Subversion of the actin cytoskeleton during viral infection

Viral infection converts the normal functions of a cell to optimize viral replication and virion production. One striking observation of this conversion is the reconfiguration and reorganization of cellular actin, affecting every stage of the viral life cycle, from entry through assembly to egress. The extent and degree of cytoskeletal reorganization varies among different viral infections, suggesting the evolution of myriad viral strategies. In this Review, we describe how the interaction of viral proteins with the cell modulates the structure and function of the actin cytoskeleton to initiate, sustain and spread infections. The molecular biology of such interactions continues to engage virologists in their quest to understand viral replication and informs cell biologists about the role of the cytoskeleton in the uninfected cell.     

Subversion of phosphoinositide metabolism by intracellular bacterial pathogens

Phosphoinositides are short-lived lipids, whose production at specific membrane locations in the cell enables the tightly controlled recruitment or activation of diverse cellular effectors involved in processes such as cell motility or phagocytosis. Bacterial pathogens have evolved molecular mechanisms to subvert phosphoinositide metabolism in host cells, promoting (or blocking) their internalization into target tissues, and/or modifying the maturation fate of their proliferating compartments within the intracellular environment.     

Dynamic remodeling of the actin cytoskeleton: Lessons learned from Listeria locomotion

The bacterial pathogen Listeria monocytogenes displays the remarkable ability to reorganize the actin cytoskeleton within host cells as a means for promoting cell-to-cell transfer of the pathogen, in a manner that evades humoral immunity. In a series of events commencing with the biosynthesis of the bacterial surface protein ActA, host cell actin and many actin-associated protein self-assemble to from rocket-tail structures that continually grow at sites proximal to the bacterium and depolymerize distally. Widespread interest in the underlying molecular mechanism of Listeria locomotion stems from the likelihood that the dynamic remodeling of the host cell actin cytoskeleton at the cell's leading edge involves mechanistically analogous interactions. Recent advances in our understanding of these fundamental cytoskeletal rearrangements have been achieved through a clearer recognition of the central role of oligo-proline sequence repeats present in ActA, and these findings provide a basis for inferring the role of analogous host cell proteins in the force-producing and position-securing steps in pseudopod and lamellipod formation at the peripheral membrane.     

Autophagy and the cytoskeleton: new links revealed by intracellular pathogens

Actin-based motility is used by various pathogens such as Listeria and Shigella for dissemination within cells: and tissues, yet host factors counteracting this process have not been identified. We have recently discovered that infected host cells can prevent actin-based motility of Shigella by compartmentalizing bacteria inside 'septin cages,' revealing a novel mechanism of host defense that restricts dissemination. Because bacterial proteins controlling actin-based motility also regulate the autophagy process, we hypothesized and then established a link between septin caging and autophagy. Together, these results unveiled the first cellular mechanism that counteracts pathogen dissemination. Understanding the role of septins, a so far poorly characterized component of the cytoskeleton, will thus provide new insights into bacterial infection and autophagy.     

Eukaryotic Lsm proteins: lessons from bacteria

Over the last five years Sm-like (Lsm) proteins have emerged as important players in many aspects of RNA metabolism, including splicing, nuclear RNA processing and messenger RNA decay. However, their precise function in these pathways remains somewhat obscure. In contrast, the role of the bacterial Lsm protein Hfq, which bears striking similarities in both structure and function to Lsm proteins, is much better characterized. In this perspective, we have highlighted several functions that Hfq shares with Lsm proteins and put forward hypotheses based on parallels between the two that might further the understanding of Lsm function.     

Microtubule cytoskeleton: navigating the intracellular landscape

Recent studies have significantly advanced our understanding of how a dividing cell asymmetrically positions the mitotic spindle--a key process in metazoan development--while maintaining a dynamic spindle state that can respond and reorient when necessary.     

Lactic acid bacteria from chicken carcasses with inhibitory activity against Salmonella spp. and Listeria monocytogenes

This study was conducted to isolate psychrotrophic lactic acid bacteria (LAB) from chicken carcasses with inhibitory activity against strains of Salmonella spp. and Listeria monocytogenes. A total of 100 broiler samples were examined for the presence of LAB. Ninety-two LAB isolates that showed antimicrobial effects against Salmonella spp. and L. monocytogenes were further analysed to examine their LAB (Gram-positive, catalase negative, oxidase negative) and psychrotrophic characteristics (ability to grow at 7 °C). Fifty isolates were further selected and identified initially using standard biochemical tests in miniature (Micro-kits API CH 50) and then by sequencing of the 16s-23s rRNA gene boundary region (Intergenic Spacer Region). By molecular identification, these isolates were classified into 5 different LAB species: Lactobacillus salivarius, Lactobacillus reuteri, Lactobacillus johnsonii, Pediococcus acidilactici, and Lactobacillus paralimentarius. None of the isolates produced tyramine or histamine.     

Introduction: microbiology and immunology: lessons learned from Salmonella.

Salmonella enterica, a Gram-negative bacterium, causes significant morbidity and mortality worldwide, and is an excellent model to study bacterial pathogenesis and cellular immune responses. With the development of powerful new technologies, there has been a fusion of research on immunology, molecular biology and cellular microbiology of S. enterica infections. This multidisciplinary research will enhance our understanding of the basic mechanisms of bacterial infections and immunity; it also provides new approaches towards therapeutic and control measures.     

