Bacteria-wise Antimicrobial Susceptibility Testing (AST): Introduction, List of Bacterial Panel, Antibiotics

Introduction of Bacteria-wise Antimicrobial Susceptibility Testing (AST)

All antimicrobial agents/ antibiotics are not applicable for all bacteria and for this reason, we should follow organism-wise antibiotics policy. As we know, antimicrobial resistance (AMR) is increasing day by day but no new antibiotic has not been discovered for two decades. AMR is not only prolonged the hospital stay of the patient but also adds an economic burden and challenges for the clinicians to treat. Misuse and overuse of antimicrobials are the two main drivers in the development of drug-resistant pathogens. To control the AMR, every country has a national AMR program and the Global Antimicrobial Resistance Surveillance System (GLASS) of WHO. We have two strong guidelines for Antimicrobial Susceptibility Testing. One is the Clinical and Laboratory Standards Institute (CLSI) and another is European Committee on Antimicrobial Susceptibility Testing (EUCAST). According to CLSI guidelines, bacterial-wise antimicrobials have been plotted below.

List of Bacterial Panels for Bacteria-wise Antimicrobial Susceptibility Testing (AST)

1. Staphylococcus species
2. Enterobacteriaceae
3. Enterococcus species
4. Pseudomonas aeruginosa
5. Streptococcus pneumoniae
6. Streptococcus species (ß-Haemolytic Group)
7. Haemophilus influenzae/ parainfluenzae
8. Acinetobacter species
9. Streptococcus species Viridans Group

Bacteria-wise Antibiotics

Staphylococcus spp.

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

1. Penicillin (10 U) : ≥29
2. Cefoxitin (30 µg): ≥ 22 ( S. aureus)
3. Cefoxitin (30 µg): ≥ 25 (CoNS)
4. Ciprofloxacin(5 µg): 16-20
5. Ofloxacin (5 µg):15-17
6. Levofloxacin (5 µg): 16-18
7. Co-trimoxazole(1.25/23.75 µg): 11-15
8. Chloramphenicol (except urine, 30 µg): 13-17
9. Nitrofurantoin (only urine,300 µg ): 15-16
10. Gentamicin (10 µg): 13-14
11. Azithromycin (15 µg): 14-17
12. Erythromycin (15 µg): 14-17
13. Doxycycline (30 µg): 13-15
14. Clindamycin( 2 µg): 15-20
15. Vancomycin (MIC≤2):4-8 (S. aureus)
16. Vancomycin (MIC≤4):8-16 (CoNS)
17. Teicoplanin (MIC≤8):16
18. Linezolid (30 µg): ≥29
19. Ceftaroline (30 µg): 21-23 (for MRSA)

Keynotes

1. Novobiocin: S. saprophyticus is intrinsically resistant to novobiocin.
2. Inducible clindamycin resistance can be detected by disk diffusion using the D-zone test.
3. MRSA/MRS: They are considered resistant to other ß-lactam agents, i.e. penicillins, ß-lactam/ ß-lactamase inhibitor combination, cephems (with the exception of cephalosporins with anti-MRSA activity), and carbapenems.
4. Cefoxitin is a screening drug and therefore does not report.

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

Extra

1. Amoxycillin clavulanate (20/10 µg): 14-17
2. Ampicillin-sulbactam (10/10 µg): 12-14
3. Piperacillin-tazobactam (100/10 µg): 18-20
4. Cefepime (30 µg): 19-24
5. Ceftazidime (30 µg): 18-20
6. Cefoperazone (75 µg): 16-20
7. Aztreonam (30 µg): 18-20
8. Imipenem (10 µg): 20-22
9. Levofloxacin(5 µg): 14-16
10. Tobramycin (10 µg): 13-14

Enterococcus spp.

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

1. Penicillin (10 U): ≥15
2. Ampicillin (10 µg): ≥17
3. Ciprofloxacin (5 µg): 16-20
4. Norfloxacin (10 µg):13-16
5. Levofloxacin (5 µg): 14-16
6. Chloramphenicol (except urine, 30 µg): 13-17
7. Nitrofurantoin (only urine,300 µg ): 15-16
8. Erythromycin (15 µg): 14-22
9. Doxycycline (30 µg): 13-15
10. Vancomycin (30 µg): 15-16
11. Teicoplanin (30 µg): 11-13
12. Linezolid (30 µg): ≥21-22
13. Gentamicin (HLG,120 µg): 7-9

Pseudomonas aeruginosa

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

1. Ciprofloxacin (5 µg): 16-20
2. Ofloxacin (5 µg):13-15
3. Levofloxacin (5 µg): 14-16
4. Gentamicin (10 µg): 13-14
5. Tobramycin (10 µg): 13-14
6. Amikacin (30 µg): 15-16
7. Aztreonam (30 µg): 16-21
8. Meropenem (10 µg): 16-18
9. Imipenem (10 µg): 16-18
10. Piperacillin (100 µg): 15-20
11. Piperacillin tazobactam (100/10 µg): 15-20
12. Ceftazidime (30 µg): 15-17
13. Cefepime (30 µg): 15-17
14. Colistin (10 µg): MIC≤2
15. Polymyxin B (300U): MIC≤2

