Roles of metallic ions in host-parasite interactions

Autophagic elimination of Trypanosoma cruzi in the presence of metals

Trypanosoma cruzi is an obligate intracellular parasite transmitted to vertebrate hosts by blood-sucking insects. Molecules present in parasites and mammalian cells allow the recognition and parasite internalization. Metallic ions play an essential role in the establishment and maintenance of host-parasite interaction. However, little is known about how parasites handle with essential and nonessential metal quotas. This study aimed to investigate the influence of metal ions on the biological processes of T. cruzi infected cells. Infected cells were incubated with ZnCl₂, CdCl₂, and HgCl₂ for 12 h and labeled with different specific dyes to investigate the cellular events related to intracellular parasite death and elimination. Infected host cells and parasite's mitochondria underwent functional and structural disorders, in addition to parasite's DNA condensation and pH decrease on host cells, which led to parasite death. Further investigations suggested that lysosomes were involved in pH decrease and the double membrane of the endoplasmic reticulum formed vacuoles surrounding damaged parasites, which indicate the occurrence of autophagy for parasite elimination. In conclusion, low concentrations of nonessential and essential metals cause a series of damage to Trypanosoma cruzi organelles, leading to its loss of viability, death, and elimination, with no removal of the host cells.

A crowdsourced analysis to identify ab initio molecular signatures predictive of susceptibility to viral infection

The response to respiratory viruses varies substantially between individuals, and there are currently no known molecular predictors from the early stages of infection. Here we conduct a community-based analysis to determine whether pre- or early post-exposure molecular factors could predict physiologic responses to viral exposure. Using peripheral blood gene expression profiles collected from healthy subjects prior to exposure to one of four respiratory viruses (H1N1, H3N2, Rhinovirus, and RSV), as well as up to 24 h following exposure, we find that it is possible to construct models predictive of symptomatic response using profiles even prior to viral exposure. Analysis of predictive gene features reveal little overlap among models; however, in aggregate, these genes are enriched for common pathways. Heme metabolism, the most significantly enriched pathway, is associated with a higher risk of developing symptoms following viral exposure. This study demonstrates that pre-exposure molecular predictors can be identified and improves our understanding of the mechanisms of response to respiratory viruses.

Intracellular development of Trypanosoma cruzi in the presence of metals

Trypanosoma cruzi is transmitted to vertebrate hosts during the feeding of blood-sucking insects. After the invasion of host cells, the parasite resides within the parasitophorous vacuole until to escape to host cytoplasm and to proliferate, establishing an infection. Studies demonstrated that some intracellular parasites have to acquire all essential nutrients as well as transition metals from the host cell to be pathogenic, to maintain the homeostasis and to replicate. The present study investigated the progressive steps of the intracellular parasite development and establishment of infection in the presence of ZnCl₂, CdCl₂ and HgCl₂. LLC-MK2 cells were infected with trypomastigotes during 6-84 h to investigate the steps of intracellular parasite development. After the host cells were infected during 12 h and treated with metals during 24 or 60 h or they were treated for 24 h and cultured for 72 h more to observe the reversibility. The results showed that the non-synchronous invasion of trypomastigotes resulted in an increasing number of intracellular parasites in intermediary forms (until 24 h post-infection), the appearance (from 36 h) and proliferation (84 h) of the amastigotes. The 24 h-treatments were not enough to impair parasite escape to the host cytoplasm and reproduction. However, 60 h of incubations led to a significant reduction in parasite numbers, as well as the reversibility assays. In conclusion, new insights about the intracellular T. cruzi development in the presence of metals were provided, and further studies should be performed to investigate the events involved in parasite death and elimination.

