Lusine Ghazaryan

Energy costs of blood digestion in a host-specific haematophagous parasite.

SUMMARY Fleas consume and digest blood from their hosts. We hypothesized that the energy costs of digestion of blood by fleas is dependent on the host species. To test this hypothesis, we studied CO2 emission, a measure of energy expenditure, during digestion of a blood meal taken by Parapulex chephrenis from a preferred (Acomys cahirinus) and a non-preferred (Gerbillus dasyurus) host. We predicted that the energy cost of digestion would be lower for A. cahirinus blood than that for G. dasyurus. Male and female fleas consumed similar amounts of blood per unit body mass, independent of host species. Our prediction was supported in that fleas expended significantly more energy digesting blood of G. dasyurus than blood of A. cahirinus. We also found CO2 emission rates of fed fleas were higher than those of unfed fleas and differed significantly among stages of blood digestion when a flea fed on G. dasyurus but not when it fed on A. cahirinus. When fed on G. dasyurus, fleas spent less energy during earlier than later stages of digestion.

Recent Advances in Bacteriocin Application as Antimicrobials

The dramatic rise in antibiotic-resistant pathogens has renewed efforts to identify, develop, and redesign antibiotics active against the resistant bacteria. This review will focus on the increasing number of patents aimed at employing the potential of antimicrobial polypeptides i.e., the bacteriocins, in vetrinary medicine and human health. Bacteriocins demonstrate enormous possibilities in treating and containing target bacteria and may be a part of novel approaches for replacing classical antibiotics at a time when many pathogens are no longer susceptible to the existing antibiotics. We will review the new knowledge-based approach to the exploitation of these bacterial produced toxins to treat human and animal infectious diseases as well as inhibit the proliferation of eukaryotic cells i.e., treating tumours. The ability to develop novel bacteriocin-based-drugs aimed at potential target cells, prokaryotic as well as eukaryotic, may open new possibilities for the design of improved antibiotics possessing refined characteristics.

The Weak Shall Inherit: Bacteriocin-Mediated Interactions in Bacterial Populations

Background Evolutionary arms race plays a major role in shaping biological diversity. In microbial systems, competition often involves chemical warfare and the production of bacteriocins, narrow-spectrum toxins aimed at killing closely related strains by forming pores in their target’s membrane or by degrading the target’s RNA or DNA. Although many empirical and theoretical studies describe competitive exclusion of bacteriocin-sensitive strains by producers of bacteriocins, the dynamics among producers are largely unknown. Methodology/Principal findings We used a reporter-gene assay to show that the bacterial response to bacteriocins’ treatment mirrors the inflicted damage Potent bacteriocins are lethal to competing strains, but at sublethal doses can serve as strong inducing agents, enhancing their antagonists’ bacteriocin production. In contrast, weaker bacteriocins are less toxic to their competitors and trigger mild bacteriocin expression. We used empirical and numerical models to explore the role of cross-induction in the arms race between bacteriocin-producing strains. We found that in well-mixed, unstructured environments where interactions are global, producers of weak bacteriocins are selectively advantageous and outcompete producers of potent bacteriocins. However, in spatially structured environments, where interactions are local, each producer occupies its own territory, and competition takes place only in “no man’s lands” between territories, resulting in much slower dynamics. Conclusion/Significance The models we present imply that producers of potent bacteriocins that trigger a strong response in neighboring bacteriocinogenic strains are doomed, while producers of weak bacteriocins that trigger a mild response in bacteriocinogenic strains flourish. This counter-intuitive outcome might explain the preponderance of weak bacteriocin producers in nature. However, the described scenario is prolonged in spatially structured environments thus promoting coexistence, allowing migration and evolution, and maintaining bacterial diversity.

The role of stress in colicin regulation

Bacteriocins produced by Enterobacteriaceae are high molecular weight toxic proteins that kill target cells through a variety of mechanisms, including pore formation and nucleic acid degradation. What is remarkable about these toxins is that their expression results in death to the producing cells and therefore bacteriocin induction have to be tightly regulated, often confined to times of stress. Information on the regulation of bacteriocins produced by enteric bacteria is sketchy as their expression has only been elucidated in a handful of bacteria. Here, we review the known regulatory mechanisms of enteric bacteriocins and explore the expression of 12 of them in response to various triggers: DNA-damaging agents, stringent response, catabolite repression, oxidative stress, growth phase, osmolarity, cold shock, nutrient deprivation, anaerobiosis and pH stress. Our results indicate that the expression of bacteriocins is mostly confined to mutagenic triggers, while all other triggers tested are limited inducers.

