Comparison of the impact of direct plating versus a short or overnight pre-enrichment on detection of methicillin-resistant Staphylococcus aureus from clinical specimens

Abstract number: P824

Van Heirstraeten L., Cortiñas Abrahantes J., Lammens C., Lee A., Harbarth S., Molenberghs G., Aerts M., Goossens H., Malhotra-Kumar S.

Objectives: Rapid detection of methicillin-resistant Staphylococcus aureus (MRSA) from screening cultures is crucial for effective infection control. While an overnight pre-enrichment (On-En) can increase chances of MRSA detection, time to result is 48 hrs. A short, 4-hour pre-enrichment (Short-En) would enable next day results, however its advantage over direct plating (DP) is not known. We compared the impact of DP to plating after Short-En or ON-En on MRSA detection from screening samples.

Methods: Fifty two nasal and groin swabs from 25 patients previously identified as MRSA carriers were collected in BHI + glycerol. 10 ml sample was spiral plated directly on a chromogenic medium, CHROMagar MRSA (BD, Belgium) or on mannitol-salt agar with 4 mg/ml cefoxitin (MSAC), or added to enrichment broth (Tryptone soya broth with 2.5% salt, 20 mg/ml aztreonam, and 3.5 mg/ml cefoxitin). 10 ml of enrichment broth was spiral plated on CHROMagar and on MSAC after a Short-En and ON-En. Colony counts were done for agar cultures after overnight incubation, and putative MRSA colonies confirmed by standard tests. Non-parametric comparisons were made using Friedman's test. Differences in readings (MRSA positive/negative) at the three time-points were modelled using a logistic approach. Generalised estimating equation was used to account for repeated measures over time and the Score test to assess differences between the three time-points.

Results: Of the 52 samples, 9 were negative for MRSA at all three time-points. Plate readings for MRSA positivity after DP or Short-En did not differ significantly (P = 0.317), and showed clear differences after ON-En in comparison to DP or Short-En (P = 0.002 and 0.004, respectively). Two MRSA negative samples gave positive results after ON-En (4% misclassification error). Colony counts differed significantly between DP (mean CFUs/ml: 3.91×10⁴, 95% CI: ±9.66×10²), Short-En (mean CFUs/ml: 6.79×10⁴, 95% CI: ±1.22×10³), and ON-En (mean CFUs/ml: 2.31×10⁵, 95% CI: ±1.16×10³) (Friedman's chi-square = 53.91, degrees of freedom = 2, P = 1.964e-12) (Figure). Of the 52 samples, 60% (n = 30) showed similar colony counts after DP and Short-En, 23% after DP and ON-En, and 31% after Short-En and ON-En.

Conclusions: A Short-En does not offer a significant increase in MRSA detection in comparison to DP and cannot replace an overnight enrichment at least when culture-based methods are used for downstream processing.

Figure: Colony count profiles and trend after DP, Short-En, and ON-En.