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AbstractBackground:Salmonellais a major bacterial pathogen transmitted commonly through food. Increasing resistance to antimicrobial agents (e.g., ceftriaxone, ciprofloxacin) used to treat serious Salmonellainfections threatens the utility of these agents. Infection with antimicrobial-resistant Salmonellahas been associated with increased risk of severe infection, hospitalization, and death. We describe changes in antimicrobial resistance among nontyphoidal Salmonellain the United States from 1996 through 2009.Methods:The Centers for Disease Control and Prevention's National Antimicrobial Resistance Monitoring System conducts surveillance of resistance among Salmonellaisolated from humans. From 1996 through 2009, public health laboratories submitted isolates for antimicrobial susceptibility testing. We used interpretive criteria from the Clinical and Laboratory Standards Institute and defined isolates with ciprofloxacin resistance or intermediate susceptibility as nonsusceptible to ciprofloxacin. Using logistic regression, we modeled annual data to assess changes in antimicrobial resistance.Results:From 1996 through 2009, the percentage of nontyphoidal Salmonellaisolates resistant to ceftriaxone increased from 0.2% to 3.4% (odds ratio [OR]=20, 95% confidence interval [CI] 6.3–64), and the percentage with nonsusceptibility to ciprofloxacin increased from 0.4% to 2.4% (OR=8.3, 95% CI 3.3–21). The percentage of isolates that were multidrug resistant (resistant to ≥3 antimicrobial classes) decreased from 17% to 9.6% (OR=0.6, 95% CI 0.5–0.7), which was driven mainly by a decline among serotype Typhimurium. However, multidrug resistance increased from 5.9% in 1996 to a peak of 31% in 2001 among serotype Newport and increased from 12% in 1996 to 26% in 2009 (OR=2.6, 95% CI 1.1–6.2) among serotype Heidelberg.Conclusions:We describe an increase in resistance to ceftriaxone and nonsusceptibility to ciprofloxacin and an overall decline in multidrug resistance. Trends varied by serotype. Because of evidence that antimicrobial resistance among Salmonellais predominantly a consequence of antimicrobial use in food animals, efforts are needed to reduce unnecessary use, especially of critically important agents.

ABSTRACTDue to emerging resistance to traditional antimicrobial agents, such as ampicillin, trimethoprim-sulfamethoxazole, and chloramphenicol, azithromycin is increasingly used for the treatment of invasive Salmonellainfections. In the present study, 696 isolates of non-Typhi Salmonellacollected from humans, food animals, and retail meats in the United States were investigated for antimicrobial susceptibility to azithromycin. Seventy-two Salmonella entericaserotype Typhi isolates from humans were also tested. For each isolate, MICs of azithromycin and 15 other antimicrobial agents were determined by broth microdilution. Among the non-Typhi Salmonellaisolates, azithromycin MICs among human isolates ranged from 1 to 32 μg/ml, whereas the MICs among the animal and retail meat isolates ranged from 2 to 16 μg/ml and 4 to 16 μg/ml, respectively. Among Salmonellaserotype Typhi isolates, the azithromycin MICs ranged from 4 to 16 μg/ml. The highest MIC observed in the present study was 32 μg/ml, and it was detected in three human isolates belonging to serotypes Kentucky, Montevideo, and Paratyphi A. Based on our findings, we propose an epidemiological cutoff value (ECOFF) for wild-type Salmonellaof ≤16 μg/ml of azithromycin. The susceptibility data provided could be used in combination with clinical outcome data to determine tentative clinical breakpoints for azithromycin and Salmonella enterica.

AbstractTo increase understanding of community-acquired resistance, stool samples from 477 nonhospitalized persons in Maryland and Michigan, from 2004 to 2008, were screened for ceftriaxone resistance. Seven (1.5%) yielded ceftriaxone-resistant Escherichia coli; one isolate was resistant to all eight antimicrobial classes routinely tested: aminoglycosides, β-lactam/β-lactamase inhibitor combinations, cephems, penicillins, folate pathway inhibitors, phenicols, quinolones, and tetracyclines. The extensively resistant isolate was from a 50-year-old woman who denied antimicrobial use, hospitalization, or international travel within 6 months. Meat (beef, chicken, and pork) and eggs were consumed within 1 month before stool collection. Further studies are warranted to understand potential sources, including the food supply, of resistant E. coli.

