AST Pattern of Bacteria: Introduction, Group of AST Pattern, List of Antimicrobial Agents, and Keynotes

AST Pattern of Bacteria

AST Pattern of Bacteria covers common bacterial pathogens groups like Enterobacteriaceae, Staphylococcus, Enterococcus, Pseudomonas aeruginosa, Streptococcus pneumoniae, beta-haemolytic streptococci (Streptococcus pyogenes, Streptococcus agalactiae), Haemophilus species, Acinetobacter, Viridans group of streptococci, Shigella, and Neisseria species.

Enterobacteriaceae

Antimicrobial Agents- Zone Diameter (Intermediate)

1. Ampicillin (10 μg) : 14-16 mm
2. Ceftriaxone (30 μg): 20-22
3. Ciprofloxacin(5 μg): 16-20
4. 0floxacin (Salmonella,5 μg):21-30
5. Co-trimoxazole(1.25/23.75 μg): 11-15
6. Chloramphenicol (except urine, 30 μg): 13-17
7. Nitrofurantoin (only urine,300 μg ): 15-16
8. Gentamicin (10 μg): 13-14
9. Amikacin (30 μg ): 15-16
10. Cefotaxime (30 μg): 23-25
11. Cefuroxime (oral, 30): 15-22
12. Cefixime (oral, 5 μg):16-18
13. Doxycycline (30 μg): 11-13
14. Nalidixic acid (30 μg): 14-18 (don’t report I)
15. Meropenem (10 μg): 20-22 (Ignore for Salmonella
    and Shigella spp.)
16. Azithromycin (Only for S. Typhi, 15 μg): ≥13

Extra

18. Amoxycillin clavulanate (20/10 μg): 14-17
19. Ampicillin-sulbactam (10/10 μg): 12-14
20. Piperacillin-tazobactam (100/10 μg): 18-20
21. Cefepime (30 μg): 19-24
22. Ceftazidime (30 μg): 18-20
23. Cefoperazone (75 μg): 16-20
24. Aztreonam (30 μg): 18-20
25. Imipenem (10 μg): 20-22
26. Levofloxacin(5 μg): 14-16
27. Tobramycin (10 μg): 13-14

Staphylococcus species

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

Penicillin (10 U) : ≥29

Cefoxitin (30 µg): ≥ 22 (S. aureus)

Cefoxitin (30 µg): ≥ 25 (CoNS)

Ciprofloxacin (5 µg): 16-20

Ofloxacin (5 µg):15-17

Levofloxacin (5 µg): 16-18

Co-trimoxazole (1.25/23.75 µg): 11-15

Chloramphenicol (except urine, 30 µg): 13-17

Nitrofurantoin (only urine,300 µg): 15-16

Gentamicin (10 µg): 13-14

Azithromycin (15 µg): 14-17

Erythromycin (15 µg): 14-17

Doxycycline (30 µg): 13-15

Clindamycin (2 µg): 15-20

Vancomycin (MIC≤2):4-8 (S. aureus)

Vancomycin (MIC≤4):8-16 (CoNS)

Teicoplanin (MIC≤8):16

Linezolid (30 µg): ≥29

Ceftaroline (30 µg): 21-23 (for MRSA)

Keynotes on AST Pattern of Bacteria

• Novobiocin (): S. saprophyticus is intrinsically resistant to novobiocin.

• Inducible clindamycin resistance can be detected by disk diffusion using the D-zone test.
• MRSA/MRS: They are considered resistant to other ß-lactam agents, i.e., penicillin, ß-lactam/ ß-lactamase inhibitor combination, cephems (with the exception of cephalosporins with anti-MRSA activity), and carbapenems.

• Cefoxitin is a screening drug and therefore do not report.

Enterococcus species

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

Penicillin (10 U): ≥15

Ampicillin (10 µg): ≥17

Ciprofloxacin (5 µg): 16-20

Norfloxacin (10 µg):13-16

Levofloxacin (5 µg): 14-16

Chloramphenicol (except urine, 30 µg): 13-17

Nitrofurantoin (only urine,300 µg): 15-16

Erythromycin (15 µg): 14-22

Doxycycline (30 µg): 13-15

Vancomycin (30 µg): 15-16

Teicoplanin (30 µg): 11-13

Linezolid (30 µg): ≥21-22

Gentamicin (HLG,120 µg): 7-9

Pseudomonas aeruginosa

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

Ciprofloxacin (5 µg): 16-20

Ofloxacin (5 µg):13-15

Levofloxacin (5 µg): 14-16

Gentamicin (10 µg): 13-14

Tobramycin (10 µg): 13-14

Amikacin (30 µg): 15-16

Aztreonam (30 µg): 16-21

Meropenem (10 µg): 16-18

Imipenem (10 µg): 16-18

Piperacillin (100 µg): 15-20

Piperacillin tazobactam (100/10 µg): 15-20

Ceftazidime (30 µg): 15-17

Cefepime (30 µg): 15-17

Colistin (10 µg): MIC≤2

Polymyxin B (300U): MIC≤2

Extrinsic Resistance

Amoxicillin-clavulanate

Ampicillin

Amoxicillin

Ampicillin-sulbactam

Azithromycin

Cefotaxime

Ceftriaxone

Trimethoprim

Cephalosporin I (Cephalothin, cefazolin)

Cephamycins (cefoxitin, cefotetan)

