Disease relevance of hla

• Background. The Staphylococcus aureus global regulators--agr, sarA, and sae--coordinately control alpha -toxin gene (hla) expression in vitro [1].
• These strain sets were used to quantify hla expression in vitro and in an experimental infective endocarditis (IE) model using flow cytometry.Results [1].
• The same mutation was present in the hla gene cloned from a human septicaemia strain (V37) isolated in Dublin. The nonsense mutation of Todd 555 was suppressed by the supE44 mutation in Escherichia coli resulting in haemolytic activity in cell lysates [2].
• The hla gene was cloned from an Hla- TSST-1+ strain, Todd 555, which had been isolated from a case of toxic shock syndrome in the USA [2].
• In Staphylococcus aureus infection hemolysis caused by the extracellular protein alpha-toxin encoded by hla is thought to contribute significantly to its multifactorial virulence [3].

High impact information on hla

• In addition, salicylic acid reduced the expression of the alpha-hemolysin gene promoter, hla, and the fibronectin gene promoter, fnbA [4].
• Hybridization with RNAIII prevents this intramolecular base-pairing and makes the hla mRNA accessible for translation initiation [5].
• Deletion of the SarA recognition motif in the promoter regions of agr and hla in shuttle plasmids rendered the transcription of these genes undetectable in agr and hla mutants, respectively [6].
• However, DNase I footprinting assays demonstrated that the SarA-binding region on the spa and hla promoter is more extensive than the predicted consensus sequence, thus raising the possibility that the consensus sequence is an activation site within a larger binding region [6].
• Mutation of agr and sarA in strain Newman and RN6390 had no consequence for hla expression in vivo [7].

Chemical compound and disease context of hla

• Effects of subinhibitory concentrations of antibiotics on alpha-toxin (hla) gene expression of methicillin-sensitive and methicillin-resistant Staphylococcus aureus isolates [8].
• The levels of expression of hla and alpha-toxin and total hemolysis did not differ significantly when amoxicillin, gentamicin, or moxifloxacin was added to cultures of S. aureus strain 8325-4 [3].
• In the present study we investigated the effects of subinhibitory concentrations of amoxicillin, gentamicin, and moxifloxacin on hla and alpha-toxin expression and total hemolysis of S. aureus strain 8325-4, a high-level alpha-toxin producer, and its alpha-toxin-negative mutant, DU 1090, in vitro and in a rat model of chronic S. aureus infection [3].

Biological context of hla

• In vitro, hla expression was positively modulated by all 3 regulons (sae > agr/sarA > agr and sarA) in both RN6390 and SH1000 backgrounds [1].
• Additionally, it was shown that mutation of msa resulted in altered transcription of the accessory gene regulator (agr) and the genes encoding several virulence factors including alpha toxin (hla) and protein A (spa) [9].
• In contrast, the mutation in sae resulted in severe downregulation of hla in vitro as well as in vivo [7].
• The majority of clinical isolates of Staphylococcus aureus that produce toxic shock syndrome toxin-1 (TSST-1) fail to express alpha-toxin, despite having a copy of the hla gene in the chromosome [2].
• Transcriptional fusions showed that expression of hla, ssp and spa was higher in both mutants than in the wild-type strain, indicating that the arl operon decreases the production of virulence factors by downregulating the transcription of their genes [10].

Anatomical context of hla

• Ribonuclease T1 digestion experiments revealed that the ribosome binding site of the hla transcript is blocked by intramolecular base-pairing [5].
• Here, we assess the regulation of hla in a guinea pig model of device-related infection by quantifying RNAIII (the effector molecule of agr) and hla directly in exudates accumulating in infected devices without subculturing of the bacteria [7].
• Despite the low level of RNAIII expression, spa was detectable only in minute amounts and an irregular transcription of hla was observed in all sputum samples [11].
• Protoplast fusion was then used to isolate double-defective (hla and coa) recombinants and recombinants with regained properties, i.e., production of alpha-hemolysin and coagulase [12].
• Coagulase was assayed by clotting of citrated rabbit plasma, and hla, hld and SLO by lysis of rabbit, human and horse erythrocytes, respectively [13].

Associations of hla with chemical compounds

• Four differently regulated genes (hla, alpha-toxin; hld, RNAIII; spa, protein A; ssp, serine protease) were analysed for binding of potential regulatory proteins to the corresponding promoter DNA fragments, linked to magnetic beads [14].
• The effect of glucose on two agr target genes, sec+ and hla, was also studied [15].
• Furthermore, Northern blot analysis of hla mRNA and Western blot (immunoblot) analysis of culture supernatants of both methicillin-sensitive and methicillin-resistant S. aureus strains revealed that methicillin-induced alpha-toxin expression is a common phenomenon of alpha-toxin-producing strains [8].
• The most striking observation was a strong induction of hla expression by subinhibitory concentrations of beta-lactams and an almost complete inhibition of alpha-toxin expression by clindamycin [8].
• Whereas glycopeptide antibiotics had no effect, the macrolide erythromycin and several aminoglycosides reduced and fluoroquinolones slightly stimulated hla expression [8].

Regulatory relationships of hla

• This has now been verified by the demonstration that sarA repressed hla transcription in an rsbU(+) derivative of strain 8325-4 (SH1000) [16].
• The effect of the sigB mutation on the enhanced expression of the alpha-hemolysin gene (hla) was primarily transcriptional [17].

Other interactions of hla

• Regulation of Staphylococcus aureus alpha -Toxin Gene (hla) Expression by agr, sarA, and sae In Vitro and in Experimental Infective Endocarditis [1].
• SarA Is a Repressor of hla ({alpha}-Hemolysin) Transcription in Staphylococcus aureus: Its Apparent Role as an Activator of hla in the Prototype Strain NCTC 8325 Depends on Reduced Expression of sarS [16].
• Upon complementation with a plasmid containing the sigB gene, hla expression returned to near parental levels in the mutant [17].
• Our results indicated that the solely active P1 promoter led to a lower SarA protein level, which has an effect on agr transcription and subsequently had corresponding effects on hla, sspA, and spa transcription, probably in both agr-independent and agr-dependent manners [18].
• Infection of D. melanogaster with reporter gene fusion strains demonstrated the in vivo expression levels of the accessory gene regulator, agr, alpha-toxin, hla, and a manganese transporter, mntA [19].

Analytical, diagnostic and therapeutic context of hla

• These observations were verified by Northern blot analysis of hla mRNA and by Western blot (immunoblot) analysis of culture supernatants of strain Wood 46 [20].
• Also, intraperitoneal injection of 1.5 x 10(9) CFU of the antisense hla-containing transformant was significantly less lethal in a murine model than that of the parent (1 of 10 versus 7 of 10 mice expired [P < 0.02]) or the transformant without hla (1 of 10 versus 7 of 7 mice expired [P < 0.001]) [21].
• Molecular characterization of CA-MRSA was performed with PCR, including staphylococcal cassette chromosome (SCCmec), pvl (lukS-PV plus lukF-PV), hla, hlb and selected microbial surface components recognizing adhesive matrix molecule genes, ie, cna, clfA, fnbA and fnbB [22].
• The agr mutant strain and the hla mutant strain showed no difference in bacterial counts in murine wounds compared to their respective parent strains [23].
• Single strain comparisons between wild-type strain 8325-4 and strain DU1090 (hla-) as well as between strain RN6911 (agr) and wild-type strain RN6390 were performed using the same three animal models of infection [23].

References