Impact of agr dysfunction on virulence profiles and infections associated with a novel methicillin-resistant Staphylococcus aureus (MRSA) variant of the lineage ST1-SCCmec IV
© Ferreira et al.; licensee BioMed Central Ltd. 2013
Received: 5 November 2012
Accepted: 22 April 2013
Published: 27 April 2013
A novel variant of the ST1-SCCmecIV methicillin-resistant Staphylococcus aureus (MRSA) lineage, mostly associated with nosocomial bloodstream infections (BSI), has emerged in Rio de Janeiro. Bacterial biofilm has been considered a major virulence factor in central venous catheter-associated BSI. The mechanisms involved in biofilm formation/accumulation are multifactorial and complex. Studies have suggested that biofilm production was affected in vitro and vivo for agr-null mutants of S. aureus.
The impact of naturally occurring inhibition of agr signaling on virulence profiles and infections associated with the ST1 variant was investigated. agr dysfunction was detected in a significant percentage (13%) of the isolates with concomitant increase in biofilm accumulation in vitro and in vivo, and enhanced ability to adhere to and invade airway cells. The biofilm formed by these ST1 isolates was ica-independent and proteinaceous in nature. In fact, the improved colonization properties were paralleled by an increased expression of the biofilm-associated genes fnbA, spa and sasG. The transcription of sarA, a positive regulator of agr, was two-times reduced for the agr-dysfunctional MRSA. Remarkably, the agr inhibition was genetically stable. Indeed, agr-dysfunctional isolates succeed to colonize and cause both acute and chronic infections in hospitalized patients, and also to effectively accumulate biofilm in a mouse subcutaneous catheter implant model.
The ability of agr-dysfunctional isolates to cause infections in humans and to form biofilm in the animal model suggests that therapeutic approaches based on agr-inactivation strategies are unlikely to be effective in controlling human-device infections caused by ST1 isolates. The increased biofilm accumulation associated with the acquisition of multiple antimicrobial resistant traits might have influenced (at least in part) the expansion of this USA400 related clone in our hospitals.
KeywordsMRSA ST1-SCCmecIV USA400 agr Biofilm Virulence factors
Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) lineage ST1- SCCmec IV was first reported in the 1980s among aborigines in Australia (WA-1 clone) and in the USA (MW2/USA400 clone) where cases of fatal infections were reported in Michigan, Minnesota and North Dakota [1–3]. Nowadays, CA-MRSA infections have been described in different countries involving a number of genetically distinct lineages [4, 5].
Many CA-MRSA isolates (including USA300, USA400 and USA1100) carry lukSF encoding for Panton-Valentine leukocidin (PVL). Despite the controversy regarding the role of the PVL, this leukocidin has been linked to severe skin infections and necrotizing pneumonia [6–8]. In the USA, USA300 has replaced USA400 as the predominant clone in many communities . However, USA400 isolates were the main cause of an outbreak of skin infections that occurred in rural southwestern Alaska, in 1996–2000 . Indeed, USA400 was the far most common CA-MRSA clone recovered from three northern remote communities of Saskatchewan, Canada . In 2005, a novel variant of the lineage ST1-SCCmecIV emerged in Rio de Janeiro city as an important cause of bloodstream infections (BSI) . It is intriguing that despite the genetic relationship with Australian WA-1 and MW2/USA400, isolates of this novel clone were PVL-negative, multiresistant and mostly involved in hospital-associated BSI . It is still poorly understood why isolates of CA-MRSA have become successful so quickly . Nevertheless, for hospital-associated MRSA (HA-MRSA), the bacterial ability to produce biofilm has been recognized as an important virulence factor for the pathogenesis of intravenous catheter-related bacteremia and infections associated with the use of medical prosthesis. In addition, the bacterial ability to adhere to, colonize and invade host tissues is considered important factor associated with bacterial virulence, adaptation and spread [14, 15]. Different surface proteins have been implicated in biofilm formation/accumulation and host colonization, including fibronectin-binding proteins A and B (FnBPAB), S. aureus surface protein G (SasG) and staphylococcal protein A (Spa) [16–19]. In addition, extracellular DNA (eDNA) has also been associated with bacterial biofilms .
