The objective of this study was to characterize 365 nontyphoidal Salmonella enterica isolates from animal feed. Among the 365 isolates, 78 serovars were identified. Twenty-four isolates (7.0%) were recovered from three of six medicated feed types. Three of these isolates derived from the medicated feed, Salmonella Newport, Salmonella Typhimurium var. O 5−(Copenhagen), and Salmonella Lexington var. 15+ (Manila), displayed antimicrobial resistance. Susceptibility testing revealed that only 3.0% (12) of the 365 isolates displayed resistance to any of the antimicrobial agents. These 12 isolates were recovered from unmedicated dry beef feed (n = 3), medicated dry beef feed (n = 3), cabbage culls (n = 2), animal protein products (n = 2), dry dairy cattle feed (n =1), and fish meal (n =1). Only Salmonella Newport and Salmonella Typhimurium var. O 5−(Copenhagen) were multidrug resistant. Both isolates possessed the IncA/C replicon and the blaCMY-2 gene associated with cephalosporin resistance. Plasmid replicons were amplified from 4 of 12 resistant isolates. Plasmids (40 kb) were Salmonella Montevideo and Salmonella Kentucky. Conjugation experiments were done using 7 of the 12 resistant isolates as donors. Only Salmonella Montevideo, possessing a plasmid and amplifying IncN, produced transconjugants. Transconjugants displayed the same antimicrobial resistance profile as did the donor isolate. Three isolates that amplified replicons corresponding to IncA/C or IncHI2 did not produce transconjugants at 30 or 37°C. The results of this study suggest that the prevalence of antimicrobial-resistant Salmonella contaminating animal feed is low in Texas. However, Salmonella was more prevalent in feed by-products; fish meal had the highest prevalence (84%) followed by animal protein products (48%). Ten of the 35 feed types had no Salmonella contamination. Further investigation is needed to understand the possible role of specific feed types in the dissemination of antimicrobial resistant bacteria.

In 2011, the U.S. Food and Drug Administration (FDA) Center for Veterinary Medicine published its annual report summarizing sales and distribution data of antimicrobial drugs approved for food-producing animals (37). This report revealed that more than 13,241 metric tons of antimicrobial drugs had been sold and distributed in the United States. However, no accurate estimate of the amount of antimicrobials used in the United States for food-producing animals is available, and no public health agency reporting system exists to acquire an accurate measurement (3). Many antimicrobial drugs and drug classes have been marketed in the United States, including aminoglycosides, amphenicols, cephalosporins, fluoroquinolones, glycolipids, ionophores, lincosamides, macrolides, penicillins, streptogramins, sulfas, and tetracyclines. Antimicrobial agents are used among food animals for growth promotion, disease prophylaxis, and disease treatment and are generally administered to the flock or herd in water or feed (21).

The Centers for Disease Control and Prevention (CDC) estimated that annually 1.028 million human foodborne illnesses, 19,000 hospitalizations, and 400 deaths occur domestically due to infection with nontyphoidal Salmonella serovars (34). Food-producing animals are a primary source of salmonellosis, and dissemination of Salmonella occurs when humans consume food such as, poultry, beef, pork, eggs, milk, seafood, and fresh produce contaminated with viable Salmonella from animal feces (14, 19, 30). The emergence of antimicrobial-resistant Salmonella continues to be an increasing concern for global public health. Many pathogens, including Salmonella, have developed antimicrobial resistance in a cumulative manor, thus becoming resistant to multiple classes of antimicrobial agents. However, illness in humans has been traced back to the handling and consumption of contaminated food from food-producing animals (41). Contaminated feed is a significant source of infection in animals; therefore to minimize human foodborne multidrug-resistant (MDR) bacterial infection, the prevalence of Salmonella in feed must be determined (14). To minimize human foodborne multidrug resistance, Salmonella-contaminated animal feed should be investigated as a possible reservoir for MDR pathogens. In 2007, the Office of the Texas State Chemist started a surveillance program to screen for nontyphoidal Salmonella enterica in animal feed in the state of Texas. From 2007 to 2011, 33 feed product types were screened for Salmonella. Six feed types (dry beef cattle feed, dry dairy cattle feed, swine feed, poultry feed, sheep feed, and goat feed and minerals) were collected in medicated and unmedicated forms (23). The first publication related to this study provided a timeline of Salmonella isolation but did not discriminate between medicated and unmedicated feed. Nine Salmonella serovars isolated from Texas feeds were among a CDC state-by-state outbreak list of pathogenic Salmonella serovars. The objective of this study was to determine (i) the prevalence of antimicrobial-resistant Salmonella contaminating ready-to-eat and by-product types of feed, (ii) the prevalence of these isolates contaminating medicated versus unmedicated animal feed, and (iii) the ability of resistant isolates to horizontally transfer resistance to other isolates.

