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CDC’s Role in Tracking Trends in Resistance

NARMS Surveillance and Laboratory Testing

CDC, through NARMS, tracks antibiotic resistance and studies patterns of emerging resistance in select bacteria transmitted commonly through food. Surveillance helps identify patterns of emerging resistance that can guide public health prevention and policy efforts to protect people from resistant infections. This information is used by a variety of stakeholders, including federal regulatory agencies, policymakers, consumer advocacy groups, industry, and the public.

National surveillance for NARMS is in all 50 states with several large metropolitan areas participating independently (New York City, Los Angeles, Houston, and Washington, D.C.)

Laboratory Testing and Isolate Submissions

The CDC reference laboratory conducts antibiotic susceptibility testing on isolates from sporadic cases and outbreaks of illness. The lab also confirms and studies bacteria that have new antibiotic resistance patterns and performs research to understand the genetic mechanisms of resistance and how they are spread.

Sporadic Infections

Public health laboratories submit every 20th non-typhoidal Salmonella, Shigella, and Escherichia coli O157 isolate received at their laboratories to CDC NARMS for antibiotic susceptibility testing. They also submit every Salmonella serotype Typhi, serotype Paratyphi A, serotype Paratyphi C, and Vibrio (other than V. cholerae) isolate received at their laboratories.

Public health laboratories of the 10 state health departments that participate in CDC’s Foodborne Diseases Active Surveillance Network (FoodNet) also forward a sample of Campylobacter isolates to CDC for susceptibility testing. The FoodNet sites are Connecticut, Georgia, Maryland, Minnesota, New Mexico, Oregon, Tennessee, and selected counties in California, Colorado, and New York.

Pathogen tested Year testing began Current sampling scheme
Salmonella, non-Typhi 1996 1 in 20
E. coli O157 1996 1 in 20
Shigella 1999 1 in 20
Campylobacter 1997 varies
Salmonella Typhi 1999 all
Salmonella Paratyphi A and C 2008 all
Vibrio species other than V. cholerae 2009 all

Outbreak Investigations

CDC NARMS tests bacteria from outbreaks for antibiotic resistance. Resistance patterns and mechanisms can help investigators identify the source of an outbreak or provide clues about the source of the outbreak. Outbreaks in 2011, 2011-2012, and 2013 of multi-resistant Salmonella traced to ground beef and poultry show how animal and human health are linked.

Antibiotics Tested by NARMS

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). CDC NARMS tests for susceptibility to 22 antibiotic agents that are in 13 classes of drugs. The names and classes of drugs and the testing methods used for susceptibility testing depend on the type of bacteria being tested:

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Salmonella, Shigella, and Escherichia coli O157

Antimicrobial agents used for susceptibility testing for Salmonella, Shigella, and Escherichia coli O157 isolates
CLSI Class Antimicrobial Agent Antimicrobial Agent
Concentration Range (g/mL)
MIC Interpretive Standard (μg/mL)
Susceptible Intermediate* Resistant
Aminoglycosides Amikacin 0.5–64 ≤16 32 ≥64
Aminoglycosides Gentamicin 0.25–16 ≤4 8 ≥16
Aminoglycosides Kanamycin 8–64 ≤16 32 ≥64
Aminoglycosides Streptomycin 32–64 ≤32 N/A ≥64
β–lactam / β–lactamase
inhibitor combinations
Amoxicillin-clavulanic acid 1/0.5–32/16 ≤8/4 16/8 ≥32/16
Piperacillin-tazobactam§ 0.5–128 ≤16 32–64 ≥128
Cephems Cefepime§ 0.06–32 ≤8 16 ≥32
Cefotaxime§ 0.06–128 ≤1 2 ≥4
Cefoxitin 0.5–32 ≤8 16 ≥32
Ceftazidime§ 0.06–128 ≤4 8 ≥16
Ceftiofur 0.12–8 ≤2 4 ≥8
Ceftriaxone 0.25–64 ≤1 2 ≥4
Cephalothin** 2–32 ≤8 16 ≥32
Folate pathway inhibitors Sulfamethoxazole†† 16–512 ≤256 N/A ≥512
Sulfisoxazole 16–256 ≤256 N/A ≥512
Trimethoprim-
sulfamethoxazole
0.12/2.38–4/76 ≤2/38 N/A ≥4/76
Macrolides Azithromycin‡‡ 0.12-16 ≤16 N/A ≥32
Monobactams Aztreonam§ 0.06–32 ≤4 8 ≥16
Penems Imipenem§ 0.06–16 ≤1 2 ≥4
Penicillins Ampicillin 1–32 ≤8 16 ≥32
Phenicols Chloramphenicol 2–32 ≤8 16 ≥32
Quinolones Ciprofloxacin§§ 0.015–4 ≤1 2 ≥4
Nalidixic acid 0.5–32 ≤16 N/A ≥32
Tetracyclines Tetracycline 4–32 ≤4 8 ≥16

