other metal ions including ions of Zn, Cu, Mo, V Cd, Ni, Pb, Mn, Al, Th, U, and Pu
can bind to siderophores [43]. That makes it possible to develop various environmental applications and bioleaching processes. In the framework of the new
approach on the development of diagnostic systems, siderophores can be applied
as biosensors and nanosensors [43, 44]. Since iron is required by all living systems, it
is also biologically competed element among the microbes. On the other hand, free
iron can be biologically harmful, and its existence must be strictly controlled. Free
soluble Fe(II) may react with oxygen and produce damaging oxygen species. In
human body, for example, extra iron ions are bound to specific protein carriers [45],
and concentration of free iron is extremely low in serum, <10
À24 M Fe(III) [46]. The
repertoire of siderophores varies among microbial species and even among strains.
There is an increasing amount of data suggesting that siderophores impact on
microbial pathogenesis with different mechanisms. Siderophores can also modulate
the host response [46]. In pathogenic interactions, siderophores are involved in iron
acquisition from the host and are sometimes necessary for full virulence. To protect
themselves against iron theft, mammalian hosts have developed siderophore-binding
molecules, siderocalins, which may also trigger immunity. As to plants, fewer data
are available, but the plant-microbe relation is surely important as it is also with
mammals. In plants, siderophores can trigger immunity in several contexts through
induced systemic resistance. However, the underlying mechanisms are not yet well
understood [47].
Bacteria are known in transporting extracellular siderophores charged with metal
ions to their intracellular environment. Thus, infection of antibiotic-resistant pathogens can be prevented through siderophores: some bacteria can link antibacterial
agent into a siderophore to generate a sideromycin. A competing bacterium,
attempting to steal iron from another one, may uptake sideromycin and get the
toxin inside the cell, acting like the “Trojan horse” [48]. To date, siderophoreproducing bacteria and fungi that thrive at normal pH are well studied and reviewed
[41, 49]. The information on the nature of iron-chelating molecules produced by
extremophiles is scanty, in particular that of alkaliphiles [50]. The biochemical and
molecular mechanisms used by alkaliphilic bacteria to acquire iron are not well
understood. One aspect is that in alkaline, environment iron and some other metal
ions precipitate as hydroxides. Therefore, especially in alkaline milieu, the availability of iron can become growth limiting. May be iron hydroxides require different
binding compounds. Siderophores should therefore be exceptionally important
for alkaliphiles. It was demonstrated that alkaliphilic Bacillus sp. strains,
Caldalkalibacillus thermarum, B. halodurans C-125, B. pseudofirmus, and
B. alcalophilus, were sensitive to artificial iron (Fe
3+ ) chelators even though
these microbes produce siderophores. The siderophores contained catechol and
hydroxamate moieties, and their synthesis was stimulated by manganese (II) salts
and suppressed by FeCl 3 addition. Purification and mass spectrometric characterization of C. thermarum siderophores failed to identify any matches to previously
observed fragmentation spectra of known siderophores, suggesting novel
structures [51].
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E. Khalikova et al.
can bind to siderophores [43]. That makes it possible to develop various environmental applications and bioleaching processes. In the framework of the new
approach on the development of diagnostic systems, siderophores can be applied
as biosensors and nanosensors [43, 44]. Since iron is required by all living systems, it
is also biologically competed element among the microbes. On the other hand, free
iron can be biologically harmful, and its existence must be strictly controlled. Free
soluble Fe(II) may react with oxygen and produce damaging oxygen species. In
human body, for example, extra iron ions are bound to specific protein carriers [45],
and concentration of free iron is extremely low in serum, <10
À24 M Fe(III) [46]. The
repertoire of siderophores varies among microbial species and even among strains.
There is an increasing amount of data suggesting that siderophores impact on
microbial pathogenesis with different mechanisms. Siderophores can also modulate
the host response [46]. In pathogenic interactions, siderophores are involved in iron
acquisition from the host and are sometimes necessary for full virulence. To protect
themselves against iron theft, mammalian hosts have developed siderophore-binding
molecules, siderocalins, which may also trigger immunity. As to plants, fewer data
are available, but the plant-microbe relation is surely important as it is also with
mammals. In plants, siderophores can trigger immunity in several contexts through
induced systemic resistance. However, the underlying mechanisms are not yet well
understood [47].
Bacteria are known in transporting extracellular siderophores charged with metal
ions to their intracellular environment. Thus, infection of antibiotic-resistant pathogens can be prevented through siderophores: some bacteria can link antibacterial
agent into a siderophore to generate a sideromycin. A competing bacterium,
attempting to steal iron from another one, may uptake sideromycin and get the
toxin inside the cell, acting like the “Trojan horse” [48]. To date, siderophoreproducing bacteria and fungi that thrive at normal pH are well studied and reviewed
[41, 49]. The information on the nature of iron-chelating molecules produced by
extremophiles is scanty, in particular that of alkaliphiles [50]. The biochemical and
molecular mechanisms used by alkaliphilic bacteria to acquire iron are not well
understood. One aspect is that in alkaline, environment iron and some other metal
ions precipitate as hydroxides. Therefore, especially in alkaline milieu, the availability of iron can become growth limiting. May be iron hydroxides require different
binding compounds. Siderophores should therefore be exceptionally important
for alkaliphiles. It was demonstrated that alkaliphilic Bacillus sp. strains,
Caldalkalibacillus thermarum, B. halodurans C-125, B. pseudofirmus, and
B. alcalophilus, were sensitive to artificial iron (Fe
3+ ) chelators even though
these microbes produce siderophores. The siderophores contained catechol and
hydroxamate moieties, and their synthesis was stimulated by manganese (II) salts
and suppressed by FeCl 3 addition. Purification and mass spectrometric characterization of C. thermarum siderophores failed to identify any matches to previously
observed fragmentation spectra of known siderophores, suggesting novel
structures [51].
172
E. Khalikova et al.