ARTÍCULOS ORIGINALES

Antibiotic susceptibility of Staphylococcus aureus isolates in oral mucosa and pockets of patients with gingivitis-periodontitis

 

Alicia I. Cuesta¹, Virginia M. Jewtuchowicz^1,2, María I. Brusca¹, María T. Mujica², Alcira C. Rosa¹

¹Department of Microbiology, Faculty of Dentistry. University of Buenos Aires, Argentina.
²Department of Microbiology, Faculty of Medicine. University of Buenos Aires, Argentina.

CORRESPONDENCE Dr. Alicia I. Cuesta M. T. de Alvear 2142. PISO 2 sector B C1122AAH. CABA Argentina. alicuest@gmail.com

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ABSTRACT

Both oral cavity and subgingival pocket are ecological niches conducive to hosting microorganisms that may act as opportunistic pathogens, such as Staphylococcus aureus and especially methicillin-resistant Staphylococcus aureus (MRSA). Early detection of MRSA is a matter of concern to Public Health. The aim of our study was to determine phenotypic and genotypic detection of methicillin resistance of S. aureus in oral mucosa and subgingival pocket in 102 patients with gingivitis-periodontitis. The prevalence of S. aureus was 10.8% (n = 11) in subgingival pocket and 19.6% (n = 20) in oral mucosa. We obtained 31 isolates of S. aureus of which 13 were mecA positive and 18 were mecA negative. Detection of mecA gene by PCR was used as the reference method to compare the results of phenotypic methods to determine methicillin resistance. Early, accurate detection of S. aureus through phenotyping and genotyping methods is crucial for assessing the colonization and preventing the spread of MRSA.

Key words: Staphylococcus aureus; Antibiotics; Periodontal diseases.

RESUMEN

Susceptibilidad a antibióticos de cepas de Staphylococcus aureus aisladas de mucosa bucal y bolsas de pacientes con enfermedad gingivoperiodontal

La cavidad bucal y el interior de la bolsa subgingival constituyen nichos ecologicos propicios para albergar microorganismos que podrian actuar como patogenos oportunistas, como el Staphylococcus aureus y en particular S. aureus resistente a la meticilina (SARM). La deteccion temprana de portadores reviste importancia para salud publica. El objetivo de nuestro trabajo fue determinar por metodos fenotipicos y genotipicos la meticilino resistencia de cepas de S. aureus aisladas de mucosa bucal y bolsa subgingival de 102 pacientes con enfermedad gingivoperiodontal. Se observo una prevalencia de S. aureus en bolsa subgingival del 10,8% (n = 11) y en mucosa bucal del 19,6% (n = 20). Se obtuvieron 31 aislamientos de S. aureus de los cuales 13 fueron mec A positivos y 18 eran mecA negativos. La deteccion del gen mec A por PCR se utilizo como metodo de referencia para comparar los resultados de metodos fenotipicos para determinar la resistencia a meticilina. La deteccion rapida y exacta de S. aureus por metodos microbiologicos fenotipicos y genotipicos es relevante para evaluar la colonizacion y prevenir la propagacion del SARM.

Palabras clave: Staphylococcus aureus; Antibioticos; Enfermedad periodontal.