Infestation experience of a rodent host and offspring viability of fleas: variation among host–parasite associations

We studied survival and development of preimagoes and the ability to withstand starvation of adults in two flea species, host-specific Parapulex chephrenis and host-opportunistic Xenopsylla ramesis, when parent fleas fed on a typical (Acomys cahirinus and Dipodillus dasyurus, respectively) or an atypical (D. dasyurus and A. cahirinus, respectively) rodent host that either had never been parasitized by fleas (pristine) or had previously been exposed to fleas. We asked whether a repeatedly infested host acquired resistance that would cause decreased viability of the next generation fleas. Survival of preimaginal P. chephrenis was similar, independent of host species or its infestation status. Preimaginal X. ramesis had a higher survival rate when their parents fed on preinfested than on pristine typical hosts, whereas no effect of infestation status of an atypical host was found. P. chephrenis developed faster if their parents fed on atypical than on typical hosts and on pristine than on preinfested hosts of either species. X. ramesis developed faster if parents fed on pristine than preinfested typical hosts, but no difference in duration of development was found for atypical hosts. Under starvation, P. chephrenis lived longer if their parents fed on preinfested than on pristine typical hosts, but their lifespan did not depend on infestation status of atypical hosts. The latter was also true for X. ramesis and both host species. We conclude that a host is constrained in its ability to cope with a parasite, whereas a parasite is able to cope with defence responses of a host.

Auto-regulation of DNA degrading bacteriocins: molecular and ecological aspects.

Colicins, proteinaceous antibiotics produced by Escherichia coli, specifically target competing strains killing them through one of a variety of mechanisms, including pore formation and nucleic acid degradation. The genes encoding colicins display a unique form of expression, which is tightly regulated, involving the DNA damage response regulatory system (the SOS response system), confined to stressful conditions and released by degradation of the producing cell. Given their lethal nature, colicin production has evolved a sophisticated system for repression and expression. While exploring the expression of 13 colicins we identified a novel means of induction unique to strains that kill by DNA degradation: these colicinogenic strains mildly poison themselves inflicting DNA damage that induces their DNA repair system (the SOS system), and their own expression. We established that among the four known DNase colicins (E2, E7, E8 and E9), three act to induce their own production. Using different stresses we show that this form of self-regulation entails high cost when growth conditions are not optimal, and is not carried out by individual cells but is a population-mediated trait. We discuss this novel form of colicins’ regulation and expression, and its possible molecular mechanism and evolutionary implications.

Dietary intake and time budget in two desert rodents : a diurnal herbivore, Psammomys obesus, and a nocturnal granivore, Meriones crassus

We studied dietary intakes and time budgets in two desert gerbillids, Psammomys obesus, a diurnal herbivore, and Meriones crassus, a nocturnal granivore. P. obesus was offered fresh leaves of Atriplex halimus while M. crassus was offered millet seeds and Atriplex halimus, mainly as a source of water. We predicted that the (1) nocturnal rodent will feed mainly at night and the diurnal rodent mainly during the day; and (2) the herbivore consuming a relatively low energy diet will spend more time feeding than the granivore consuming a relatively high energy diet. The latter prediction was confirmed in that P. obesus spent more time feeding than M. crassus. Number of feeding sessions in M. crassus was the same as in P. obesus but each feeding session was shorter. However, the former prediction was only partially confirmed. Feeding during the dark phase was significantly longer than the light phase in the nocturnal M. crassus, as predicted; but feeding did not differ between diel phases in the diurnal P. obesus. The large amount of forage required by P. obesus necessitated consumption throughout the light and dark phases at the same rates of intake.