Horizontal gene transfer and the origin of species: lessons from bacteria

In bacteria, horizontal gene transfer (HGT) is widely recognized as the mechanism responsible for the widespread distribution of antibiotic resistance genes, gene clusters encoding biodegradative pathways and pathogenicity determinants. We propose that HGT is also responsible for speciation and sub-speciation in bacteria, and that HGT mechanisms exist in eukaryotes.     

Sterols and intracellular vesicular trafficking: lessons from the study of NPC1

Cholesterol is an important structural component of membranes as well as a precursor for steroid hormone, bile acid and regulatory oxysterol biosynthesis. Recent observations revealed that cholesterol plays an important role in signaling and the regulation of intracellular vesicular trafficking. Studies on Niemann-Pick type C disease, a fatal neuro-visceral cholesterol storage disorder, led to the elucidation of a sterol-modulated vesicular trafficking pathway. Mutations in the NPC1 gene, which cause the majority of cases of Niemann-Pick type C disease, result in the accumulation of free cholesterol in lysosomes and associated defects in glycolipid sorting. NPC1 has a sterol-sensing domain that presumably recognizes free sterols in the protein's environment and participates in the movement of cholesterol out of lysosomes. The compartment containing NPC1 is a subset of late endosomes; it is highly mobile, travels along microtubules, emitting flexible tubules. The movements of this compartment require an intact NPC1 sterol-sensing domain and are dramatically suppressed when free cholesterol accumulates in the late endosomes. Two other proteins involved in sterol trafficking enter into the NPC1 compartment, NPC2 also known as HE1, a secreted sterol-binding glycoprotein, and MLN64, a StAR-related lipid transfer (START) domain protein, which can bind cholesterol and promote its movement from donor to acceptor membranes. Mutations in NPC2 cause a rarer form of Niemann-Pick type C disease, establishing its importance in intracellular sterol movement. NPC2, NPC1 and MLN64 may act in an ordered sequence to sense cholesterol, effect sterol movement, and consequently, influence the process of vesicular trafficking.     

Protozoa and pathogenic bacteria: lessons learned from Legionella pneumophila

Bacterivorous protozoa and bacteria have been in co-existence since the origin of life. This co-existence has led unequivocally to the evolution of many different co-interactions. Most bacteria are ingested and digested, but many escape ingestion for various reasons. Others are ingested but evade digestion, and a few, notoriously Legionella pneumophila , even have the capacity of multiplying within the protozoan host. The aims of this study were to elucidate the interactions of various multi-drug-resistant Staphylococcus aureus (MRSA) strains, Listeria monocytogenes sv4b, and Escherichia coli K12 with the amoeba, Acanthamoeba polyphaga . To evaluate the interactions, we set up co-cultures in Neffs' amoebic saline, at a multiplicity of invasion (MOI) of 1:100 of amoeba to bacteria, and a temperature of 37°C, although the effects of MOI and temperature were also assessed. Survival of bacteria and amoeba was checked at regular intervals, coupled with microscopy. It was discovered under our test conditions, that E. coli was ingested and digested by A. polyphaga , but in contrast, L. monocytogenes , had the capacity to flourish in the presence of A. polyphaga . We also report, for the first time, that all six MRSA isolates tested, survived and replicated in association with A. polyphaga , in comparison to conditions where amoebae were absent. Indeed, we also have evidence suggesting that increases in MRSA, in the presence of A. polyphaga , may be attributable to intracellular survival and replication. These findings have profound implications for the hospital environment, where Acanthamoeba sp., are also commonly isolated. In conclusion, this study illustrates the significance of protozoa as vehicles augmenting the survival of MRSA and L. monocytogenes in the environment.     

Pathogenicity of bacteria and the actin cytoskeleton

Actin system of eukaryotic cells creates the driving force for alteration of the phagocytic cytoplasmatic membrane shape, which is needed for cell movement in the space and for microorganism capturing. Manipulation by actin cytoskeleton mediated through specialized bacterial products can promote proliferation of bacteria in the host. Published reports indicate that bacterial regulation of the actin system activity can be carried out by two modes: 1) by bacterial interactions with surface receptors regulating the cytoskeleton status and 2) by introduction of bacterial products targeted to the cytoskeleton components into the cells. Intracellular pathogens (Legionella) possess ligands which interact with eukaryotic receptors and type IV secretion system fit for translocation of heretofore unknown effector molecules into the cytoplasm. This can result in stimulation of actin polymerization activity and accelerated phagocytosis of the bacteria with rapid multiplication in tissues. By contrast, representatives of extracellular pathogens (Clostridium) produce substances penetrating inside the eukaryotic cells and destroying the actin network, thus making capturing and intracellular digestion of these microorganisms impossible.     

Optimizing the balance between host and environmental survival skills: lessons learned from Listeria monocytogenes

Environmental pathogens - organisms that survive in the outside environment but maintain the capacity to cause disease in mammals - navigate the challenges of life in habitats that range from water and soil to the cytosol of host cells. The bacterium Listeria monocytogenes has served for decades as a model organism for studies of host-pathogen interactions and for fundamental paradigms of cell biology. This ubiquitous saprophyte has recently become a model for understanding how an environmental bacterium switches to life within human cells. This review describes how L. monocytogenes balances life in disparate environments with the help of a critical virulence regulator known as PrfA. Understanding L. monocytogenes survival strategies is important for gaining insight into how environmental microbes become pathogens.