Antibiotics Tested by NARMS

At a glance

This page provides detailed information on the antibiotics NARMS uses for susceptibility testing of Salmonella, Shigella, E. coli, Campylobacter, and Vibrio species other than V. cholerae.

Two Petri dish culture plates growing bacteria in the presence of discs containing various antibiotics. The isolate on the left plate is susceptible to the antibiotics on the discs, and is therefore unable to grow adjacent to the discs. The plate on the right was inoculated with a bacterium that proved to be resistant to all of the antibiotics tested, and is therefore, able to grow near the discs.

Overview

NARMS tests isolates to determine their antibiotic susceptibility. This task is accomplished by finding the lowest concentration of a particular antibiotic that will inhibit the growth of the bacteria, which is called the minimum inhibitory concentration (MIC).

Currently, CDC NARMS routinely tests for susceptibility to 17 antibiotics in 12 classes of drugs. The names and classes of antibiotics and the testing methods used for susceptibility testing depend on the type of bacteria being tested.

What data we collect

Antibiotics used for susceptibility testing of Salmonella isolates

Antimicrobial agents used for susceptibility testing for Salmonella isolates
CLSI Class Antibiotic Years
Tested		 Antimicrobial Agent
Concentration Range
(μg/mL)		 MIC Interpretive Standard (μg/mL)
Susceptible Intermediate* or S-DD† Resistant
Aminoglycosides Amikacin 1997–2010 0.5–64 ≤16 32 ≥64
Gentamicin 1996–present 0.25–16 ≤4 8 ≥16
Kanamycin 1996–2013 8–64 ≤16 32 ≥64
Streptomycin‡ 1996–2013 32–64 ≤32 N/A* ≥64
2014–2019 2–64 ≤16 N/A* ≥32
β–lactam combination agents Amoxicillin-clavulanic acid 1996–present 1/0.5–32/16 ≤8/4 16/8 ≥32/16
Piperacillin-tazobactam§ 2011–2015 0.5–128 ≤16/4 32/4–64/4 ≥128/4
Cephems Cefepime†,§ 2011–2015 0.06–32 ≤2 4-8† ≥16
Cefotaxime§ 2011–2015 0.06–128 ≤1 2 ≥4
Cefoxitin 2000–present 1–32 ≤8 16 ≥32
Ceftazidime§ 2011–2015 0.06–128 ≤4 8 ≥16
Ceftiofur 1996–2015 0.12–8 ≤2 4 ≥8
Ceftriaxone 1996–present 0.25–64 ≤1 2 ≥4
Cephalothin 1996–2003 2–32 ≤8 16 ≥32
Folate pathway antagonists Sulfamethoxazole 1996–2003 16–512 ≤256 N/A* ≥512
Sulfisoxazole 2004–present 16–256 ≤256 N/A* ≥512
Trimethoprim-
sulfamethoxazole		 1996–present 0.12/2.38–4/76 ≤2/38 N/A* ≥4/76
Lipopeptides Colistin 2020–present 0.25–8 See note ≤2 ≥4
Macrolides Azithromycin** 2011–present 0.25–64 ≤16 N/A* ≥32
Monobactams Aztreonam§ 2011–2015 0.06–32 ≤4 8 ≥16
Penems Imipenem§ 2011–2015 0.06–16 ≤1 2 ≥4
Meropenem 2016–present 0.06–4 ≤1 2 ≥4
Penicillins Ampicillin 1996–present 1–32 ≤8 16 ≥32
Phenicols Chloramphenicol 1996–present 2–32 ≤8 16 ≥32
Quinolones Ciprofloxacin 1996–present 0.015–4 ≤0.06 0.12–0.5 ≥1
Nalidixic acid 1996–present 0.5–32 ≤16 N/A* ≥32
Tetracyclines Tetracycline 1996–present 4–32 ≤4 8 ≥16
  • *N/A indicates that no MIC range of intermediate susceptibility exists
  • †Cefepime MICs above the susceptible range, but below the resistant range are designated by CLSI to be susceptible-dose dependent (S-DD)
  • ‡CLSI breakpoints are not established for streptomycin; interpretive standards used are NARMS-established breakpoints for resistance monitoring and should not be used to predict clinical efficacy. During 1996–2013 resistance was defined as ≥64 µg/mL; the breakpoint was updated to ≥32 µg/mL in 2014. The 2014 breakpoint could not be applied to previous years due to limited concentrations tested.
  • § Broad-spectrum β-lactam antibiotic only tested for nontyphoidal Salmonella isolates displaying ceftriaxone and/or ceftiofur resistance during 2011–2015.
  • **CLSI breakpoints for azithromycin are only established for Salmonella ser. Typhi. Interpretive criteria for Salmonella ser. Typhi are based on MIC distribution data and limited clinical data. The azithromycin interpretive standards used for Salmonella serotypes other than ser. Typhi are NARMS-established breakpoints for resistance monitoring and should not be used to predict clinical efficacy.
  • Note: According to the Clinical and Laboratory Standards Institute M100, colistin results for the Enterobacterales are interpreted either as intermediate (I) (MIC ≤ 2 µg/mL) or resistant (R) (MIC ≥4 µg/mL). There is no susceptible category, even for isolates with a low minimum inhibitory concentration (MIC). Some isolates with resistant MICs may be of a group or serotype that has naturally higher MIC distribution compared with other organisms, or isolates may have an acquired mechanism of resistance such as mcr-1. Presence of acquired mechanisms of colistin resistance can be determined by whole genome sequencing or other DNA-based detection methods.

