In the 1880s the German scientist Franz Hofmeister showed that some ions were more effective that others in causing proteins to precipitate. For example, chloride ions are more effective than nitrate ions in precipitating egg proteins. Hofmeister went on to show the existence of a "sequence of effectiveness" by which the same ions were more effective for a whole range of chemical and biological processes.

"Hofmeister's experiments on the relative effectiveness of different salts on the precipitation of proteins stand in the scheme of things as Mendel's did to genetics", write Pierandrea Lo Nostro and colleagues at the University of Florence and the Australian National University, "except that Mendel's experiments are understood and Hofmeister's are not."

In the latest experiments Lo Nostro and colleagues cultivated two types of bacteria, Staphylococcus aureus and Pseudomonas aeruginosa, in solutions that contained varying concentrations of different salts including sodium chloride, fluoride, bromide, iodide, thiocyanate, nitrate and acetate.

Lo Nostro and co-workers measured the growth rate of the microorganisms and found that the bacteria grew more quickly in almost all dilute salt solutions. However, at higher concentrations, some ions continued to cause an increase in bacterial growth while others significantly slowed down or even inhibited the process.

The scientists say that the results can be explained by two mechanisms that involve disruption of the hydrogen-bonding network in water and the absorption of salts at interfaces. For instance fluoride ions readily break hydrogen bonds, which reduces bacterial growth rates, while chloride atoms do not break bonds, which leads to higher growth rates. The different amounts of absorption influence the growth rate through the ions activating or deactivating the enzyme sites responsible for the growth of the bacteria.

This latter mechanism could be used in biological and medical applications to enhance or reduce cell processes as required. "For example, moderately concentrated salt solutions could be used as antimicrobial agents and to switch cellular mechanisms on or off in a reversible way," says Lo Nostro.