Comparison of two chromogenic media

Vancomycin-resistant enterococci (VRE) are a group of microorganisms that has caused serious public health problems in local and international healthcare agencies for the past ten years or more (7).

Aside from the threat that is attributed to the antibiotic resistance properties of this specific group of infectious agents, the rampant distribution of VREs in clinical settings was also observed to escalate for the past 15 years (7).Hence, the importance of devising a novel way of detecting the existence of VREs in patients and other potential reservoir is an important task that must be performed in the soonest possible time. In connection to this, the goal of this paper is to test two newly released selective chromogenic media for VRE, chromID VRE (C-ID) medium and CHROM-agar VRE (CHR) medium, based on their selectivity, stability of colony colour and growth characteristics, and effectiveness in recovering VRE particles from clinical stool specimens.Materials and Methods ChromID VRE (C-ID) was derived from BioMerieux (Nurtingen, Germany) while CHROM-agar VRE (CHR) was manufactured by CHROMagar (Paris, France). The methodology section of this study was divided into two parts: the checking of the selectivity of the two media by inoculating them with pure VRE strains of Enterecoccus faecium (VREfm) and Enterecoccus faecalis (VREfs), and the routine evaluation of 259 stool specimens derived from gastroenterology and oncology wards, and intensive care units for the presence of VRE (7).

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Part 1 of the experimentation procedure was done by streaking pure and mixed cultures of reference strains VREfm and VREfs on C-ID and CHR. These reference strains were derived from the National Reference Centre for Streptococci culture collection (7). Inoculated media were incubated for 24-48 hrs at 370C and the evaluation of the colony colours were evaluated through the analysis of pictures that were taken during the 24th hr of incubation.

The expected appearance of the colonies was patterned according to the description provided by the manufacturer of the two media. Part 2 started with the enrichment of the stool specimen, followed by parallel inoculation of the stool specimens to the two media, incubation for 24-48 hrs. , Gram staining of the suspected VRE colonies, subjecting the suspected VRE colonies to pyrrolidonyl arylamidase test (PYRase) for confirmation, administration of susceptibility testing to 50? g furazolidine and 10?g muporicin with Rosco Neo-Sensitabs on Mueller-Hinton agar (MHA) and rapid fermentation test with Rosco Diatabs for further differentiation to species level, and lastly, the utilization of vancomycin and teicoplanin for the establishment of antimicrobial resistance patterns of the isolated VRE strains, an event that served as the final confirmatory test for the presence of VRE along with the PCR determination of the vancomycin resistant genes that is possessed by the isolated VRE strains from stool specimens (7). Results and DiscussionFor part A, results showed that C-ID is the more effective media in allowing the selective growth and differentiation of VREfm and VREfs as compared to CHR (7). This observation is supported by the production on C-ID of two distinct colony colours of rosy brown and green for VREfm and VREfs respectively as compared to the weak and non-differentiated growth of the two VRE pure strains on CHR (7). In contrary to this, part B results showed that CHR allowed a more colour differentiated growth of VRE strains from a mixed culture against the pale colour segregation on C-ID (7).

Growth on CHR after 48 hrs of incubation revealed mauve VREfs and white yeasts while growth on C-ID revealed green VREfs and dull green yeasts. VREfm colonies have also grown on both CHR and C-ID plates but were also characterized of limited colour differentiation. From this event it can be inferred that the two media used were not able to live up to the promise of rapid VRE detection from clinical samples but it must be emphasized that stool specimens do possess an overwhelming abundance of varied microbiological agents.Hence, the importance of stool specimen enrichment to enable selective selection of VRE strains from a mixed culture must be implemented prior to the inoculation of the samples on chromogenic media (7). Results have shown that the fulfilment of the previously mentioned step proves to be effective in detecting the presence of vancomycin-resistant E.

faecium and E. faecalis as indicated by the parallel results obtained from PYRase testing, Gram staining, antibiotic resistance patterns, PCR screening of the antibiotic resistant gene, and multiple-locus variable tandem repeat analysis (MLVA) of the isolated VREfm and VREfs particles (7).Consequently, the sensitivity of the two media was found to be 98.

