Table 2 Results of the correlation analysis between the standard broth microdilution method and the zone diameter distribution, and the results of AST of clinical isolates performed with the disk diffusion method.

From: Evaluation and comparison of antibiotic susceptibility profiles of Streptomyces spp. from clinical specimens revealed common and region-dependent resistance patterns

 

Antimicrobials

Abbreviation

Disk content

Zone diameter breakpoints (mm)

MIC breakpoints (mg/L)

Correlation parameters

% clinical isolates (n = 84)

COWT of cluster C ≥ (mm)

Pearson´s r

No. of errors (%)

R ≤ 

I

S ≥ 

R

I

S

VM

M

m

R

I

S

Zone diameter breakpoints corelated with MICs

Ampicillin

AMP

10 µg

21

22–24

25

≥ 16

–

≤ 8A

−0.96

0

0

5(20)

82

6

12

11

Amikacin

AKN

30 µg

–

–

30①

≥ 16B

–

≤ 8B

–

–

–

–

–

–

100

28③

Chloramphenicol

CMP

30 µg

23

24–26

27

> 8C

–

≤ 8C

−0.9

2(7)

0

1(4)

13

2

85

24

Ciprofloxacin

CIP

5 µg

24

25–29

30

≥ 4B

2B

≤ 1B

−0.89

0

0

9(21)

6

46

48

26

Erythromycin

ERY

15 µg

22

23–28

29

≥ 8B

4B

≤ 2B

−0.94

0

1(5)

6(29)

87

7

6

9

Gentamycin

GEN

10 µg

–

–

28①

≥ 16A, D

8D, F

≤ 4A, D

–

–

–

–

–

–

100

29

Penicillin

PEN

10UI

44

45–49

50

> 0.1252E

–

≤ 0.12E

−0.75

–

–

–

100

–

–

–

Tetracycline

TET

30 µg

27

28–31

32

> 2E

–

≤ 2E

−0.93

0

0

2(29)

8

1

91

34

Trimethoprim-sulfamethoxazole

SXT

1.25 µg + 23.75 µg

14

–

15

≥ 4/76B

–

≤ 2/38B

−0.87

0

1(9)

–

79②

–

21②

–

Vancomycin

VAN

30 µg

–

–

24①

–

–

≤ 2D

–

–

–

–

–

–

100

25

Tentative breakpoints according ZDs distribution

Amoxicillin

AMX

25 µg

27

28–30

31

–

–

–

–

–

–

–

81

6

13

18

Amoxicillin-clavulanic acid

AMC

20 µg + 10 µg

20

21–22

23

–

–

–

–

–

–

–

7

6

87

20

Cefazolin

CZN

30 µg

23

24–29

30

–

–

–

–

–

–

–

98

0

2

12

Ceftriaxone

CRO

30 µg

23

24–29

30

–

–

–

–

–

–

–

88

5

7

10

Clarithromycin

CLR

15 µg

21

22–25

26

–

–

–

–

–

–

–

7

4

89

22

Doxycycline

DOX

30 µg

27

28–31

32

–

–

–

–

–

–

–

7

1

92

35

Minocycline

MNO

30 µg

27

28–31

32

–

–

–

–

–

–

–

4

2

94

36

Arbitrary breakpoints

Rifampicin

RIF

5 µg

20

21–29

30

–

–

–

–

–

–

–

11

27

62

24③

Streptomycin

SMN

10 µg

20

21–29

30

–

–

–

–

–

–

–

54

39

7

11

Ofloxacin

OFX

5 µg

20

21–29

30

–

–

–

–

–

–

–

17

76

7

18

Only BM method performed

Linezolid

LIZ

–

–

–

–

–

–

≤ 8B

–

–

–

–

–

–

100④

–

Ampicillin-sulbactam

AMS

–

–

–

–

≥ 16

–

≤ 8A

–

–

–

–

48④

–

52④

–

  1. List of antibiotics, their abbreviations, and disk concentration used in the study are provided. Antibiotics are grouped into categories according to zone diameter breakpoint settings. Table shows the proposed zone diameter breakpoints based on ZD-MIC correlation analysis altogether with ZDs distribution among clinical strains, proposed MIC breakpoints according to available breakpoints values in CLSI and EUCAST guidelines and according to ZDs distribution among clinical strains. Parameters of the correlation analysis are provided: Pearson correlation coefficient (r), VM – very major error, false-susceptible by DD; M – major error, false-resistant by DD; m- minor error, one of the test results is intermediate and the other is susceptible or resistant. Antibiotic susceptibility distribution of clinical strains based on proposed ZD breakpoints and wild type cut-off (COWT) calculated for clinical strains in the cluster C. Abbreviations: R = resistant, I = intermediate susceptible, S = susceptible; ① minimum ZD values were proposed as susceptibility breakpoints (type strain DSM 40783 was excluded for vancomycin and BCCO 10_1638 for amikacin), since predominantly susceptible strains (BM method) were present; ② only 78 clinical isolates were included, cluster D was excluded; ③ abnormal standard deviation; ④ only 29 clinical isolates were tested. References: A—Larruskain, J., Idigoras, P., Marimón, J. M., Pérez-Trallero, E. Susceptibility of 186 Nocardia sp. isolates to 20 antimicrobial agents. Antimicrob. Agents. Chemother. 55, 2995–8 (2011); B—CLSI. Susceptibility testing of mycobacteria, and other aerobic actinomycetes. 2nd Ed. CLSI Guideline M24 (2011); C—Dragomirescu, C. C. et al. Antimicrobial susceptibility testing for Corynebacterium species isolated from clinical samples in Romania. Antibiotics 9, 1–9 (2020); D—CLSI. Methods for antimicrobial dilution and disk susceptibility testing of infrequently isolated or fastidious bacteria. 3rd Ed. CLSI Guideline M45 (2015); E—EUCAST. Testing Breakpoint tables for interpretation of MICs and zone diameters. Available from: https://www.eucast.org/fileadmin/src/media/PDFs/EUCAST_files/Breakpoint_tables/v_10.0_Breakpoint_Tables.pdf (2020).
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