Antibiotics, which kill bacteria, are no longer called miracle drugs because of complications in their use.

Bacteria are complex bits of life, complex enough to have mechanisms that allow them over time to develop resistance to their antibiotic killers. The hope of researchers now is that man's ingenuity in using the old antibiotics and creating new ones will avert the ultimate disaster in bacterial disease: the time when bacteria are immune to antibiotics. Some strains of bacteria are now resistant to five antibiotics, and resistance is spreading.

Swedish scientists reported last week that they have produced a new antibiotic made synthetically rather than by the usual method of extracting natural cell-killing chemicals from living things, such as the penicillium mold.

The new class of synthetic antibiotics, reported in the June 25 issue of the journal Nature, uses a "Trojan horse" strategy to kill a variety of bacteria called "Gram-negative."

The strategy of the researchers from the Astra Alab pharmaceutical company in Sodertalje, Sweden, was to prevent the bacteria from making molecules needed to construct an outer skin.

To do so, it was necessary to design a chemical that would get inside the bacterial cell, a difficult matter because the cell's outer membrane allows few molecules through. The researchers found that they could attach a toxic chemical to a small molecule that is normally allowed through the outer walls.

Even after the molecule is inside, the bacterial cell does not recognize the ruse and quickly goes about its usual business of breaking the incoming molecules into useful bits. But when the Trojan molecule is broken up, it releases the toxic part of the molecule that quickly gums up important manufacturing processes that make the substances, called lipopolysaccharides, necessary to maintain the bacterium's outer skin.

This new mechanism may soon give rise to a whole class of antibiotics of similar design.

The first use will be to combat infection by Gram-negative bacteria, which represent a large class of sometimes difficult-to-treat infections. Many of the common infections caught in hospitals are of the Gram-negative type and are often resistant to common antibiotics.

But the design could also deliver a variety of different substances to the inside of certain cells, giving the system great potential as a method of delivering herbicides discretely or chemotherapy agents directly into cancer cells.