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INTRODUCTION

The oral cavity and the interior of the subgingival pocket are ecological niches conducive to hosting microorganisms that may act as opportunistic pathogens. Numerous epidemiological studies suggest that periodontal disease may be a risk factor for systemic infectious disease^1,2. Over the past decade, Staphylococcus aureus has become increasingly methicillin-resistant^3,4. Methicillin- resistant Staphylococcus aureus (MRSA) is an emerging pathogen that causes hospital and community- acquired infections with high rates of morbidity and mortality. In particular, there has been an increase in community-acquired methicillin-resistant S. aureus infections (CA-MRSA) with no apparent identified risk factors⁵. According to data from the World Health Organization Net (WHONET), the prevalence of MRSA in clinical isolates in Argentina is very high (40 to 50%)^6,7. Early, accurate detection of MRSA is a matter of great concern to public health, so that carriers may be identified and the spread of MRSA prevented. According to international literature, only phenotypic analytical techniques are used to detect Staphylococcus aureus in samples from oral mucosa and periodontal pocket, using a single solid selective culture medium such as mannitol salt agar or Baird- Parker agar to isolate it, and traditional biochemical tests such as catalase and coagulase to identify it^8,9. The presence of the nuc gene allows S. aureus to be distinguished from other Staphylococcus species. The main mechanism in methicillin resistance is the expression of a penicillin-binding protein (PBP) called PBP2a, encoded by the mecA gene and characterized by its very low affinity for β-lactam antibiotics. Detection of the mecA gene or its product, penicillin binding protein (PBP2a), is considered the gold standard^10,11 for MRSA confirmation. Recent investigations suggest that disk diffusion using cefoxitin is superior to most previously recommended phenotypic methods, including oxacillin disk diffusion and oxacillin screen agar testing¹². Borderline oxacillin-resistant Staphylococcus aureus (BORSA) isolates are characterized by MICs to oxacillin close to or just above resistance breakpoints. The aims of this study were to: a) determine the presence of Staphylococcus aureus strains isolated from oral mucosa and pocket samples from patients with gingivitis-periodontitis, and their sensitivity to antimicrobial agents b) evaluate the results obtained by molecular detection methods and the methicillinresistance of those isolates, using the respective phenotypic methods for identification and determination of antibiotic susceptibility.

MATERIALS AND METHODS

Source of isolates
The S. aureus isolates evaluated in this study were from 102 immunocompetent adult patients with gingivitis-periodontitis, who visited the outpatient offices at the School of Dentistry of the University of Buenos Aires. Subjects were aged 18 to 70 years, and average age was 42.6 ± 15.1 (54.1% female and 45.9% male). The gingival and periodontal evaluation included clinical and radiographic examinations and measurement of clinical indicators with a pressure-controlled periodontal probe: attachment loss (AL), probing depth (PD), plaque index¹³ and gingival index¹⁴. The measurements were taken on all teeth except the third molar, at ⁴ sites per tooth (mesial, vestibular, distal and palatal/ lingual). The following exclusion criteria were applied: patients with systemic diseases altering the gingival- periodontal state; patients who had taken antibiotics, anti-inflammatory or anti-fungal drugs within six months prior to the study, and patients who had received periodontal treatment within six months prior to the study. Samples were taken before basic periodontal therapy was performed¹⁵. The reference strains used were S. aureus ATCC 43300 (methicillin-resistant) and S. aureus ATCC 25923 (methicillin-sensitive).

Sampling and microbiological method
Oral cavity and subgingival samples were taken after the patient had rinsed his/her mouth with distilled water. Samples from the subgingival pocket were taken with a Gracey 7/8 curette, after relatively isolating the zone with cotton rolls and high-power suction and removing supragingival biofilm. A sample of oral mucosa (mucosa, tongue and check) was taken from each patient with a sterile cotton swab. All samples were placed in sterile PBS (phosphate buffer solution, pH 7.4) transport medium, and kept at 4o C until processing. The material was immediately sent to the microbiology laboratory. The samples were cultured in selective hyper-salted broth (containing 5 g proteose- peptone (Merck, Darmstadt, Germany), 10 g meat extract (Merck, Darmstadt, Germany), 65 gNaCl (Merck, Darmstadt, Germany) and distilled water to 1 liter, pH 7.5) and selective, differential solid media such as mannitol salt agar (Biokar Diagnostics, Beauvais, France), CHROMagar MRSA (CHROMagar Company, Paris, France) and CHROMagar Staph aureus (CHROMagar Company, Paris, France). Microscope studies with Gram coloring and biochemical tests were used to identify genus and species. A coagulase test (bioMerieux, Marcy-L’Etoile, France) and a Microgen TM STAPH-ID System kit (Microgen Bioproducts, Cambaley, UK) were used.