Feed product types.

Samples (n = 2,622) were individually collected from 2007 through 2011 by state inspectors and shipped to the laboratory. Eleven processed ready-to-eat (intended feed) and 21 unprocessed by-product (unintended feed) feed types are referred to here as ready to eat and by-products, respectively. The number of medicated and unmedicated feed samples from each type of feed are given in Table 1. The active antimicrobial agents used were lasalocid (dry beef cattle feed and sheep and goat feed), monensin (minerals, dry beef cattle feed, and sheep and goat feed), rumensin (dry beef cattle feed), decoquinate (sheep and goat feed), salinomycin (poultry feed), and chlortetracycline (dry beef cattle feed).

TABLE 1.

Prevalence of Salmonella in ready-to-eat or by-product feed types

Prevalence of Salmonella in ready-to-eat or by-product feed types
Prevalence of Salmonella in ready-to-eat or by-product feed types

Identification of Salmonella.

Analysis of 2,622 feed samples yielded 365 Salmonella isolates belonging to 78 serovars. Twenty-five grams of each sample was screened for Salmonella (23). Serotyping of all 365 isolates was conducted by the National Veterinary Services Laboratory (NVSL; Ames, IA). Pulse-field gel electrophoresis (PFGE) analysis of the Salmonella serovars represented by more than one isolate was also done by NVSL and entered into the CDC PulseNet Database. The data were analyzed by the Office of the State Chemist.

Antimicrobial susceptibility testing.

Susceptibility testing was performed according to Clinical and Laboratory Standards Institute (CLSI) methods (13). The Sensititre automated antimicrobial susceptibility system (Trek Diagnostic Systems, Westlake, OH) was used according to the manufacturer's instructions. The National Antibiotic Resistance Monitoring System (NARMS) panel CMV2AGNF for gram-negative Enterobacteriaceae was used with the Sensititre system. Escherichia coli ATCC 25922, E. coli ATCC 35218, and Enterococcus faecalis ATCC 29212 were used as quality control organisms. Data were interpreted using CLSI break points (13) unless these were unavailable, in which case breakpoints from the NARMS 2004 annual report (36) were used. MICs for rifampin were determined manually by broth microdilution using the methods described by the CLSI (13).

PCR-based replicon typing.

PCR-based replicon typing (PBRT) of the Salmonella isolates was performed as previously described (7). The replicon types included were B/O, K, FIIAs, FIA, FIB, FIC, HI1, HI2, Y, I1, Frep, X, L/M, N, P, W, T, and A/C. Positive controls for these replicons were provided by Istituto Superiore di Sanità (Rome, Italy) (7). The PCR to identify the blaCMY-2 gene was done as previously described (29).

Large plasmid isolation and agarose gel electrophoresis.

E. coli isolates were streaked onto Luria-Bertani (LB; BD, Sparks, MD) plates and incubated at 37°C for 18 to 20 h. Five or six colonies were picked from each LB plate and suspended in the lysis buffer (50 mM tris, pH 12.6, and 3% sodium dodecyl sulfate). The cell suspension was incubated at 55°C for 1.5 h and then swiftly extracted with phenol–chloroform–isoamyl alcohol (25:24:1). The mixture was centrifuged at 14,000 × g at 4°C for 5 min, and 50 to 70 μl of the upper aqueous layers was stored at −20°C. A 20-μl aliquot of each sample was electrophoresed on a 0.8% agarose gel in Tris-borate-EDTA buffer at 100 V and 4°C for 300 min (24). Gels were stained with ethidium bromide (1 μg/ml) for 30 min, destained in water for 20 min, and digitally photographed.

Plasmid transfer via conjugation.

