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)

A possible role for bacteria in the pathogenesis of insulin dependent diabetes mellitus

The hypothesis proposed is that insulin dependent diabetes mellitus (IDDM) is caused by an autoimmune response precipitated by common bacteria which have antigens in common with pancreatic beta cells. If the bacteria are enteric, but IDDM arises when first exposure occurs in the nasopharynx following a viral respiratory tract infection, then the various epidemiological features of IDDM can be explained. These include a latent interval between disease onset and clinical presentation, seasonal variation in presentation, an increased incidence in temperature climates and technologically advanced countries, and the lack of an effect of incidence on age distribution. The hypothesis points to new avenues of research and should be pursued.

Relation of streptococcal M protein with human and rabbit tropomyosin: the complete amino acid sequence of human cardiac alpha tropomyosin, a highly conserved contractile protein

Partial sequences of group A streptococcal M proteins exhibit up to 50% sequence identity with segments of rabbit skeletal tropomyosin. It is well recognized that rheumatic fever and rheumatic heart disease in humans are sequelae of group A streptococcal infection. To examine whether the human cardiac tropomyosin would exhibit greater homology with the streptococcal M proteins, we have now determined its complete amino acid sequence. The amino acid sequence of human cardiac tropomyosin was established from sequence analyses of its peptides derived by enzymic and chemical cleavages, and comparison of these sequences to the reported sequence of rabbit skeletal tropomyosin. These studies have revealed that the amino acid sequence of human cardiac alpha tropomyosin is identical to that of the rabbit skeletal alpha tropomyosin, but for a single conservative substitution of Arg/Lys at position 220. This observation increases the significance of the previously observed sequence homology between streptococcal M protein and rabbit skeletal tropomyosin and may have relevance to the pathogenesis of rheumatic fever. Furthermore, these results rank tropomyosin as one of the most highly conserved contractile proteins between vertebrate species reported thus far.

Autoimmunity: a decision theory model

Concepts from statistical decision theory were used to analyse the detection problem faced by the body's immune system in mounting immune responses to bacteria of the normal body flora. Given that these bacteria are potentially harmful, that there can be extensive cross reaction between bacterial antigens and host tissues, and that the decisions are made in uncertainty, there is a finite chance of error in immune response leading to autoimmune disease. A model of ageing in the immune system is proposed that is based on random decay in components of the decision process, leading to a steep age dependent increase in the probability of error. The age incidence of those autoimmune diseases which peak in early and middle life can be explained as the resultant of two processes: an exponentially falling curve of incidence of first contact with common bacteria, and a rapidly rising error function. Epidemiological data on the variation of incidence with social class, sibship order, climate and culture can be used to predict the likely site of carriage and mode of spread of the causative bacteria. Furthermore, those autoimmune diseases precipitated by common viral respiratory tract infections might represent reactions to nasopharyngeal bacterial overgrowth, and this theory can be tested using monoclonal antibodies to search the bacterial isolates for cross reacting antigens. If this model is correct then prevention of autoimmune disease by early exposure to low doses of bacteria might be possible.

Sequence homology of group A streptococcal Pep M5 protein with other coiled-coil proteins

Group A streptococcal Pep M5 protein, an antiphagocytic determinant of the bacteria, is an alpha-helical coiled-coil molecule, and exhibits significant sequence homology with tropomyosin and myosin, but to a lesser degree with other coiled-coil proteins. Moreover, Pep M5 is more homologous to myosin than to tropomyosin, and the homologies are more numerous between the C-terminal domain of the Pep M5 protein and the S2 fragment of myosin. The C-terminal domain of the Pep M5 protein exhibits extensive sequence identity with the C-terminal region of Pep M6 molecule, another M protein serotype. Thus, regions within two M protein serotypes are homologous to the S2 region of the myosin molecule. These observations are consistent with the immunological findings of other investigators and thus may explain some of the previously reported immunological cross-reactions between antigens of the group A streptococcus and mammalian heart tissue.

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.