Antibiotics used for susceptibility testing of Shigella isolates

Antimicrobial agents used for susceptibility testing for Shigella isolates
CLSI Class Antibiotic Years
Tested		 Antimicrobial Agent
Concentration Range
(μg/mL)		 MIC Interpretive Standard (μg/mL)
Susceptible Intermediate* Resistant
Aminoglycosides Amikacin 1999–2010 0.5–64 ≤16 32 ≥64
Gentamicin 1999–present 0.25–16 ≤4 8 ≥16
Kanamycin 1999–2013 8–64 ≤16 32 ≥64
Streptomycin† 1999–2013 32–64 ≤32 N/A* ≥64
2014–2019 2–64 ≤16 N/A* ≥32
β–lactam combination agents Amoxicillin-clavulanic acid 1999–present 1/0.5–32/16 ≤8/4 16/8 ≥32/16
Cephems Cefoxitin 2000–present 1–32 ≤8 16 ≥32
Ceftiofur 1999–2015 0.12–8 ≤2 4 ≥8
Ceftriaxone 1999–present 0.25–64 ≤1 2 ≥4
Cephalothin 1999–2003 2–32 ≤8 16 ≥32
Folate pathway antagonists Sulfamethoxazole 1999–2003 16–512 ≤256 N/A* ≥512
Sulfisoxazole 2004–present 16–256 ≤256 N/A* ≥512
Trimethoprim-
sulfamethoxazole		 1999–present 0.12/2.38–4/76 ≤2/38 N/A* ≥4/76
Lipopeptides Colistin 2020–present 0.25–8 See note ≤2 ≥4
Macrolides Azithromycin 2011–present 0.25–64 ≤8 16 ≥32
Penems Meropenem 2016–present 0.06–4 ≤1 2 ≥4
Penicillins Ampicillin 1999–present 1–32 ≤8 16 ≥32
Phenicols Chloramphenicol 1999–present 2–32 ≤8 16 ≥32
Quinolones Ciprofloxacin 1999–present 0.015–4 ≤0.25 0.5 ≥1
Nalidixic acid 1999–present 0.5–32 ≤16 N/A* ≥32
Tetracyclines Tetracycline 1999–present 4–32 ≤4 8 ≥16
  • *N/A indicates that no MIC range of intermediate susceptibility exists
  • †CLSI breakpoints are not established for streptomycin; interpretive standards used are NARMS-established breakpoints for resistance monitoring and should not be used to predict clinical efficacy. During 1999–2013 resistance was defined as ≥64 µg/mL; the breakpoint was updated to ≥32 µg/mL in 2014. The 2014 breakpoint could not be applied to previous years due to limited concentrations tested.
  • Note: According to the Clinical and Laboratory Standards Institute M100, colistin results for the Enterobacterales are interpreted either as intermediate (I) (MIC ≤ 2 µg/mL) or resistant (R) (MIC ≥4 µg/mL). There is no susceptible category, even for isolates with a low minimum inhibitory concentration (MIC). Some isolates with resistant MICs may be of a group or serotype that has naturally higher MIC distribution compared with other organisms, or isolates may have an acquired mechanism of resistance such as mcr-1. Presence of acquired mechanisms of colistin resistance can be determined by whole genome sequencing or other DNA-based detection methods.