Further aspects of Toxoplasma gondii elimination in the presence of metals

Toxoplasma gondii, the etiological agent of toxoplasmosis, infects nucleated cells and then resides and multiplies within a parasitophorous vacuole. For this purpose, the parasite secretes many virulence factors for the purpose of invading and subverting the host microbicidal defenses in order to facilitate its survival in the intracellular milieu. Essential metals are structural components of proteins and enzymes or cofactors of enzymatic reactions responsible for these parasitic survival mechanisms. However, an excess of non-essential or essential metals can lead to parasite death. Thus, infected host cells were incubated with 20 μM ZnCl₂ in conjunction with 3 μM CdCl₂ or HgCl₂ for 12 h in order to investigate cellular events and organelle damage related to intracellular parasite death and elimination. In the presence of these metals, the tachyzoites undergo lipid uptake and transport impairment, functional and structural mitochondrial disorders, DNA condensation, and acidification of the parasitophorous vacuole, thus leading to parasite death. Additional research has suggested that lysosome-vacuole fusion was involved in parasite elimination since acid phosphatases were found inside the parasitophorous vacuole, and vacuoles containing parasites were also positive for autophagy. In conclusion, low concentrations of CdCl₂, HgCl₂, and ZnCl₂ can cause damage to Toxoplasma gondii organelles, leading to loss of viability, organelle death, and elimination without causing toxic effects to host cells.

Mechanism of Tuberculostasis in Mammalian Serum III. Neutralization of Serum Tuberculostasis by Mycobactin

Tubercle bacilli exposed to an iron-poor medium multiplied at a slower rate but released more of the serum-tuberculostasis neutralizing factor (TNF) than the bacilli in an iron-rich medium. This growth-promoting factor found in spent medium exhibited characteristics which suggested its relationship to or identity with mycobactin. The identity of these two bacillary products was established by showing that both iron-free mycobactin and TNF promoted bacillary multiplication in tuberculostatic serum. This study resolved a long-standing controversy as to whether mycobactin serves as a growth factor or as a carrier of iron for tubercle bacilli. It was found that the tuberculostasis in mycobactin-neutralized serum was reconstituted by the addition of iron-free transferrin (Tr). The investigation of the interplay between mycobactin and Tr revealed that mycobactin does not serve as a growth factor but as a carrier of growth-essential iron which mycobactin (as contrasted to Tr) provides to tubercle bacilli in a utilizable form.

Effect of farnesol on a mouse model of systemic candidiasis, determined by use of a DPP3 knockout mutant of Candida albicans

This work extends our previous observation that the fungus Candida albicans secretes micromolar levels of farnesol and that accumulation of farnesol in vitro prevents the yeast-to-mycelium conversion in a quorum-sensing manner. What does farnesol do in vivo? The purpose of this study was to determine the role of farnesol during infection with a well-established mouse model of systemic candidiasis with C. albicans A72 administered by tail vein injection. This question was addressed by altering both endogenous and exogenous farnesol. For endogenous farnesol, we created a knockout mutation in DPP3, the gene encoding a phosphatase which converts farnesyl pyrophosphate to farnesol. This mutant (KWN2) produced six times less farnesol and was ca. 4.2 times less pathogenic than its SN152 parent. The strain with DPP3 reconstituted (KWN4) regained both its farnesol production levels and pathogenicity. These mutants (KWN1 to KWN4) retained their full dimorphic capability. With regard to exogenous farnesol, farnesol was administered either intraperitoneally (i.p.) or orally in the drinking water. Mice receiving C. albicans intravenously and farnesol (20 mM) orally had enhanced mortality (P < 0.03). Similarly, mice (n = 40) injected with 1.0 ml of 20 mM farnesol i.p. had enhanced mortality (P < 0.03), and the onset of mortality was 30 h sooner than for mice which received a control injection without farnesol. The effect of i.p. farnesol was more pronounced (P < 0.04) when mice were inoculated with a sublethal dose of C. albicans. These mice started to die 4 days earlier, and the percent survival on day 6 postinoculation (p.i.) was five times lower than for mice receiving C. albicans with control i.p. injections. In all experiments, mice administered farnesol alone or Tween 80 alone remained normal throughout a 14-day observation period. Finally, beginning at 12 h p.i., higher numbers of C. albicans cells were detected in kidneys from mice receiving i.p. farnesol than in those from mice receiving control i.p. injections. Thus, reduced endogenous farnesol decreased virulence, while providing exogenous farnesol increased virulence. Taken together, these data suggest that farnesol may play a role in disease pathogenesis, either directly or indirectly, and thus may represent a newly identified virulence factor.

