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Chapter 2
2.6. Results
2.6.1. Assessment of removal techniques
Table 2.2 displays the density of cells recovered from the five test surfaces
after sonication at different output levels (measured in KHz) and two time
intervals (30 and 7 minutes). As displayed in Table 2.2 no culturable cells
were detected after sonication on high power for 30 minutes. No viable
cells were recovered from any of the five surfaces using the high power
settings.
Viable cells were recovered from all five surfaces when sonicated at lower
power settings of 10, 15, 20 and 25 kHz. The cell counts were lowest at 10
kHz with a slight increase at 15 kHz and highest recovery of cells at 20 kHz.
When the sonication conditions were increased to 25 kHz there was a
decline in the number of viable cells recovered from the surfaces. As a
result, sonication at 20 kHz was performed for the remainder of the
experiments described in this thesis. The results displayed in Table 2.2 also
indicate that scraping the surface of the coupons with a pre-moistened
cotton swab was not an effective method for removing all viable cells.
Table 2.2: Log 10 density of cells recovered from the surfaces
Strain Sonicatio Time Glass Steel Polycarbonate Tile Concrete
n (kHz) (min)
S08-0601 10 7 2.70 2.87 3.00 3.91 3.99
S08-0601 15 7 2.87 3.24 3.30 4.12 4.17
S08-0601 20 7 6.47 6.57 6.63 7.27 7.34
S08-0601 25 7 5.70 5.87 6.00 7.04 7.22
27853 50 30 ND ND ND ND ND
S08-0601 Swabbed 2.17 2.30 2.24 2.57 2.47
Table 2.2 illustrates the log 10 density of cells recovered from the surface
after sonication at 10, 15, 20, 25 and 50 kilohertz (kHz). The strains
examined include 1 Salmonella enterica subspecies enterica serovar Agona
(S08-0601) and 1 Pseudomonas aeruginosa strain (27853). The mean log 10
density of cells recovered after scraping the surfaces with a sterile swab
are also displayed. ND indicates that no cells were detected.
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