Conjugation experiments using Salmonella donor isolates were done on solid supports (0.45-μm-pore size, 13-mm syringe filter; Millipore, Billerica, MA) with recipient strains resistant to nalidixic acid and rifampin. E. coli DH5α F (Invitrogen, Carlsbad, CA) and E. coli CVM1572 were used as the recipient strains (4, 32). E. coli DH5α and CVM1572 were negative for all replicons tested by PBRT. Each E. coli recipient was grown overnight in brain heart infusion broth (BD) with 32 mg/ml nalidixic acid and rifampin. Putative transconjugant colonies were confirmed as E. coli using an indole spot test, and four indole-positive transconjugant colonies were subcultured to tryptic soy agar with 5% sheep blood (BD) or used for susceptibility testing and molecular analysis.

Prevalence of S. enterica from ready-to-eat feeds and animal feed by-products.

Eleven types (744 samples) of ready-to-eat animal feed and 22 types of animal feed byproducts (1,878 samples) were collected. From these samples, 365 S. enterica isolates from 78 serovars were identified from 28 unmedicated and 6 medicated feed types (Table 1). Fish meal products had the highest percentage (84%) of Salmonella-positive feed samples (Table 1). Animal protein products had the second highest prevalence (48%) of Salmonella contamination. Among the medicated feed samples, medicated sheep and goat feed had the highest prevalence (30%) of Salmonella contamination. Samples from 10 feed types had no Salmonella contamination.

Individual S. enterica serovars from animal feed.

A complete list of Salmonella serovars from corresponding feed types (unmedicated and medicated) is given in Table 2. Salmonella Mbandaka (52 isolates) was the most prevalent serovar, followed by Salmonella Montevideo (33 isolates). Twenty-four isolates were recovered from three medicated feed types: dry beef cattle feed (13 isolates), sheep and goat feed (10 isolates), and poultry feed (1 isolate). Among these 24 Salmonella isolates, 16 serovars were identified (Table 2).

TABLE 2.

Salmonella serovars isolated from various feed types and corresponding serovar PFGE similarities

Salmonella serovars isolated from various feed types and corresponding serovar PFGE similarities
Salmonella serovars isolated from various feed types and corresponding serovar PFGE similarities
TABLE 2.

Extended

Extended
Extended
TABLE 2.

Continued

Continued
Continued
TABLE 2.

Continued, Extended

Continued, Extended
Continued, Extended

Prevalence of antimicrobial-resistant serovars.

Of the 16 serovars identified in medicated feed, 3 displayed resistance to antimicrobial agents: Salmonella Lexington var. 15+ (Manila), Salmonella Typhimurium var. O 5− (Copenhagen), and Salmonella Newport; all 3 were found in medicated dry beef cattle feed (Table 3). Rumensin was used in feed that harbored Salmonella Typhimurium var. O 5− (Copenhagen). Salmonella Newport and Salmonella Lexington var. 15+ (Manila) were isolated from feed treated with monensin. In total, roughly 3% (12) of Salmonella isolates were resistant to antimicrobial agents representing 11 serovars. Eight were resistant to one antimicrobial agent, and two were resistant to two agents. Salmonella Newport and Salmonella Typhimurium var. O 5−(Copenhagen) were the only MDR serovars, and both were resistant to nine antimicrobial agents (Table 3). These two serovars originated from the same medicated dry beef cattle feed sample. Of the MDR isolates, 58% (7) were recovered from ready-to-eat feed and 42% (5) were recovered from feed by-products.

TABLE 3.

Profiles of Salmonella isolates resistant to antimicrobial agents

Profiles of Salmonella isolates resistant to antimicrobial agents
Profiles of Salmonella isolates resistant to antimicrobial agents

Molecular typing by PFGE.

Thirty-four serovars were represented by multiple isolates; PFGE was performed on these serovars with a total of 331 isolates. When grouping all isolates, the similarity index was 53.1 to 100%. High similarity indexes for the Salmonella isolates clustered primarily based on serovar. PFGE grouping by serovar is shown in Supplementary Figure S1 (www.dropbox.com/s/kgnumhnwcjbs5aj/Supplemental%20file1.pdf?oref=e&n=267022451).

