Abstract
Purpose: Urinary tract infection (UTI) is the most common nosocomial infection among hospitalized patients. Meanwhile, most frequent infections involving enterococci affect the urinary tract. The aims of this study were to investigate the susceptibility pattern of isolated enterococci from UTI and the prevalence of virulence genes. Methods: The study used enterococci isolated from urinary tract infections obtained from 3 university teaching hospitals in Northwest Iran. The antimicrobial susceptibility of the strains was determined using the disc diffusion method. Multiplex PCR was performed for the detection of genus- species specific targets, and potential virulence genes. Results: Of 188 enterococcal isolates, 138 (73.4%) and 50 (26.6%) were Enterococcus faecalis and E. faecium, respectively. Antibiotic susceptibility testing showed high resistance to amikacin (86.2%), rifampicin (86.2%) and erythromycin (73.9%), irrespective of species. In total, 68.1% were positive for gelE, and 57.4%, 53.2%, 56.4%, and 52.1% of isolates were positive for cpd, asa1, ace, and esp, respectively. Conclusion: The study revealed that most of UTI isolates were multidrug resistance against the antibiotics tested and antibiotic resistance was more common among E. faecium isolates than E. faecalis. A significant correlation was found between UTI and the presence of gelE among E. faecalis strains (p < 0.001).
Keywords: Enterococcus faecalis, Enterococcus faecium, Urinary tract infection, virulence genes
Introduction
The enterococci are a dominant bacterial group in the intestinal flora of human and animals1 and it is recognized that they cause serious infections such as endocarditis, septicemia and UTI.2 The natural ability of enterococci to acquire, accumulate, and share extra chromosomal elements encoding virulence traits or antibiotic resistance genes , in part, explains their increasing importance as nosocomial pathogens.3 Acquired resistance to various antimicrobial agents and available antibiotics currently limits the therapeutic options. It is believed that nosocomial enterococci might have virulence elements that increase their ability to colonize hospitalized patients.4 The aims of this study were to investigate the susceptibility pattern of isolated E. faecalis and E. faecium from UTI and the prevalence of genes encoding gelatinase (gelE), aggregation substance (asa1), enterococcal Surface protein (esp), collagen adhesine (ace), and sex pheromones (cpd).
Materials and Methods
Bacterial isolates
One hundred and eighty eight enterococcal isolates obtained from urine specimens of patients with urinary tract infections in 3 university teaching hospitals located in Tabriz (Imam Reza and Sina Hospitals) and Orumieh (Imam Khomeini Hospital), Iran, from April 2008 to June 2010. All isolates were phenotypically identified to the species level using conventional methods5 and their identities were later confirmed by PCR.6
Antibiotic susceptibility testing
The antimicrobial susceptibility of the strains was determined using the disk diffusion method, according to the Clinical and Laboratory Standards Institute (CLSI)7 guidelines for the following antimicrobial agents: penicillin G, imipenem, erythromycin, rifampicin, teicoplanin, ampicillin, streptomycin, ciprofloxacin, and amikacin. Considering to an increasing rate of vancomycin and high level gentamicin resistant enterococci in clinical isolates worldwide, the minimum inhibitory concentrations (MICs) of vancomycin and gentamicin were determined using the agar dilution method based on the CLSI (2006) guidelines. E. faecalis ATCC 29212 was used as a quality control strain for performing antimicrobial tests.
DNA extraction and molecular approach
The DNA of clinical isolates was extracted using a commercial kit (DNGTM-Plus; CinnaGen, Iran). Multiplex PCRs were performed on enterococcal isolates for simultaneous detection of potential virulence genes (esp, gelE, and asa1), as described previously but conditions have been optimized for detection cpd9 and ace.10 Briefly, the 25 µL PCR mixture contained; 2.5 µL of bacterial DNA, 10 pM of each primer for cpd and 4 pM of each primer for ace, 1.5 mM MgCl2, 0.2 mM of each dNTP, and 2.5 U of Taq DNA polymerase (CinnaGen).
Reactions were performed on thermal cycler (ASTEC-Japan) with an initial denaturation at 95 °C for 10 min, followed by 30 cycles of denaturation (94 °C for 1 min), annealing (56 °C for 1 min), extension (72 °C for 1 min), and a final extension step at 72 °C for 10 min. PCR products were analyzed by electrophoresis on a 1.5% agarose gel stained with ethidium bromide and photographed under UV light. Each PCR assay was accompanied with a negative control, containing all of the reagents without template DNA.
