Public Service Review: - Issue 14
An anti-bac alert
25 February 2008
As resistance to antibiotics spreads, Professor Peter W Taylor urges the conversion of research into new drugs if infection is to be fought.
February 2009 sees the bicentenary of Charles Darwin, whose profound insights into evolution via genetic variation and natural selection have become the central organising principle in biology.
Darwin would have understood that the rapid increase in resistance of human pathogens to antimicrobial agents is perhaps the most prescient and well documented example of his evolutionary principles in action. The early widespread optimism that antibiotics would banish infectious disease to the 'dustbin of history' has proven to be premature and infections remain the second leading cause of mortality worldwide.
The reasons for the failure to defeat the threat from infection are complex: socio-cultural change, political and economic upheavals, and environmental damage all provide new opportunities for the spread of infectious agents; 'new' infectious agents with the capacity to cause novel infections seem to emerge with depressing regularity; therapeutic advances have resulted in new groups of individuals, such as transplant patients, susceptible to microbes that do not normally cause disease. The biggest threat, however, comes from the emergence of bacteria refractory to currently useful antibiotics: the evolution of antibacterial drug resistance has resulted in significant increases in mortality of hospitalised patients, increased the length of hospital stays and dramatically increased the costs of treatment.
From the perspective of evolutionary biology, the 'Darwinian' selective pressure applied to bacterial populations by antibiotics is extreme: the target cell population either survive or are killed – there is little incremental change. Bacteria possess effective mechanisms for mobilising and disseminating resistance genes, sometimes to distantly related species, and conventional antibacterial chemotherapy leads inevitably to the emergence of drug resistance. Most antibiotics inhibit or kill bacteria by disrupting biosynthetic pathways; such discrete modes of action present the bacteria with an opportunity to bypass the susceptible metabolic step, to prevent the antibiotic reaching its target or to produce an enzyme that breaks down the antibiotic before it can inhibit cellular processes, and many such mechanisms have evolved through the heavy selective pressure of antibiotic overuse.
As resistance to antibiotics continues to spread, new antibacterial drugs with novel modes of action are desperately needed in order to continue the fight against infection. Unfortunately, the upturn in incidence of drug resistance has coincided with a marked reduction by pharmaceutical companies in efforts to develop new agents. When the industry was last surveyed, of over 500 molecules in development, only five were antibiotics. By current commercial criteria, antibiotics are not profitable in comparison to other classes of drugs and the in-built redundancy of antibiotics due to resistance reinforces this view.
Can we devise fundamentally different ways to treat infection? Traditionally, we look at infection as invasion of the host by pathogens that overwhelm the body's defences, but this is a simplistic view. Even 'professional' pathogens such as Staphylococcus aureus, a bacterium equipped with an impressive array of virulence effectors for damaging the host, requires a reduction in the host's defences in order to colonise, enter tissue and cause infection. In the words of the pioneering microbiologist René Dubos: “Most bacteria are only opportunistic invaders of tissues already weakened by crumbling defences.” We are attempting to correct this imbalance between host and pathogen by either shoring up the host's defences or modifying the bacteria at the site of infection to render them less able to survive in the host – making them, in Darwinian terms, 'less fit' through alteration of the phenotype. Bacteria survive, multiply and cause infection because they are either virulent (producing factors that enable them to survive and cause the symptoms of disease) or resistant to antibiotics (producing factors that enable them to survive the onslaught of antibiotic therapy). Therapeutic agents that modify the properties of pathogens in vivo but do not kill them should apply far less selective pressure on bacterial populations in comparison to conventional antibiotics.
We have established that this concept of 'modification of the bacterial phenotype' can lead to rapid and effective resolution of experimental lethal infections. Many pathogenic bacteria produce hydrated, negatively charged polysaccharide capsules external to the cell wall; these structures confer resistance to immune mechanisms such as engulfment by phagocytes and killing by complement. Encapsulated bacteria cause infections as diverse as pneumonia, osteomyelitis, septic arthritis, pyelonephritis and anthrax. Often, the capsule is the major determinant of survival of the pathogen in the host. We surmised that removal in situ of the capsule could lead to resolution of the infection, as the bacteria are converted to the immune susceptible phenotype. We employed an enzyme from a bacteriophage (bacterial virus) that selectively destroys the protective capsule surrounding strains of Escherichia coli K1: these bacteria are frequently recovered from neonates with bacterial sepsis and meningitis, infections with high mortality and morbidity. The enzyme was administered systemically to neonatal rats with K1 bacteraemia and meningitis, and single small doses of the recombinant enzyme were able to protect the animals from the neuropathogen. This concept of selective capsule depolymerisation as a means of therapy is currently being extended to other infections caused by encapsulated bacteria.
Can we modify drug resistant bacteria to prevent the expression of resistance determinants? Such an approach could revive the use of older antibiotics that have become less effective due to the emergence and spread of resistance. There is little doubt that compounds that restore the sensitivity of major pathogens to established antibiotics would be invaluable therapeutic aids and could have significant impact on the cost of treatment. With this in mind, we investigated the capacity of components of Japanese green tea to modulate the expression of proteins critical for the determination of ß-lactam resistance in S. aureus. Resistance to methicillin and other ß-lactam drugs arises in MRSA isolates due to the acquisition of a modified penicillin binding protein (PBP2a) to which ß-lactams bind only poorly. Epicatechin gallate (ECg), a major component of green tea, is able to sensitise MRSA to ß-lactam antibiotics by a subtle mechanism that involves insertion of the modifier into the bacterial membrane, a process that compromises the synthesis of the cell wall that is in part mediated by PBP2a. Further, insertion of ECg into the membrane inhibits the export of a variety of toxins and other proteins that MRSA requires in order to cause infection. Thus, ECg compromises both drug resistance and bacterial virulence through a common mechanism and we are designing derivatives of ECg that have pharmacologically acceptable properties.
Natural products may also provide tools to increase the efficacy of cellular immune functions should they compromise the capacity of the host to ward off infection. We have found that root extracts of plants belonging to the genus Pelargonium (geraniums) contain compounds with activities against Mycobacterium tuberculosis. Extensive use of these extracts in Southern African native medicine has indicated efficacy against a range of respiratory conditions. Extracts contain a number of compounds that have little or no direct mycobactericidal capacity but are able to induce macrophages to engulf and kill the tubercule bacillus, raising the possibility that such compounds may stimulate the removal of the pathogen from the lungs of infected patients by a mechanism that is unlikely to be selected for drug resistance. New TB drugs are badly needed and such mixtures, given as supplements to conventional chemotherapy, could find a role in the therapy of a disease that is notoriously difficult to control. The challenge remains to convert these promising laboratory approaches into effective medicines to preserve the efficacy of our dwindling antibacterial armamentarium in a way that would meet with the approval of 'Darwinists' such as myself.