Abstract
The virulence factor α-toxin (hla) is needed by Staphylococcus aureus in order to cause infections in both animals and humans. Although the complicated regulation of hla expression has been well studied in human S. aureus isolates, the mechanisms of of hla regulation in bovine S. aureus isolates remain undefined. In this study, we found that many bovine S. aureus isolates, including the RF122 strain, generate dramatic amounts of α-toxin in vitro compared with human clinical S. aureus isolates, including MRSA WCUH29 and MRSA USA300. To elucidate potential regulatory mechanisms, we analyzed the hla promoter regions and identified predominant single nucleotide polymorphisms (SNPs) at positions −376, −483, and −484 from the start codon in α-toxin hyper-producing isolates. Using site-directed mutagenesis and hla promoter-gfp-luxABCDE dual reporter approaches, we demonstrated that the SNPs contribute to the differential control of hla expression among bovine and human S. aureus isolates. Using a DNA affinity assay, gel-shift assays and a null mutant, we identified and revealed that an hla positive regulator, SarZ, contributes to the involvement of the SNPs in mediating hla expression. In addition, we found that the bovine S. aureus isolate RF122 exhibits higher transcription levels of hla positive regulators, including agrA, saeR, arlR and sarZ, but a lower expression level of hla repressor rot compared to the human S. aureus isolate WCUH29. Our results indicate α-toxin hyperproduction in bovine S. aureus is a multifactorial process, influenced at both the genomic and transcriptional levels. Moreover, the identification of predominant SNPs in the hla promoter region may provide a novel method for genotyping the S. aureus isolates.
Highlights
S. aureus is an important pathogen capable of causing both animal and human infections such as pneumonia, endocarditis, toxic shock syndrome and bovine mastitis
QPCR analysis identified that RF122 has a 7-fold higher level of hla transcript compared to WCUH29, suggesting the up-regulation of a-toxin is at the transcription level (Table 1)
Our studies revealed that the hla promoter region possesses predominant single nucleotide polymorphisms (SNPs) that contribute to the modulation of hla overexpression in some bovine S. aureus isolates
Summary
S. aureus is an important pathogen capable of causing both animal and human infections such as pneumonia, endocarditis, toxic shock syndrome and bovine mastitis. The continuous increase of infections associated with both hospital- and community-acquired methicillin resistant Staphylococcus aureus (HA-MRSA and CA-MRSA) has caused serious public health concerns [1]. The ability of this organism to cause a wide variety of infections partially depends on the coordinated and regulated expression of multiple cell- and surface-associated virulence factors [2,3] and exported proteins including various proteases and toxins [4]. Alpha-toxin plays a critical role in the modulation of S. aureus– induced cytotoxicity in Jurkat T-lymphocytes, human peripheral blood lymphocytes, monocytes [5] and epithelial cells [6], even though multiple virulence factors are required for the bacterium to induce apoptosis in endothelial cells [7]. It has been demonstrated that a-toxin is an important virulence factor in experimental brain abscesses and pneumonia [14,15,16] and intraperitoneal infection [17,18], whereas the overproduction of a-toxin significantly reduces virulence in experimental endocarditis [19]
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