PFGE comparison of the 24 Salmonella isolates recovered from medicated feed had similarity indexes of 51.9 to 100% (Fig. 1). Twenty-one of the isolates from medicated feed were susceptible to all antimicrobial agents tested (Fig. 1). Salmonella 3,19:–:z27 and Salmonella Senftenberg were 95.2% identical based on PFGE analysis, and both were isolated from sheep and goat feed. Three Salmonella Mbandaka isolates from sheep and goat feed were identical based on PFGE analysis. One Salmonella Mbandaka isolate from dry beef cattle feed was 96.3% identical to the three Salmonella Mbandaka isolates from sheep and goat feed. Four Salmonella Mbandaka isolates recovered from dry beef cattle feed also were similar. Salmonella Cubana from different feed types were 86.5% similar. The remaining isolates from medicated feed were not genetically similar based on PFGE analysis.

FIGURE 1.

PFGE comparison of 24 Salmonella isolates recovered from medicated feed.

FIGURE 1.

PFGE comparison of 24 Salmonella isolates recovered from medicated feed.

Close modal

When antimicrobial-resistant Salmonella serovars were compared, similarity indexes were 56 to 100%. Two Salmonella Muenster isolates from cabbage culls had identical PFGE profiles, although they had different antimicrobial resistance profiles (Fig. 2). These Salmonella Muenster isolates differed from pansusceptible Salmonella Muenster isolates from dry beef cattle feed and animal protein by-products, with similarity indexes of 88 and 78%, respectively. PFGE comparison within the 11 remaining antimicrobial-resistant Salmonella serovars indicated no genetic similarity associated with antimicrobial resistance (Fig. 2).

FIGURE 2.

PFGE comparison of 12 antimicrobial-resistant Salmonella serotypes.

FIGURE 2.

PFGE comparison of 12 antimicrobial-resistant Salmonella serotypes.

Close modal

Plasmid isolation, replicon typing, and PCR amplification for blaCMY-2.

Large plasmids (≥30 kb) were found in only two antimicrobial-resistant Salmonella serovars, Salmonella Kentucky and Salmonella Montevideo. The plasmids were both approximately 40 kb in size. Plasmid replicons were amplified from 4 of 12 resistant isolates. Salmonella Newport and Salmonella Typhimurium var. O 5− (Copenhagen) had the A/C replicon, Salmonella Brandenburg had replicon HI2, and Salmonella Montevideo had the N replicon.

Amplification by PCR for the blaCMY-2 resistance gene that confers resistance to cephalosporins revealed that MDR Salmonella Newport and Salmonella Typhimurium var. O 5− (Copenhagen) possessed blaCMY-2 (Table 3).

Conjugation with resistant isolates.

Conjugation of drug-resistant Salmonella isolates was performed using two recipients, E. coli CVM1572 and E. coli DH5α. Only 7 of the 12 drug-resistant isolates were resistant to tetracycline, so conjugation using tetracycline as a counterselection agent was done with those isolates. Salmonella Newport and Salmonella Typhimurium var. O 5−(Copenhagen) that were positive by PCR for blaCMY-2 were nonconjugative. Salmonella Kentucky possessed a plasmid but did not produce transconjugants. Salmonella Montevideo was the only conjugative serovar. The isolate possessed the N replicon and produced transconjugants when E. coli CVM1572 or E. coli DH5α was used as the recipient. All three transconjugants characterized for each recipient possessed the N replicon and the same susceptibility profile as the Salmonella Montevideo donor, regardless of the recipient used. Six other drug-resistant Salmonella serovars did not produce transconjugants at detectable levels under the conditions used in this study. Salmonella Brandenburg possessed IncHI2; therefore, conjugations were performed at 37 and 30°C. Transconjugants were not produced at either temperature.

Food animal management practices from “farm to fork” are under scrutiny in an effort to improve global food safety. Animal feed should be monitored as an early step in the food chain that might harbor pathogens, particularly MDR pathogens.

S. enterica can contaminate animal feed and may lead to infection or colonization of the animals by ingestion (14, 38, 39). Feed manufacturing has, by necessity, expanded over several decades and involves the inclusion of plant and animal by-products and miscellaneous ingredients (33). Contamination of feed with Salmonella is dependent on many factors, including the source of the feed. Contamination may occur during harvesting, processing, and storage of plant products (39) or during rendering of animal byproducts (14, 33). An international Salmonella Agona infection outbreak was estimated to have caused more than one million cases of salmonellosis in several countries between 1968 and 1972. The source was determined to be fish meal (12, 14). What has been described as protein-based animal waste has also been used in feed (33). Animal waste includes dried ruminant and swine waste and dried poultry litter. The FDA does not endorse this practice because of the likelihood that viable pathogens may contaminate the feed and subsequently be introduced into food animals (33).