It is also well known that virulence in S. aureus is modulated by an intricate regulatory network . The accessory gene regulator (agr), the major S. aureus quorum sensing system, down-regulates a number of genes encoding for cell-surface proteins involved in colonization processes, and up-regulates (by an indirect mechanism involving RNAIII dependent down-regulation of Rot) different exoproteins associated with host-cell damages . Previous works have suggested that inactivation of Agr could be very effective at inhibiting S. aureus infections , including those associated with implantable medical devices [24, 25]. Studies have demonstrated that biofilm production, host cell adhesion and invasion as well as other mechanisms involved in the establishment and course of staphylococcal diseases were affected by knockout of the agr locus [26–28]. Despite the improvements achieved in staphylococcal virulence, most of the investigations have been carried out using relatively few laboratory constructions or clinical isolates . In addition, those results have not been validated using current clinical isolates of MRSA. In this paper we characterized the biofilm formed by USA400-related (ST1-SCCmecIV) MRSA emergent in Rio de Janeiro, investigated the adhesive and invasive properties of naturally agr-dysfunctional isolates and analyzed the impact of the agr inhibition on S. aureus infections associated with the use of medical device. Our results suggest that strategies based on agr inactivation approaches may not be effective as an anti-biofilm strategy in the management of device-associated infections caused by these MRSA isolates.
Proteinaceous nature of the biofilm
Role of eDNA in ST1 biofilm
Expression of agr-regulated genes
Expression of biofilm-associated genes fnbAB, sasG and spa
The agr-dysfunctional isolate 08–008, which showed increased biofilm accumulation in vitro and in vivo, had a significant increase (p=0.02) in fnbA transcripts (RQ fnbA =10.08±0.18) when compared with the isolate 96/05 RQ fnbA =4.91±0.19; Figure 8). However, no significant difference was detected when fnbB expression were analyzed (RQ96/05 =0.11±0.04; RQ08-008 =0.18±0.05; Figure 8). Similarly to fnbA, the expression of sasG (Figure 8; p=0.03) and spa (Figure 8; p<0.001) was also increased in 08–008 (RQ sasG =1.13±0.11; RQ spa =52.8±0.17) compared with 96/05 isolate (RQ sasG =0.65±0.14; RQ spa =0.8±0.20).
Adherence and invasion
The great majority of the USA400-related isolates (50/60; 83.3%) were able to accumulate strong/moderate biofilms on polystyrene surfaces. The isolates remaining produced weak biofilms. The ability to accumulate biofilm increased when the surfaces were covered with human fibronectin, as also reported by others [19, 29]. In opposition to our results, it was reported that MW2 MRSA had a weak biofilm phenotype [30, 31]. Similarly, a slight biofilm accumulation (OD=0.25-0.3) was observed for another USA400 strain called BAA-1683 . In addition, recent data from our laboratory (Ramundo MS & Figueiredo AMS, 2012; unpublished observations) showed that another SCCmecIV isolates (ST30 CA-MRSA) accumulated much lower amount of biofilm compared with ST1-SCCmecIV isolates.
Previous data from our group  have also demonstrated that the ST1 isolates from Rio de Janeiro do not carry lukSF genes and have acquired a number of antimicrobial resistance traits. Thus, it is possible that the enhanced ability to accumulate biofilm, associated with the biological cost of acquired resistance and the absence of PVL, might have been the results (at least in part) of the microevolutionary events that accounted for changes in a previously community pathogen, promoting enhanced bacterial fitness to spread in hospitals and cause health-care associated diseases. The ica-independent nature of the biofilm formed by USA 400-related isolates was revealed by the disruption of bacterial film by proteinase K. Similar results were also observed by others using different MRSA isolates [33, 34]. Some researchers have suggested that the bacterial autolysis increases eDNA concentration and, consequently, enhances the level of biofilm accumulation . In fact, in our study, we observed a moderate correlation between biofilm accumulation and autolysis. In addition, we detected threefold increase in eDNA for the ST1 MRSA displaying enhanced ability to accumulate biofilm. Indeed, the addition of DNase I (56U/Well) caused a significant reduction (about 30%) in biofilm accumulation, suggesting eDNA cooperatively contributes to the biofilm architecture of ST1 isolates.
The statistical analysis showed that the group of clinical isolates with no hemolytic activity (agr-dysfunctional) had significant increase in the level of biofilm accumulation when compared with agr-functional isolates. These data are in agreement with previous studies for agr-laboratory knockouts [27, 35, 36], which have indicated that some agr mutants can display increased levels of biofilm accumulation. In spite of that, using another S. aureus strain it was reported that inhibition of agr reduced biofilm accumulation significantly [24, 25]. In fact, agrRNAIII is a negative regulator of different surface proteins [22, 23], and consistent with this regulation, amplified expression of genes encoding for biofilm-associated proteins FnBPA, SasG and Spa was found for the agr-dysfunctional variant. Both FnBPA and B have been implicated as major proteins for biofilm formation/accumulation in S. aureus[19, 33]. However, despite the detection of an enhanced expression of fnbA, we could not find a significant increase in the transcription of fnbB-mRNA for the agr-dysfunctional ST1-MRSA. Equally, a study from Wolz and collaborators suggested that fnbB was not significantly affected by agr.
