Rapid optical determination of β-lactamase and antibiotic activity

BMC Microbiology

Table 3 Cefazolin disk diffusion results

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?

*The Antibiotic Susceptibility (AS) was determined using the CLSI Zone Diameter Interpretive Criteria for Cefazolin Disk Diffusion [41].
≤ 14 mm: Resistant (R); 15–17 mm: Intermediate (I); ≥ 18 mm: Susceptible (S)
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].
^&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.

ISSN: 1471-2180