The role of iron in the growth and hemolysin (Streptolysin S) production in Streptococcus pyogenes

Four strains of Streptococcus pyogenes were propagated at 37 degrees C in a reduced iron medium supplemented with Fe3+-citrate to give concentrations of 1 through 11 micrograms per milliliter, in order to observe the effects of iron on growth and on the vitro production of Streptolysin S. Both growth and hemolysin production were observed to be influenced by medium iron concentration of which 1.2 micrograms per ml of iron was critical. Hemolysin was produced during the exponential phase of the growth cycle with maximum yield as the organism entered the stationary phase. Hemolytic activity (which was accepted as the ability of the hemolysin to lyse sheep erythrocytes) fell below detectable levels as the organisms entered fully into the stationary phase (9-10 hours post incubation). Serum (bovine, human, chicken) was observed to have a high stabilizing effect on the hemolysin.

[Demonstration of iron transport activity in Pasteurella multocida cultures]

It has been established that Pasteurella multocida cultures possess pronounced iron transport activities to accumulate the iron necessary for growth. Experiments with Fe-59 confirmed that the bacterial cells are able to acquire iron without direct contact from high molecular iron substrates, such as iron dextrane, ferritine or transferrine. Microbial siderophores of the hydroxamate and phenolate types, such as desferrioxamin B and enterobactine as well as other iron chelators (phenanthroline, citrate and nitrilotriacetate) decrease the bacterial cell growth or iron incorporation and are not relevant for iron transport in P. multocida. The direct analytical identification of siderophores using the reactions by Csaky (hydroxamate type) and Arnow (phenolate type) has proved unsuccessful. The importance of the mannan cell wall polysaccharide is discussed with respect to the iron transport. Thus in terms of iron accumulation, P. multocida is similar to Yersinia, which also possess an efficient transport system for iron not involving siderophores.

Hydroxamate siderophore production by opportunistic and systemic fungal pathogens

It has been suggested that siderophores may function as virulence factors. There have been few studies on production of siderophores by opportunistic and pathogenic fungi. We examined siderophore production by Absidia corymbifera, Aspergillus niger, Rhizopus arrhizus, Rhizopus oryzae, Blastomyces dermatitidis, Histoplasma capsulatum, Sporothrix schenickii, Candida albicans, and Trichophyton mentagrophytes. Fungi were cultured at 37 and 27 degrees C in a chemically defined low-iron media (0.2 microM Fe). Culture supernatants were assayed for siderophores by two nonspecific methods [FeCl3 and Fe(ClO4)3] and three chemically specific assays (catechol, 2,3-dihydroxybenzoate, and hydroxamate). All fungi secreted siderophores. Only siderophores of the hydroxamate type were found. More siderophore was produced at 27 degrees C than at 37 degrees C. The present study adds eight fungi to the list of known siderophore producers and confirms siderophore production by H. capsulatum.

Roles of metallic ions within an area endemic for the plague bacillus (Yersinia pestis)

There are observations that strongly suggest that the synthesis of specific factors of virulence in the plaque bacillus (Yersinia pestis) depend on the level of a specific metallic ion(s). The varied roles of metallic ions in host-plague interactions are quite similar to the structural and catalytic roles of such ions in free-living macro-and micro-organisms. The antimicrobial power of mammalian fluids is depressed by low levels of Fe and enhanced by an increase in the Fe binding capacity of a system. The bactericidal power of serum as well as endotoxin components are affected by levels of Ca and Mg. Se also plays a role in the defense mechanism of animals to certain diseases. Yet, there is sufficient uniqueness of roles of key metallic ions in many specific host systems so that the balance could be tipped in favor of either host or the plague bacillus by subtle alteration of the metallic ion environment, specifically within an area endemic for the disease.

