#enterococcus

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The first outbreak of VRE in Singapore occurred among patients on a haematology ward in 2004. There were 2 cases of bacteraemia and during the outbreak investigation we picked up a further 4 cases of gastrointestinal colonization, all with a similar molecular fingerprint by pulsed-field gel electrophoresis (PFGE). The isolates were phenotypically Van B and confirmed to have the vanB gene (Chlebicki MP et al, 2006). The clone belonged to the novel multi-locus sequence type (MLST ) ST280 and was positive for the virulence gene esp (Enterococcal surface protein) but negative for hyl (hyaluronidase). The multi-locus variable number tandem repeat (MLVA) profile was 5-7-3-3-2-3. 

Because this outbreak was detected early, it was quickly contained, and we were able to write up our experience and get it published. The manuscript concluded with the following sentence “Hospitals in the region should anticipate the possibility of outbreaks of VRE colonization and infection and should consider developing a program for active surveillance for VRE.” This unfortunately turned out to be prophetic.

Isolate from the first VRE outbreak. Note that the zone around the nitrofurantoin disk (F/M 300) is fuzzy indicating this is likely to be an Enterococcus faecium. Also the vancomycin resistance seems to be more obvious with the low content (5 mg) vancomycin disk used by EUCAST versus the high content (30 mg) vancomycin disk as used by CLSI. 

On the morning of 3 March 2005, one of our senior technologists showed me a susceptibility testing plate she had some concerns about.

This was an Enterococcus faecium that had been isolated from the urine of a patient. There  were apparently large zones of inhibition around the vancomycin disk (VA 30) when viewed with reflected light.

However, when closely examined with transmitted light, small colonies (arrow) could be seen extending into the zone of inhibition. (CLSI actually makes it a point that Enterococcus disk susceptibility testing for vancomycin needs to be incubated for the full 24 hours and viewed with transmitted light).

The isolate had a low MIC of 8 mcg/ml that actually falls within the intermediate range of vancomycin susceptibility. However molecular testing confirmed that the vanB gene was present. So this was a true VRE that was quite difficult to detect because it was not fully vancomycin resistant, and would have been missed if the technologist had not been alert. This sparked off a bout of VRE screening. We were dismayed to find that the more we screened, the more VRE cases we picked up!

Stacks of VRE screening plates from the outbreak investigation.

In an effort to control the spread of VRE, the infection control team consulted Dr Keryn Christiansen who had successfully controlled a similar large outbreak of Van B E. faecium in Western Australia (Christiansen KJ, 2004). The hospital was eventually even closed to non-urgent operations, a rare occurrence that warranted a report in the mass media. More than 100 VRE carriers were identified (Kurup A, 2008). The outbreak was eventually controlled but VRE could not be eradicated.

Pulsed-field gel electrophoresis of the VRE isolates revealed that there was one main clone responsible (ST17, vanB, esp+/hyl-, 5-7-3-3-2-3), but in addition, there were a few other minor clones in the background (ST18, vanB, esp+/hyl-, 5-7-3-3-2-3;  ST117, vanB, esp-/hyl-, 5-7-3-2-2-2; ST280, vanA, esp+/hyl+, 5-7-3-3-2-3) (Koh TH, 2009). All outbreak clones, including the one in 2004, belonged to complex-17, a lineage of E. faecium associated with oubreaks worldwide.

We also picked up a few sporadic vanA E. faecium that did not have any virulence genes (which may account for their lack of success spreading). These had very diverse molecular fingerprints and were also unusual in being phenotypically Van B (susceptible to teicoplanin despite having the vanA gene). The strains had point mutations in vanS, a gene coding for the sensor member of the two-component regulatory system involved in the expression of glycopeptide resistance (Hashimoto Y, 2000). 

In collaboration with our colleagues in the Agri-Food and Veterinary Authority of Singapore, we were also able to study some VRE isolated from chickens. What was interesting was that the Chicken VRE had the same characteristics as the sporadic human isolates (Van B phenotype/vanA genotype, absence of virulence genes). It is possible that the sporadic human isolates were acquired from chickens. I would assess the risk of these VRE strains to humans as minimal, though there is always the risk of the vanA gene spreading to potentially more pathogenic bacteria.

Van B phenotype/vanA genotype Enterococcus faecium

Our failure to completely eliminate VRE from the hospital in 2005 has continued to haunt us as VRE are now isolated on a regular basis. Significantly, vanA E. faecium have increased since 2011, and are almost on par with vanB E. faecium. Preliminary typing data by our trainee Dr Karrie Ko indicates that this is not due to any predominant clones. A potential threat we must be alert for, given our relatively high prevalence of MRSA, is the emergence of VRSA due to the transfer of vanA from E. faecium to Staphylococcus aureus. 

P.S. An unusual VRE with the Van B phenotype was recently described by our colleagues at the Tan Tock Seng and National University Hospitals. This isolate which belonged to ST78 and was esp+ had vanM, a resistance determinant recently described in E. faecium in China (Teo J, 2011).

 Chlebicki MP, Ling ML, Koh TH, Hsu LY, Tan BH, How KB, Sng LH, Wang GC, Kurup A, Kang ML, Low JG. First outbreak of colonization and infection with vancomycin-resistant Enterococcus faecium in a tertiary care hospital in Singapore. Infect Control Hosp Epidemiol. 2006 Sep;27(9):991-3. (no free access)

 Christiansen KJ, Tibbett PA, Beresford W, Pearman JW, Lee RC, Coombs GW, Kay ID, O'Brien FG, Palladino S, Douglas CR, Montgomery PD, Orrell T, Peterson AM, Kosaras FP, Flexman JP, Heath CH, McCullough CA. Eradication of a large outbreak of a single strain of vanB vancomycin-resistant Enterococcus faecium at a major Australian teaching hospital. Infect Control Hosp Epidemiol. 2004 May;25(5):384-90. (no free access)

 Kurup A, Chlebicki MP, Ling ML, Koh TH, Tan KY, Lee LC, Howe KB. Control of a hospital-wide vancomycin-resistant Enterococci outbreak. Am J Infect Control. 2008 Apr;36(3):206-11. (no free access)

 Koh TH, Low BS, Leo N, Hsu LY, Lin RT, Krishnan P, Chan D, Nadarajah M, Toh SL, Ong KH. Molecular epidemiology of vancomycin-resistant enterococci in Singapore. Pathology. 2009;41(7):676-80. (no free access)

Hashimoto Y, Tanimoto K, Ozawa Y, Murata T, Ike Y. Amino acid substitutions in the VanS sensor of the VanA-type vancomycin-resistant Enterococcus strains result in high-level vancomycin resistance and low-level teicoplanin resistance. FEMS Microbiol Lett. 2000 Apr 15;185(2):247-54. (free access)