Statistical analysis
The data were analysed using the chi-squared test by the SPSS statistical software (version 18.0). A P-value < 0.05 was considered statistically significant.
Results
Bacterial isolates and susceptibility testing
In total, 138 (73.4%) E. faecalis and 50 (26.6%) E. faecium were collected from urine specimens. Antibiotic susceptibility testing by the disk diffusion showed high resistance to rifampicin and amikacin (86.2%) and erythromycin (73.9%), followed by penicillin G (68.6%), ciprofloxacin (65.4%), streptomycin (47.3), ampicillin (28.2%) and teicoplanin (18.6%), irrespective of species concern. The resistance patterns of two species were shown in Table 1.
Table 1. Results of Disk Diffusion tests of Isolated Enterococci.
| Antibiotics | E. faecalis (%) | E. faecium (%) | Total (%) | ||||||
| S | I | R | S | I | R | S | I | R | |
| Ampicillin | 132(95.7) | - | 6(4.3) | 3(6) | - | 47(94) | 135(71.8) | - | 53(28.2) |
| Amikacin | 15(10.9) | 9(6.5) | 114(82.6) | 1(2) | 1(2) | 48(96) | 16(8.5) | 10(5.3) | 162(86.2) |
| Ciprofloxacin | 15(10.9) | 47(34.1) | 76(55.1) | 0(0) | 3(6) | 47(94) | 15(8) | 50(26.6) | 123(65.4) |
| Erythromycin | 20(14.5) | 26(18.8) | 92(66.7) | 2(4) | 1(2) | 47(94) | 22(11.7) | 27(14.4) | 139(73.9) |
| Imipenem | 132(95.7) | 1(0.7) | 5(3.6) | 3(6) | 0(0) | 47(94) | 135(71.8) | 1(0.5) | 52(27.7) |
| Penicillin G | 58(42) | - | 80(58) | 1(2) | - | 49(98) | 59(31.4) | - | 129(68.6) |
| Rifampicin | 16(11.6) | 10(7.2) | 112(81.2) | 0(0) | 0(0) | 50(100) | 16(8.5) | 10(5.3) | 162(86.2) |
| Streptomycin | 69(50) | 2(1.4) | 67(48.6) | 28(56) | 0(0) | 22(44) | 97(51.6) | 2(1.1) | 89(47.3) |
| Teicoplanin | 130(94.2) | - | 8(5.8) | 23(46) | - | 27(54) | 153(81.4) | - | 35(18.6) |
The agar dilution method indicated that 35 (18.6 %) strains were vancomycin resistant (MIC ≥ 256 µg/mL excluding 1 isolate with MIC = 8 µg/mL) and 113 (60.1%) isolates were high-level gentamicin- resistant with MICs ≥ 512 µg/mL. Of vancomycin resistance strains, 33 (94.3%) were associated with high-level resistance to gentamicin. Of these, 7 (21.2%) were E. faecalis and 26 (78.8%) were E. faecium strains.
Presence of virulence genes in E. faecalis and E. faecium
The presence of genes encoding for potential virulence factors were studied by multiplex PCR. Of 188 isolates, 68.1% were positive for gelE, and 57.4%, 53.2%, 56.4%, and 52.1% of isolates were positive for cpd, asa1, ace, and esp, respectively (Figure 1). The percentages of E. faecalis and E. faecium isolates harboring virulence genes were demonstrated in Table 2. All E. faecalis strains carried 2 or more virulence determinants, whereas, 14 E. faecium strains did not harbor any of the genes tested and 4 isolates positive for two or more virulence determinants.
Figure 1.Agarose gel electrophoresis of amplified asa1, esp and gelE by multiplex PCR.