Antibiotics used for susceptibility testing of E. coli isolates

Antimicrobial agents used for susceptibility testing for E. coli isolates
CLSI Class Antibiotic Years
Tested		 Antimicrobial Agent
Concentration Range
(μg/mL)		 MIC Interpretive Standard (μg/mL)
Susceptible Intermediate* Resistant
Aminoglycosides Amikacin 1997–2010 0.5–64 ≤16 32 ≥64
Gentamicin 1996–present 0.25–16 ≤4 8 ≥16
Kanamycin 1996–2013 8–64 ≤16 32 ≥64
Streptomycin† 1996–2013 32–64 ≤32 N/A* ≥64
2014–2019 2–64 ≤16 N/A* ≥32
β–lactam combination agents Amoxicillin-clavulanic acid 1996–present 1/0.5–32/16 ≤8/4 16/8 ≥32/16
Cephems Cefoxitin 2000–present 1–32 ≤8 16 ≥32
Ceftiofur 1996–2015 0.12–8 ≤2 4 ≥8
Ceftriaxone 1996–present 0.25–64 ≤1 2 ≥4
Cephalothin 1996–2003 2–32 ≤8 16 ≥32
Folate pathway antagonists Sulfamethoxazole 1996–2003 16–512 ≤256 N/A* ≥512
Sulfisoxazole 2004–present 16–256 ≤256 N/A* ≥512
Trimethoprim-
sulfamethoxazole		 1996–present 0.12/2.38–4/76 ≤2/38 N/A* ≥4/76
Lipopeptides Colistin 2011–present 0.25–8 See note ≤2 ≥4
Macrolides Azithromycin§ 2011–present 0.25–64 ≤16 N/A* ≥32
Penems Meropenem 2016–present 0.06–4 ≤1 2 ≥4
Penicillins Ampicillin 1996–present 1–32 ≤8 16 ≥32
Phenicols Chloramphenicol 1996–present 2–32 ≤8 16 ≥32
Quinolones Ciprofloxacin 1996–present 0.015–4 ≤0.25 0.5 ≥1
Nalidixic acid 1996–present 0.5–32 ≤16 N/A* ≥32
Tetracyclines Tetracycline 1996–present 4–32 ≤4 8 ≥16
  • *N/A indicates that no MIC range of intermediate susceptibility exists
  • †CLSI breakpoints are not established for streptomycin; interpretive standards used are NARMS-established breakpoints for resistance monitoring and should not be used to predict clinical efficacy. During 1996–2013 resistance was defined as ≥64 µg/mL; the breakpoint was updated to ≥32 µg/mL in 2014. The 2014 breakpoint could not be applied to previous years due to limited concentrations tested.
  • §CLSI breakpoints are not established for azithromycin and E. coli; interpretive standards used are NARMS-established breakpoints for resistance monitoring and should not be used to predict clinical efficacy.
  • Note: According to the Clinical and Laboratory Standards Institute M100, colistin results for the Enterobacterales are interpreted either as intermediate (I) (MIC ≤ 2 µg/mL) or resistant (R) (MIC ≥4 µg/mL). There is no susceptible category, even for isolates with a low minimum inhibitory concentration (MIC). Some isolates with resistant MICs may be of a group or serotype that has naturally higher MIC distribution compared with other organisms, or isolates may have an acquired mechanism of resistance such as mcr-1. Presence of acquired mechanisms of colistin resistance can be determined by whole genome sequencing or other DNA-based detection methods.

