By Dean Haycock
Special To BioWorld Today
Bacteria have been around for 3 billion years or so. Homo sapiens go back approximately 200,000. With that much more life experience, should we be surprised that bacteria are outwitting our efforts to destroy them?
"Virtually everything we can develop to work on microorganisms eventually is going to get evaded except for things like acids and heat. The organism — sooner or later — is going to get around anything we are going to use against it therapeutically," said Roger Cunningham, director of the Center of Immunology at the University of Buffalo School of Medicine, in Buffalo, N.Y.
First, we learned that germs develop resistance to antibiotics. Now there is proof that bacteria as a group may be no more inconvenienced by our efforts to kill them with antibacterial soaps than with antibiotics.
The first evidence that germs develop resistance to a widely used ingredient in antibacterial soaps and other products appears in the Aug. 6, 1998, issue of Nature in a letter entitled "Triclosan targets lipid synthesis." Triclosan is found in many soaps, lotions and even fabrics and plastics, in the home.
There was a widespread assumption that the active ingredients in antibacterial products kill bacteria by nonspecific mechanisms such as disruption of their cell membranes. In this sense, chemicals such as triclosan were viewed much like alcohol.
"It would be very unusual for an organism to become resistant to alcohol. That is such a fundamental attack [ the bacterial membrane] and there is no alternative to that membrane," Cunningham said.
Like alcohol, triclosan kills a wide range of microorganisms, including different types of bacteria and fungi. This led some scientists to conclude that its mechanism of action was nonspecific. That would make it unlikely for bacteria to develop resistance to it. But triclosan, and perhaps other biocides, are not alcohol. It now seems that such compounds have specific biochemical targets. Such specificity means that germs may have the potential to develop specific mutations that confer resistance.
Specific Resistance Detected
Laura McMurry, senior research associate at the Tufts University School of Medicine, in Boston, and her colleagues previously showed bacteria can develop resistance to pine oil. "They were also resistant to other antibiotics," McMurry told BioWorld Today.
This was all to a rather low degree, however. Their latest research depicts a more striking microbial adaptation to human attempts at hygiene. They found that bacteria develop specific resistance to triclosan just as they do to antibiotics.
The Boston researchers began by isolating five independent triclosan-resistant mutants of Escherichia coli bacteria. Using cloning techniques, they showed that one gene, fab1, was linked to triclosan resistance. Fab1 encodes an enzyme, enoyl-acyl carrier protein reductase, which plays a crucial role in the synthesis of fatty acids. Fatty acids are important components of fat-soluble molecules in microbial as well as plant and animal cells. Sequencing studies showed that any of three mutations, involving the substitution of amino acids at positions 93, 159 or 203 in the fab1 gene product, can account for triclosan resistance.
These three positions line the cleft in the enzyme at which an enzymatic cofactor, NADH, normally binds. The mutations that confer triclosan resistance may do so by interfering with the interaction between the enzyme and the cofactor it needs to function properly.
The specificity of triclosan's antibacterial effects is indicted by results of experiments measuring lipid synthesis in E. coli cells. Triclosan inhibited fatty acid synthesis, and subsequently lipid synthesis, in non-resistant cells by 92 percent. In resistant cells, the same concentration of triclosan inhibited synthesis by only 2 percent. The results support the suggestion that triclosan targets the reductase enzyme encoded by the fab1 gene.
Research May Indicate New Targets
The results raise the possibility that other "nonspecific biocides" might also have specific targets. So far, however, no other chemicals have been tested. But by pointing to a specific mechanism of action, the work could point to new targets for developing other bacteria-killing compounds.
"You might be able to devise a 'modified triclosan,'" McMurry suggested.
The results also raise the possibility that biocide-resistant bacteria might emerge in this new era of antibiotic resistant bacteria. Recently, "superbugs," germs resistant to vancomycin and other antibiotics that physicians depended on to knock out bacteria resistant to more common antibiotics, have appeared. This microbial trend threatens to return infectious disease treatment to the pre-antibiotic era unless new antibiotics are developed.
"It (triclosan) is a perfectly usable material. Now we will have to rethink how it is used," Cunningham said, "Since there are bacteria resistant to it, you can't use it use it as a panacea."
McMurry notes that triclosan may be overused and is not necessarily needed.
"It seems kind of whimsical to add these things to give maybe a little bit of antibacterial activity where we really don't need to kill off all the bacteria that surround us. Unless you are working in a hospital or have a sick person at home, just for everyday life, soap is fine." *