Figure 1.
Schematic of swab transfer experiments.
Seven commercially-available clinical swabs (labeled A, B, C, D, E, F, and G) were tested for volume recovery and organism recovery. Organisms were applied to swabs in three ways: pipetting a low-volume sample onto the swab, dipping the swab into excess-volume sample, or rubbing the swab across dried sample on a surface. Selected cases included variation in sample concentration, addition of simulated nasal matrix, and comparison of dry and pre-wet swabs. Different manual swab agitation methods, manual twirling and forced flow, were tested for their effects on swab transfer efficiency compared to vortexing (gold standard method).
Figure 2.
(A) Schematic of the experimental setup. The tube containing 128 µL TE was weighed (W1), and 15 µL TE was pipetted onto the swab, which was then transferred into the tube using 10 second 1 Hz side twirl, and removed. The tube containing the leftover buffer (eluate) was weighed (W2). The % volume available for analysis (% Volume Recovery) was calculated using Equation 3 in the text. (B) Mean TE volume (µL) absorbed by each type of swab (N = 5). (C) Comparison of the % Volume Recovery (mean ± SE; N = 5) from each swab. Calcium alginate swabs were resuspended in 1% w/v sodium citrate buffer to dissolve fibrous tip materials, the % Volume Recovery was not reported here due to density change of the buffer during (A). * indicates significant differences (Tukey-Kramer, α = 0.05).
Figure 3.
Organism recovery for low-volume samples.
(A) Schematic of the experimental set up. 15 µL S. aureus/TE (∼100, ∼104, or ∼106 CFU, equivalent to 500, 6×104, or 4×106 ldh1 copies, respectively, as measured by qPCR) was spiked onto the swab, which was then agitated in 128 µL lysis buffer using 10 second 1 Hz side twirl, and removed. (B) Comparison of the % Organism Recovery in four swabs at three different organism input numbers (mean ± SE, N = 5), which was calculated using Equation 5 in the text. (C) Comparison of the % Organism Recovery (mean ± SE; N = 5) using ∼104 CFU/swab of S. aureus in the presence and absence of simulated nasal matrix (SNM).
Figure 4.
Organism recovery for high-volume samples.
(A) Schematic of the experimental set up. Either a dry or pre-wet swab was dipped into 1 mL ∼106 CFU/mL S. aureus solution (equivalent to 6×106 ldh1 copies/mL, as measured by qPCR) and agitated by 10 second 1 Hz side twirl. The swab was then inserted into 128 µL lysis buffer, agitated by 10 second 1 Hz side twirl, and removed. (B) Comparison of the absolute number of organisms recovered for dry and pre-wet swabs. Absolute organism recovery was reported (rather than %) since the uptake of sample volume was different for each swab; absolute recovery was calculated using Equation 4 in the text. In all cases, recovery was larger than would be expected based on swab volume and sample concentration by colony counts due to presence of multiple target copies per CFU. (C) The number of organisms recovered from each swab from panel (B) normalized by the number of organisms expected based solely on the sample concentration and volume capacity of the swab (estimated number of organisms collected by the swab = swab volume capacity (µL) x bacterial stock concentration (copies/µL from qPCR)).
Figure 5.
Organism recovery for dried samples.
(A) Schematic of the experimental set up. 15 µL of S. aureus solution (∼104 CFU, equivalent to 6×104 ldh1 copies, as measured by qPCR) was spotted on a 25/64-inch diameter PDMS punch and left to dry. A dry or pre-wet swab was rubbed on the PDMS surface (10 times), agitated in 128 µL lysis buffer using 10 second 1 Hz side twirl, and removed. (B) Comparison of % organism recovery for pre-wet and dry swabs based on a control sample and an assumption of 100% collection efficiency.
Figure 6.
Comparison of manual agitation methods for swab transfer.
(A) Schematic of action performed over a period of 1 second for different manual twirling methods. (B) Comparison of % organism recovery of PUR swabs using different twirling methods, which was calculated using Equation 5 in the text. (C) Schematic of the new forced flow syringe method. (D) Comparison of % organism recovery for PES and rayon swabs, using different twirling methods and the forced flow syringe method. * indicates statistically significant differences (Tukey-Kramer, α = 0.05).