Chloramphenicol

Clarithromycin

Ertapenem

Erythromycin

Tetracycline

Trimethoprim-sulfamethoxazole

Cephalosporin II (Cefuroxime)

Streptococcus pneumoniae

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

Oxacillin (1 µg): ≥20 (screening drug, do not report)

0floxacin (5 µg):13-15

Levofloxacin (5 µg): 14-16

Chloramphenicol (30 µg): ≥ 21

Erythromycin (15 µg): 16-20

Azithromycin (15 µg):  14-17

Doxycycline (30 µg): 25-27

Vancomycin (30 µg): ≥ 17

Clindamycin (2 µg): 16-18

Co-trimoxazole (1.25/23.75 µg): 16-18

Linezolid (30 µg): ≥21

Keynotes on AST pattern of bacteria-

1. For non-meningitis isolates, oxacillin zone ≥20 mm can predict susceptibility to the following lactams- ampicillin, ampicillin-sulbactam, Amoxycillin, amoxicillin-clavulanate, cefaclor, cefdinir, cefditoren, cefepime, cefotaxime, cefpodoxime, cefprozil, ceftaroline, ceftizoxime, ceftriaxone, cefuroxime, doripenem, ertapenem, imipenem, loracarbef, meropenem, and penicillin.
2. For isolates with oxacillin zones less than 19 mm, do not report penicillin as resistant without performing a penicillin MIC test.

Streptococcus species (ß-Haemolytic Group)

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

Penicillin (10 U): ≥24 or

Ampicillin (10 µg): ≥24

Cefotaxime (30 µg): ≥24 or

Ceftriaxone (30 µg): ≥24

Vancomycin (30 µg): ≥ 17

Azithromycin (15 µg): 14-17

Erythromycin (15 µg): 16-20

Tetracycline (30 µg):19-22

Levofloxacin (5 µg): 14-16

0floxacin (5 µg):13-15

Chloramphenicol (30 µg): 18-20

Clindamycin (2 µg): 16-18

Linezolid (30 µg): ≥21

Keynotes on AST Pattern of Bacteria-

• Penicillin and ampicillin are drugs of choice for the treatment of beta-hemolytic streptococcal infection.

• Non-susceptible isolates (i.e., penicillin MICs>0.12 µg/mL and ampicillin MICs >0.25 µg/mL) are extremely rare in any ß-haemolytic streptococcus and have not been reported for Streptococcus pyogenes.

• This type of isolate should be re-identified and retested in such a scenario. For confirmation, it should be submitted to the national public health laboratory.

Haemophilus influenzae/ parainfluenzae

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

Ampicillin (10 µg): 19-21

Ampicillin-sulbactam (10/10 µg): ≥ 20

Amoxycillin clavulanate (20/10 µg): 14-17

Piperacillin tazobactam (100/10 µg): ≥21

Ceftazidime (30 µg): ≥26

Cefotaxime (30 µg): ≥26

Ceftriaxone (30 µg): ≥26

Cefuroxime (oral, 30): 17-19

Cefepime (30 µg): ≥26

Aztreonam (30 µg): ≥26

Imipenem (10 µg): ≥16

Meropenem (10 µg): ≥20

Levofloxacin (5 µg): ≥17

Ciprofloxacin (5 µg): ≥21

Ofloxacin (5 µg): ≥16

Co-trimoxazole (1.25/23.75 µg): 11-15

Chloramphenicol (except urine, 30 µg): 26-28

Acinetobacter spp.

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

Ciprofloxacin (5 µg): 16-20

Levofloxacin (5 µg): 14-16

Co-trimoxazole (1.25/23.75 µg): 11-15

Doxycycline (30 µg): 10-12

Gentamicin (10 µg): 13-14

Tobramycin (10 µg): 13-14

Amikacin (30 µg): 15-16 Colistin (10 µg): MIC≤2

Polymyxin B (300U): MIC≤2

Meropenem (10 µg): 15-17

Imipenem (10 µg): 19-21

Piperacillin (100 µg): 18-20

Piperacillin tazobactam (100/10 µg): 18-20

Ceftazidime (30 µg): 15-17

Cefepime (30 µg): 15-17

Cefotaxime (30 µg): 15-22

Ceftriaxone (30 µg):  14-20

Ampicillin-sulbactam (10/10 µg): 12-14

Extrinsic Resistance

Amoxicillin-clavulanate

Ampicillin

Amoxicillin Azithromycin Aztreonam Fosfomycin

Cephalosporin I (Cephalothin, cefazolin)

Cephalosporin II (Cefuroxime)

Chloramphenicol

Clarithromycin

Ertapenem

Erythromycin

Trimethoprim

Cephamycins (cefoxitin, cefotetan)

Streptococcus spp. Viridans Group

Antimicrobial Agents Zone Diameter (Intermediate, mm)

Penicillin (MIC) :≤0.12 or
Ampicillin (MIC): ≤ 0.25
Cefotaxime (30 μg): 26-27 or
Ceftriaxone (30 μg): 25-26
Vancomycin (30 μg): ≥ 17
Azithromycin (15 μg): 14-17
Erythromycin (15 μg): 16-20
Tetracycline (30 μg):19-22
Levofloxacin (5 μg): 14-16
0floxacin (5 μg):13-15
Chloramphenicol (30 μg): 18-20
Clindamycin(2 μg): 16-18
Linezolid (30 μg): ≥21
Meropenem (MIC): ≤0.5