Sensitivity to antimicrobial agents tests
To evaluate the susceptibility of Staphylococccus aureus to antimicrobial agents, diffusion tests and MIC were used according to the Clinical Laboratory Standard Institute (CLSI) standards.¹⁶ The diffusion method in Mueller-Hinton agar (Merck, Darmstadt, Germany) supplemented with 2% sodium chloride (NaCl) was performed using disks (Laboratorios Britania, Buenos Aires, Argentina) with 1μg oxacillin (OXA) and 30 μg cefoxitin (FOX)^15,16. To determine the MIC for OXA in solid medium against S. aureus isolates, we used Mueller-Hinton agar (Merck, Darmstadt, Germany) supplemented with 2% de NaCl, to which increasing concentrations of OXA were added. The presence of colonies was considered positive, and lack of growth, negative. The S. aureus isolates with oxacillin MIC 2 to 8 μg / ml and without the mec A gene were considered as oxacillin borderline (BORSA)¹¹.

Molecular methods
A multiple-PCR was performed to evaluate the presence of the nuc gene, in order to distinguish Staphylococcus aureus from other Staphylococcus species, and the methicillin-resistant gene mecA was amplified^20,21.
DNA was extracted from a strain cultured in brain heart broth (Merck, Darmstadt, Germany) incubated for 18 hours at 37oC²² S. aureus ATCC 259²³ and S. aureus ATCC 43300 were used as negative and positive controls, respectively, for the mecA gene. For the Multiple-PCR we used primers MecA 1 (AGT TCT GCA GTA CCG GAT TTG C), MecA 2 (AAA ATC GAT GGT AAA GGT TGG C )14, nuc- 1 (GCG ATT GAT GGT GAT ACG GTT ) and nuc- 2 (AGC CAA GCC TTG ACG AAC TAA AGC )23 following the protocol described by Geha G et al.²⁴ The PCR products were separated by electrophoresis on 1.5% agarose gel containing ethidium bromide, for viewing under UV light.

Statistical analysis
Sensitivity, Specificity and Essential Correlation were calculated for the different phenotypic methods used to determine methicillin resistance. Sensitivity: number of resistant strains detected by each method divided by total strains showing presence of the mecA gene by PCR, as a percentage. Specificity: number of sensitive strains shown by each method divided by total strains that did not show presence of the mecA gene by PCR. Essential Correlation (C): determined using the presence of the mecA gene as a reference, by means of the following formula:

where N is the number of strains studied, Fs are the false sensitive results and Fr are the false resistant results.

RESULTS

Table 1 shows the means, standard deviations and 95% confidence intervals for average probing depth, attachment loss, gingival index and plaque index for the 102 subjects at the time of sampling.

Table 1: Patients’ periodontal clinical parameters (Mean ± SD and CI 95%) at the time of sampling according to periodontal health condition*.

Average clinical indices were compared by means of the ANOVA test. All patients with chronic gingivitis and periodontitis were positive for bleeding on probing. The correlation between identification of S. aureus with traditional phenotypic tests and detection of the nuc gene was 100%. Among the 102 patients with gingivitis-periodontitis, S.aureus had a prevalence of 10.8% (n = 11, CI 95%: 5.8-18.9) in pocket and 19.6% (n = 20, CI 95%: 12,7-28,9) in oral mucosa; methicillin-resistant S. aureus (MRSA) was isolated from the pocket in 3.9% (n = 4, CI 95%: 1.1-9.7) and from oral mucosa in 8.8% (n = 9, CI 95%: 4.1-16.1).
Table 2 shows total patients with gingivitis-periodontitis who were positive for culture of the genus Staphylococcus. Among the 20 patients with gingivitis, prevalence was 45.0% (n = 9) in pocket and 65.0% (n = 13) in oral mucosa; while for the 82 patients with chronic periodontitis, prevalence was 32.9% (n = 27) in the pocket and 47.6% (n = 39) in oral mucosa. For both sites there was predominance of coagulase-negative Staphylococcus (CNS) over S. aureus; CNS prevalence was 25.5% (n = 26, CI 95%: 17.6-35.3) in the pocket and 34.3% (n = 35, CI 95%: 25.4-44.4) in oral mucosa. Association between S. aureus and CNS was found at both sites, and its prevalence was 1.0% (n = 1, CI 95%: 0.0- 5.3) in the pocket and 3.0% (n = 3, CI 95%: 0.6-8.4) in oral mucosa.

