TABLE 3.
Average lag phase duration, maximum growth rate, and maximum optical density of wild type (WT) and cQAC-adapted (qAD) L. monocytogenes strains exposed to sublethal concentration of cQAC (2 ppm) in tryptic soy broth with yeast extract (TSB-YE), with and without reserpine (R; 20 μg/ml), at 30°C for 24 h.
| Isolate and treatment | Lag-phase duration (h)a,b |
Maximum growth rate (increase in OD600/h)a,c |
Maximum OD600a |
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| WTd | qADe | WT | qAD | WT | qAD | |
| WRLP354 | ||||||
| TSB-YE | 8.43 ± 0.36 | 8.95 ± 0.19 | 0.17 ± 0.00 | 0.13 ± 0.01*a | 0.59 ± 0.02 | 0.56 ± 0.03 |
| TSB-YE + Rf | 10.49 ± 0.06 | 10.53 ± 0.05 | 0.12 ± 0.01#c | 0.12 ± 0.01 | 0.49 ± 0.07 | 0.49 ± 0.06 |
| 2 ppm cQAC | –g | 10.50 ± 1.30 | – | 0.14 ± 0.01 | – | 0.50 ± 0.01 |
| 2 ppm cQAC + R | – | – | – | – | – | – |
| WRLP380 | ||||||
| TSB-YE | 8.34 ± 0.38 | 8.37 ± 0.43 | 0.16 ± 0.01 | 0.12 ± 0.02 | 0.60 ± 0.03 | 0.60 ± 0.03 |
| TSB-YE + R | 9.96 ± 0.46 | 10.32 ± 0.38 | 0.13 ± 0.01 | 0.13 ± 0.01 | 0.52 ± 0.03 | 0.52 ± 0.03 |
| 2 ppm cQAC | – | 10.48 ± 1.86 | – | 0.13 ± 0.02 | – | 0.55 ± 0.0 |
| 2 ppm cQAC + R | – | – | – | – | – | – |
| WRLP394 | ||||||
| TSB-YE | 8.08 ± 0.40 | 8.01 ± 1.09 | 0.17 ± 0.01 | 0.11 ± 0.03 | 0.65 ± 0.02 | 0.67 ± 0.02 |
| TSB-YE + R | 10.40 ± 0.19 | 10.46 ± 0.16 | 0.14 ± 0.01# | 0.14 ± 0.01 | 0.54 ± 0.04 | 0.54 ± 0.03 |
| 2 ppm cQAC | – | 9.95 ± 2.48 | – | 0.13 ± 0.04 | – | 0.59 ± 0.03 |
| 2 ppm cQAC + R | – | – | – | – | – | – |
| WRLP483 | ||||||
| TSB-YE | 8.24 ± 0.40 | 8.41 ± 0.34 | 0.16 ± 0.01 | 0.15 ± 0.02 | 0.59 ± 0.00 | 0.56 ± 0.03 |
| TSB-YE + R | 9.85 ± 0.23 | 9.58 ± 0.19 | 0.13 ± 0.02 | 0.13 ± 0.02 | 0.48 ± 0.02 | 0.48 ± 0.01 |
| 2 ppm cQAC | – | 10.41 ± 0.25$b | – | 0.15 ± 0.02 | – | 0.50 ± 0.02 |
| 2 ppm cQAC + R | – | – | – | – | – | – |
| WRLP530 | ||||||
| TSB-YE | 8.58 ( 0.25 | 9.04 ( 0.16 | 0.16 ( 0.01 | 0.14 ( 0.01 | 0.64 ( 0.03 | 0.59 ( 0.04 |
| TSB-YE + R | 10.29 ± 0.37 | 10.07 ± 0.30 | 0.13 ± 0.02 | 0.13 ± 0.01 | 0.52 ( 0.01 | 0.52 ( 0.01 |
| 2 ppm cQAC | – | 11.66 ± 0.11$ | – | 0.14 ± 0.02 | – | 0.53 ± 0.00 |
| 2 ppm cQAC + R | – | – | – | – | – | – |
| WRLP533 | ||||||
| TSB-YE | 8.41 ( 0.41 | 8.99 ( 0.12 | 0.16 ( 0.01 | 0.14 ( 0.01 | 0.65 ( 0.02 | 0.61 ( 0.06 |
| TSB-YE + R | 10.62 ± 0.23 | 10.73 ± 0.19 | 0.13 ± 0.02 | 0.13 ± 0.01 | 0.51 ± 0.02 | 0.51 ± 0.01 |
| 2 ppm cQAC | – | 10.98 ± 0.32$ | – | 0.12 ± 0.05 | – | 0.54 ± 0.03 |
| 2 ppm cQAC + R | – | – | – | – | – | – |
aValues represent mean values ± standard deviation for three independent assays, with each sample and treatment measured in duplicate. Statistically significant values between WT and qAD strains within each treatment are indicated by asterisk (*P < 0.05, unpaired two-tailed t test).
bStatistically significant values of lag phase duration of qAD strains grown in the presence of 2 ppm cQAC compared to TSB-YE are indicated by $ (P < 0.05, paired two-tailed t test).
cStatistically significant values of maximum growth rate of WT strains grown in TSB-YE compared to TSB-YE + R are indicated by # (P < 0.05, paired two-tailed t test).
dWT, wild type.
eqAD, strains adapted to 3 ppm cQAC, a commercial quaternary ammonium compound sanitizer.
fR, reserpine added at 20 μg/ml concentration.
gNo growth is indicated by “–”.
The bold values represent statistically significant results.