A dependence on CO2 for optimal CBR-5884 biological activity leucocidin expression (290). A CO2 percentage as high as 40 was used for the generation of preparative cultures of S. aureus for PVL purification (290). While CO2 may enhance the production of toxins, it does not appear to be required for abundant secretion of PVL, as most other studies cultured S. aureus under normal atmospheric conditions. Interestingly, DuMont et al. used luciferase reporter strains of S. aureus to demonstrate that the lukAB (lukHG) promoter is significantly upregulated over other leucocidins when cultured in the presence of primary human neutrophils. Such findings indicate that additional host-derived signals may directly influence leucocidin gene expression (234). In all cases, the media and growth conditions described above are relatively complex; thus, it is unknown what specific factors may be promoting the production of one leucocidin over another. These studies do highlight a number of critical considerations: (i) in vitro growth conditions must be carefully considered when evaluating in vitro leucocidin activity, (ii) the perceived dominance of any leucocidin under one growth condition is not necessarily the same under another, (iii) extrapolation of the relevance of one leucocidin to pathogenesis based solely on in vitro studies should be made with caution, and (iv) it is likely that complex environmental cues within an infected host may have effects similar to those seen in broth culture experiments, as the diversity of leucocidins in one infectious site may vary considerably from that in another. The described variations in toxin gene expression in complex media are likely a direct consequence of the diverse regulatory inputs known to exist in S. aureus, as many of these systems are believed to be highly responsive to environmentalmmbr.asm.orgMicrobiology and Molecular Biology ReviewsS. aureus LeucocidinsFIG 7 Leucocidin gene regulation. The molecular details of leucocidin gene regulation have not been extensively defined, but a number of master regulators and external signals provide major inputs into their altered gene expression in various environments. Some major regulatory inputs are shown. (1) The Agr quorum-sensing system is activated upon reaching a high bacterial density, leading to AgrA-dependent activation of the P3 promoter, which encodes the regulatory RNA, RNAIII. RNAIII negatively regulates the SCR7 chemical information translation of the leucocidin repressor Rot, leading to increased leucocidin production. (2) The global regulator SarA indirectly facilitates leucocidin expression by positively regulating the expression of the P3 promoter, leading to a similar repression of Rot translation and increased leucocidin production. (3) Rot is believed to bind directly to leucocidin promoters to inhibit toxin gene expression. (4) The SaeRS two-component system recognizes external stimuli from the environment, leading to direct binding of SaeR to leucocidin promoters and subsequent enhancement of gene expression. (5) Other environmental stimuli positively and negatively regulate leucocidin gene expression, although the precise stimuli and their mechanism(s) of activation/repression are yet to be defined.stimuli. Some major regulatory systems known to influence leucocidin gene expression are discussed below.Regulation at the Transcriptional LevelBacterial gene regulation of the bicomponent leucocidins is complex (Fig. 7). Multiple inputs from a variety of global regulatory systems feed into o.A dependence on CO2 for optimal leucocidin expression (290). A CO2 percentage as high as 40 was used for the generation of preparative cultures of S. aureus for PVL purification (290). While CO2 may enhance the production of toxins, it does not appear to be required for abundant secretion of PVL, as most other studies cultured S. aureus under normal atmospheric conditions. Interestingly, DuMont et al. used luciferase reporter strains of S. aureus to demonstrate that the lukAB (lukHG) promoter is significantly upregulated over other leucocidins when cultured in the presence of primary human neutrophils. Such findings indicate that additional host-derived signals may directly influence leucocidin gene expression (234). In all cases, the media and growth conditions described above are relatively complex; thus, it is unknown what specific factors may be promoting the production of one leucocidin over another. These studies do highlight a number of critical considerations: (i) in vitro growth conditions must be carefully considered when evaluating in vitro leucocidin activity, (ii) the perceived dominance of any leucocidin under one growth condition is not necessarily the same under another, (iii) extrapolation of the relevance of one leucocidin to pathogenesis based solely on in vitro studies should be made with caution, and (iv) it is likely that complex environmental cues within an infected host may have effects similar to those seen in broth culture experiments, as the diversity of leucocidins in one infectious site may vary considerably from that in another. The described variations in toxin gene expression in complex media are likely a direct consequence of the diverse regulatory inputs known to exist in S. aureus, as many of these systems are believed to be highly responsive to environmentalmmbr.asm.orgMicrobiology and Molecular Biology ReviewsS. aureus LeucocidinsFIG 7 Leucocidin gene regulation. The molecular details of leucocidin gene regulation have not been extensively defined, but a number of master regulators and external signals provide major inputs into their altered gene expression in various environments. Some major regulatory inputs are shown. (1) The Agr quorum-sensing system is activated upon reaching a high bacterial density, leading to AgrA-dependent activation of the P3 promoter, which encodes the regulatory RNA, RNAIII. RNAIII negatively regulates the translation of the leucocidin repressor Rot, leading to increased leucocidin production. (2) The global regulator SarA indirectly facilitates leucocidin expression by positively regulating the expression of the P3 promoter, leading to a similar repression of Rot translation and increased leucocidin production. (3) Rot is believed to bind directly to leucocidin promoters to inhibit toxin gene expression. (4) The SaeRS two-component system recognizes external stimuli from the environment, leading to direct binding of SaeR to leucocidin promoters and subsequent enhancement of gene expression. (5) Other environmental stimuli positively and negatively regulate leucocidin gene expression, although the precise stimuli and their mechanism(s) of activation/repression are yet to be defined.stimuli. Some major regulatory systems known to influence leucocidin gene expression are discussed below.Regulation at the Transcriptional LevelBacterial gene regulation of the bicomponent leucocidins is complex (Fig. 7). Multiple inputs from a variety of global regulatory systems feed into o.
