Staphylococcus aureus isolated from patients with recurrent furunculosis carrying Panton-Valentine leukocidin genes represent agr specificity group IV

ARTICLE

Auteur(s) : Katarzyna GARBACZ¹ kasgab@amg.gda.pl, Lidia PIECHOWICZ¹, Wioletta BARAŃSKA-RYBAK², Maria DąBROWSKA-SZPONAR¹

¹ Department of Medical Microbiology, Medical University of Gdańsk, Poland

² Department of Dermatology, Venereology and Allergology, Medical University of Gdańsk, ul. Do Studzienki 38, 80-227 Gdańsk, Poland

Reprints: K. GARBACZ

Furuncles are acute, usually necrotic infections of hair follicles caused by Staphylococcus aureus. Furunculosis is diagnosed when multiple furuncles at different stages of development are found on the patient [1]. The clinical presentation varies greatly, from small follicular nodules to large nodules with varying degrees of erythema, swelling, and central necrosis [2]. The infection tends to be recurrent in many patients and often spreads to other family members [3]. Recurrent furunculosis (RF) constitutes a difficult clinical problem because it may require incision, drainage and antibiotic therapy [4]. The tendency for certain individuals to develop RF is not fully understood [5].

The pathogenicity of S. aureus is associated with the production of a number of extracellular toxins, enzymes and cell-surface-associated proteins. The expression of most of these virulence factors is controlled by the locus agr (accessory gene regulator). Based on the polymorphism of agrC and agrD genes, S. aureus strains may be divided into four major groups (I-IV) [6]. Associations between a peculiar agr type and specific staphylococcal syndromes have been shown for toxic shock syndrome (TSS) and staphylococcal scalded-skin syndrome (SSSS). Toxic shock syndrome toxin-1 (TSST-1) produces isolates belonging to agr specificity group III, whereas most exfoliatin-producing strains responsible for SSSS belong to the agr group IV [7, 8].

The aim of the study was to characterise agr groups and detect toxin genes among S. aureus strains isolated from patients with recurrent furunculosis.

Materials and methods

Bacterial strains

Forty-four patients suffering from RF, reporting to four dermatological clinics within the city of Gdańsk (Poland) between 2004 and 2005, were included in the study. The control group comprised 150 healthy volunteers with no history of RF. These were students of the Medical University of Gdansk, examined during their 2008/2009 classes at the Department of Medical Microbiology. Microbiological material was obtained from evacuated furuncules and the swabs were inoculated directly onto 5% sheep blood agar (Oxoid, England). One strain of S. aureus was isolated from every subject. Additionally, for the purpose of S. aureus carrier state identification, nasal swabs were obtained both from RF patients and from the controls. S. aureus identification was performed on the basis of colony morphology, Gram staining, Staphyslide agglutination test (BioMerieux, France) and by the API ID32Staph system (BioMerieux, France). Control strains for PCR detection of genes of virulence factors and agr groups were obtained from the National Institute of Public Health (NIPH, Warsaw, Poland) and from the National Staphylococcus Reference Centre (Gdańsk, Poland). Both the control and examined strains were stored in Triptic Soy Broth (Oxoid, England) at –70 ̊C and supplemented with 15% glycerol.

Toxin gene detection by PCR technique

DNA was isolated according to Barski et al. [9]. DNA amplification was carried out in Perkin Elmer 2400 thermocycler (Norwalk, USA). Primers for enterotoxins (SEA, SEB, SEC and SED), TSST-1 and exfoliative toxins (ETA, ETB) were used as described previously [10]. Bacterial DNA (50-100 ng) of S. aureus isolates was amplified in two sets of multiplex PCR. Set A contained 20 pmol (each) of sea, seb, and sec primers and 40 pmol of sed primer, while set B contained 50 pmol of eta and 20 pmol (each) of etb, and tst primers. Detection of the genes was performed in 50 μL of a mixture consisting of: 5 μL bacterial DNA (5 ng/μL), 5μl 10X Reaction Buffer (Fermentas, Germany), primers sea, seb, sec, sed or eta, etb and tst (Sigma-Proligo, USA), 200 μM (each) dNTP, 2.5 U of Taq polymerase (Fermentas, Germany) and sterile water. DNA amplification was carried out with the following thermal cycling profile: an initial denaturation at 94 ̊C for 5 min was followed by 35 cycles of amplification (denaturation at 94 ̊C for 2 min, annealing at 57 ̊C for 2 min, and extension at 72 ̊C for 1 min) and a final extension at 72 ̊C for 7 min. The amplification of PVL genes (lukS/lukF-PV) was performed as described by Lina et al. [11]. Agr specificity groups were identified by PCR amplification of the hypervariable domain according to Shopsin et al. [12]. The PCR products were analysed on 2% agarose gel (Sigma, USA) in the presence of ethidium bromide and photographed under UV illumination.

Antimicrobial susceptibility test

The antibiotic-susceptibility test included eight antimicrobial agents: clindamycin, ciprofloxacin, co-trimoxazole, erythromycin, gentamicin, oxacillin, penicillin, and tetracycline (Becton Dickinson, USA), and was performed by the disk diffusion method in accordance with the guidelines set by the Clinical and Laboratory Standards Institute (CLSI) [13]. Interpretation of the results was performed according to CLSI recommendations [13]. Methicillin susceptibility was verified by mecA gene amplification [9].

Statistical analysis

Continous variables were presented as medians and ranges. Association between the presence of lukS/lukF-PV genes and agr group IV specificity was verified with Fisher exact test, similar to the significance of differences in S. aureus nasal carriage rates between RF patients and the controls. Calculations were performed using Statistica 8 (StatSoft^®, Poland) software, and statistical significance was defined as p ≤ 0.05.