Bacteriocin expression in sessile and planktonic populations of Escherichia coli

INTRODUCTION When competing for nutrients and space, most bacterial species produce bacteriocins, which are ribosomally synthesized peptides with antibacterial activity. In fact, dozens of bacteriocins have been found within a single bacterial species,1 leading researchers to suggest that they have a critical role in mediating microbial interactions and maintaining microbial diversity.2–4 Colicins—bacteriocins produced by Escherichia coli—have been studied for almost a century and are a model system for bacteriocins produced by Gram-negative bacteria.5 Most colicins are encoded on operons regulated by stress, specifically DNA damage,5 and are comprised of three genes: a toxin gene encoding the colicin protein, an immunity gene encoding a protein that protects the cell from its produced toxin and a lysis gene encoding a protein that lyses the producing cell to release the colicin.5 Due to the lethality of colicin expression, the clonal colicinogenic population exhibits heterogeneity such that only a small subset of cells produces and releases colicins through lysis. This phenotypic heterogeneity has been reported in a number of colicinogenic populations with only 0.5–3% of the cells expressing colicins.6–8 All the cells within these populations express the immunity protein, regulated by a constitutive promoter, thus protected from the colicin. This behavior, carried by a handful of cells within a given population, might be explained by potential benefit to clonal siblings; by killing their adversaries, nutrients and space are thereby freed for their use. Colicins have been identified in up to 50% of E. coli species isolated from mammals’ gastrointestinal (GI) tract.5 In the GI tract, bacteria adhere to the colon epithelial cells and form a stable biofilm.9 Hence, in their natural environment, colicinogenic E. coli cells reside in a spatially structured environment in which cell–cell interactions are localized.10 Nevertheless, most studies of colicin function, expression, ecology and evolution have been performed in well-mixed environments in which the cells are free swimming.5 Many studies have compared transcription or translation profiles in planktonic versus biofilm cultures of an E. coli strain, showing differential expression of a large number of genes.9 A geneexpression study in E. coli strain K-12 showed that 5–10% of the genome is differentially expressed in these two types of culture and that genes induced in biofilm settings relate to stress responses, energy production and envelope biogenesis.9 These differences in expression profiles may also affect bacteriocin production. Studies describing bacteriocin-mediated interactions all report a disparity in competition outcome when bacteria are sessile rather than planktonic.3,4,11 This disparity was mostly attributed to changes in the type of cell–cell interactions, from local in a spatially structured environment to global in a well-mixed environment.3 Here we suggest that the reported disparities may result not only from changes in the cells’ interactions but also from differential bacteriocin expression. We present a first attempt at comparing colicin expression in biofilm and planktonic environments. To that end, we tagged a colicinogenic E. coli strain with green fluorescent protein (GFP) to monitor colicin expression in real time at the single-cell level in planktonic and biofilm settings. We predicted that in a spatially structured environment, the population would increase its bacteriocin expression, corresponding to an elevation in genes reported to be associated with stress response in E. coli biofilm.9,12

Does Acquired Resistance of Rodent Hosts Affect Metabolic Rate of Fleas

We studied whether (a) previous infestation of a rodent host with fleas and (b) the reproductive effort of fleas affect the rate of CO(2) emission in two flea species, host-specific Parapulex chephrenis and host-opportunistic Xenopsylla ramesis when feeding on their typical and atypical rodent hosts. We measured the rate of CO(2) emission in preovipositing and ovipositing female fleas fed on either pristine or previously infested Acomys cahirinus (typical host of P. chephrenis) and Dipodillus dasyurus (typical host of X. ramesis). When P. chephrenis fed on a typical host, its mass-specific rate of CO(2) emission was not affected by previous infestation of a host, whereas when this flea fed on the atypical host, its rate of CO(2) emission was higher when a host was previously infested. This was manifested, however, mainly during the oviposition period. The rate of CO(2) emission by X. ramesis feeding on pristine hosts was significantly lower than in conspecifics feeding on previously infested hosts, independent of host species. Both flea species feeding on their typical hosts emitted CO(2) at similar mass-specific rates during preoviposition and oviposition, except for P. chephrenis feeding on D. dasyurus, which increased its rate during oviposition. There was no effect of the number of eggs produced per female on the rate of CO(2) emission during oviposition.