Are multiply resistant enterococci a common phenomenon in Hong Kong?

A. F. Cheng*, T. S. Char and J. M. Ling

Department of Microbiology, The Chinese University of Hong Kong, The Prince of Wales Hospital, Shatin, New Territories, Hong Kong SAR, China

Keywords: enterococci, multiply resistant, resistance mechanisms

Sir,

Enterococci have become increasingly important as nosocomial pathogens during the past 20 years in many parts of the world.1 This is due, in particular, to their intrinsic and acquired resistance to many antimicrobial agents. The choice of drugs for serious enterococcal infections has thus become limited, and it is therefore important to undertake surveillance of enterococci to provide data that can guide treatment choices. As such information is lacking in Hong Kong, we aimed to study the antimicrobial susceptibilities of enterococci isolated in the Prince of Wales Hospital to determine their level of resistance.

A total of 498 enterococcal isolates from our previous study in 1997–19982 were tested. They were identified by the API 20 Strep system (bioMérieux SA, Montalieu Vercieu, France), PCR3 and conventional biochemical methods. The MICs of antimicrobial agents were determined by an agar dilution method4 using a multipoint inoculator (MIC 2000; Dynatech Laboratories, Alexandria, VA, USA). Enterococci with vancomycin MICs >= 4 mg/L were tested for the presence of van genes by PCR.3 Control strains included Enterococcus faecium AIB40 (vanA), Enterococcus faecalis ATCC 29212 and ATCC 51299 (vanB), Enterococcus gallinarum AIB39 (vanC-1) and Enterococcus casseliflavus ATCC 25788 (vanC-2). All enterococci with gentamicin MICs >= 1024 mg/L were subjected to PCR for the detection of the gene coding the aminoglycoside-modifying enzyme (AME) AAC6'/APH2''.5 Staphylococcus aureus ATCC 29213 was used as a negative control, 16S rRNA primers as an internal control and S. aureus 77040101, which produces AAC6'/APH2'', was used as a positive control. Twenty strains with gentamicin MICs <= 256 mg/L were also randomly selected for testing. Enterococci with ampicillin MICs > 8 mg/L were screened for the presence of ß-lactamase with a cefinase disc (BBL Microbiology Systems, Cockeysville, MD, USA) according to the manufacturer’s instructions. Multiply resistant enterococci were selected for molecular typing by PFGE.6 The PFGE patterns were compared using the GelCompar 4.0 software (Applied Maths, Kortrijk, Belgium) and interpreted.6

The susceptibilities of enterococci to 12 antimicrobial agents are summarized in Table 1. Antimicrobial resistance was particularly prevalent in E. faecium. Multiple resistance was also common, with 59% being resistant to more than three antimicrobials. In general, the percentage of enterococci resistant to various antimicrobials in this study was higher compared with that in other countries.


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Table 1.  Antimicrobial susceptibilities of 498 isolates of Enterococcus spp. collected during 1997–1998
 
Although vancomycin-resistant enterococci (VRE) are a major problem in hospitals worldwide, we detected only 12 VRE isolates from the Prince of Wales Hospital in 1998. This may be due to the fact that our hospital has an efficient infection control team that is responsible for preventing the spread of infections within the hospital.

It was surprising to find in this study that 9% of E. faecium isolates were resistant to 1 mg/L quinupristin/dalfopristin, as the drug was not yet available in Hong Kong at the time.

Although enterococci are intrinsically resistant to aminoglycosides, the treatment of serious enterococcal infections is the synergic combination of a ß-lactam or glycopeptide with an aminoglycoside. However, this synergy would be lost if high-level aminoglycoside resistance developed, and such therapy would not be effective. Our study showed that an average of 15% of the enterococcal isolates studied were resistant to ampicillin and 25% were resistant to high concentrations of gentamicin. Thus, there would be a one in four chance of treatment failure if the standard regimen for treating serious enterococcal infections was used. However, vancomycin and teicoplanin remained effective, as only a very small proportion of enterococci were resistant to them. Only four of 124 isolates with gentamicin MICs >= 1024 mg/L did not produce AAC6'-APH2'', the most common AME produced by enterococci with high-level gentamicin resistance. However, we have yet to investigate, by PCR using more primers, whether our isolates produced only a single enzyme, and to further investigate the mechanism of high-level gentamicin resistance in the four strains with no amplification product after PCR. The resistance might be due to APH(2''), novel enzymes or resistance mechanisms other than enzyme inactivation.

None of the 76 ampicillin-resistant enterococcal isolates was found to produce a ß-lactamase. This is not surprising as ß-lactamases have only rarely been detected in enterococci. Resistance to ß-lactams is usually due to decreased affinity of the drug to penicillin-binding proteins. Thus, further work needs to be carried out to determine the mechanism of resistance to ß-lactams.

In summary, although VRE were rare in Hong Kong in the time period covered in this study, antimicrobial resistance and multiple resistance were common amongst enterococci. Various patterns were obtained after total DNA fingerprinting by PFGE, indicating that our multiply resistant enterococcal infections were most probably sporadic cases and not due to spread of a few related strains.

Acknowledgements

We thank Dr D. F. Sahm for the gift of E. faecium AIB40 and E. gallinarum AIB39.

Footnotes

* Corresponding author. Tel: +852-2632-3339; Fax: +852-2647-3227; E-mail: a-cheng{at}cuhk.edu.hk Back

References

1 . Leclercq, R. (1997). Enterococci acquire new kinds of resistance. Clinical Infectious Diseases 24, Suppl. 1, S80–4.[ISI][Medline]

2 . Ling, J. M., Char, T. S. & Cheng, A. F. (2002). Distribution of enterococci in Hong Kong. Journal of Infection, in press.

3 . Dutka-Malen, S., Evers, S. & Courvalin, P. (1995). Detection of glycopeptide resistance genotypes and identification to the species level of clinically relevant enterococci by PCR. Journal of Clinical Microbiology 33, 24–7.[Abstract]

4 . National Committee for Clinical Laboratory Standards. (1997). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically: Approved Standard M7-A4. NCCLS, Villanova, PA, USA.

5 . van de Klundert, J. A. M. & Vliegenthart, J. S. (1993). PCR detection of genes coding for aminoglycoside-modifying enzymes. In Diagnostic Molecular Microbiology, Principles and Applications (Persing, D. H., Smith, T. F., Tenover, F. C. & White, T. J., Eds), pp. 547–52. American Society for Microbiology, Washington, DC, USA.

6 . Ling, J. M., Koo, I. C. & Cheng, A. F. (2001). Epidemiological analysis of Salmonella enterica serotype typhimurium from Hong Kong by ribotyping and pulsed-field gel electrophoresis. Scandinavian Journal of Infectious Diseases 33, 272–8.





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