will require constant innovation

While enterococci are always quick to devise mechanisms for developing resistance to novel antibiotics, staphylococci seem to take longer. The differential pace of the development of resistance is fortunate in that enterococci have a tendency to be less virulent than staphylococci, but it only delays the inevitable — sooner or later staphylococci catch up.Cases of methicillin resistant Staphylococcus aureus (MRSA) first arose in the 1980s, plaguing hospital wards and more recently moving out to attack people living in the community. Latest UK statistics show a 2% increase in reports of MRSA between 1999 and 2000.The problem did not really initially cause widespread concern because it was thought the glycopeptide vancomycin could cope. But in May 1996 this assumption came under threat with the first reported case of S. aureus resistant to vancomycin. This first patient was in Japan, and it took a couple of years before one showed up in the USA. Now the problem has also been recorded in the UK. The message is clear. Vancomycin-intermediate S. aureus (VISA) has arrived.Five years later and everyone is still holding their breath. While vancomycin resistance has become firmly established in enterococci, there have still been only a handful of recorded isolates in S. aureus. "Fortunately there have not been any reported cases where isolates have spread from one patient to another," says Alan Johnson of the Public Health Laboratory Service in London. "Whether that is due to the skill and efficiency of infection control teams, or whether these strains are not good at spreading, we just don't know. That's not to say that this time next week the story may have changed."Intriguingly' it appears that the molecular mechanism of resistance to vancomycin in staphylococci is different to the vanA and vanB gene-mediated glycopeptide resistance in enterococci. Data presented at the American Society for Microbiology earlier this year showed that resistance in S. aureus is multifactorial, and not simply due to acquisition of genes from enterococci. In some cases there are alterations in cell wall synthesis, suggesting that S. aureus generate multiple false targets. In others production of penicillin-binding proteins and a thickening of the cell wall accompanies the development of glycopeptide resistance.The uncertainty about the future trajectory of resistance to glycopeptides is fueling the drive to find novel antimicrobial agents. Linezolid seemed likely to become a candidate compound as it attached to the central loop of domain V of the 23S ribosomal subunit of the bacterial ribosome. While enterococci quickly evolved to evade it, developing resistance during clinical trials, staphylococci were again slow on the uptake. That was until Mary Jane Ferraro and colleagues at Beth Israel Deaconess Medical Center, Boston, Massachusetts, reported in 21 July 2001 Lancet that they had found a S. aureus isolate with high-level resistance to linezolid (Lancet 2001, 358:207-208).Once more the need for more novel agents became apparent.With the Sanger Centre in Cambridge recently posting a complete copy of the genome of MRSA in the public domain there is the possibility that this could trigger a new phase of research. Tim Foster, Professor of Molecular Microbiology at Trinity College Dublin, who worked on the sequencing team, does not, however, believe the immediate impact of this public sequence will be that great, pointing out that most antibiotic development is carried out by the large pharmaceutical companies who already had their own versions of the data.Identifying the weak points in S. aureus genes should, however, be very useful and is the approach being taken by Microcide Pharmaceuticals Inc, in Mountain View, California. Using rich medium and a temperature of 43°C they have identified 286 strains of S. aureus that can require the support of an additional gene if they are going to grow in these unfavorable conditions. This has revealed a total of 96 genes that are essential to the in vitro growth of S. aureus. Associate Director of Discovery Biology Scott Hecker says that many of these show distinct promise as new targets for antibiotics.Another approach is to use a protein-based strategy. An exponent of this is Cameron Macdonald, CEO of PanTherix, a technology start-up company in Glasgow, Scotland. They use gene knock-out experiments to validate new potential targets, clone the gene and express its protein, generating enough so they can fully characterize its three dimensional structure and its functionality.Standard screening techniques have been used to test the protein against a battery of compounds to see if they give even the remotest interaction between chemicals and the protein target. "If we can get enough low level hits we can build up a picture of the sort of chemical we need to design to inhibit the function of the enzyme," said Macdonald. They then use an iterative process to build novel best-fit chemicals with anti-bacterial properties. The hope is that the more novel the compound, the greater the chance that bacteria will find it difficult to develop resistance against it.PanTherix is currently working on the chorismate pathway, a key pathway in the formation of essential amino acids in bacteria. "We have one target in that pathway that we have fully characterized. There are currently no antibiotics that address this pathway and we think that we are the only company in the world that has this particular enzyme structure," said Macdonald. They are already working on at least one specific inhibitor.Another pathway of interest is that involving bacterial Co-enzyme A. This is a pathway that most antibiotic developers have avoided because there is a mammalian equivalent. "We have shown at a genetic level that the two pathways are very different – while they operate in a similar fashion the actual structure of the enzymes are sufficiently different that we can design compounds that selectively target the bacterial pathway," said Macdonald, who now has two targets of interest within that pathway.Macdonald's approach enables his team to look at the older targets in a novel way. The company is also looking for new targets in the folate pathway, a pathway that is currently affected by antibiotics such as the sulphonamides and trimethoprim.There is no single answer to the problems of bacterial infection. "What we need to do is give the clinician a new tool kit and keep looking ahead and not sit back once you have found something because the likelihood is that resistance will develop so you have to come up with something else — a never ending battle," concluded Macdonald.

will require constant innovation

resistant to vancomycin and now linezolid illustrates the urgent need for novel antibiotic design strategies.

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