The abundance of bacteriocins has lead researchers to suggest that they play a critical role in mediating microbial interactions, community dynamics, and in maintaining microbial diversity. An additional role has recently been proposed for bacteriocins produced by Gram-positive bacteria, in which they mediate quorum sensing [1, 5, 6]. From an ecological point of view, bacteriocins are anticompetitor molecules that enable bacteria to respond to environmental challenges by reducing competition from sensitive bacterial strains that share the same ecological niche and have similar nutritional requirements [2, 6, 7].
As a family, bacteriocins are classified into two main groups: the toxins produced by Gram-positive and Gram-negative bacteria. Gram-positive bacteria produce bacteriocins which are selfregulated by specific transport mechanisms; a good example are the bacteriocins produced by lactic acid bacteria that are used in the preservation of meat and milk [5]. The toxins produced by Gram-negative bacteria are usually released through cell lysis and are dependent on the host regulatory pathways, like the SOS regulation; the most extensively studied example of this group are the colicins [1, 6, 8].
Colicins represent a heterogeneous group of antimicrobial, high molecular weight proteins [2, 6, 9, 10], which have a limited killing spectrum: they are toxic only to competing E. coli strains and Enterobacteriaceae, such as Salmonella and Citrobacter [5, 6, 11, 12]. The relative abundance of colicinogenic E. coli strains in natural and clinical isolates was shown to be about 15-50% [2, 13-15]. To date, 34 types of colicins have been identified [2, 10]; they were divided into two groups based on their encoding plasmid, their release system, and their mode of action: Group A is comprised of colicins that are encoded on small plasmids, are released into the medium through the lysis of the producing cell, and are translocated across the membrane through the Tol system. This group includes colicins A, E1 to E9, K, L, N, S4, U, and Y [6, 9, 16-18]. Group B includes colicins that are encoded on large plasmids, colicins whose secretion mechanism is not through lysis, and that require the TonB system for translocation. This group includes colicins B , D, Ia, Ib, M, 5, and 10 [6, 9, 10, 17]. However, some colicins might belong to one group and share homologies with colicins of the other group, which is the case of colicins D and 10 [1, 6].
Colicins are encoded on operons that carry at least two, usually three genes: the colicin structural gene that encodes the toxin (cxa), followed by the immunity gene (cxi), which encodes a protein that provides the producing cell with specific protection against its own toxin. The third gene is the lysis protein encoding gene (cxl), or the bacteriocin release protein that facilitates the release of colicin through lysis of the cytoplasmic membrane of the producing cell [1-3, 5, 9, 10] .
The colicin toxins have three linearly organized functional domains: the central part (R-domain) is the colicin receptor-binding domain that is involved in recognition of specific receptors on the target cell outer membrane, whereas the N-terminal portion (T-domain) is involved in translocation across the outer membrane; finally, the C-terminus (P-domain) contains both the specific toxic activity and a binding site to the immunity protein [2, 5, 6, 9, 10, 19, 20].
In general, colicin killing of sensitive bacteria follows three steps: (i) binding of a colicin molecule to a specific receptor, mostly to ones involved in nutrient uptake, located outside the target bacterial outer membrane; (ii) translocating through the cell envelope using either the Ton or Tol systems; and (iii) having gained access to the cell interior, the toxin kills the target cell by one of a variety of ways, including: pore formation in the cytoplasmic membrane, nonspecific degradation of cellular DNA by nuclease, inhibition of protein biosynthesis by the cleaving of 16s ribosomal RNA, or degradation of the bacterial cell wall resulting from inhibition of peptidoglycan synthesis [1, 2, 5, 9, 10].
Unlike bacteriocins produced by Gram-positive bacteria, the induction of the colicin gene clusters is controlled and regulated by the host cell’s pathways, mainly by the SOS system [1, 2, 6, 9]. Production of the toxins is induced under conditions of stress such as DNA-damage, increasing population density or nutrient depletion. Once the colicin is produced, it is released to the extracellular environment by the lysis protein, causing the host cell’s death [5, 18, 22]. However, only a small fraction (less than 3%) of colicinogenic cells of the population "commits suicide” [6, 11, 13, 15, 21, 22].
The SOS response has been studied extensively in E. coli; the SOS genes are normally repressed and the SOS genes’ expression is the interplay of the LexA and RecA proteins that function as repressor and activator, respectively. Under standard conditions, colicin synthesis is switched off in most cells of a bacterial population as the SOS system is repressed by the LexA protein
by binding to a conserved sequence upstream the operon. Under DNA damage, the LexA is derepressed by the activation of the RecA protein, which induces the cleavage of the LexA repressor, consequently provoking the induction of an ensemble of DNA repair proteins as well as the colicin encoding genes [2, 6, 10, 11, 13, 23-26].So far, the regulation of only two colicins have been the subject of extensive study: Colicin E1 was found to be produced in response to a variety of assaults, including mutagenic agents, anaerobiosis, nutrient depletion and catabolite repression, suggesting that regulators other than SOS agents interfere with this colicin transcription [27, 28]. Colicin K [29], on the other hand, was found to be mainly induced by guanosine tetraphosphate (ppGpp), while the SOS response was found to be a mild signal for its expression. Amino acid starvation induced colicin K production [30, 31] while the expression invoked by DNA damage was not prominent. Interestingly, both colicin E1 and K belong to group A colicins; therefore, what is known so far about colicins has been obtained from seemingly similar proteins [30].
Previous studies have explored the role that SOS triggers play in colicin production, investigating the regulatory elements involved in their expression; the results showed that certain triggers such as, bile salts and β-lactams antibiotics induce the production of colicins though not through the promoter region located upstream to the initiation codon [32], suggesting that different mechanisms are involved in regulating colicin expression [33].
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