Towards New Antibiotics

Bacterial resistance to antibiotics is on the increase worldwide. This major threat to human health is driving scientists to search for new antibiotics and inhibitors of antibiotic-degrading enzymes. At least five projects financed under FP5 (Quality of Life) are using genetically engineered micro-organisms and the cell-factory approach to develop new or modified antibiotics and inhibitors. In addition, researchers will seek out clean and environmentally friendly synthesis procedures.

Discovery of a new kind of drug

When Alexander Fleming, returning to his Oxford laboratory after summer vacations in 1928, discovered that one of his bacterial colonies had been invaded by a mould, he did not realise the consequences of this ‘accident’. He misunderstood the phenomenon and, after some research, gave up. About the same time, at Rutgers University the French researcher René Dubos was studying the activities of an enzyme that removed the polysaccharide coating of bacteria. Although this enzyme was never used on humans, he proved that the rational approach to chemotherapy using microbiological techniques could yield stunning results. In 1939, he published a first report on the isolation of a bacteria-attacking microbe, Bacillus brevis, which produced gramicidin, a natural substance able to kill bacteria. Gramicidin was never used in humans because of its toxicity, but it proved effective in animals as, in fact, the first antibiotic.
Howard Florey and Ernst Chain picked up Fleming’s discovery (he named the mould penicillin) and developed it into the first antibiotic used in humans (1941). Contrary to the sulphates or metal-based drugs used until then, antibiotics were not toxic to humans and only attacked specific bacterial strains.

The development of antibiotics

Antibiotics are produced by colonies of mould, i.e. micro-fungi such as the Streptomyces and Streptococci, which engineer these products to defend themselves against bacteria. Today, more than 5 000 antibiotic substances are known. The former problems of continuous production and stability have been solved, making antibiotics cheap products with safe supplies. The cultures are produced in large bioreactors (fermenters) containing up to 200 000 litres. Normally, the
cultured strains are genetically modified to guarantee high yields at high degrees of purity by maintaining culture stability as long as possible. World production now exceeds 30 000 tonnes, with a total market value of $24 billion (1999).
The first class of antibiotics, based on enzymatic activity, was followed in 1944 by a second one, composed mainly of amino acids: the peptide antibiotics. In the meantime, several other
classes have followed: methicillins, vancomycins, aminoglycosides, macrolides, cephalosporins, quinolones, lipopeptides, glycopeptides, etc. They are all based on a mere 15 compounds, such as the beta-lactams to which penicillin and the cephalosporins belong. All currently used antibiotics were introduced between 1940 and 1962 then, after a gap of 38 years, the new class of oxazolidinones followed in 2000. These newcomers function by blocking protein synthesis in bacteria.

The threat to antibiotics

The concept of evolution stipulates that selective pressure favours adaptive behaviour. It could have been anticipated, therefore, that bacteria, too, would adapt to antibiotics by developing resistance. In fact, between 1990 and 1998 the numbers of reports of bacterial resistance increased from 30 000 to 50 000. Resistance development was helped considerably by the fact that since 1962 until recently only modifications of existing antibiotic classes had been launched. Bacteria resistance to one product could more easily adapt to the whole class. In the US alone, 50 to 60% of nosocomial infections involve antibiotic-resistant bacteria, adding at least $4-5 billion to the cost of healthcare.
This threat pushed researchers to search for completely different compounds able to attack bacteria in ways not yet exploited by man: fluoroquinolones, quinopristin, dalfopristin, linezolid, ketolides and glycylcyclines. Interesting compounds have been found in animals – for example, various groups of antimicrobial peptides – such as magnainin in frogs.
An alternative is the search for specific genes in the sequenced genomes of major pathogenic
bacteria. If the exact sequence of a gene coding for key processes in the microbial metabolism could be found, it might be possible to engineer an inhibitor molecule. Bacteria would then face greater difficulty in developing resistance to such a new weapon. Experts estimate that genomics has already given us about 500 to 1 000 new broad spectrum antibacterial targets. In addition, bacteriophages (bacterium-invading viruses) are making a comeback and can be of significant help for a few specific applications.

An integrated approach

Bacteria attack classical antibiotics, such as the beta-lactams, with specific enzymes which destroy the key part of the drug. A promising approach has been the combining of such antibiotics with a so-called ‘guardian-angel’ compound that neutralises the beta-lactamase by inhibition. Clavulanic acid is one such compound. Until now, relatively little resistance has been observed in the combination therapy. Unfortunately, clavulanic acid is ineffective in protecting the cephalosporins, frequently used in hospitals. The beta-lactamase inhibitors already developed have not yet progressed to clinical use because of the high cost of production.
One of the projects dealing with antibiotic synthesis and currently receiving funding under FP5 is using the cell factory approach (TNA = Towards New Antibiotics, QLK3-2000-00513). This approach aims at using engineered micro-organisms to provide efficient routes for the production of templates for modification into antibiotics and beta-lactamase inhibitors. It also seeks clean synthesis routes to existing antibiotics to avoid costly and environmentally unfriendly production procedures. Furthermore, it intends to make possible the large-scale production of broad-spectrum beta-lactamase inhibitors. This integrated approach could help us to win the next round of the fight against antibiotic-resistant bacteria.

Other projects are:

• GENOVA Glycosylation engineering for novel antibiotics (QLK3-1999-00095)
  
• SANITAS screening for compounds able to inhibit bacterial cell division (QLK3-2000-00079)
  
• CYANOMYCES Generation of therapeutics by combining genes from actinomycetes and cyanobacteria (QLK3-2000-00131)
  
• BAS ANTI-MICROBIALS Developing novel anti-bacterials by targetting bacterial cell death (QLK3-2001-00277)

CO-ORDINATOR
Prof. Christopher J. Schofield
The Oxford Centre for Molecular Sciences and The Dyson Perrins Laboratory
Oxford University
Oxford OX1 3QY - UK
Tel: +44 1865 275625
E-mail: christopher.schofield@chem.ox.ac.uk