Strategies for Reducing Virulence andor Influencing Pathogenesis

The general progress in bacterial physiology combined with the rapidly increasing knowledge of the molecular biology of infection and bacterial genomics have given an enormous boost to our understanding of the bacterial infection process. As a consequence we have gained deeper insight into the factors that enable certain bacteria to induce disease in particular hosts, while others behave as innocent bystanders or are even beneficial for the host. The recent literature and the present volume are full of examples about the diverse molecular factors expressed by various pathogens which are required to establish an infection, to promote its further progress, and to cause later sequelae. Based on this almost revolutionary gain in knowledge, and fuelled by the threat of recent resistance developments, the concept of disarming a pathogen of its disease-inducing, disease promoting, and/or disease-worsening factors [93] might be a viable alternative to classical antibiotics, which aim at killing or at least suppressing the growth of the entire pathogen. Of course, like most of the alternative strategies discussed before, a drastic improvement in ultrarapid diagnostic measures is important, as bacterial evolution has invented a bewildering range of diverse virulence and pathogenicity factors, which usually differ strongly between different species and even between distinct strains of the same species. Thus, depending on the exact set of such factors, an E. coli strain may behave as a harmless commensal, an enteropathogen, or might be predisposed to cause uroseptic illness. Additionally, many predisposing host factors are usually equally important in determining whether and to what extent a certain disease is induced or how it will progress. The general immune status of the host, in particular, but also other predisposing factors such as organ abnormalities (e.g., in urinary tract infections) have a decisive role to play. It is also compelling that several important hospital pathogens in today's highly developed countries would have been classified as almost nonpathogenic 50 years before and owe their present role to the immune deficiencies of an ever-aging population and to other treatments that drastically impair immune status, such as aggressive surgery, organ transplantation, and cancer therapy. The strategy of targeting virulence and/or pathogenicity factors carries the additional difficulty of establishing simple in vitro susceptibility tests, comparable to a standard MIC test for classical antibiotics, in order to predict the potential success or failure of the therapy. Thus, because a clinical proof of concept still needs to be demonstrated, the approach has been the subject of both great enthusiasm and strong criticism [94-97].

One often-discussed aspect of targeting virulence and/or pathogenicity factors is the presumed low selection pressure for development of resistance. It should be noted, however, that the experimental basis of this assumption is poor at present, and that, in general terms, mutation frequencies in genes coding for nonessential targets should be much higher than in essential genes, because mutations that interfere with the function of the gene product are also tolerated.

In spite of all these open questions, it is important to follow up this approach and to arrive at experimentally proven conclusions to decide whether and under what circumstances the new treatment paradigm would be useful. It goes without saying that in other important areas of medically oriented microbiological research, such as prophylactic and diagnostic approaches, the value of this strategy has already become obvious. An interesting example of the kind of investigations needed to come closer to an answer to these open questions is provided by a recent industrially sponsored study to exploit the type III protein secretion systems (TTPS) of gram-negative bacteria [98]. TTPS was selected as a virulence target because (a) these systems are present and structurally conserved among many clinically relevant gram-negative species including the special problem pathogen P. aeruginosa, (b) they are not present in eukaryotes, and (c) they are expected to be essential for virulence under in vivo conditions, as they translocate a variety of bacterial effector proteins which interfere with eukaryotic signal transduction into host cells. Potential inhibitors of TTPS were identified in a whole-cell high-throughput screen measuring the secretion of a reporter protein into the medium [99] and were further optimized by standard medicinal chemistry for specificity of interference with TTPS at low micromolar concentrations as well as low general cytotoxicity. A series of substituted azoles and dipeptides were reported to be especially active against TTPS from P. aeruginosa and Salmonella. As expected, in vitro antibacterial activity of the compounds was minimal or absent, but in an in vivo animal model of Pseudomonas sp. murine lung infection one selected compound showed some initial activity. While the compound alone did not influence the course of the disease under the experimental conditions chosen, a combination of the compound with suboptimal doses of ciprofloxacin resulted in slightly better protection of the animals from death than the same (suboptimal) doses of cipro-floxacin alone [100]. However, it must be mentioned that only the time of death was retarded; the death rate (100%) was not reduced. More such experiments are clearly needed to define the potential therapeutic or prophylactic value of such approaches.


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