ABSTRACTNontyphoidal salmonellae (NTS) are important causes of community-acquired bloodstream infection. We describe patterns of antimicrobial resistance among invasive NTS in the United States. We compared bloodstream NTS isolates with those from stool submitted to the National Antimicrobial Resistance Monitoring System (NARMS) from 1996 to 2007. We describe antimicrobial resistance among invasive strains by serogroup and serotype. Of the 19,302 NTS isolates, 17,804 (92.2%) were from stool or blood. Of these, 1,050 (5.9%) were bloodstream isolates. The median ages (ranges) of patients with and without bacteremia were 36 (<1 to 97) years and 20 (<1 to 105) years, respectively (P< 0.001). Males (odds ratio [OR], 1.21; 95% confidence interval [CI], 1.06 to 1.38) and those ≥65 years of age were at greater risk for invasive disease. Salmonella entericaserotypes Enteritidis, Typhimurium, and Heidelberg were the most common serotypes isolated from blood; S. entericaserotypes Dublin, Sandiego, and Schwarzengrund were associated with the greatest risk for bloodstream isolation. Of invasive isolates, 208 (19.8%) were resistant to ampicillin, 117 (11.1%) to chloramphenicol, and 26 (2.5%) to trimethoprim-sulfamethoxazole; 28 (2.7%) isolates were resistant to nalidixic acid and 26 (2.5%) to ceftriaxone. Antimicrobial resistance to traditional agents is common. However, the occurrence of nalidixic acid and ceftriaxone resistance among invasive NTS is cause for clinical and public health vigilance.

Purpose:Multidrug-resistant (MDR) Salmonellastrains are associated with excess bloodstream infections, hospitalizations, and deaths compared with pansusceptible strains. Bovine products are sometimes a source of MDR Salmonella. To generate hypotheses for regional differences in risk factors for human infection, we analyzed distributions of the two most prevalent MDR Salmonellaphenotypes in the United States, 2003–2005: (i) MDR-ACSSuT (resistant to at least ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracycline) Typhimurium; (ii) MDR-AmpC (resistant to at least ampicillin, chloramphenicol, streptomycin, sulfonamides, tetracycline, amoxicillin/clavulanic acid, and ceftiofur, and with decreased susceptibility to ceftriaxone) Newport.Materials and Methods:Participating public health laboratories in all states forwarded every 20th Salmonellaisolate from humans to the National Antimicrobial Resistance Monitoring System for Enteric Bacteria for antimicrobial susceptibility testing. Among the serotypes Typhimurium and Newport isolates submitted 2003–2005, pansusceptible, MDR-ACSSuT Typhimurium, and MDR-AmpC Newport were identified. Patterns of resistance, demographic factors, and cattle density were compared across regions.Results:Of 1195 serotype Typhimurium isolates, 289 (24) were MDR-ACSSuT. There were no significant differences in region, age, or sex distribution for pansusceptible versus MDR-ACSSuT Typhimurium. Of 612 serotype Newport isolates, 97 (16) were MDR-AmpC, but the percentage of MDR-AmpC isolates varied significantly across regions: South 3, Midwest 28, West 32, and Northeast 38 (p< 0.0001). The South had the lowest percentage of MDR-AmpC Newport isolates and also the lowest density of milk cows. More Newport isolates were MDR-AmpC in the 10 states with the highest milk cow density compared with the remaining states. Overall, 22 of pansusceptible Newport isolates but only 7 of MDR-AmpC Newport isolates were from patients <2 years of age. For both serotypes, MDR phenotypes had less seasonal variation than pansusceptible phenotypes.Conclusion:This is the first analysis of the distribution of clinically important MDR Salmonellaisolates in the United States. MDR-ACSSuT Typhimurium was evenly distributed across regions. However, MDR-AmpC Newport was less common in the South and in children <2 years of age. Information on individuals' exposures is needed to fully explain the observed patterns.

ABSTRACTPlasmid-mediated quinolone resistance determinants are emerging among gram-negative pathogens. Here we report results of a retrospective study investigating the prevalence of aac(6′)-Ib-cr, qepA, and qnrgenes among 19,010 human isolates of non-Typhi Salmonella entericacollected in the United States from 1996 to 2006.

ABSTRACTFluoroquinolones commonly are used to treat adult Salmonellainfections. Fluoroquinolone treatment has failed for persons infected with nalidixic acid-resistant Salmonella. From 1996 to 2003, state public health laboratories forwarded 12,252 non-Typhi Salmonella entericaisolates to the Centers for Disease Control and Prevention for antimicrobial susceptibility testing; 203 (1.6%) of the isolates were nalidixic acid resistant, and 14 (7%) of those were ciprofloxacin resistant. Resistance to nalidixic acid significantly increased from 0.4% in 1996 to 2.3% in 2003. All ciprofloxacin-resistant isolates had at least one point mutation in the quinolone resistance determining region (QRDR) of gyrAand did not harbor qnror have point mutations in the QRDR of gyrB, parC, or parE. Continued surveillance of antimicrobial resistance among non-Typhi S. entericaisolates is needed to mitigate the increasing prevalence of nalidixic acid resistance.