Lane 1: 1-kb DNA ladder
Lanes 2, 3 and 7: isolates positive for esp (510 bp)
Lane 4: isolate positive for gelE (213 bp)
Lane 5: isolate positive for asa1 (375 bp), esp and gelE
Lane 6: isolate positive for asa1 and gelE
Lane 8: negative control (without DNA)
Table 2. The percentages of virulence genes among isolates.
| Virulence Genes | No. (%) | Total No. (%) |
||
| E. faecalis | E. faecium | |||
| gelE | 124(89.9) | 4(8) | 128(68.1) | |
| asa1 | 96(69.6) | 4(8) | 100(53.2) | |
| cpd | 106(76.8) | 2(4) | 108(57.4) | |
| ace | 104(75.4) | 2(4) | 106(56.4) | |
| esp | 65(47.1) | 33(66) | 98(52.1) | |
Moreover, esp-positive E. faecium strains recovered from urine samples, exhibited high resistance to gentamicin (90.9 %), ampicillin (97%) and penicillin (100 %), ciprofloxacin (100%), rifampicin (100 %), amikacin (100%), imipenem (100 %). Nearly 64% of the isolates were also resistant to vancomycin.
Discussion
In this study, most (79.3 %) of UTI enterococcal isolates were resistant to at least three of the antibiotics tested, possibly, these reflect miss-using of antibiotics and selective pressure in our setting. While E. faecalis was clearly the predominant species in urine samples, E. faecium showed a much higher incidence of resistance to antibiotics tested. E. faecium strains displayed resistance to ampicillin, imipenem, teicoplanin, vancomycin and high level gentamicin (p<0.001). With an exception, resistance against streptomycin was more common among E. faecalis than E. faecium strains.
Detection of multidrug resistance enterococci in this study, particularly VRE and HLGR is an alarming situation, since these organisms limit the number of therapeutic options available to the clinician.11
Antibiotic resistance alone cannot explain the virulence of enterococci. The pathogenesis of most infections follows a common sequence of events involving colonization of and adhesion to host tissues, invasion of the tissue and resistance to defense mechanisms of the host. The pathogen must produce pathological changes either directly by toxin production or indirectly by inflammation.12 However, each of virulence traits may be associated with one or more of the stages of infection mentioned above.
According to results from the present study, the major differences on the incidence of virulence determinants found both in E. faecalis and E. faecium isolates from the urinary tract infection, involved a remarkably higher average number of traits in E. faecalis isolates and a much lower average number of traits in E. faecium isolates. The E. faecalis strains tested all harbor multiple virulence determinants. The gelE gene (codes for gelatinase is an extracellular zinc metalloendopeptidase) was the most widespread virulence determinant. In this study, gelE enriched in E. faecalis isolates in comparison with E. faecium and may involve in the creation of a urinary tract infection (p<0.001). Likewise, previous studies in E. faecalis demonstrate the presence of this gene in high incidence among their isolates.9,13 Consistent with our findings, gelE-positive E. faecium isolates has been found in less frequency in clinical isolates.14 In contrast, some studies did not found gelE gene in any E. faecium isolates.8
The present study revealed higher frequency of the esp gene (coding enterococcal surface protein) among E. faecium isolates. In agreement with our research, other studies revealed higher incidence in E. faecium isolates.4,15 In contrast, a research study showed the higher prevalence of this gene in E. faecalis isolates from urine samples16 but the results of other study17 was mostly the same as our findings.
Furthermore, esp-positive E. faecium strains, showed high resistance (> 90 %) to the most of tested antibiotics as mentioned in results. Nearly 64% of E. faecium strains were also resistant to vancomycin. Considering these results, it seems that the presence of esp may facilitate for esp-positive E. faecium isolates to obtain more antibiotic-resistance genes. The prevalence of this gene between two species indicated possible role for esp in E. faecium strains to cause urinary tract infection (p<0.05), although an earlier study on E. faecalis, has been demonstrated a role for esp gene product for E. faecalis isolates causing UTI.18
In present investigation, the asa1 gene, (which encodes aggregation substance), was found in high frequency among E. faecalis strains. A high incidence of this gene in E. faecalis was reported in previous studies.19 Results of studies on clinical E. faecium isolates are contradictory. In some studies, asa1 was not found in E. faecium4 but in contrast, in our study and some other studies14 this gene was detected in less frequency among E. faecium isolates. In total, the rate of asa1 gene in urine isolates did not indicate significant association between the presences of asa1 and emergence of UTI.
Gene cpd encoding for sex pheromone peptides showed a lower incidence among E. faecium isolates whereas it was in higher incidence among E. faecalis. Other studies also reported higher frequency of this gene among clinical E. faecalis isolates.20 Presence of multidrug resistance among our isolates may be related to the higher incidence of cpd gene, since the presence of sex‑pheromone genes facilitate the acquisition of the relevant sex‑pheromone plasmid and therefore the associated virulence and resistance determinants.21
The ace gene (codes for collagen-binding protein) has been detected in high frequency in E. faecalis strains that is in agreement with previous studies.22 Although, Ace has been suggested as a valuable drug target against human UTI,23 but in this investigation, the presence of ace gene was not statistically significant.