Confirming the agr inhibition detected, the expression of two genes up-regulated by RNAIII, hla and psmα, was lower compared with the agr-functional MRSA. Both cytolysins (HLA and PSMα) seem to have remarkable roles in the pathogenesis of S. aureus. HLA has been associated with lethal pneumonia in USA400 and USA300 strains [37, 38]. It was also previously found that psmα-deleted mutant of CA-MRSA exhibited attenuated virulence in animal models . In this study, we detected a superior expression of pmsα by the agr-functional isolates of USA400-related clone detected in Rio de Janeiro. In fact, it was shown by others that the transcription of psmα-mRNA was increased in most prevalent CA-MRSA lineages, including MW2, compared with other S. aureus isolates . However, the molecular mechanisms involved with the enhanced expression of PSMα were not clarified . Despite the importance of these virulence factors for S. aureus pathogenicity, it is remarkable that among the agr-dysfunctional variants, 4 were recovered from cases of BSI, 2 from colonization, 1 from pneumonia and 1 from infected prosthesis, showing that these variants were able to colonize and cause both severe acute (pneumonia and BSI) and chronic (foreign-body infection) staphylococcal diseases in humans. These data demonstrated that regardless the reduced virulence of agr-laboratory knockouts in some animal models , the virulence of naturally dysfunctional agr variants was confirmed for hospitalized patients. In contrast to the assumption that agr-dysfunctional isolates may not be able to initiate infections , the isolate 08–008 was able to colonize polyurethane endovenous catheter in a foreign-body mouse model, forming a denser biofilm accumulation when compared with the agr-functional isolate. It is important to state that because the ST1 isolates studied were not isogenic, it is possible that factors other than the inhibition of agr might also have accounted for the increased biofilm accumulation observed. Nevertheless, supporting our data, similar increase of the biofilm formed on catheters implanted in mice was previously reported for an agr laboratory knockout . In opposition to the results obtained by Traber et al. , all individual colonies formed by the agr-dysfunctional MRSA remained non-hemolytic before and after passages in mice, strongly suggesting the genetic stability of the phenotype. This stability was confirmed for all agr-dysfunctional isolates from our collection. Corroborating our findings, while we were finishing this manuscript, we noticed the work by Park et al.  that found agr dysfunction in S. aureus significantly associated with persistent bacteremia with eradicated foci, even though the predominant MRSA isolates showed SCCmecII, agrII (possible belonging to USA100-New York/Japan clone) while the isolates studied here displayed SCCmecIV, agrIII and clustered in USA400-MW2/WA-1 clone. In fact, the bacterial ability to adhere to and invade epithelial cells, and consequently evade host defense mechanisms, has already been associated with persistence in host cells and development of disseminated infections [43, 44]. In the present study, the differential expression of agrRNAIII in MRSA clinical isolates had a significant impact on adherence and invasion at 3h30min incubation. The same impact was observed for the agr isogenic knockout, as previously showed by others using different cell lines and mostly laboratory mutants [26, 45].
Recently, Pozzi et al. demonstrated that high level of PBP2a expression by the homogeneous methicillin-resistant derivative of the strain 8325–4 induced a proteinaceous biofilm and significant repression of the agr locus . In addition, excision of the SCCmec element from the MRSA strain BH1CC, with consequent loss of oxacillin resistance, had the opposite effect on biofilm and lead to an increase of the agrRNAIII transcription. In addition, Rudkin et al. showed that methicillin resistance reduced the virulence of HA-MRSA by interfering with agr. The great majority of ST1 isolates studied had MIC of 128 µg/mL (agr-functional or -dysfunctional), which is compatible with heterogeneous resistance to this drug. Indeed, mecA overexpression was not detected in the agr-dysfunctional isolates tested. SarA, a global transcriptional regulator of S. aureus, was previously found to be a positive regulator of agr and of biofilm formation/accumulation [21, 48]. Thus, aiming to understand the mechanism involved in agr impairment in these clinical isolates, the level of sarA transcripts was also examined. It was observed that sarA expression was significantly diminished in the agr-dysfunctional compared with the agr-functional MRSA, suggesting the defect was upstream agr. Beeken et al. indicated that sarA repression inhibited biofilm accumulation due to SarA inhibition of both proteases and nucleases activity either in the presence or absence of agr mutations . In contrast, the results obtained here demonstrated that agr-dysfunctional isolates showed increased biofilm accumulation, despite the fact that sarA-mRNA transcripts were reduced. In fact, other studies have showed that sarA or agr-sarA laboratory mutants had lower ability to bind to fibronectin due to sarA down-regulation of fnbA transcription . Possible explanations for this apparent divergence could be the fact that the agr-dysfunctional ST1 studied showed only partial sarA inhibition, or may display strain-dependent variation in the genetic background affecting other genes apart to those studied.
