them store food for long periods of time. This was obviously important in places where winters are long and fresh food isn't available all year round, or humid places where food spoiled quickly. In arid climates, food can preserved by drying. But where the humidity is higher, fungus and bacteria can rapidly destroy stored food. Even where food can be stored in ice during a winter, it quickly rots when it thaws in the spring. Documents from Sweden in the 1500s clearly show how severe the problem of food preservation was and what strategies were used in ensuring adequate supplies of meat and grains for upcoming winter and early spring.
Epilogue: Salt, Bacteria, and Decay

We have seen how salt, toxic in high concentrations to food-spoiling bacteria, has been used as a preservative. In some of the activities that accompany this introduction (lab activity, Cell Membranes) you will be able to see how it is that salt can act as a preservative, how it actually prevents biological activity. The decay of dead plants and animals is fundamentally important for recycling chemical elements and compounds in the biosphere. One organism's waste is another's food! Without bacteria and other microorganisms, higher forms of life including human beings could not exist. Elements and compounds would be forever “locked up” in the remains of once-living things, unable to be used by the next generation.

While we want bacteria to decompose the dead animals and plants that die in the forest, we don't want bacteria to decompose the dead plants and animals that we plan to eat! The bacteria that efficiently rid the world of dead organisms also can cause disease if people eat rotting food, especially rotting meat. In this module we will trace the history of drugs that counter bacteria, so called “antibiotics” in pharmaceutical language. While salt was good at keeping bacteria from attacking meat, it's not really useful for treating human disease. For the story of how we came up with ways to fight disease-causing bacteria once they're already in the body, keep reading!
For more information, at other Web Sites...

Salt of the Earth — a brief look at the modern-day salt trade in the West African nation of Mali, part of On the Line's Virtual Journey of Mali.
The Road to Timbuktu — learn more about the African kingdoms whose wealth was built on the salt trade, part of Henry Louis Gates, Jr.'s Wonders of the African World from PBS online.
Timbuktu — a short history of the city that was built on the salt trade, with links to several other Web sites, by Lynn Garry Salmon.

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References

The Importance of Salt — from the University of California at Davis.

History of Salt — from the Salt Institute.

Image credit

Salt being traded in Mali: Courtesy Dr. Galen R. Frysinger, www.galenfrysinger.com.

Copyright ©2002 The Chemical Heritage Foundation