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Evaluation of the increase rate of MRSA detection using enrichment

Abstract number: 1732_71

Curry A., Walsh M., Cahill N., Collins E., Knowles S.

The objective of the work carried out was to determine the increase rate of MRSA detection using an enrichment step compared to direct culture on chromogenic agar.

MRSA screening swabs from neonates in the NICU were compared: each swab was first cultured directly onto chromogenic MRSA ID agar and then incubated in Brain Heart Infusion broth for 24 hours. The broths were subsequently subcultuerd onto MRSA ID agar. All plates were incubated for 48 hours aerobically at 37°C and checked for the presence of green colonies, which is indicative for MRSA, at 24 and 48 hours. All green colonies had confirmatory tests for MRSA. The limit of detection was determined for both direct and enriched cultures on MRSA ID agar using doubling dilutions up to 1/4096 from an initial 0.5 McFarland suspension. Plates were incubated for 24 hours aerobically at 37°C after which colony counts were performed.

A total of 1181 swabs were tested using direct and enriched culture methods. With direct culture, 16 isolates of MRSA were detected after 24 hours incubation and a further 6 after 48 hours incubation (22 in total). Following enrichment, 34 isolates were detected after 24 hours incubation and one more following 48 hours incubation (35 in total). Enrichment yielded a 59% increase in the number of isolates of MRSA detected compared to direct culture. The limit of detection for direct culture was 69,000 cfu/mL at 1/128 dilution with 84,000 cfu/mL at 1/2048 dilution following enrichment.

Results showed that the limit of detection was higher following enrichment indicating a 16-fold increase in sensitivity compared to direct culture. The comparison of patient samples correlated with this by showing an increase of 59% in the rate of detection of MRSA following enrichment. In conclusion enrichment has proven to dramatically increase the sensitivity and rate of detection of MRSA compared to direct culture alone and in order to ensure maximum sensitivity in the detection of MRSA enrichment should be employed as the method of choice in routine laboratories.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Subject:
Location: ICC, Munich, Germany
Presentation type:
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