The revival of interest in mechanisms of bacterial pathogenicity

1. After a long barren period, the study of bacterial pathogenicity is now one of the most popular subjects in microbiology. This is because bacterial diseases remain a major problem in public health despite the advent of antibiotics, and the subject is a fertile field for the application of genetics and molecular biology. 2. Pathogenicity is a multifactorial property. The biological requirements are abilities to: infect mucous surfaces; enter the host through those surfaces; multiply in the environment of the host; interfere with host defences; and damage the host. Each requirement has many facets all of which can be accomplished by a variety of processes. 3. The molecular determinants of the five requirements for pathogenicity can be identified and the relation between their structure and function obtained by a seven step procedure. Genetic manipulation and observations on organisms grown in vivo play major roles in this procedure. Other vital aspects are the availability of good animal models and the design of biological tests for virulence determinants in vitro that are pertinent to the situation in vivo. 4. A survey of the state of studies on bacterial pathogenicity has highlighted some areas of immense erudition and exposed others that need more attention in the future. Research is often at the highest level of molecular biology for: adherence to and entry of epithelial cells; interference with humoral and phagocytic defences; toxins; and direct induction of cytokines and inflammation. The major gaps are: the determinants of competition with commensals on mucous surfaces; spread into deeper tissues; the host supplied nutrients and metabolism underlying growth rate in vivo; the determinants of interference with the immune response in important chronic diseases and carrier states; the determinants of immunopathological reactions that cause damage in chronic disease; and the determinants of change from carrier to invasive state. Areas that are receiving some attention but are worthy of more are: moving through mucus to gain access to mucous surfaces; opportunistic infections; the determinants of mixed infections; and the determinants of host and tissue susceptibility to infection. 5. Current interest in the regulation of production of virulence determinants and the influence on it of environmental factors has raised speculation on the role these factors play in vivo. However, it has not yet provided much information on the host factors specifically involved in particular bacterial infections. The individualistic concept of community, as a relative latecomer to discussions of animal community, is sometimes misconstrued as holding that communities are random assemblages of organisms without biotic interactions among species. Nevertheless, it has increasingly been accepted as supported by studies of diverse taxa and habitats. However, many other ecologists continue to argue for integrated, biotically controlled and evolved communities.(ABSTRACT TRUNCATED AT 400 WORDS)

Inactivation of the alpha-haemolysin gene of Staphylococcus aureus 8325-4 by site-directed mutagenesis and studies on the expression of its haemolysins

S. aureus strain 8325-4 was shown to produce alpha-, beta-, delta- and gamma-haemolysins by haemolytic assays and immunoblotting. Hybridization experiments indicated that a single copy of the alpha-haemolysin gene (hla) resides in the chromosome. Site-directed mutagenesis was used to inactivate the hla gene. This gene, which had previously been cloned in E. coli, was inactivated in vitro by inserting a fragment carrying an erythromycin resistance marker. Shuttle plasmids were constructed and transformed into 8325-4 and non-haemolytic recombinants enriched by a plasmid incompatibility technique. A previously isolated Tn551 insertion defective in alpha-haemolysin was not located in hla. It had pleiotropic defects in expression of alpha-, beta- and delta-haemolysins. Expression of alpha-haemolysin from a plasmid-located hla gene was very low. In contrast, hla-erm mutants were deficient only in alpha-haemolysin and allowed high level expression of the plasmid-borne hla gene. The Tn551 insertion is probably located in a gene encoding a positive regulatory element required for expression of several exoproteins. An hla-erm mutant was less virulent than the otherwise isogenic 8325-4 hla+ strain in a mouse peritonitis model, confirming that alpha-haemolysin is an important virulence factor.

The biochemical challenge of microbial pathogenicity

In the past decade there has been a revival of interest in microbial pathogenicity. The reasons for this revival are two-fold. First, infectious disease is still with us despite the impact of the antibiotic era; for example, the rise of bacterial and fungal infections in compromised patients and the lack of a good general antiviral drug. Second, the subject of microbial pathogenicity is ripe for application in techniques of biochemistry, molecular biology and genetics that have developed in other areas of biology over the past twenty years; and the potential of these techniques is particularly attractive to young people, who are entering the field in increasing numbers. In this lecture I shall survey the methods and difficulties of investigating microbial pathogenicity and what we know of the main aspects of the subject at the molecular level. I shall use bacteria as examples because more is known about them than other types of microbes. Lack of space prevents quoting original papers in such a wide-ranging task; in most cases reference is made to authoritative reviews.