Antibiotics used for susceptibility testing of Campylobacter isolates

Antimicrobial agents used for susceptibility testing of Campylobacter isolates
CLSI Class Antibiotic Years Tested Antimicrobial Agent
Concentration Range (μg/mL)		 MIC Interpretive Standard (μg/mL)†
C. jejuni C. coli
Susceptible Resistant Susceptible Resistant
Aminoglycosides Gentamicin 1998–present 0.12–32
0.016–256*		 ≤2 ≥4 ≤2 ≥4
Ketolides Telithromycin‡ 2005–2019 0.015–8 ≤4 ≥8 ≤4‡ ≥8‡
Lincosamides Clindamycin 1997–present 0.03–16
0.016–256*		 ≤0.5 ≥1 ≤1 ≥2
Macrolides Azithromycin 1998–present 0.015–64
0.016–256*		 ≤0.25 ≥0.5 ≤0.5 ≥1
Erythromycin 1997–present 0.03–64
0.016–256*		 ≤4 ≥8 ≤8 ≥16
Penems Meropenem 2020–present 0.004–16 No EUCAST or NARMS interpretations
Phenicols Chloramphenicol 1997–2004 0.016–256* ≤16 ≥32 ≤16 ≥32
Florfenicol 2005–present 0.12–64 ≤4 ≥8 ≤4 ≥8
Quinolones Ciprofloxacin 1997–present 0.015–64
0.002–32*		 ≤0.5 ≥1 ≤0.5 ≥1
Nalidixic acid 1997–present 4–64
0.016–256*		 ≤16 ≥32 ≤16 ≥32
Tetracyclines Tetracycline 1997–present 0.12–64
0.016–256*		 ≤1 ≥2 ≤2 ≥4
  • *Etest dilution range used before 2005
  • †MIC interpretative standard is based on epidemiological cutoff values (ECVs) established by the European Committee on Antimicrobial Susceptibility Testing. This approach was adopted in 2012 and applied to all years. EUCAST uses the terms "wild-type" and "non-wild-type" instead of susceptible and resistant, respectively, to reflect the nature of the populations of bacteria in each group and to highlight that these categories are not to be used to predict clinical efficacy.
  • ‡ A telithromycin ECV for Campylobacter coli is not currently published by EUCAST. We apply the previously published [PDF – 3 pages] ECV of 4 µg/mL to all C. coli isolates, designating "wild-type" isolates (MIC ≤4 µg/mL) as sensitive and "non-wild-type" isolates (MIC ≥8 µg/mL) as resistant.

Antibiotics used for susceptibility testing of Vibrio species other than V. cholerae isolates

Antimicrobial agents used for susceptibility testing of Vibrio species other than V. cholerae isolates
CLSI Class Antibiotic Years Tested Antimicrobial Agent
Concentration Range (μg/mL)		 MIC Interpretive Standard (μg/mL)
Susceptible Intermediate* Resistant
Aminoglycosides Gentamicin 2013–present 0.25–16
0.064–1024†		 ≤4 8 ≥16
Kanamycin 2009–2012 0.016–256† No CLSI or NARMS interpretations
Streptomycin 2009–2012;
2015–2019		 2–64
0.064–1024†		 No CLSI or NARMS interpretations
 β–lactam combination agents Amoxicillin-clavulanic acid  2015–present  1/0.5–32/16  ≤8/4  16/8  ≥32/16
Cephems Cefotaxime 2013–2014 0.016–256† ≤1 2 ≥4
Cefoxitin 2015–present 1–32 ≤8 16 ≥32
Ceftazidime 2013–2014 0.016–256† ≤4 8 ≥16
Ceftiofur 2015 0.12–8 No CLSI or NARMS interpretations
Ceftriaxone 2015–present 0.25–64 No CLSI or NARMS interpretations
Cephalothin 2009–2012 0.016–256† No CLSI or NARMS interpretations
Folate pathway antagonists Sulfisoxazole 2015–present 16–256 No CLSI or NARMS interpretations
Trimethoprim-sulfamethoxazole 2009–present 0.12/2.38–4/76
0.002–32†		 ≤2/38 N/A* ≥4/76
Lipopeptides Colistin 2020–present 0.25–8 No CLSI or NARMS interpretations
Macrolides Azithromycin 2015–present 0.25–64 See footnote§
Penems Imipenem 2013–2014 0.002–32† ≤1 2 ≥4
Meropenem 2016–present 0.06–4 ≤1 2 ≥4
Penicillins Ampicillin 2009–present 1–32
0.016–256†		 ≤8 16 ≥32
Phenicols Chloramphenicol 2009–present 2–32
0.016–256†		 No CLSI or NARMS interpretations
Quinolones Ciprofloxacin 2009–present 0.015–4
0.002–32†		 ≤1 2 ≥4
Nalidixic acid 2009–present 0.5–32
0.016–256†		 No CLSI or NARMS interpretations
Tetracyclines Tetracycline 2009–present 4–32
0.016–256†		 ≤4 8 ≥16
  • *N/A indicates that no MIC range of either intermediate or resistant susceptibility exists
  • †Etest dilution range used before 2015
  • §CLSI has only established a susceptible breakpoint (≤2 µg/mL) for azithromycin and cautions that the utility of this interpretation for Vibrio species other than V. cholerae is uncertain due to limited clinical or in vitro MIC data. Because of this, NARMS will not apply any interpretive criteria to azithromycin MICs for non-choleraeVibrio until further data are available.
Print
Content Source:
About the National Center for Emerging and Zoonotic Infectious Diseases