Growth requirements of pathogenic Leptospira

Nutritional requirements for growth at 30 C of Leptospira pomona and L. canicola have been determined. Both pathogenic serotypes initially required bovine serum albumin (BSA) for growth in a medium (SM-4) which permitted growth of the water isolate B-16. Requirement for BSA was eliminated by (i) removing much of the apparent toxicity of free fatty acids in Tween 80 on an anion exchange column, (ii) decreasing extended lag periods observed from small inocula by incorporation of pyruvate into the medium, (iii) the addition of acetate to permit full utilization of substrate fatty acids in Tween 80, and (iv) the addition of glycerol to decrease generation times. Physiologic significance of these findings is discussed, and the possibility is suggested that apparent toxicity of fatty acids for leptospires may result from their auto-oxidation products. The resulting protein-free medium (SM-5) permitted the growth of pathogens at 30 C to high cell yields in low inocula. Highly virulent and avirulent strains from the same clone of L. canicola Moulton were used to determine additional growth requirements associated with virulence. As incubation temperatures were increased from 30 C to those of mammalian hosts, virulent cells required biotin at 35 C and higher levels of K(+) and Mg(2+) at 37 C. Additional Fe(2+) eliminated the necessity for removing the toxicity of Tween 80 by anion exchange. Significance of these physiologic studies are discussed in relationship to virulence. The final protein-free medium (SM-6) grew highly virulent L. canicola from tissue to high yields from low inocula at 37 C with no loss in virulence over several transfers.

Cell attachment and penetration by influenza virus

Attachment and penetration of influenza virus into clone 1-5C-4 cells were quantitatively determined by the immunofluorescent cell-counting assay. Aided by centrifugal force, more than 95% of virus inocula of five representative influenza virus strains (A(0)/PR8, A(1)/Ann Arbor, A(2)/Japan, B/Lee, B/Great Lakes) were attached to cells at a linear rate within 10 min, in contrast to approximately 35% after stationary incubation at 35 C for 2 h. By the former procedure, a proportionality between the number of infected cells and volume of inoculum was revealed which was not evident when stationary incubation was employed. Maximal binding of virus to cells occurred at 0.2 M NaCl. The salt requirement, added to evidence of pH dependence and temperature independence, indicated that the initial virus-cell union involved electrostatic forces. Virus penetration into cells, measured by the insensitivity of virus-cell complexes to antiviral serum, was linear and complete within 15 min at 35 C for all five virus strains tested. Maximal virus penetration occurred at 0.1 to 0.2 M NaCl; the process was pH- and temperature-dependent. Both virus attachment and penetration processes were partially inhibited in the presence of diethylaminoethyl-dextran.

Employment of tuberculostasis in serum-agar medium for the study of production and activity of Mycobactin

Mycobactin (M), an iron-chelating product of tubercle bacilli, neutralized serum tuberculostasis by removing growth-essential iron from transferrin (Tr) and supplying the metal to the bacteria. The competition for iron between Tr and M has been demonstrated by the agar-plate diffusion test. This test is suitable not only for the study of Tr-iron-M interplay but also for the evaluation of serum tuberculostasis. Extremely poor solubility of M in water and consequently its association with lipoidal cell wall of tubercle bacillus was overcome by the use of water-dispersible and surface-active Tween 80. The addition of Tween 80 to culture media insured the presence of M in spent media; otherwise M was extracted from bacillary cells with a solution of Tween 80 or a mixture of ethanol and Tween 80. Although M was produced irrespective of the amount of iron present in culture medium, its production in iron-poor medium was more prolific than in iron-rich medium. M extracted from BCG or H(37)Rv cells neutralized serum tuberculostasis as effectively for the homologous as for heterologous strains. However, the extract of virulent bacilli was much more active in the neutralization than similar extract prepared from attenuated cells; whether this difference is of quantitative or qualitative nature remains to be determined.