Salmonella was isolated from 14% of the samples in the present study. Qualitative analysis of the data suggested that fish meal might pose the greatest risk of Salmonella transmission because of the high prevalence of Salmonella (84%) in samples of this product. Similar data have been obtained internationally by other researchers. Additional protein by-products contaminated by Salmonella include animal protein, peanut, cottonseed, rice, and soybean products (12, 17, 39). In Norway, many imported soybeans were contaminated with Salmonella, but when hazard analysis critical control point programs were implemented in the crushing mill, Salmonella was significantly reduced if not eliminated from the final product (39).

The present study included 11 ready-to-eat feed types and 22 feed by-products. In the by-product category, animal protein products had a Salmonella contamination prevalence of 48%. By-products often consist of rendered animal protein, and pretreatment of this source is difficult to ascertain (33). Grain sorghum products, rice products, and minerals were the only other feed ingredients with a Salmonella prevalence above 25%. Overall, the data suggest that by-products pose a greater risk of Salmonella transmission to animals when they make up a significant percentage of the feed. Medicated sheep and goat feed was the only ready-to-eat feed that was contaminated with Salmonella at a level above 25%.

Surveys by others have been conducted to examine the prevalence of Salmonella in commercial feed (35, 38). Whyte et al. (38) surveyed a small poultry feed mill and recovered Salmonella from the preheat and postheat areas with an incidence of 18.8 and 22.6%, respectively. In Spain, Torres et al. (35) found that only 4.8% of the 3,844 samples from 523 mills were contaminated with Salmonella. Dargatz et al. (16) examined two mixer-feeder cattle feedlots that by definition produced their own feed. Overall, the incidence of Salmonella in their feed was only 5.3% (57 of 1,070 samples). Those results are similar to those previously published by our laboratory (23), even though the methodology of Salmonella isolation differed. In the study of Dargatz et al. (16), 54% (31 of 57) of the Salmonella isolates were susceptible to all the antimicrobial agents tested. Most of the resistant Salmonella isolates (24%) were recovered from mixed feed, and the incidence of resistant isolates among the other feed types was low. Because of the possibility of fecal contamination, a higher incidence of resistant Salmonella isolates may be expected when rendered beef products are part of a mixed feed (1, 5, 15). The incidence of resistance in mixed feed found by Dargatz et al. (16) is high compared with that found in the present study. However, a direct comparison of results from feed samples collected in the present study and from the mixer-feeder cattle feedlots may not be accurate because of differences in the source of the feed and the bacterial isolation methods. In a total of 201 feed ingredient samples, Ge et al. (18) found a Salmonella incidence of 22.9% (n = 46); the animal-derived by-products had an incidence of 34.4%, and the plant-derived by-products had an incidence of 5.1%.

Genetic relatedness as determined by PFGE was higher among serovars than it was among isolates that were derived from medicated feed or those that displayed resistance to antimicrobial agents. Salmonella 3,19:–:z27 and Salmonella Senftenberg derived from sheep and goat feed were 95.2% similar even though they represented different serovars. This result may indicate that the lack of PFGE typing for serovars that had only one representative isolate may have missed closely related isolates.

Of the antimicrobial agents used in feed in this study, only chlortetracycline has a human equivalent. The other antimicrobial agents used were ionophores. The Salmonella isolates were further characterized phenotypically by examining antimicrobial resistance profiles. Overall, the 12 antimicrobial-resistant Salmonella isolates originated from six types of feed, and only three of the 12 isolates originated from medicated feed. Eight and two Salmonella isolates displayed resistance to one and two antimicrobial agents, respectively. Although only two MDR Salmonella serovars, Salmonella Newport and Salmonella Typhimurium var. O 5− (Copenhagen), were obtained from 365 Salmonella isolates in the past 5 years, these isolates displayed resistance to seven clinically important antimicrobial agents that could potentially be transferred to other bacterial genera. The resistance of Salmonella Newport to cephalosporins is significant because this serovar has been recognized as epidemic in humans and animals in the United States (10, 20, 42). Other researchers have examined the susceptibility or genetic profiles of the Salmonella serovars isolated from animal feed (1, 16, 18). Li et al. (25) found that 21% of the isolates collected were resistant to at least one antimicrobial agent. Dargatz et al. (16) found a 46% incidence of resistance among 57 Salmonella isolates tested. In the study by Ge et al. (18), none of the 74 Salmonella isolates were resistant to the 17 antimicrobial agents tested.

