Prior observations of phage-host systems in vitro have led to the conclusion that susceptible host cell populations must reach a critical density before phage replication can occur. Such a replication threshold density would have broad implications for the therapeutic use of phage. In this report, we demonstrate experimentally that no such replication threshold exists and explain the previous data used to support the existence of the threshold in terms of a classical model of the kinetics of colloidal particle interactions in solution. This result leads us to conclude that the frequently used measure of multiplicity of infection (MOI), computed as the ratio of the number of phage to the number of cells, is generally inappropriate for situations in which cell concentrations are less than 10 ⁷ /ml. In its place, we propose an alternative measure, MOI _actual , that takes into account the cell concentration and adsorption time. Properties of this function are elucidated that explain the demonstrated usefulness of MOI at high cell densities, as well as some unexpected consequences at low concentrations. In addition, the concept of MOI _actual allows us to write simple formulas for computing practical quantities, such as the number of phage sufficient to infect 99.99% of host cells at arbitrary concentrations.

The emergence and increasing prevalence of multidrug-resistant bacterial pathogens emphasizes the need for new and innovative antimicrobial strategies. Lytic phages, which kill their host following amplification and release of progeny phage into the environment, may offer an alternative strategy for combating bacterial infections. In this study, however, we describe the use of a nonlytic phage to specifically target and deliver DNA encoding bactericidal proteins to bacteria. To test the concept of using phage as a lethal-agent delivery vehicle, we used the M13 phagemid system and the addiction toxins Gef and ChpBK. Phage delivery of lethal-agent phagemids reduced target bacterial numbers by several orders of magnitude in vitro and in a bacteremic mouse model of infection. Given the powerful genetic engineering tools available and the present knowledge in phage biology, this technology may have potential use in antimicrobial therapies and DNA vaccine development.

Over the past few years, increasing evidence has accumulated to implicate infectious agents in the etiology of systemic sclerosis (SSc) and Raynaud phenomenon. Infection rates in patients with SSc compared with those in control populations do not provide clear support for any specific pathogen. However, increased antibody titers, a preponderance of specific strains in patients with SSc, and evidence of molecular mimicry inducing autoimmune responses suggest mechanisms by which infectious agents may contribute to the development and progression of SSc. Here we review studies examining the potential involvement of Helicobacter pylori, cytomegalovirus, and parvovirus B19 in SSc pathogenesis.

Autographa californicaM nucleopolyhedrovirus (AcMNPV) is the prototypical member of theNucleopolyhedrosisgenus of theBaculoviridae, a family of large, double-stranded DNA viruses that are highly diverse. Nucleocapsid morphogenesis of AcMNPV and others in theNucleopolyhedrovirusgenus takes place within the nuclei of infected host cells. Previously, we showed that filamentous actin (F-actin) is essential for this process to occur in AcMNPV-infected cells, an unprecedented finding for a DNA virus that replicates within the nucleus. Because of the fundamental importance of this requirement to our understanding of virus–host interactions, and because of the diversity of viruses included within theNucleopolyhedrovirusgenus, we were compelled to determine whether the replication of other nucleopolyhedroviruses was also F-actin dependent. We report here that progeny virus production of six other lepidopteran nucleopolyhedroviruses, representing both phylogenetic groups I and II within the genus, is also F-actin dependent. The six viruses studied (Spodoptera frugiperdaMNPV,Bombyx moriNPV,Orgyia pseudotsugataMNPV,Lymantria disparMNPV,Anticarsia gemmatalisMNPV andHelicoverpa zeaSNPV) were unable to produce progeny in the presence of either cytochalasin D or latrunculin A, two actin-binding agents that interfere with F-actin-dependent processes but differ in their modes of action. F-actin-dependent progeny morphogenesis, therefore, appears to be a characteristic common among viruses in this genus that have lepidopteran hosts.