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AST Pattern for Bacteria 

AST: Antimicrobial Sensitivity Testing

Since not all antibiotics are applicable in a single bacterium and there is a need for the AST pattern of each bacterium or family of bacteria or genus. Some common bacteria that AST pattern should be discussed later are-

1. Staphylococcus species
2. Streptococcus spp. ( beta-hemolytic group)
3. Streptococcus species- viridans group
4. Streptococcus pneumoniae
5. Enterococcus species
6. Enterobacteriaceae
7. Pseudomonas aeruginosa
8. Haemophilus influenzae/ parainfluenzae
9. Shigella species
10. Acinetobacter species
11. Neisseria gonorrhoeae
12. Neisseria meningitidis

List of common antimicrobial agents( antibiotics)

• Penicillin
• Ampicillin
• Cefoxitin
• Teicoplanin
• Gentamycin
• Erythromycin
• Doxycycline
• Ciprofloxacin
• Ofloxacin 
• Levofloxacin 
• Nitrofurantoin 
• Chloramphenicol
• Cotrimoxazole
• Clindamycin 
• Linezolid 
• Vancomycin (disk and E-strip) 
• Ceftriaxone 
• Cefotaxime
• Cefixime
• Azithromycin 
• Tetracycline 
• Nalidixic acid
• Meropenem 
• Amikacin 
• Aztreonam 
• Ampicillin sulbactam
• Ampicillin clanuvic acid
• Penicillin tazobactam 
• Ceftazidime 
• Cefepime 
• Imipenem 
• Tobramycin
• Colistin (E-strip) 
• Polymyxin B (E-strip) 
• Colistin E strip
• Oxacillin (1 µg) 

Diagnostic disks

• Novobiocin:Staphylococcus saprophyticus is an intrinsic resistance. This disc uses to differentiate S. saprophyticus from other coagulase-negative staphylococci by the overnight incubation disc test method.
• ONPG disks: The ONPG test is useful in differentiating members of the family  Enterobacteriaceae and genera Neisseria based on b-D-galactosidase activity. ONPG stands for o-Nitrophenyl-b-D-galactopyranoside. Positive –Escherichia coli ATCC 25922 and Neisseria lactamica ATCC 23971 whereas negative Proteus mirabilis ATCC 12453 and  Neisseria gonorrhoeae ATCC 43069.
• X, V, and XV disks: For Haemophilus and its species identification.
• Bacitracin (0.04 U): It uses to differentiate Streptococcus pyogenes from other ß hemolytic streptoccci, Streptococcus agalactiae . S. pyogenes is sensitive whereas S. agalcatiae is resistant.
• Bacitracin 10 U: It is applied in sputum culture plate ( chocolate agar) to inhibit normal flora, including gram-positive bacteria, such as streptococci, and most species of Neisseria whereas to support the growth of Haemophilus which is resistant.
• Oxidase disks: To differentiate oxidase-positive organisms like Pseudomonas, Aeromonas, Vibrio, Haemophilus, Neisseria, Brucella, etc.
• Optochin: It uses for the presumptive identification of Streptococcus pneumoniae, which is optochin-sensitive, from other alpha-hemolytic streptococci such as viridans streptococci, which are resistant.

AST pattern for staphylococcus species

Staphylococcus antimicrobial sensitivity testing || AST|| antibiotics use according to CLSI-

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

0floxacin (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:

1. Novobiocin: Staphylococcus 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 thus does not report.

AST pattern for Streptococcus ( beta-hemolytic group)

Streptococcus pyogenes identification: Simplest and quickest way- Blood agar having pin-point and beta-hemolytic colonies Catalase test-Negative, Bacitracin (0.04 U)-Sensitive and Gram stain- Gram-positive cocci in chains-

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:

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, retested. For confirmation, it should be submitted to the national public health laboratory.

AST pattern for Streptococcus species- viridans group

Viridans streptococci: The viridans streptococci are a large group of commensal streptococcal Gram-positive bacteria species that are α-hemolytic, producing a green coloration on blood agar plates (hence the name “viridans”, from Latin “vĭrĭdis”, green). Optochin is resistant. This group contains- Streptococcus mutans, Streptococcus salivarius, Streptococcus anginosus,Streptococcus sanguinis, and Streptococcus mitis.

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

AST pattern for Strptococcus pneumoniae

Know the antibiotics uses for Streptococcus pneumoniae-

Optochin susceptibility test for  Streptococcus pneumoniae: Zone of inhibition of optochin is equal and greater than 14 mm, Sensitive i.e. S. pneumoniae and less than 14 mm is resistance-

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

1. For non-meningitis isolates, oxacillin zone ≥20 mm can predict susceptibility to the following lactams-ampicillin, ampicillin-sulbactam, Amoxycillin, Amoxycillin-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.

AST pattern for Enterococcus species

Staphylococcus and Enterococcus on blood agar and use of catalase test-

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

AST pattern for Enterobacteriaceae (E . coli, Klebisella, Proteus, Citrobacter, etc.)

Antibiotics used for the Enterobacteriaceae family of bacteria-Antimicrobial sensitivity testing (AST) for Enterobacteriaceae family e.g. E. coli, Klebsiella Citrobacter, Enterobacter, Proteus, Serratia, Salmonella, Shigella, Hafnia, etc

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

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

AST pattern for Pseudomonas aeruginos

Antimicrobial sensitivity testing of Pseudomonas aeruginosa: List of antibiotics uses- Ciprofloxacin, Ofloxacin, Levofloxacin, Piperacillin, Pilperacillin -Tazobactum, Ceftazidime, Cefepime, Aztreonam, Meropenem Imipenem, Gentamycin, Amikacin, Tobramycin, Colistin and  Polymyxin B-Pseudomonas aeruginosa antimicrobial sensitivity testing || AST||antibiotics use according to CLSI-

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

0floxacin (5 µg): ≥16

Co-trimoxazole(1.25/23.75 µg): 11-15

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

AST pattern for Acinetobacter species

Acinetobacter calcoaceticus baumanii complex growth and its biochemical tests-

 Acinetobacter calcoaceticus baumanii complex on MacConkey agar Acinetobacter calcoaceticus baumanii complex on blood agar

Acinetobacter calcoaceticus baumanii complex on Nutrient agar Acinetobacter calcoaceticus baumanii complex on TSI test

Acinetobacter calcoaceticus baumanii complex on SIM test

Acinetobacter calcoaceticus baumanii complex on Citrate utilization test Acinetobacter calcoaceticus baumanii complex on urea hydrolyzation test

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)

AST pattern for Neisseria gonorrhoeae

Gonorrhea and isolation of its causative agent Neisseria|| STI ||Neisseria gonorrhoeae–

||Gonorrhoea|| A sexually transmitted disease caused by bacteria Neisseria gonorrhoeae which is Gram-negative diplococci. Catalase positive, Oxidase positive, Non-hemolytic small colonies on blood agar, Gram stain of Urethral discharge: Gram-negative diplococci intracellular as well extracellular seen Antimicrobial sensitivity testing was also performed blood agar as shown in the video. Selective media for Neisseria gonorrhoeae are Modified Newyork city medium and Thayer Martin Medium with VCN.

AST pattern for Neisseria meningitidis

Neisseria menigitidis growth on blood agar and antibiotic sensitivity test-

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