One mechanism that leads to the rapid development of multidrug resistance is horizontal transfer of self-transferable plasmids (6, 27). These large plasmids are often transferred among Enterobacteriaceae and can result in multidrug resistance in Salmonella (2, 28). The present study included examination of the conjugative transferability of antimicrobial resistance to tetracycline from Salmonella to E. coli. Only the N replicon in Salmonella Montevideo was conjugative. Transconjugants were positive for the N replicon and possessed the same antimicrobial resistance phenotype as the original Salmonella Montevideo donor. This result suggests the presence of the entire IncN plasmid rather than only a small fragment of the N replicon. Recombination of DNA sequences can lead to changes in the location of replicon sequences, such translocation to an area of the genome where they would be much less likely to be transferred by the process of conjugation.

Multiple Salmonella serovars isolated from food animals and humans have acquired plasmid-mediated AmpC-like β-lactamase (CMY-2), which confers cephalosporin resistance (2, 29, 40). This acquisition has led to Salmonella serovars with resistance to nine or more antimicrobial agents (40). Resistance to β-lactams has also become an increasing concern for human health in the last decade. Two plasmid-mediated enzyme families of importance are extended-spectrum β-lactamases and AmpC β-lactamases (26). Among AmpC enzymes, blaCMY-2 is the most prevalent in the United States, especially in Salmonella species (8, 9). The blaCMY-2 gene that encodes an AmpC β-lactamase has been predominately found on a few incompatibility plasmids, in particular IncA/C and IncI, that are common among foodborne pathogens (29). In the present study, both MDR Salmonella isolates that displayed resistance to the β-lactam antimicrobial agents and cephalosporins, ampicillin, amoxicillin–clavulanic acid, cefoxitin, ceftiofur, and ceftriaxone were positive for the A/C replicon that often carries the blaCMY-2 gene. The two A/C-positive Salmonella isolates were recovered from medicated feed in this study. These two isolates were nonconjugative, and neither had large plasmids that could be visualized after plasmid purification. Some A/C plasmids carried by Salmonella may be nonconjugative but may be transferred when a coresident conjugative plasmid is present (31). Plasmids may not be visualized when the plasmid has integrated into the genome.

In the present study, the populations of resistant Salmonella isolates probably were not high enough in the feed ecosystem to support gene transfer. Horizontal gene transfer between bacteria occurs when bacterial cells establish a cell-to-cell bridge structure. This interaction is mediated by a family of conjugative transfer proteins, encoded by tra genes, which are a subset of bacterial type IV secretion systems (11). Thus, the resistant Salmonella cells must reach a certain density to put them spatially within reach for a conjugative bridge to form. Because only 12 of 365 Salmonella isolates displayed resistance and only 1 of those isolates was conjugative, the animal feed types sampled for this study were an unlikely reservoir for conjugative Salmonella isolates carrying antimicrobial resistance genes.

Medicated feed may be a more likely source for MDR Salmonella isolates in some situations; however, in this study the MDR Salmonella isolates were derived from feed treated with ionophores. Ionophore antimicrobial agents are not used in human medicine. Currently, no known genes confer ionophore resistance, and none are transferrable by conjugation (22).

In the gut ecosystem of an animal given medicated feed, clonal expansion of resistant isolates may allow establishment of a population that could support plasmid transfer to commensal or pathogenic Enterobacteriaceae. Clonal expansion of Salmonella populations could also lead to specific serovar infection outbreaks, as occurred with Salmonella Agona decades ago (12).

The authors thank Denise Caldwell for laboratory technical support.