Isolates of this novel hospital-associated USA400 clone were able to accumulate moderate/strong amount of biofilms, in vitro and in vivo, and could efficiently adhere to and invade human airway cells. Moreover, agr inhibition was an ordinary phenomenon among those isolates, which seems to have impacted the expression of some important virulence genes studied. Although it is difficult to interpret in vitro studies in the light of what occurs in an infected human host, it follows logical that the enhanced adhesive properties combined with the acquisition of multiple drug resistance traits by ST1 isolates could have provided fitness advantages for spreading in hospital environments. Indeed, agr-dysfunctional isolates were recovered from cases of hospital pneumonia, bacteremia and infected prosthesis. Finally, our results strongly suggest that strategies for controlling MRSA biofilm based on agr inhibition approaches are unlikely to be effective, at least for ST1 MRSA isolates.
Sixty USA400-related isolates were obtained from patients located in different hospital wards in Rio de Janeiro as part of standard clinical care. Thirty isolates were recovered from BSI (50%) and 8 from catheter tips (CT; 13.3%). The remaining were from colonization (C; 13.3%), pneumonia (P; 6.7%), skin/soft tissue infections (SSTI; 5%), urinary tract infections (UTI; 3.3%) and prosthesis fragment (PF; 1.7%). The infection sites had not been reported for 4 isolates. The agr-knockout MNY474 (Δagr::tetM) and the rnaIII-trans-complemented mutant CMNY474 (Δagr::tetM, pbla-rnaIII) were previously constructed from the clinical S. aureus isolate NY474 . BMB9393 (ST239-SCCmecIII) was used as positive control for biofilm and gene expression experiments . The S. aureus RN4220 and RN6390B, a gift from Richard Novick (New York University), were used for hemolytic activity and gene expression analyses; respectively. This study was approved (#1055/09) by the Human Research Ethics Committee from Federal University of Rio de Janeiro, RJ, Brazil.
Minimal inhibitory concentration (MIC)
Oxacillin MIC was determined using Müller Hinton plates and performed in accordance with the Clinical Laboratories Standards Institutes (CLSI) guidelines .
In vitrobiofilm assay
For all 60 isolates, biofilm was tested using 96-well inert polystyrene microtiter plates (Nunclon; Nunc A/S, Roskilde, Denmark) as previously described . The biofilm unit (BU) was defined as indicated by Amaral et al.  and the isolates were classified as non-producers (BU≤0.230), weak (BU>0.230 and ≤0.460), moderate (BU>0.460 and ≤0.920) or strong producers (BU>0.920), as suggested . For 19 isolates, biofilm assays were also carried out on surfaces covered with human fibronectin (Merck; Darmstadt, Germany) as previously described .
In some experiments, before treatment with crystal violet, the biofilm was treated with sodium metaperiodate (10mM/well; Sigma; St. Louis, MO, USA) or proteinase K (6U/well, Invitrogen; Carlsbad, California, EUA) . Confocal laser scanning microscopy (CLSM) was employed to record and contrast structural images of the biofilm as described . eDNA was quantified in biofilm supernatants using Qubit® 2.0 Fluorometer (Invitrogen; Eugene, Oregon, USA), after ethanol precipitation. For some experiments, biofilms were formed in the presence of DNase I (28U/well or 56U/well Invitrogen; Carlsbad, California, EUA).
A pair of isolates showing differential agr expression (08–008, agr-dysfunctional, obtained from BSI and 96/05, agr-functional, from CT) was used. The mouse subcutaneous catheter implant model was described in detail by Ferreira et al. . Briefly, two intravenous polyurethane catheter segments (C-UDLM-953J model; Cook Medical, Bloominaton, USA) were implanted in the back of each anesthetized young-adult BALB/c male mice. Infection was induced 24 h after the implantation procedure by injecting a mid-exponential growth phase culture (106 CFU/10 µL) into the lumen of the implanted catheter segment. The animal was euthanized after three days post-infection, and the catheter segments were surgically removed to assess the biofilm by counting catheter-adherent bacteria by CFU determination. Three independent experiments were performed. The animal study was approved (#IMPPG013) by The Ethics Committee for Animal Care and Use from Federal University of Rio de Janeiro, RJ, Brazil.
