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Table 3 Cefazolin disk diffusion results

From: Rapid optical determination of β-lactamase and antibiotic activity

S. aureusisolate # Zone of inhibition diameter (mm) AS* Zone edge Interpretation as per zone edge test criteria&
1 21.5 ± 1.0 S Sharp β
2 31.0 ± 1.0 S Fuzzy  
3 33.5 ± 0.5 S Fuzzy  
4 33.0 ± 2.0 S Fuzzy  
5 32.5 ± 0.5 S Fuzzy  
6 36.5 ± 0.5 S Sharp β
7 32.0 ± 0.5 S Fuzzy  
8 39.5 ± 1.5 S Fuzzy  
9 29.5 ± 1.5 S Fuzzy  
10 41.5 ± 0.5 S Fuzzy  
11 34.5 ± 2.5 S Little fuzzy Weak β?
12 41.0 ± 1.6 S Fuzzy  
13 32.5 ± 0.5 S Fuzzy  
14 33.0 ± 0.0 S Fuzzy  
15 35.5 ± 2.5 S Fuzzy  
16 36.5 ± 0.5 S Fuzzy  
17 36.5 ± 0.5 S Fuzzy  
18 33.5 ± 0.5 S Sharp β
19 31.0 ± 0.0 S Sharp β
20 20.5 ± 0.3 S Sharp β
21 38.0 ± 1.0 S Fuzzy  
22 34.0 ± 1.1 S Little fuzzy Weak β?
23 33.5 ± 1.5 S Fuzzy  
24 34.5 ± 1.5 S Fuzzy  
25 30.5 ± 0.5 S Fuzzy  
26 34.0 ± 0.0 S Fuzzy  
27 36.0 ± 2.0 S Little fuzzy/sharpish Weak β?
  1. *The Antibiotic Susceptibility (AS) was determined using the CLSI Zone Diameter Interpretive Criteria for Cefazolin Disk Diffusion [41].
  2. ≤ 14 mm: Resistant (R); 15–17 mm: Intermediate (I); ≥ 18 mm: Susceptible (S)
  3. The results shown are averages of at least two independent experiments, and are presented as Average ± Standard Error. The CLSI recommended quality control strain ATCC 25923 (#2) was included each time and gave zone of inhibition diameter within the expected range (29-35 mm) [41].
  4. &The zone edge test was also applied and the edge of the zone of inhibition was observed. S. aureus ATCC 29213 (#1) was used as a positive control for the zone edge test (sharp edge), and ATCC 25923 (#2) as a negative control (fuzzy edge). ‘β’ denotes β-lactamase producing strain.