Microbial parasites adapted to thrive at mammalian mucosal surfaces have evolved multiple times from phylogenetically distant lineages into various extracellular and intracellular life styles. Their symbiotic relationships can range from commensalism to parasitism and more recently some host–parasites interactions are thought to have evolved into mutualistic associations too. It is increasingly appreciated that this diversity of symbiotic outcomes is the product of a complex network of parasites–microbiota–host interactions. Refinement and broader use of DNA based detection techniques are providing increasing evidence of how common some mucosal microbial parasites are and their host range, with some species being able to swap hosts, including from farm and pet animals to humans. A selection of examples will illustrate the zoonotic potential for a number of microbial parasites and how some species can be either disruptive or beneficial nodes in the complex networks of host–microbe interactions disrupting or maintaining mucosal homoeostasis. It will be argued that mucosal microbial parasitic diversity will represent an important resource to help us dissect through comparative studies the role of host–microbe interactions in both human health and disease.

A DNA polymerase activity has been detected and characterized in crude extracts from tachzoites of Toxoplasma gondii. The enzyme has a sedimentation coefficient of 6·4 S, corresponding to an approximate molecular weight of 150000 assuming a globular shape. Like mammalian DNA polymerase α, the DNA polymerase of T. gondii was sensitive to N-ethylmaleimide and inhibited by high ionic strength. However, the enzyme activity was not inhibited by aphidicolin which is an inhibitor of mammalian DNA polymerases α, δ and ε and also cytosine-β-D-arabinofuranoside-5′-triphosphate which is an inhibitor of α polymerase. The activity was inhibited by 2′,3′-dideoxythymidine-5′-triphosphate which is an inhibitor of mammalian DNA polymerase β and γ. Magnesium ions (Mg2+) were absolutely required for activity and its optimal concentration was 6 mM. The optimum potassium (K+) concentration was 50 mM and a higher concentration of K+ markedly inhibited the activity. Activity was optimal at pH 8. Monoclonal antibodies against human DNA polymerase did not bind to DNA polymerase of T. gondii. Thus the T. gondii enzyme differs from the human enzymes and may be a useful target for the design of toxoplasmacidal drugs.

Throughout evolution, enzymes and their metabolites have been highly conserved. Parasites are no exception to this and differ most markedly by the absence of metabolic pathways that are present in the mammalian host. In general, parasites are metabolically lazy and rely on the metabolism of the host both for a supply of prefabricated components such as purines, fatty acids, sterols and amino acids and for the removal of end-products. Nonetheless, parasites are metabolically highly sophisticated in that (1) they retain the genetic capacity to induce many pathways, when needed, and (2) they have developed complex mechanisms for their survival in the host. Certain unique features of the metabolism of trypanosomes, leishmania, malaria and anaerobic protozoa will be discussed. This will include (1) glycolysis and electron transport with reference to the unique organelles: the glycosome and the hydrogenosome, (2) purine salvage, pyrimidine biosynthesis and folic acid metabolism and (3) polyamine and thiol metabolism with special reference to the role of the unique metabolite of trypanosomes and leishmanias, trypanothione.

The complete intracellular cycle of the Leishmania mexicana mexicana G. S. strain was quantified in human macrophages and in the mouse IC-21 macrophage line utilizing a culture system that allows the direct observation of individual intracellular parasites. A wide range of pre-replicative lag periods exists, implying that promastigotes may be in any phase of their DNA synthetic cycle when phagocytosed by the macrophage. Amastigotes replicated 2–3 times, after which the host cell died and liberated amastigotes that were taken up by other macrophages and continued to replicate. The mean amastigote population-doubling time in human macrophages (17.5 h) was not statistically different from promastigotes growing in axenic culture (16.4 h), but was nearly 2-fold less than amastigotes growing in mouse-derived IC-21 macrophages (33.7 h). These observations are markedly different from cover-glass culture assays of Leishmania-macrophage interactions and provide an unambiguous description of the intracellular cycle of Leishmania mexicana mexicana.

Proteolytic activities of 5 strains of Leishmania (Viannia) braziliensis isolated from Brazilian and Colombian patients, presenting distinct clinical manifestations, were characterized and compared using whole-promastigote extracts and extracellular secretions. Zymographic assays concerning whole-cell extracts and supernatants resulted in the detection of high molecular weight bands, ranging from 50 to 125 kDa. Proteolytic activities from both whole-cell extracts and supernatants were optimal in a pH range 5·5 to 9·0 for all analysed strains. Such protease activities were inhibited when 10 mM 1,10-phenanthroline was assayed, strongly suggesting that the enzymes responsible for hydrolysis of the substrate belong to the metalloproteases class. Distinct profiles of metalloproteases were observed among the studied L. (V.) braziliensis strains. Differences among the microorganisms might be related to the geographical origin of the strains and/or to the clinical presentation.

Endocytosis is the process by which a cell ingests material (solute, paniculate, living cells) and encloses it in a membrane-limited vacuole, the endosome. Protozoa may ingest or they may become ingested. In the latter case they may serve as food for the ingesting cell or they may, as parasites, survive and multiply at the expense of the ingesting cell. The present symposium was concerned with two topics: (1) endocytosis by free living protozoa, and (2) endocytosis of parasitic protozoa. In both cases the initial result of endocytosis is the formation of an endosome, whereas the further fate of the ingested contents differs (Fig. 1); in the first instance the contents are digested by the action of hydrolytic (lysosomal) enzymes and in the second instance the ingested parasite avoids digestion and becomes adapted to parasitism.