The effect of iron and transferrin on the killing of Escherichia coli in fresh serum

Iron in excess of the binding capacity of transferrin allows Escherichia coli to grow in fresh serum. This appears to be due to two mechanisms. Firstly iron stimulates growth as it is an essential metabolite which is not available to E. coli in serum. Secondly, it interferes with bacterial killing, a process normally mediated by natural antibody and complement.

Mechanism of tuberculostasis in mammalian serum. II. Induction of serum tuberculostasis in guinea pigs

The growth of tubercle bacilli in serum samples of untreated animals depends upon the availability of ionic iron which serves as a growth factor in supporting bacillary multiplication. The amount of available iron in serum is determined by the ratio between iron-saturated and iron-free transferrin; a low value for the ratio is associated with tuberculostasis (e.g., human serum, 0.4), whereas a high value is associated with the growth-supporting quality (e.g., guinea pig serum, 5.6). The treatment of guinea pigs with lipopolysaccharide of Escherichia coli or tuberculous cell wall material consistently and significantly reduced serum iron levels; a similar but less striking effect was observed in BCG-vaccinated animals. Pronounced differences were observed in the time of appearance and duration of serum hypoferremia; in lipopolysaccharide-treated animals, it appeared in 1 day and lasted for several days, whereas in BCG-vaccinated animals it appeared in about 2 weeks and lasted for much longer time periods. The induced hypoferremia was always associated with the concomitant development of serum tuberculostasis which could be neutralized by the addition of iron. These results indicate, therefore, that the mechanism of induced serum tuberculostasis in lipopolysaccharide- or tuberculous cell wall-treated and BCG-vaccinated guinea pigs is the same as that present in tuberculostatic sera of untreated animals.

Primary virus-cell interactions in the immunofluorescence assay of Venezuelan equine encephalomyelitis virus

The conditions under which Venezuelan equine encephalomyelitis (VEE) virus attached to host cells markedly influenced the assay of virus by the fluorescent cell-counting technique. When virus inoculum was centrifuged onto McCoy cell monolayers, approximately 97% of virus was attached to cells within 10 min, in contrast to 34% after stationary incubation at 35 C for 2 hr. Maximal binding of virus occurred only in the presence of 0.1 to 0.15 m NaCl. This salt requirement, added to evidence of (p)H dependence and temperature independence of VEE virus attachment to cells, indicated that the initial union involved electrostatic forces. Virus penetration, measured by the insensitivity of virus-cell complexes to viral antiserum, was complete in 30 min at 35 C. The process was temperature-dependent and un-affected by the ionic content of medium. For assay of VEE virus by the fluorescent cell-counting technique, infected cells may be enumerated as early as 12 hr after infection of cell monolayers. The relationship between virus concentration and cell-infecting units was linear; the distribution of fluorescent cells was random. The virus assay was equivalent in sensitivity but more precise and rapid than that of intracerebral inoculation of mice.

Bacteriostatic effects of horse sera and serum fractions on Clostridium welchii Type A, and the abolition of bacteriostasis by iron salts

Under a variety of conditions of concentration, Eh, and pH, horse anti-Clostridium welchii serum and normal horse serum exerted similar bacteriostatic effects against Cl. welchii Type A. Ferric iron abolished the bacteriostatic effect when added during the first 2 hours of incubation at Eh+60 mV. Ferrous iron abolished the bacteriostatic effect when added after 3 hours. Ferric iron abolished the bacteriostatic effect at—140 mV. A mixture consisting of horse β₂- and γ-globulins together with human transferrin exerted a bacteriostatic effect similar to that of whole serum. This system responded in the same way as whole serum to the addition of iron. A mixture of horse β₂- and γ-globulins exerted an immediate bactericidal effect which could not be prevented by ferric iron.