Nasal screening for MRSA: different swabs--different results!

Abstract

Objectives: Swab-based nasal screening is commonly used to identify asymptomatic carriage of Staphylococcus aureus in patients. Bacterial detection depends on the uptake and release capacities of the swabs and on the swabbing technique itself. This study investigates the performance of different swab-types in nasal MRSA-screening by utilizing a unique artificial nose model to provide realistic and standardized screening conditions.

Methods: An anatomically correct artificial nose model was inoculated with a numerically defined mixture of MRSA and Staphylococcus epidermidis bacteria at quantities of 4×10(2) and 8×10(2) colony forming units (CFU), respectively. Five swab-types were tested following a strict protocol. Bacterial recovery was measured for direct plating and after elution into Amies medium by standard viable count techniques.

Results: Mean recovered bacteria quantities varied between 209 and 0 CFU for MRSA, and 365 and 0 CFU for S. epidermidis, resulting swab-type-dependent MRSA-screening-sensitivities ranged between 0 and 100%. Swabs with nylon flocked tips or cellular foam tips performed significantly better compared to conventional rayon swabs referring to the recovered bacterial yield (p<0.001). Best results were obtained by using a flocked swab in combination with Amies preservation medium. Within the range of the utilized bacterial concentrations, recovery ratios for the particular swab-types were independent of the bacterial species.

Conclusions: This study combines a realistic model of a human nose with standardized laboratory conditions to analyze swab-performance in MRSA-screening situations. Therefore, influences by inter-individual anatomical differences as well as diverse colonization densities in patients could be excluded. Recovery rates vary significantly between different swab-types. The choice of the swab has a great impact on the laboratory result. In fact, the swab-type contributes significantly to true positive or false negative detection of nasal MRSA carriage. These findings should be considered when screening a patient.

Figure 1. Recovery of bacteria in absolute numbers after direct plating.

Viable counts of bacteria by direct plating of the displayed swab-types were determined by CFU counting as described in the methods section. CFU = colony forming units. Results from statistical analysis are shown in Table S1.

Figure 2. Recovery of bacteria in absolute numbers after elution into Amies medium.

Viable counts of bacteria after elution of the displayed swab-types into Amies medium were determined by CFU counting as described in the methods section. CFU = colony forming units. Results from statistical analysis are shown in Table S1.

Figure 3. Relative recovery of bacteria compared to inoculation dose after direct plating.

Ratios were determined by comparison of viable counts of released bacteria to the inoculation dose, which was defined as 100%. Results from statistical analysis are shown in Table S2.

Figure 4. Relative recovery of bacteria compared to inoculation dose after elution into Amies medium.

Ratios were determined by comparison of viable counts of released bacteria to the inoculation dose, which was defined as 100%. Results from statistical analysis are shown in Table S2.