National Antimicrobial Resistance Monitoring System (NARMS)
1997 Annual Report
Report Summary

Table of Contents

• Summary
• Methods
• Results
  • Salmonella
  • E. coli O157:H7
  • Campylobacter

SUMMARY

In 1997, there were 1314 Salmonella isolates, 171 E. coli O157:H7 isolates, and 250 Campylobacter isolates submitted to the National Antimicrobial Resistance Monitoring System (NARMS). Thirty-four percent of Salmonella isolates were resistant to one or more antimicrobial agents. Among Salmonella Typhimurium isolates, 62.7% were resistant to one or more antimicrobial agents. Thirty-five percent of Salmonella Typhimurium isolates had the multi-drug resistant pattern characteristic of DT104. No Salmonella isolates were resistant to ciprofloxacin; however, the percentage of Salmonella isolates with minimum inhibitory concentrations (MICs) >0.25 increased from 0.4% in l996 to 0.6% in l997. Among E. coli O157:H7 isolates, 12.4% were resistant to one or more antimicrobial agents. Among Campylobacter isolates, 85.7% were resistant to one or more antimicrobial agents; 13.4% were resistant to ciprofloxacin.

METHODS

NARMS was launched in l996, within the framework of CDC’s Emerging Infections Program’s Epidemiology and Laboratory Capacity Program as a collaboration between CDC and 14 state and local health departments (CA, CO, CT, FL, GA, KS, Los Angeles County, MN, MA, NJ, New York City, OR, WA, and WV), to prospectively monitor the antimicrobial resistance of human non-typhoid Salmonella and Escherichia coli O157:H7 isolates. In July 1997, Maryland was added as the 15^th NARMS site, bringing the population in NARMS to 83.5 million persons (32.1% of the United States population). In 1997, five states (CA, CT, GA, MN, OR) also began monitoring antimicrobial resistance among human Campylobacter isolates.

NARMS participating public health laboratories select every tenth Salmonella and every fifth E. coli O157:H7 isolate received at their laboratory, and forward the isolates to CDC for susceptibility testing. At CDC, a semi-automated system (Sensititre, Accumed, Westlake, OH) is used to determine the MICs for 17 antimicrobial agents: amikacin, ampicillin, amoxicillin-clavulanic acid, apramycin, ceftiofur, ceftriaxone, cephalothin, chloramphenicol, ciprofloxacin, gentamicin, kanamycin, nalidixic acid, streptomycin, sulfamethoxazole, tetracycline, trimethoprim-sulfamethoxazole, and ticarcillin (Table 1). Public health laboratories from five states also select and forward Campylobacter isolates to CDC for susceptibility testing. For Campylobacter, the Etest system (AB BIODISK, Solna, Sweden) is used to determine the MICs for 7 antimicrobial agents: chloramphenicol, ciprofloxacin, clindamycin, erythromycin, nalidixic acid, tetracycline, and trimethoprim-sulfamethoxazole (Table 1). For all three pathogens in this report, MIC results are dichotomized, and isolates with intermediate susceptibility are categorized as sensitive.

RESULTS

Salmonella
A total of 1314 Salmonella isolates were received at CDC in 1997; 1301/1314 (99.0%) were tested for antimicrobial susceptibility (Table 2, Figure 1). Among Salmonella isolates, 443/1301 (34.1 %) were resistant to one or more agents, and 345/1301 (26.5%) were resistant to two or more agents. Among Salmonella, 328/1301 (25.2%) isolates were resistant to sulfamethoxazole, 284/1301 (21.8%) were resistant to tetracycline, 282/1301 (21.7%) were resistant to streptomycin, and 240/1301 (18.5%) were resistant to ampicillin. Correlation between ampicillin resistance and ticarcillin resistance was very high; 235/241 (97.5%) of isolates resistant to ampicillin were resistant to ticarcillin. All ticarcillin-resistant Salmonella isolates were also resistant to ampicillin. Ten (0.8 %) Salmonella isolates were resistant to nalidixic acid.

Five (0.4%) Salmonella isolates were resistant to ceftriaxone. No Salmonella isolates tested were resistant to amikacin, apramycin, or ciprofloxacin (Table 3, Figure 2). MICs of these agents for Salmonella are shown in Figures 3 and 6.

Of Salmonella isolates received which were serotyped, 301/1221 (24.7%) were serotype Enteritidis and 326/1221 (26.7%) were serotype Typhimurium (includes serotype Typhimurium var. Copenhagen) (Table 4, Figure 4). Among S. Enteritidis isolates, 78/301 (26.0%) were resistant to at least one or more antimicrobial agents. Among S. Typhimurium isolates, 202/326 (62.7%) were resistant to one more antimicrobial agents.

In recent years, a multidrug-resistant strain of S. Typhimurium has been identified, called S. Typhimurium DT104. Among 326 S. Typhimurium isolates tested, 115 (35.3%) were resistant to the five antimicrobial agents, ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline (ACSSuT), to which S. Typhimurium DT104 is commonly resistant (Table 5, Figure 5). Of the 115 S. Typhimurium isolates with the ACSSuT resistance pattern, 13 (11.3%) were also resistant to amoxicillin-clavulanic acid, 10 (8.7%) were also resistant to ceftiofur, and 9 (7.8%) were also resistant to kanamycin (Table 6). S. Typhimurium with the ACSSuT resistance pattern were more commonly isolated from blood (9/115 or 7.8%) than were other S. Typhimurium isolates (6/211 or 2.8%) and other Salmonella (47/975 or 4.8%) (Table 7).

The percentage of Salmonella isolates with ciprofloxacin MICs >0.25 increased from 0.4% (5/1326) in 1996 to 0.6% (8/1301) in 1997 (Figure 6). None had MICs >1.0. The percentage of Salmonella isolates resistant to nalidixic acid (MIC >32) increased from 0.4% (5/1326) in 1996 to 0.8% (11/1301) in 1997 (Figure 7).

E. coli O157
A total of 171 E. coli O157:H7 isolates were received at CDC in 1997; 161/171 (94.2%) were tested for antimicrobial sensitivity (Table 2, Figure 1). Among E. coli O157:H7 isolates, 20/161 (12.4%) were resistant to one or more antimicrobial agents and 11/161 (6.8%) were resistant to two or more agents. The most common resistance among E. coli O157:H7 isolates was to sulfamethoxazole (18/161 or 10.6%) or cephalothin (6/161 or 3.7%). None of the E. coli O157:H7 isolates tested were resistant to amikacin, amoxicillin/clavulanic acid, ampicillin, apramycin, ceftiofur, ceftriaxone, chloramphenicol, ciprofloxacin, gentamicin, kanamycin, nalidixic acid, trimethoprim-sulfamethoxazole, or ticarcillin (Table 8, Figure 8). The MICs for E. coli O157:H7 are shown in Figure 9.

Campylobacter
A total of 250 Campylobacter jejuni isolates were collected in 1997 and forwarded to CDC; 217/250 (86.8%) were tested for antimicrobial susceptibility (Table 2, Figure 1). Among Campylobacter jejuni isolates, 186/217 (85.7%) were resistant to one or more antimicrobial agents, and 108/217 (49.8%) were resistant to two or more agents. The most common resistance among Campylobacter jejuni isolates was to trimethoprim/sulfamethoxazole 149/217 (68.7%), followed by tetracycline 104/217 (47.9%), nalidixic acid 52/217 (23.9%), and ciprofloxacin 29/217 (13.4%) (Table 9, Figure 10). The MICs for Campylobacter jejuni are shown in Figure 11.