Table 2: Distribution of absolute (n) and relative (%) frequencies of the genus Staphylococcus from isolates from oral mucosa and subgingival pocket in patients with gingivitis-periodontal disease (n =102).

The Staphylococcus strains isolated were identified and their susceptibility to antibiotics evaluated by Multiplex-PCR (Fig. 1).

Fig. 1: Electrophoresis on agarose gel containing 533-bp amplicons corresponding to the mecA gene and 270-bp amplicons corresponding to the nuc gene. Lanes: a, strain MRSA-42 oral cavity; b, strain MSSA-13 oral cavity; c, CNS mecA positive oral cavity; d, strain MSSA-3 oral cavity; e, strain MSSA-3 periodontal pocket; f, strain MRSA-89 periodontal pocket; g, strain MSSA-71 periodontal pocket; h, strain MSSA-71 oral cavity; i, strain MRSA-20 periodontal pocket; j, strain MSSA-20 oral cavity; l, strain MRSA-73 oral cavity; M, molecular weight marker (100bp Marker); n, strain MRSA-87 periodontal pocket; o, negative reagent control; p, S. aureus ATCC 43300.

Table 3 shows the results of susceptibility to antimicrobial agents using phenotypic and genotypic methods for the 31 S. aureus isolates.

Table 3: Sensitivity to antimicrobial agents of Staphylococcus aureus strains (nuc gene positive). Results of the evaluation by disc diffusion method, MIC in solid oxacillin medium, and detection of the Mec A gene.

DISCUSSION

The oral cavity and subingival pockets of patients with gingivitis-periodontitis are a possible reservoir of opportunistic pathogens such as S. aureus, the genus Candida and Pseudomona aeruginosa, among others^25,26. S. aureus and Staphylococcus epidermidis are frequently reported as pathogens responsible for infections. However, they are seldom or not at all studied from the oral cavity and periodontal pockets. Subgingival Staphylococcus does not necessarily represent an infection, but it does form part of the indigenous microbiota. Slots et al.²⁷ found Staphylococcus spp. in refractory periodontitis 19 years ago in 28.3% of individuals aged 25 to 60 years. In this study, which analyzed a large number of samples, S. aureus was the second most frequent sub-gingival Staphylococcus species following S. epidermidis belonging to the CNS group. Our methodology enabled a higher percentage of S. aureus to be recovered from the periodontal pocket than the methodology used by Loberto J et al.²⁸ This might be explained by the different prevalence values according to the geographic area of the population studied, but may also be due to the fact that the method we used for recovering S. aureus employs more than one culture medium for its isolation. We found similar percentages to those reported in the literature for CNS and S. aureus in the oral cavity. In a study on patients with treated periodontitis, Rams et al.^21,30 report about 50% of subgingival Staphylococcus isolates and 22.3% S. aureus. In our study, all the results of the biochemical tests for identifying S. aureus match the results of the evaluation of the nuc gene that encodes thermonuclease.
According to the literature, and in agreement with our results, cefoxitin disk diffusion can accurately predict the presence of the S. aureus mecA gene^7,29. Detection of the mecA gene is considered to be the reference method for evaluating S. aureus methicillin resistance^30,31. It allows MRSA to be identified and distinguished from borderline oxacillin resistant strains (BORSA). Clinical evidence for failure with lactam therapy has been lacking for patients with borderline oxacillin resistant S. aureus (BORSA) infections. Skinner et al. describe a failure of cloxacillin for a patient with endocarditis due to BORSA³².
This is the first report of local prevalence of S. aureus and MRSA in samples from oral mucosa and subgingival pocket in patients with periodontal disease. Moreover, a significative amount of MRSA was found. Today there are microbiological tools that allow the relative frequency of these pathogens in different niches of the oral mucosa to be established, in particular, in periodontal pocket in patients with periodontal disease. Prompt and accurate identification of MRSA is essential in order to manage infections they cause effectively and important to prevent them from spreading.¹²

ACKNOWLEDGEMENTS

This study was funded by UBACYT O016 from the University of Buenos Aires.

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