Article Figures & Data
Figures
- Fig. 1.
Scatterplots of oxacillin MICs versus oxacillin disk diffusion zone diameters for 50 CoNS tested in 11 laboratories. Proposed MIC and disk diffusion breakpoints are indicated by horizontal and vertical lines, respectively. (A) mecA-positive strains; (B) mecA-negative strains.
- Fig. 2.
Scatterplots of oxacillin MICs versus oxacillin disk diffusion zone diameters for 200 CoNS tested at CDC. Proposed MIC and disk diffusion breakpoints are indicated by horizontal and vertical lines, respectively. (A) mecA-positive strains; (B)mecA-negative strains.
Tables
- Table 1.
Inoculum counts performed on a single well of a broth microdilution plate
Species (no. of strains) Mean (105) CFU/ml (range)a on a: CDC plate MicroScan plate S. epidermidis(27) 1.78 (1.1–3.5) 1.63 (0.6–3.6) S. hominis(8) 0.89 (0.3–2.1) 0.92 (0.3–2.6) S. warneri(6) 1.60 (0.6–2.6) 1.53 (0.6–3.6) S. haemolyticus (3) 1.34 (0.6–2.2) 1.17 (0.2–2.7) S. lugdunensis(2) 4.43b (0.9–10.6) 4.14b (0.9–12.1) S. saprophyticus(2) 2.57 (0.8–4.1) 2.31 (0.7–5.1) S. capitis(1) 2.92 (0.98–5.4) 2.58 (0.7–4.8) S. simulans(1) 2.02 (0.5–3.5) 1.66 (0.2–3.2) ↵a The means represent the average counts from both MIC plates for each organism for each species counted in each laboratory. Each range consists of the mean from the laboratory with the lowest average counts per species and the mean from the laboratory with the highest average counts per species.
↵b Counts within allowable range for final inoculum in MIC plates.
- Table 2.
Effect of medium source on oxacillin MICs expressed as dilution difference of MICs
Organism group and media No. of results with a dilution difference of: % of results within a dilution difference of ±1 ≥−3 −2 −1a 0 +1b +2 ≥+3 mecA negative S. epidermidis CDC Difco vs. MicroScan Difco 5 76 16 100 CDC Difco vs. CDC Acumedia 13 77 9 1 99.0 MicroScan Difco vs. MicroScan BDMS 1 37 58 2 99.0 Other species CDC Difco vs. MicroScan Difco 8 32 71 18 93.8 CDC Difco vs. CDC Acumedia 19 38 54 6 83.8 MicroScan Difco vs. MicroScan BDMS 8 44 77 1 93.8 mecA positive S. epidermidis CDC Difco vs. MicroScan Difco 29 50 77 29 5 1 58.1 CDC Difco vs. CDC Acumedia 1 7 18 45 54 46 11 64.3 MicroScan Difco vs. MicroScan BDMS 2 3 12 45 91 33 11 75.1 Other species CDC Difco vs. MicroScan Difco 2 18 20 24 15 2 76.5 CDC Difco vs. CDC Acumedia 26 12 9 22 6 49.3 MicroScan Difco vs. MicroScan BDMS 1 6 21 39 12 3 87.8 - Table 3.
Comparison of results with different reference plates and medium manufacturers by mecA and oxacillin MIC breakpoint
Plate/medium No. (%) of results with the indicated MIC (μg/ml) for: mecA-positive strains (n = 318) mecA-negative strains (n = 230) ≥4 ≥2 ≥1 ≥0.5 ≤2 ≤1 ≤0.5 ≤0.25 CDC/Difco 182 (57) 260 (82) 290 (91) 298 (94) 224a (99) 211 (93) 195 (86) 166a (73) MicroScan/Difco 82 (26) 145 (46) 255 (80) 299 (94) 230 (100) 218 (95) 197 (86) 171 (74) CDC/Acumedia 198 (62) 240 (76) 257 (81) 267 (84) 222b (98) 216 (96) 202 (89) 175b (77) MicroScan/BDMS 143 (45) 209 (66) 272 (86) 297 (93) 230 (100) 227 (99) 211 (92) 186 (81) - Table 4.
Correlation of mecA PCR test results with MIC category results for current and proposed oxacillin breakpoints
Organism group (no. of tests) % Correct values with the indicated medium by MIC breakpointa Proposed (≤0.25 = S; ≥0.5 = R) Current (≤2.0 = S; ≥4.0 = R) CDC Difco MScan Difco CDC Acumedia MScan BDMS CDC Difco MScan Difco CDC Acumedia MScan BDMS mecApositive S. epidermidis(198) 95.5 96.0 94.4 93.4 52.5 8.1 72.3 40.9 Other species (120) 90.8 90.8 66.7 93.3 65.0 55.0 40.8 51.7 All strains 93.7 94.0 84.0 93.4 57.2 25.8 62.3 45.0 mecA negative S. epidermidis(99) 100 (97)b 100 (98) 99.0 (97) 100 (99) 100 (97) 100 (98) 100 (97) 100 (99) Other species (132) 52.7 (129) 55.0 (131) 60.9 (126) 66.4 (131) 97.7 (129) 100 (131) 96.9 (126) 100 (131) All strains 73.1 74.3 77.4 80.9 98.7 100 98.2 100 ↵a For mecA-positive strains, MICs must be ≥0.5 μg/ml for proposed breakpoints or ≥4.0 μg/ml for current breakpoints to be considered correct; formecA-negative strains, MICs must be ≤0.25 μg/ml for proposed breakpoints or ≤2.0 μg/ml for current breakpoints to be considered correct. MScan, MicroScan; S, susceptible; R, resistant.
↵b Number given in parentheses is the total number of results used for calculations. Differences were due to growth failures in some MIC plates.
- Table 5.
Correlation of mecA PCR test results with results of the oxacillin agar screen test
Organism group (no. of test results) % Correct results by oxacillin screen plate manufacturer and inoculation methoda Remel BDMS Wet Expressed Wet Expressed 24 h 48 h 24 h 48 h 24 h 48 h 24 h 48 h S. epidermidis, mecA-positive (198) 62.6 81.3 52.0 74.2 22.2 54.5 15.7 38.9 Other species, mecA-positive (120) 59.2 73.3 50.0 64.2 51.7 73.3 45.0 61.7 S. epidermidis, mecA-negative (99) 100 100 100 100 100 100 100 99.0 Other species,bmecA-negative (129) 98.5 97.7 99.2 99.2 100 100 100 100 ↵a Percent correct results observed for each medium (growth for mecA-positive strains; no growth formecA-negative strains). Wet, liquid was not expressed from the swab prior to inoculation of the agar screen plate; expressed, liquid was expressed from the swab prior to inoculation of the agar screen plate.
↵b Data from three strains were deleted due to testing problems at one site.
