Balancing hygienization and anaerobic digestion of raw sewage sludge.

The anaerobic digestion of raw sewage sludge was evaluated in terms of process efficiency and sludge hygienization. Four different scenarios were analyzed, i.e. mesophilic anaerobic digestion, thermophilic anaerobic digestion and mesophilic anaerobic digestion followed by a 60 °C or by an 80 °C hygienization treatment. Digester performance (organic matter removal, process stability and biogas yield) and the hygienization efficiency (reduction of Escherichia coli, somatic coliphages and F-specific RNA phages) were the main examined factors. Moreover, a preliminary economical feasibility study of each option was carried out throughout an energy balance (heat and electricity). The obtained results showed that both thermophilic anaerobic digestion and mesophilic anaerobic digestion followed by a hygienization step were able to produce an effluent sludge that fulfills the American and the European legislation for land application. However, higher removal efficiencies of indicators were obtained when a hygienization post-treatment was present. Regarding the energy balance, it should be noted that all scenarios have a significant energy surplus. Particularly, positive heat balances will be obtained for the thermophilic anaerobic digestion and for the mesophilic anaerobic digestion followed by 60 °C hygienization post-treatment if an additional fresh-sludge/digested sludge heat exchanger is installed for energy recovery.

Gene 19 of plasmid R1 is required for both efficient conjugative DNA transfer and bacteriophage R17 infection.

F-like plasmids require a number of genes for conjugation, including tra operon genes and genes traM and traJ, which lie outside the tra operon. We now establish that a gene in the "leading region," gene 19, provides an important function during conjugation and RNA phage infection. Mutational inactivation of gene 19 on plasmid R1-16 by introduction of two nonpolar stop codons results in a 10-fold decrease in the conjugation frequency. Furthermore, infection studies with the male-specific bacteriophage R17 revealed that the phage is not able to form clear plaques in Escherichia coli cells carrying an R1-16 plasmid with the defective copy of gene 19. The total number of cells infected by phage R17 is reduced by a factor of 10. Both the conjugation- and infection-attenuated phenotypes caused by the defective gene 19 can be complemented in trans by introducing gene 19 alleles encoding the wild-type protein. Restoration of the normal phenotypes is also possible by introduction of the pilT gene encoded by the unrelated IncI plasmid R64. Our functional studies and similarities of protein 19 to proteins encoded by other DNA transfer systems, as well as the presence of a conserved motif in all of these proteins (indicative for a putative muramidase activity) suggest that protein 19 of plasmid R1 facilitates the passage of DNA during conjugation and entry of RNA during phage infection.

Reversible pressure dissociation of R17 bacteriophage. The physical individuality of virus particles.

In the absence of urea, pressures up to 2.5 kbar promote only 10% dissociation of the whole particles of R17 bacteriophage. In the presence of concentrations of urea between 1.0 and 5.0 M, pressure promotes complete, reversible dissociation of the virus particles. At the lower urea concentrations reversible dissociation of R17 virus particles shows no dependence on protein concentration indicating a high degree of heterogeneity of the particles, but higher urea concentrations, 2.5 to 5.0 M, result in progressive restoration of the protein concentration dependence of the pressure dissociation. At still higher urea concentrations, 5.0 to 8.0 M, irreversible dissociation of virus takes place at atmospheric pressure. In contrast, the dissociation of the isolated dimers of the capsid protein was dependent on protein concentration to the extent predicted for a stochastic equilibrium, and dimers were much less stable than the whole virus both to dissociation by pressure or urea. In contradistinction, the reversible whole-virus dissociation observed at urea concentrations below 2.5 M appears to be a typical deterministic equilibrium, without appreciable dynamic exchange between whole particle and subunits during the lengthy experiments. The experiments demonstrate that the "thermodynamic individuality" of the virus particles arises in conformational differences in the assembled viruses, and that there is a direct relation between the stability of the particles and their heterogeneity.

The ecophysiology of bacteriophage infecting halophilic archaebacteria

An ecophysiological study of halophages, isolated from the Yallahs Salt Ponds, Jamaica, has been performed to determine (i) what effect, if any, the single most important environmental parameter, salinity, has on phage-bacterium interactions, and (ii) which phage functions and phage-bacterium interactions are most likely to be of long term significance to the co-existence of halophage and halobacteria in nature. Results indicate that the NaCl concentration governs the interactions between halophages and the extremely halophilic strains of halobacterium. Phage growth is invariably attentuated at high salinity. A more detailed study on a single phage isolate, S5100, shows that at high salinities the maturation of phage is repressed, and lytic phage infections give way to persistent infections. In most instances, the frequency of productive infections initiated by each phage on each sensitive host strain is a consistent function of the host rather than the phage. Closer inspection revealed that the intracellular stages of the phage infection process have a higher degree of host specificity than the cellular receptor sites. It has been demonstrated that mutations to higher virulence and/or extended host range occur in the morphological group A1 phages. Phage absorption is implicated as the stage of the phage infection process that is affected by these mutations. The group B1 halophage, S45, is restricted and modified in vivo by strains of Halobacteriums. Three strain-specific activities have been observed. Two of these occur in the same bacterial strain and appear to be due to different kinds of enzymatic activities. One of the restriction specificities is shown to be associated with a strain-specific endonuclease active on unmodified halobacterial DNA. These phenomena that have been observed are discussed with respect to possible implications for phage ecology and evolution (AU)