Conclusion
Our study demonstrated that E. faecalis is more common among our isolates than E. faecium, but E. faecium strains had a great ability to show drug resistance. The distribution of virulence genes were more common in E. faecalis than in E. faecium strains and the high incidence of multiple virulence factors could potentially contribute to bacterial colonization and pathogenesis of E. faecalis in the urinary tract.
The higher prevalence of esp determinant in E. faecium, may explain the role of this gene in emergence of resistance to the tested antibiotics.
Acknowledgments
This work was supported fully by Research Center of Infectious Diseases and Tropical Medicine (grant No.89/3 and 89/5), Tabriz University of Medical Sciences, Tabriz, Iran. We wish to thank all colleagues in the three hospitals for their assistance in specimen collection.
Conflict of Interest
The authors report no conflicts of interest.
References
- 1.Fabretti F, Theilacker C, Baldassarri L, Kaczynski Z, Kropec A, Holst O. et al. Alanine esters of enterococcal lipoteichoic acid play a role in biofilm formation and resistance to antimicrobial peptides. Infect immun . 2006;74(7):4164–71. doi: 10.1128/IAI.00111-06. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Murray BE. The life and times of the Enterococcus. Clin Microbiol Rev . 1990;3(1):46–65. doi: 10.1128/cmr.3.1.46. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Klibi N, Gharbi S, Masmoudi A, Ben Slama K, Poeta P, Zarazaga M. et al. Antibiotic resistance and mechanisms implicated in clinical enterococci in a Tunisian hospital. J Chemother . 2006;18(1):20–6. doi: 10.1179/joc.2006.18.1.20. [DOI] [PubMed] [Google Scholar]
- 4.Hällgren A, Claesson C, Saeedi B, Monstein HJ, Hanberger H, Nilsson LE. Molecular detection of aggregation substance, enterococcal surface protein, and cytolysin genes and in vitro adhesion to urinary catheters of Enterococcus faecalis and Efaecium of clinical origin. Int J Med Microbiol. 2009;299(5):323–32. doi: 10.1016/j.ijmm.2008.10.001. [DOI] [PubMed] [Google Scholar]
- 5.Manero A, Blanch AR. Identification of Enterococcus sppwith a biochemical key. Appl Environ Microbiol. 1999;65(10):4425–30. doi: 10.1128/aem.65.10.4425-4430.1999. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Kariyama R, Mitsuhata R, Chow JW, Clewell DB, Kumon H. Simple and reliable multiplex PCR assay for surveillance isolates of vancomycin-resistant enterococci. J Clin Microbiol . 2000;38(8):3092–5. doi: 10.1128/jcm.38.8.3092-3095.2000. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Clinical and Laboratory Standards Institute. Performance standards for antimicrobial susceptibility testing; 16th informational supplement. M100-S16. Wayne, PA: Clinical and Laboratory Standards Institute; 2006.