Difco™ DNase Test Agar (BD; Becton, Dickinson and Company, Sparks, USA) was used to screen 17 USA400-related MRSA, as recommended by the manufacturer.
Autolysin activity was measured in 8 selected isolates as previously described , except that cells were grown in TSB 1% Glc.
The δ-hemolysin (Hld), encoded by the hld gene, is codified within the rnaIII region and, consequently, the detection of δ-hemolysin is an indicative of agr expression. Sixty USA400-related isolates were screened for hemolytic activity on sheep red blood (5%) agar plates (Plast Labor, RJ, Brazil) as previously described .
Primers used in Real Time qPCR
Amplicon length (bp)
Adherence and invasion kinetics
Bacterial adherence and invasion were investigated using human bronchial epithelial cells (16HBE14o- cell line) as described , except that monolayers were prepared using Dulbecco´s Modified Eagle Medium (DMEM, Low Glucose 1X; Gibco, Invitrogen, Grand Island, USA) and 10% Fetal Bovine Serum (Gibco, Invitrogen). For determining the colony forming units (CFU) of the total adhered and invasive bacteria (CFUAI), infected monolayers were washed twice in DMEM (to remove non-adherent bacteria), incubated (5 min/37°C) with 0.25% (wt/vol) trypsin (11,000 U/mg; Sigma; St. Louis, MO USA), lysed (5 min/37°C) with 0.025% (vol/vol) Triton X-100 (Sigma) and plated in TSA. For determining the CFU of invasive bacteria (CFUI), infected monolayers were washed twice in DMEM and incubated (20 min/37°C) with 100 µg/mL lysostaphin (500 U/mg; Sigma) to lyse adherent bacteria. Monolayers were washed twice and incubated (5 min/37°C) with 0.25% (wt/vol) trypsin. The epithelial cells were lysed (5 min/37°C) with 0.025% (vol/vol) triton X-100 and plated. For each aliquot, the total CFU in the supernatant was also determined (CFUS). The CFU of adherent bacteria (CFUA) was obtained by the formula: CFUA = CFUAI - CFUI. The percentages of invasive or adherent bacteria were calculated considering as 100% the total CFU obtained by the sum of CFUAI + CFUS for each aliquot. In addition to the USA400-related isolates, the wild-type HC474, and the isogenic Δagr::tetM and rnaIII-trans-complemented constructions were also used for investigating bacterial invasion.
Student’s t-test (unpaired data) was used to compare the means of the biofilm values and of the data from gene expression experiments. In addition, correlation coefficient (r) was used to test the relationship between the autolysis and the ability of ST1 isolates to accumulate strong or weaker biofilms. This last test was also used to determine the occurrence of linear correlation between mecA and agr expressions . Data were expressed in terms of mean values obtained from at least three independent experiments and three repetitions of each set.
Accessory gene regulator
Community-acquired Methicillin-resistant Staphylococcus aureus
Confocal laser scanning microscopy
Dulbecco´s Modified Eagle Medium
Genes encoding for fibronectin-binding protein A and B
Fibronectin-binding protein A and B
Operon encoding for enzymes involved in the synthesis of the polysaccharide of intercellular adhesion (PIA)
Hospital-associated methicillin-resistant Staphylococcus aureus
Genes involved in the synthesis of the subunits S and F of the Panton Valentine leukocidin
Minimal inhibitory concentration
Gene encoding for penicillin-binding protein 2A
Methicillin-resistant Staphylococcus aureus
Penicillin-binding protein 2A
Polymerase chain reaction
Repressor of toxins
Real time quantitative polymerase chain reaction
- rrna 16S:
Gene encoding for ribosomal RNA 16S
Transcriptional regulator SarA
S. aureus surface protein G
Staphylococcal cassette chromosome mec
Skin and soft tissue infections
Tryptic soy broth
- TSB 1% Glu:
Tryptic soy broth supplemented with 1% glucose
Urinary tract infections.
This work was supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Rio de Janeiro (FAPERJ), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and by European Commission's Seventh Framework Programme (FP7), through the Marie Curie International Research Staff Exchange Scheme NANO_GUARD (PIRSES-GA-2010-269138).
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