Results

The median age of patients was 29 years (range 5-45 years). Most patients (14/44) studied were aged 21 to 30, whereas only 2 patients were between the ages of 0 and10. There were 24 males and 20 females. Median time from RF diagnosis was 24 months (range 4-120 months). Furuncules were present on the buttocks (20/44), lower limbs (19/44), chest (14/44), upper limbs (11/44), back (8/44) and face (1/44). Nearly half of RF patients (21/44) had furuncules in more than one location (usually in two or three anatomical regions). Nine subjects declared familial history of RF (two or three relatives affected). Only two patients in the examined group suffered from predisposing factors of RF: diabetes type 2 or chronic myeloid leukaemia.

The strains isolated from furuncules were resistant to penicillin (42/44), clindamycin (3/44), erythromycin (9/44), and tetracycline (5/44). The number of the strains simultaneously resistant to two (penicillin, erythromycin) or three antibiotics (penicillin, erythromycin, clindamycin) totalled 7/44 and 3/44, respectively. All strains showed susceptibility to methicillin, confirmed either with the use of the disc diffusion method or by means of PCR.

Thirty-five out of 44 strains tested were positive for leukocidin lukS/lukF-PV genes and 12/44 for enterotoxin seb gene (figure 1). The coexistence of PVL genes and seb gene concerned 7/44 strains. The remaining toxin genes were not found.

Forty-three strains belonged to agr specificity group IV and one strain was classified as agr group II (figure 2). All strains with lukS/lukF-PV genes belonged to agr specificity group IV (35/43 vs. 0/1 in agr group II, p = 0.205).

Nasal carriage of S. aureus was observed in 27/44 (61.3%) RF patients and in 43/150 (28.6%) controls (p = 0.001). Characteristics of the nasal strains of S. aureus isolated from RF patients and the controls are summarized in table 1. In all the RF subjects, nasal strains did not differ from those isolated from furuncules in terms of lukS/lukF-PV gene status and agr specificity.

Table 1 Agr and toxin gene characteristics of the nasal strains of S. aureus isolated from patients with recurrent furunculosis and from healthy controls

^* Fischer exact test

Discussion

Recurrent furunculosis frequently occurs among young people and is characterised by a complex pathogenesis which has not yet been fully explained [2, 5]. Many risk factors contribute to the recurrence of furuncles, including: obesity, alcoholism, bad dietary habits, haematological disorders, immunosupression, AIDS, and diabetes [1]. However, only two patients in the group we examined suffered from predisposing factors of RF. Similar to our study, other authors have also shown that young adults without underlying systemic diseases predominate among furunculosis patients [2, 3, 5].

Similar to previous experiments [1, 5, 14], this study showed that the fraction of S. aureus nasal carriers is higher amongst RF patients than in healthy controls. According to Toshkova et al. [15], nasal carriage may occur in up to 100% of RF subjects. Many previous studies showed that staphylococci isolated from invasive infections, bacteremia, pneumonia, bone infections or meningitis belong to agr groups I-III, and group IV specificity is sporadic [8, 16-18]. However, S. aureus strains causing SSS or impetigo belong to group IV and are characterised by their ability to produce exfoliative toxin [7, 19]. In the present study, we found that 43/44 strains isolated from recurrent furunculosis and only 6/43 strains from nasal carriers of S. aureus who were free of RF belonged to agr group IV. It has been previously reported that only 39.6% of strains found in skin abscesses or furuncles belong to this group [20]. To the best of our knowledge, we were the first study to observe such a predominance of agr group IV strains in RF patients.

Additionally, a high percentage of the strains from RF patients (but none from the controls) encoded Panton-Valentine leucocidin and all of them belonged to agr group IV. PVL is a cytotoxin that causes leukocyte destruction and tissue necrosis. The PVL gene has been detected in S. aureus strains associated with community-acquired, severe, necrotizing pneumonia and also with furunculosis [11]. The frequency of PVL gene positive strains isolated from RF patients varies with geography. The high fraction (35/44) that we reported is similar to the results of other European studies [11]. According to the literature, PVL gene-positive strain occurrence ranges from 30% in Western Africa, 40% in Japan, up to 93% in France [2, 11, 21].

Producing leukocidin also has an impact on the recurrence of furuncles. Couppie et al. [22] reported that 92% of S. aureus strains isolated from persons with recurrent furunculosis produced PVL, compared to only 33% of strains found in sporadic furuncles. These results are consistant with the high frequency of PLV positive strains found during our study on RF patients. The occurrence of lukS/F-PV genes constitutes a risk of a more severe course of furunculosis. According to Yamasaki et al. [2], furuncles caused by PVL gene-positive S. aureus have been associated with more-intense erythema around the lesions, reflecting a strong inflammation caused by polymorphonuclear cells and capillary dilation.

A few studies have shown that the PVL locus is a stable genetic marker of community-acquired methicillin-resistant S. aureus (MRSA) [23]. However, resistance to methicillin is also typical of PVL positive strains isolated from pneumonia, septicemia or other systemic infection and not from furunculosis. Along with our investigations, Nolte et al. [20], Wiese-Posselt et al. [24], and Perez-Roth et al. [25] also found that PVL positive S. aureus isolated from furunculosis were susceptible to methicillin (MSSA), in contrast to strains isolated from severe infections. However, our study revealed a high frequency of strains encoding PVL toxin among methicillin-susceptible staphylococci isolated from RF patients. This finding proves that the diagnosis of such infections should not be limited to antibiotic susceptibility testing.

Conclusion

Nasal carriage of S. aureus in RF patients is significantly more frequent than in healthy individuals. Methicillin-susceptible strains of agr specificity group IV, most of them with lukS/lukF-PV genes, predominate amongst S. aureus strains isolated from RF patients.

Disclosure

Financial support: none. Conflict of interest: none.

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