*N/A indicates that no MIC range of intermediate susceptibility exists
Amikacin was tested from 1997 to 2010 for Salmonella, Shigella, and E. coli O157
No CLSI breakpoints; resistance breakpoint used in NARMS is ≥64 μg/mL
§ Broad-spectrum β-lactam antimicrobial agent only tested for 2011 non-typhoidal Salmonella isolates displaying ceftriaxone and/or ceftiofur resistance
CLSI updated the ceftriaxone interpretive standards in January, 2010. NARMS Human Isolate reports for 1996 through 2008 used susceptible ≤8 μg/mL, intermediate 16-32 μg/mL, and resistant ≥64 μg/mL.
** Cephalothin was tested from 1996 to 2003 for Salmonella, Shigella, and E. coli O157
†† Sulfamethoxazole, which was tested during 1996–2003 to represent sulfonamides, was replaced by sulfisoxazole in 2004
‡‡ CLSI breakpoints are not established for azithromycin. The azithromycin breakpoints used in this report are NARMS- established breakpoints for resistance monitoring and should not be used to predict clinical efficacy.
§§ CLSI breakpoints for invasive Salmonella infections were updated, effective January 2012. For Salmonella, ciprofloxacin susceptibility is defined as ≤0.06 μg/mL; the intermediate category is defined as 0.12 to 0.5 μg/mL; and resistance is defined as ≥1 μg/mL.
 

Campylobacter

Antimicrobial agents used for susceptibility testing of Campylobacter isolates
CLSI Class Antimicrobial Agent Antimicrobial Agent
Concentration Range (μg/mL)
MIC Interpretive Standard (μg/mL)
Susceptible Intermediate Resistant
Aminoglycosides Gentamicin 0.12–32
0.016–256*
≤2 4 ≥8
Ketolides Telithromycin 0.015–8 ≤4 8 ≥16
Lincosamides Clindamycin 0.03–16
0.016–256*
≤2 4 ≥8
Macrolides Azithromycin 0.015–64
0.016–256*
≤2 4 ≥8
Erythromycin 0.03–64
0.016–256*
≤8 16 ≥32
Phenicols Chloramphenicol‡‡ 0.016–256* 0.016–256* 16 ≥32
Florfenicol§ 0.03–64 ≤4 N/A N/A
Quinolones Ciprofloxacin§§ 0.015–64
0.002–32*
≤1 2 ≥4
Nalidixic acid 4–64
0.016–256*
≤16 32 ≥64
Tetracyclines Tetracycline 0.06–64
0.016–256*
≤4 8 ≥16

N/A indicates that no MIC range of either intermediate or resistant susceptibility exists
* Etest dilution range used from 1997–2004
Telithromycin added to NARMS panel in 2005 Chloramphenicol, tested from 1997–2004, was replaced by florfenicol in 2005
§ Only a susceptible breakpoint (≤4 μg/mL) has been established by CLSI. In this report isolates with a MIC ≥8 μg/mL are categorized as resistant.

Vibrio species other than V. cholerae

Antimicrobial agents used for susceptibility testing of Vibrio species other than V. cholerae isolates
CLSI Class Antimicrobial Agent Antimicrobial Agent
Concentration Range (μg/mL)
MIC Interpretive Standard (μg/mL)
Susceptible Intermediate Resistant
Aminoglycosides Kanamycin 0.016-256      
Streptomycin 0.064-1024      
Cephems Cephalothin 0.016-256      
Folate pathway inhibitors Trimethoprim-sulfamethoxazole 0.002-32 ≤2/38 N/A ≥4/76
Penicillins Ampicillin 0.016-256 ≤8 16 ≥32
Phenicols Chloramphenicol 0.016-256      
Quinolones Ciprofloxacin 0.002-32 ≤1 2 ≥4
Nalidixic acid 0.016-256      
Tetracyclines Tetracycline 0.016-256 ≤4 8 ≥16

N/A indicates that no MIC range of either intermediate or resistant susceptibility exists
No CLSI or NARMS breakpoints established

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