- 8.Vankerckhoven V, Van Autgaerden T, Vael C, Lammens C, Chapelle S, Rossi R. et al. Development of a multiplex PCR for the detection of asa1, gelE, cylA, esp, and hyl genes in enterococci and survey for virulence determinants among European hospital isolates of Enterococcus faecium. J Clin Microbiol . 2004;42(10):4473–9. doi: 10.1128/JCM.42.10.4473-4479.2004. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Eaton TJ, Gasson MJ. Molecular screening of Enterococcus virulence determinants and potential for genetic exchange between food and medical isolates. Appl Environ Microbiol . 2001;67(4):1628–35. doi: 10.1128/AEM.67.4.1628-1635.2001. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Creti R, Imperi M, Bertuccini L, Fabretti F, Orefici G, Di Rosa R. et al. Survey for virulence determinants among Enterococcus faecalis isolated from different sources. J Med Microbiol . 2004;53(Pt 1):13–20. doi: 10.1099/jmm.0.05353-0. [DOI] [PubMed] [Google Scholar]
- 11.Cetinkaya Y, Falk P, Mayhall CG. Vancomycin-resistant enterococci. Clin Microbiol Rev . 2000;13 (4):686–707. doi: 10.1128/cmr.13.4.686-707.2000. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Johnson AP. The pathogenicity of enterococci. J Antimicrob Chemother . 1994; 33(6):1083–9. doi: 10.1093/jac/33.6.1083. [DOI] [PubMed] [Google Scholar]
- 13.Sabia C, De Niederhäusern S, Guerrieri E, Messi P, Anacarso I, Manicardi G. et al. Detection of bacteriocin production and virulence traits in vancomycin-resistant enterococci of different sources. J Appl Microbiol . 2008;104 (4):970–9. doi: 10.1111/j.1365-2672.2007.03612.x. [DOI] [PubMed] [Google Scholar]
- 14.Billstrom H, Lund B, Sullivan A, Nord CE. Virulence and antimicrobial resistance in clinical Enterococcus faecium. Int J Antimicrob Agents . 2008;32 (5):374–7. doi: 10.1016/j.ijantimicag.2008.04.026. [DOI] [PubMed] [Google Scholar]
- 15.Whitman RL, Przybyla-Kelly K, Shively DA, Byappanahalli MN. Incidence of the enterococcal surface protein (esp) gene in human and animal fecal sources. Environ Sci Technol . 2007;41(17):6090–5. doi: 10.1021/es070817t. [DOI] [PubMed] [Google Scholar]
- 16.Giridhara Upadhyaya PM, Umapathy BL, Ravikumar KL. Comparative study for the presence of enterococcal virulence factors gelatinase, hemolysin and biofilm among clinical and commensal isolates of enterococcus faecalis. J lab physicians . 2010;2(2):100–4. doi: 10.4103/0974-2727.72159. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.Cosentino S, Podda GS, Corda A, Fadda ME, Deplano M, Pisano MB. Molecular detection of virulence factors and antibiotic resistance pattern in clinical Enterococcus faecalis strains in sardinia. J Prev Med Hyg . 2010;51(1):31–6. [PubMed] [Google Scholar]
- 18.Shankar N, Lockatell CV, Baghdayan AS, Drachenberg C, Gilmore MS, Johnson DE. Role of Enterococcus faecalis surface protein Esp in the pathogenesis of ascending urinary tract infection. Infect immune . 2001;69 (7):4366–72. doi: 10.1128/IAI.69.7.4366-4372.2001. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19.Waar K, Muscholl-Silberhorn AB, Willems RJ, Slooff MJ, Harmsen HJ, Degener JE. Genogrouping and incidence of virulence factors of Enterococcus faecalis in liver transplant patients differ from blood culture and fecal isolates. J Infect Dis . 2002;185(8):1121–7. doi: 10.1086/339682. [DOI] [PubMed] [Google Scholar]
- 20.Abriouel H, Omar NB, Molinos AC, Lopez RL, Grande MJ, Martinez-Viedma P. et al. Comparative analysis of genetic diversity and incidence of virulence factors and antibiotic resistance among enterococcal populations from raw fruit and vegetable foods, water and soil, and clinical samples. Int J Food Microbiol . 2008;123(1-2):38–49. doi: 10.1016/j.ijfoodmicro.2007.11.067. [DOI] [PubMed] [Google Scholar]
- 21.Klibi N, Ben Slama K, Saenz Y, Masmoudi A, Zanetti S, Sechi LA. et al. Detection of virulence factors in high-level gentamicin-resistant Enterococcus faecalis and Enterococcus faecium isolates from a tunisian hospital. Can J Microbiol . 2007;53(3):372–9. doi: 10.1139/W06-136. [DOI] [PubMed] [Google Scholar]
- 22.Cariolato D, Andrighetto C, Lombardi A. Occurrence of virulence factors and antibiotic resistances in Enterococcus faecalis and Enterococcus faecium collected from dairy and human samples in North Italy. Food Control . 2008;19(9):886–92. [Google Scholar]
- 23.Lebreton F, Riboulet-Bisson E, Serror P, Sanguinetti M, Posteraro B, Torelli R. et al. ace, which encodes an adhesin in Enterococcus faecalis, is regulated by Ers and is involved in virulence. Infect immune . 2009;77(7):2832–9. doi: 10.1128/IAI.01218-08. [DOI] [PMC free article] [PubMed] [Google Scholar]