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In this review, available data on the structure (diversity, abundance, biomass) and functional imprints (bacteriolysis, lysogeny, gene transfers, regulation of prokaryotic diversity) of natural viruses in the context of food webs in aquatic microbial ecology, and the related biogeochemical cycles, are summarized. Viruses are the most abundant, and probably the most diverse, biological entities in aquatic ecosystems and in the biosphere (i.e., viriosphere). Aquatic viruses typically exceed 107 particles/mL in mesotrophic conditions, the majority being represented by phages without tails and by tailed-phages such as members of the family Siphoviridae. Both types of phages have a small capsid and a small genome size, which is considered an evolutionary adaptation to planktonic life. Their contribution to microbial mortality is significant. There is strong evidence that phages exert a significant pressure on the community structure and diversity and on the diversification of potential hosts, mainly through two major pathways: biogeochemical catalysis from lysis products and horizontal gene transfers. In turn, phages are sensitive to environmental factors, both in terms of integrity and of infectivity. Some phages contain typical viral genes that code for biological functions of interest, such as photosynthesis. In general, development in viral ecology is a source of new knowledge for the scientific community in the domain of environmental sciences, but also in the context of evolutionary biology of living cellular organisms, the obligatory hosts for viruses. For example, the recent discovery of a giant virus that becomes ill through infection by another virus (i.e., a viriophage) is fuelling debate about whether viruses are alive. Finally, future research directions are identified in the context of general aquatic ecology, including ecological researches on cyanophages and other phytoplanktonic phages as a priority, primarily in freshwater lakes.

Forty-eight strains of Bacillus thuringiensis and 12 strains of B. cereus were treated with ultraviolet light and mitomycin C. The former agent was the more effective inducer. Bacillus thuringiensis produces at least seven different phage particles with long, non-contractile tails. The frequencies of lysogeny and polylysogeny are 83 and 25% respectively. Morphologically defective phages occur in 25% of strains, whereas five of them produce low molecular-weight bacteriocins. One strain of B. cereus harbors "killer-particles." There is no apparent correlation between the presence of phage-like particles, phage senstivity, and serotypes, biotypes, or the origin of B. thuringiensis strains.

Back to our technique of phage sensitization by protein fixation with the aid of coupling reagents, and of their inactivation by the specific anti-protein antibodies, comparative studies were made on phages having a morphologically different tail. Here we report the results concerning the ultramicorscopic observation of four phages, the efficiency and the rate of their sensitization, and the possibility of their respective neutralization.

Tailed phages of enterobacteria are classified by morphological and serological properties and by physico-chemical parameters of the virion and its nuecleic acid. Twenty-four species are described; they include 250 phages belonging to seven morphological groups. Type species are proposed. About 120 poorly known phages have not been classified. A case of phage evolution, the reliability of data and the value of taxonomical criteria are discussed.

Micrococcus, Staphylococcus, and Streptococcus phages are classified by morphology, serological properties and physical and chemical parameters of the virion and its nucleic acid. Fourteen species are defined; they include 75 tailed phages belonging to 6 morphological groups. Type species are proposed. Three species seem to have phylogenetic relationships with Bacillus phages.

Since the discovery of the first Listeria bacteriophage by Schultz in 1945, more than 219 phages were isolated for L. monocytogenes, L. invanovii, L. innocua, L. seeligeri and L. welshimeri. To date, no phage for L. murrayi and L. grayi is described in the literature. Unless two phages found by Jasińska in a water sample, all the other phages, when the origin is mentioned, were isolated from lysogenic strains, with or without induction. Electron microscopic examination of 55 Listeria phages revealed that two L. innocua phages belonged to the Myoviridae family (tail with contractile sheath) whereas the 53 other phages of L. monocytogenes, L. ivanovii and L. innocua belonged to the Siphoviridae family (tail without contractile sheath). According to the length of the tail, phages of this second group were divided into three species (species 2685, 2671 and 2389). DNA/DNA hybridizations, performed with the method of Southern, after digestion of the DNAs with Eco RI, pointed out three genomic groups: one corresponded to a single phage of Myoviridae and the two others included phages of the Siphoviridae species 2671 and 2685, thus demonstrating a good correlation between ultrastructure and DNA relatedness. Serology of Listeria phages remained unclear. Host range studies showed that these phages are genus specific and allowed to type 34% to 80% Listeria strains. Phage typing of Listeria strains isolated during listeriosis epidemics and cross infections in nursery proved to be useful in order to elucidate epidemiology of this disease. Listeria phages were used to detect Listeria strains in various samples by phage fluorescent antiphage staining system and phage titer increase test. Some phages seem to induce prophylaxis against listeriosis when tested in animal listeric pneumonia and conjunctivitis models. The possible role of phages in the virulence of L. monocytogenes needs further investigations. Bacteriocins of Listeria, called monocins, were first described by Sword and Pickett in 1961. Monocins isolated by Hamon and Péron were resistant to trypsin, inactivated at 50 degrees C and at pH 4,5 and sedimented at low speed, suggesting that these bacteriocins might be in fact defective phages. This was confirmed by Bradley and Dewar by electron microscopic study. Host range of monocins included Listeria, Staphylococcus and Bacillus.

A virulent phage specific for Pseudomonas testosteroni is described. This phage have a regular icosahedral head (52 nm between opposite angles) and a contractile tail (165 X 8 nm) but no fibers on. The buoyant density is 1,51 +/- 0,01 g/ml. The nucleic acid is an desoxyribonucleic acid with a density of 1,696 +/- 0,03 g/ml and a GC% between 33,7 and 39,7.

Fifteen L. monocytogenes phages are isolated from lysogenic strains without induction and are selected for phage-typing. Four of these phages are produced by strains belonging to serotype 1/2a and 1/2b, eight by strains 4b and three other phages respectively by strains 4ab, 4g and 3c. Electron micrographs of 5 of these phages show isometric heads with non contractile tails. They belong to morphological group B of Bradley. Rabbits are immunized with 7 bacteriophages and all the phages are studied by neutralization tests with these 7 antisera. This study seems complex nevertheless it is possible to distinguish four antigenic groups: A, B, C, and D. The lytic spectrum of each phage is usually restricted to strains of the serotype from which that phage is obtained. It appears that a good correlation exists between phage sensitivity and serological type of the strains. With this first set of fifteen phages, 159 of 214 (74,3%) strains of L. monocytogenes can be classified in more than 21 types but it appears that some new phages obtained from strains of serotype 1/2 must be isolated in the future. Our data indicate that these phages can be used as a phage-typing scheme for L. monocytogenes.

All phages studied are tailed, belong to six morphological groups, and are classified by morphological and serological properties and by physico-chemical parameters of the virion and its nucleic acid. Four species of Agrobacterium and 12 species of Rhizobium phages are described, and their relationship with enterobacteria phages discussed. They include 68 viruses, whereas 60 poorly known phages have not been classified.

No morphological differences were found among the 10 Listeria phages suspensions examined. The polyhedric head appears to be about 62-66 nm in diameter, the tail devoided of contractile sheath measures 260-280 X 10 nm. The phage adsorption is observed on the cell-wall of Listeria.