To determine whether a similar tendency would be seen in fresh clinical isolates, we collected a total of 353 strains of independently isolated MRSA from 11 regionally distant hospitals. Twenty-five strains were classified as BIVR, which was equivalent to 7.0% of the total, while 328 strains (92.9%) were non-BIVR. All these strains were subjected to the blaZ test by PCR and a qualitative ß-lactamase test using a nitrocefin-impregnated disk. Among the learn more 25 BIVR strains, 21 (84.0%) were blaZ-negative and 23 (92.0%) yielded negative

results for the nitrocefin test (Table 4). Among the non-BIVR strains, 310 (94.5%) were blaZ-positive and only 18 (5.5%) were blaZ-negative. Similarly, 223 strains (61.0%) yielded positive results for the nitrocefin test and the remaining 128 (39.0%) gave negative results (Table 4). A statistically significant difference in the occurrence of the blaZ gene and ß-lactamase activity between the BIVR and non-BIVR strains was found with a probability <0.01 by the χ2 and Fisher’s tests. These results clearly showed a trend for BIVR cells to lack the ß-lactamase gene and not produce

active ß-lactamase, whereas most non-BIVR cells possessed the blaZ gene and a significant fraction (61.0%) produced ß-lactamase. click here It should be noted that the nitrocefin test is a qualitative assay and might not be sensitive enough to detect low levels of ß-lactamase. To investigate this possibility, we randomly selected 10 non-BIVR strains that were blaZ-positive and -negative for the nitrocefin

test and carried out a quantitative ß-lactamase assay. All cells produced a low level of ß-lactamase ranging from 2.74×10–3 to 2.1×10–2 U with an average of 7.25×10–3 ± 1.25×10–2 U (Table 5). Therefore, the number of ß-lactamase-positive strains must be much higher. Table 4 Presence of blaZ gene and β-lactamase very activity in clinical selleck screening library isolates of BIVR and non-BIVR strains   blaZ Nitrocefin test   + – + – BIVR 4 (16.0%) 21 (84.0%) 2 (8.0%) 23 (92.0%) Non-BIVR 310 (94.5%) 18 (5.5%) 200 (61.0%) 128 (39.0%) Table 5 Quantitative β-lactamase activity, nitrocefin test and presence of blaZ in randomly selected clinical isolates of BIVR and non-BIVR Phenotype blaZ Nitrocefin test ß-lactamase (μmol/min/mg protein) Range Average ± STD BIVR (n = 5) – - <1 × 10-4 <1 × 10-4 Non-BIVR (n = 10) + + 1.03 × 10-3 – 4.48 0.79 ± 1.84 Non-BIVR (n = 10) + – 2.76 × 10-4– 2.13 × 10-2 7.28 × 10-3  ± 1.25 × 10-2 Ten randomly selected non-BIVR strains that were blaZ-positive and positive for the nitrocefin test were subjected to the quantitative ß-lactamase assay. The activity ranged from 0.103 to 0.103×10–3 U with an average of 0.79 ± 1.84 U. Thus, it is likely that most non-BIVR cells produced ß-lactamase. Activity in BIVR cells (blaZ-negative and nitrocefin-test-negative) was undetectable.