2% as signified by the detection of 56 VRE strains out of the expected 57 strains. In general, this study revealed that the utilization of chromogenic media is a cost-effective step for the detection of VRE from clinical samples provided that enrichment culture step, antibiotic resistance patterns, and other related tests will be employed to counter-check the identity of the colonies derived. Article CritiqueThe emergence of vancomycin-resistant Enterococci (VRE) strains is definitely a foremost concern in the clinical setting. In actual cases wherein the presence of the aforesaid microorganisms is suspected to be present, the greatest challenge arises in ensuring that identification and isolation procedures are accomplished appropriately to provide reliable results (6). Thus, testing and improving chromogenic media for such purposes are without doubt worthwhile endeavours.Peltroche-Llacsahuanga and colleagues evaluated both Chromagar VRE and chromID VRE in terms of effectiveness in detecting VRE acquired from faecal samples; the overall results of the abovementioned undertaking has been favourable albeit the fact that difficulties are still present in terms of colony colour determination (7).

In order to further establish the validity of the aforesaid results, it would be necessary to provide comparisons between the study presented above and other studies published throughout scholarly or scientific literature.Asir and colleagues accomplished a study wherein stool samples have also been used to assess the effectiveness of three different chromogenic media. Chromogenic VRE Agar, chromID VRE, and VRE Agar have been selected as the media to be tested for isolation purposes (1). Interestingly, the results gained and emphasized in the study are generally favourable as well.The established sensitivity of all the media tested has been favourable in both direct and enriched culture setups; however, it has also been noted that Chromogenic VRE Agar and chromID VRE may be more efficient to utilize as problems regarding colour diffusion have not manifested in such, unlike in Oxoid VRE Agar (1). It is quite essential to note that the results regarding the reliability and efficiency of using Chromagar VRE and chromID VRE have both been positive for the two studies even though gram-staining has not been noted in the study of Asir and colleagues.It is also vital to emphasize that the same point of concern has been expressed despite not necessarily for the same agar or media: colour identification. Delmas and colleagues on the other hand focused entirely upon the effectiveness of chromID VRE for purposes of isolation.

ChromID was directly compared with BEAv, incorporating procedures for enrichment and staining so as to facilitate an easier means for identification; in general, the chromID provided superior results as the distinction of colour allowed for lessened events of confusion in colony isolation as highlighted by the fact that E.faecalis and E. faecium may be distinguished from other Enterococci through the use of chromID (3). In this sense, such results once again coincide with the findings delineated in the study of Peltroche-Llacsahuanga and colleagues.

Furthermore, it is also rather important to note that while Peltroche-Llacsahuanga and colleagues have tested the chromID VRE only for stool samples (7), Delmas and colleagues on the other hand considered the use of both stool samples and rectal swabs and still gaining favourable results for the reliability of chromID (3).However, rectal swabs in general have been known to have a higher rate of false-negative results (2). In providing a holistic critique of the study, it would be necessary to determine whether other chromogenic media would be able to provide similarly beneficial and reliable results. CHROMagar Orientation is another medium which may provide similar results to those already discussed; CHROMagar Orientation has been determined to be rather effective in terms of the pursuit to differentiate microorganisms regardless of whether mixed-growths are highly present (5).Regardless, there are notable limitations and concerns which may be identified in the study pertaining to CHROMagar Orientation. The most notable of such being the fact that the samples used in the study are not necessarily of clinical origin but rather have been acquired through previous isolates (5). Hence, it may be appropriately assumed that the media tested by Peltroche-Llacsahuanga and colleagues have been assessed through a manner focused more upon its real-world application.

Also, in another study, a prototype medium VRE-BMX has been tested which highlighted specificity and detection results ranging from 70% to 95% (4).Considering that chromogenic media are becoming increasingly essential in the isolation of clinical microorganisms in areas other than the faecal-rectal regions, such as the urinary tract (9), it is undeniable that further attempts to evaluate and improve conventional chromogenic media as well as those still under development is a worthwhile endeavour. While decades of advances, such as the development of novel substrates and the integration of such into other media, have resulted in considerable improvements for specificity (8) further improvements are still necessary as pathogens continuously change and evolve as well.Therefore, while the Peltroche-Llacsahuanga and colleagues’ study indeed highlights the effectiveness of chromID VRE in a reliable manner further studies so as to establish a more encompassing comparison between available chromogenic media, most importantly delineating the economic aspect of its use, is still necessary to lessen if not eliminate current VRE-related concerns. References 1.

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