Extrinsic Resistance

1. Amoxicillin-clavulanate
2. Ampicillin
3. Amoxicillin
4. Ampicillin-sulbactam
5. Azithromycin
6. Cefotaxime
7. Ceftriaxone
8. Trimethoprim
9. Cephalosporin I (Cephalothin, cefazolin)
10. Cephamycin (cefoxitin, cefotetan)
11. Chloramphenicol
12. Clarithromycin
13. Ertapenem
14. Erythromycin
15. Tetracycline
16. Trimethoprim-sulfamethoxazole
17. Cephalosporin II (Cefuroxime)

Streptococcus pneumoniae

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

1. Oxacillin (1 µg): ≥20 (screening drug, do not report)
2. 0floxacin (5 µg):13-15
3. Levofloxacin (5 µg): 14-16
4. Chloramphenicol (30 µg): ≥ 21
5. Erythromycin (15 µg): 16-20
6. Azithromycin (15 µg):  14-17
7. Doxycycline (30 µg): 25-27
8. Vancomycin (30 µg): ≥ 17
9. Clindamycin (2 µg): 16-18
10. Co-trimoxazole (1.25/23.75 µg): 16-18
11. Linezolid (30 µg): ≥21

Streptococcus spp. (ß-Haemolytic Group)

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

1. Penicillin (10 U): ≥24 or
2. Ampicillin (10 µg): ≥24
3. Cefotaxime (30 µg): ≥24 or
4. Ceftriaxone (30 µg): ≥24
5. Vancomycin (30 µg): ≥ 17
6. Azithromycin (15 µg):  14-17
7. Erythromycin (15 µg): 16-20
8. Tetracycline (30 µg):19-22
9. Levofloxacin (5 µg): 14-16
10. 0floxacin (5 µg):13-15
11. Chloramphenicol (30 µg): 18-20
12. Clindamycin (2 µg): 16-18
13. Linezolid (30 µg): ≥21

Keynotes

1. Penicillin and ampicillin are drugs of choice for the treatment of beta-hemolytic streptococcal infection.
2. Non-susceptible isolates (i.e. penicillin MICs>0.12 µg/mL and ampicillin MICs >0.25 µg/mL) are extremely rare in any ß-hemolytic streptococcus and have not been reported for Streptococcus pyogenes.
3. In such a scenario, this type of isolate should be re-identified and retested. For confirmation, it should be submitted to the national public health laboratory.

Haemophilus influenzae/ parainfluenzae

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

1. Ampicillin (10 µg): 19-21
2. Ampicillin-sulbactam (10/10 µg): ≥ 20
3. Amoxycillin clavulanate (20/10 µg): 14-17
4. Piperacillin tazobactam (100/10 µg): ≥21
5. Ceftazidime (30 µg): ≥26
6. Cefotaxime (30 µg): ≥26
7. Ceftriaxone (30 µg): ≥26
8. Cefuroxime (oral, 30): 17-19
9. Cefepime (30 µg): ≥26
10. Aztreonam (30 µg): ≥26
11. Imipenem (10 µg): ≥16
12. Meropenem (10 µg): ≥20
13. Levofloxacin (5 µg): ≥17
14. Ciprofloxacin (5 µg): ≥21
15. 0floxacin (5 µg): ≥16
16. Co-trimoxazole (1.25/23.75 µg): 11-15
17. Chloramphenicol (except urine, 30 µg): 26-28

Acinetobacter spp.

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

1. Ciprofloxacin (5 µg): 16-20
2. Levofloxacin (5 µg): 14-16
3. Co-trimoxazole (1.25/23.75 µg): 11-15
4. Doxycycline (30 µg): 10-12
5. Gentamicin (10 µg): 13-14
6. Tobramycin (10 µg): 13-14
7. Amikacin (30 µg): 15-16
8. Colistin (10 µg): MIC≤2
9. Polymyxin B (300U): MIC≤2
10. Meropenem (10 µg): 15-17
11. Imipenem (10 µg): 19-21
12. Piperacillin (100 µg): 18-20
13. Piperacillin tazobactam (100/10 µg): 18-20
14. Ceftazidime (30 µg): 15-17
15. Cefepime (30 µg): 15-17
16. Cefotaxime (30 µg): 15-22
17. Ceftriaxone (30 µg):  14-20
18. Ampicillin-sulbactam (10/10 µg): 12-14

Extrinsic Resistance

1. Amoxicillin-clavulanate
2. Ampicillin
3. Amoxicillin
4. Azithromycin
5. Aztreonam
6. Fosfomycin
7. Cephalosporin I (Cephalothin, cefazolin)
8. Cephalosporin II (Cefuroxime)
9. Chloramphenicol
10. Clarithromycin
11. Ertapenem
12. Erythromycin
13. Trimethoprim
14. Cephamycin (cefoxitin, cefotetan)

Streptococcus spp. Viridans Group

Antimicrobial Agents     Zone Diameter (Intermediate, mm)

1. Penicillin (MIC): ≤0.12 or
2. Ampicillin (MIC): ≤ 0.25
3. Cefotaxime (30 µg): 26-27 or
4. Ceftriaxone (30 µg): 25-26
5. Vancomycin (30 µg): ≥ 17
6. Azithromycin (15 µg):  14-17
7. Erythromycin (15 µg): 16-20
8. Tetracycline (30 µg):19-22
9. Levofloxacin (5 µg): 14-16
10. 0floxacin (5 µg):13-15
11. Chloramphenicol (30 µg): 18-20
12. Clindamycin (2 µg): 16-18
13. Linezolid (30 µg): ≥21
14. Meropenem (MIC): ≤0.5

Contd…