TABLE 4.
Properties of sVISA strains compared with those of other MRSA strains with different susceptibilities to vancomycin
| Straina | Category of vancomycin susceptibility | Colony sizeb | Doubling time (min) | Vancomycin MIC (mg/liter) evaluated by Etest after incubation time (h) ofc: |
rpoB genotyped |
No. of LC/total no. of colonies (%) observed after the indicated time (days) of serial passagee |
||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 48 | 72 | 96 | 120 | 144 | Amino acid change | Nucleotide change | 1f | 2 | 3 | 7 | ||||
| 1-4d | sVISA | PC | 64.8 | 6 | 8 | 8 | 8 | 8 | WT | 32/44 (72.7) | NT | NT | NT | |
| 1-5d | sVISA | PC | 70.7 | 6 | 8 | 8 | 8 | 8 | WT | 488/490 (99.6) | NT | NT | NT | |
| 2-6d | sVISA | PC | 106.6 | 8 | 12 | 12 | 16 | 16 | WT | 6/45 (13.3) | NT | NT | NT | |
| 3-4d | sVISA | PC | 123.8 | 8 | 12 | 12 | 16 | 16 | WT | 6/66 (9.1) | NT | NT | NT | |
| 4-4d | sVISA | PC | 123.8 | 8 | 12 | 12 | 12 | 12 | G744R | GGA→AGA | 217/231 (93.9) | NT | NT | NT |
| 4-5d | sVISA | PC | 68.6 | 8 | 8 | 8 | 12 | 12 | WT | 56/74 (75.7) | NT | NT | NT | |
| 5-4d | sVISA | SC | 60.2 | 8 | 12 | 12 | 12 | 12 | WT | 7/51 (13.7) | NT | NT | NT | |
| 5-5d | sVISA | PC | 58.3 | 8 | 12 | 12 | 12 | 12 | S746F | TCT→TTT | − | 17/6.3 × 104 | NT | NT |
| 6-4d | sVISA | PC | 71.5 | 24 | 24 | 32 | 32 | 32 | WT | − | 2/6.8 × 106 | NT | NT | |
| 11-6d | sVISA | PC | 111.8 | 8 | 16 | 16 | 16 | 16 | WT | 330/335 (98.5) | NT | NT | NT | |
| 12-5d | sVISA | PC | 66 | 8 | 12 | 12 | 12 | 12 | WT | 50/51 (98.0) | NT | NT | NT | |
| 14-4d | sVISA | PC | 64.2 | 8 | 8 | 8 | 8 | 8 | WT | 13/2.3× 104 | NT | NT | NT | |
| 16-4d | sVISA | PC | 61.9 | 6 | 8 | 8 | 8 | 8 | WT | − | − | − | 47/47 (100) | |
| 17-4df | sVISA | PC | 123.8 | 8 | 12 | 12 | 12 | 12 | WT | − | − | − | 4/5.2 × 104 | |
| 17-6d | sVISA | PC | 87.7 | 6 | 8 | 8 | 12 | 12 | S746F | TCT→TTT | 4/43 (9.3) | NT | NT | NT |
| 18-4d | sVISA | PC | 153.9 | 6 | 12 | 12 | 12 | 12 | G977V | GGT→GTG | 25/1,410 (1.8) | NT | NT | NT |
| 18-5d | sVISA | PC | 144.4 | 6 | 12 | 12 | 12 | 12 | WT | 2/1.4 × 104 | NT | NT | NT | |
| 18-6df | sVISA | PC | 182.4 | 4 | 4 | 6 | 6 | 8 | WT | − | − | − | 3/320 (0.9) | |
| 19-4d | sVISA | PC | 141.5 | 8 | 12 | 16 | 24 | 24 | WT | − | 3/1.0× 104 | NT | NT | |
| 19-5d | sVISA | PC | 63 | 8 | 12 | 12 | 12 | 12 | WT | 2/361 (0.5) | NT | NT | NT | |
| 20-4d | sVISA | PC | 64.8 | 12 | 24 | 24 | 24 | 24 | WT | 7/41 (17.0) | NT | NT | NT | |
| 20-6d | sVISA | PC | 77.3 | 8 | 8 | 8 | 8 | 8 | WT | 2/2,600 | NT | NT | NT | |
| 21-4d | sVISA | PC | 83.5 | 6 | 12 | 12 | 12 | 12 | H929T | CAT→TAT | − | 14/2.6× 105 | NT | NT |
| 22-4d | sVISA | SC | 59.2 | 8 | 12 | 12 | 12 | 12 | WT | 3/5,100 | NT | NT | NT | |
| 24-4d | sVISA | SC | 58.2 | 8 | 8 | 8 | 8 | 8 | WT | 2/5,100 | NT | NT | NT | |
| 25-4d | sVISA | PC | 80.6 | 8 | 8 | 12 | 12 | 12 | H929T | CAT→TAT | − | 2/640 (0.3) | NT | NT |
| Mu3 | hVISA | LC | 36.9 | 3 | 3 | 3 | 3 | 3 | WT | NT | NT | NT | NT | |
| Mu50 | VISA | LC | 37.1 | 12 | 12 | 12 | 12 | 12 | H481Y | CAT→TAT | NT | NT | NT | NT |
| 6R-P | sVISA | PC | 62.2 | 12 | 16 | 16 | 16 | 16 | R512P | CGT→CCT | 3/1.0× 107 | 8/93 (8.6) | 8/312 (41.0) | 65/65 (100) |
| L1 | hVISA | LC | 38.2 | 3 | 3 | 3 | 3 | 3 | P512L | CCT→CTT | NT | NT | NT | NT |
| L2 | hVISA | LC | 39.7 | 3 | 4 | 4 | 4 | 4 | P512S | CCT→TCT | NT | NT | NT | NT |
| L3 | VISA | LC | 36.9 | 4 | 6 | 6 | 6 | 6 | P512H | CCT→CAT | NT | NT | NT | NT |
| ΔIP | VSSA | LC | 26.7 | 1 | 1 | 1.5 | 1.5 | 1.5 | WT | NT | NT | NT | NT | |
| ΔIPrpoB(R512P) | hVISA | SC | 41.2 | 4 | 4 | 4 | 4 | 4 | R512P | CGT→CCT | − | − | 2/4.9× 105 | NT |
| H14 | pre-hVISAg | SC | 28.3 | 3 | 3 | 3 | 3 | 3 | WT | − | − | − | NT | |
| H14rpoB(R512P) | VISA | PC | 45.2 | 8 | 8 | 12 | 12 | 12 | R512P | CGT→CCT | − | 2/3.4 × 104 | NT | NT |
The first number in the name of the sVISA strains denotes the culture tube from which the strain was isolated. The suffix denotes the day of appearance of the strain as a visible colony.
Observed at 20 h after streaking of the strain on BHI agar plate without vancomycin. LC, large colony; PC, pinpoint colony; SC, small colony.
If the colony size of the strain was heterogeneous, the smaller colony was picked for the test.
Entire ORF of rpoB gene was sequenced. The Mu3 rpoB sequence was referred to as the wild-type sequence. The underlined base is the changed nucleotide. WT, wild type.
Day 1, the starting culture before propagation; NT, not tested; −, large colonies were not observed (<1 × 108).
The strain seems to have been converted to VISA by a mutation(s). It may not be regarded as sVISA because of its rather stable phenotype throughout the drug-free propagation.
Pre-hVISA, a VSSA strain whose vancomycin-resistant subpopulation does not reach the level of hVISA; i.e., the frequency of the subpopulations of cells that can grow on the plate containing 4 mg/liter of vancomycin is >1 × 10−7 but <1 × 10−6 (see Fig. 2C).