1.
Alam
,
M. J.
,
D.
Renter
,
E.
Taylor
,
D.
Mina
,
R.
Moxley
, and
D.
Smith
.
2009
.
Antimicrobial susceptibility profiles of Salmonella enterica serotypes recovered from pens of commercial feedlot cattle using different types of composite samples
.
Curr. Microbiol
.
58
:
354
359
.
2.
Alcaine
,
S. D.
,
S. S.
Sukhnanand
,
L. D.
Warnick
,
W.-L.
Su
,
P.
McGann
,
P.
McDonough
, and
M.
Wiedmann
.
2005
.
Ceftiofur-resistant Salmonella strains isolated from dairy farms represent multiple widely distributed subtypes that evolved by independent horizontal gene transfer
.
Antimicrob. Agents Chemother
.
49
:
4061
4067
.
3.
Angulo
,
F. J.
,
N. L.
Baker
,
S. J.
Olsen
,
A.
Anderson
, and
T. J.
Barrett
.
2004
.
Antimicrobial use in agriculture: controlling the transfer of antimicrobial resistance to humans
.
Semin. Pediatr. Infect. Dis
.
15
:
78
85
.
4.
Bischoff
,
K. M.
,
D. G.
White
,
M.
Hume
,
T. L.
Poole
, and
D. J.
Nisbet
.
2005
.
The chloramphenicol resistance gene cmlA is disseminated on transferable plasmids that confer multiple-drug resistance in swine Escherichia coli
.
FEMS Microbiol. Lett
.
243
:
285
291
.
5.
Brichta-Harhay
,
D. M.
,
T. M.
Arthur
,
J. M.
Bosilevac
,
N.
Kalchayanand
,
S. D.
Shackelford
,
T. L.
Wheeler
, and
M.
Koohmaraie
.
2011
.
Diversity of multidrug-resistant Salmonella enterica strains associated with cattle at harvest in the United States
.
Appl. Environ. Microbiol
.
77
:
1783
1796
.
6.
Carattoli
,
A.
2009
.
Resistance plasmid families in Enterobacteriaceae
.
Antimicrob. Agents Chemother
.
53
:
3112
3114
.
7.
Carattoli
,
A.
,
A.
Bertini
,
L.
Villa
,
V.
Falbo
,
K. L.
Hopkins
, and
E. J.
Threlfall
.
2005
.
Identification of plasmids by PCR-based replicon typing
.
J. Microbiol. Methods
63
:
219
228
.
8.
Carattoli
,
A.
,
V.
Miriagou
,
A.
Bertini
,
A.
Loli
,
C.
Colinon
,
L.
Villa
,
J. M.
Whichard
, and
G. M.
Rossolini
.
2006
.
Replicon typing of plasmids encoding resistance to newer β-lactams
.
Emerg. Infect. Dis
.
12
:
1145
1148
.
9.
Carattoli
,
A.
,
F.
Tosini
,
W. P.
Giles
,
M. E.
Rupp
,
S. H.
Hinrichs
,
F. J.
Angulo
,
T. J.
Barrett
, and
P. D.
Fey
.
2002
.
Characterization of plasmids carrying CMY-2 from expanded-spectrum cephalosporin-resistant Salmonella strains isolated in the United States between 1996 and 1998
.
Antimicrob. Agents Chemother
.
46
:
1269
1272
.
10.
Centers for Disease Control and Prevention
.
2002
.
Outbreak of multidrug-resistant Salmonella Newport
.
Morb. Mortal. Wkly. Rep
.
51
:
545
548
.
11.
Christie
,
P. J.
,
K.
Atmakuri
,
V.
Krishnamoorthy
,
S.
Jakubowski
, and
E.
Cascales
.
2005
.
Biogenesis, architecture, and function of bacterial type IV secretion systems
.
Annu. Rev. Microbiol
.
59
:
451
485
.
12.
Clark
,
M. C.
,
A. F.
Kaufmann
, and
E. J.
Gangarosa
.
1973
.
Epidemiology of an international outbreak of Salmonella Agona
.
Lancet
ii
:
490
493
.
13.
Clinical and Laboratory Standards Institute
.
2014
.
Performance standards for antimicrobial susceptibility testing
.
24th informational supplement, M100-S24
.
Clinical and Laboratory Standards Institute
,
Wayne, PA
.
14.
Crump
,
J. A.
,
P. M.
Griffin
, and
F. J.
Angulo
.
2002
.
Bacterial contamination of animal feed and its relationship to human foodborne illness
.
Clin. Infect. Dis
.
35
:
859
865
.
15.
Dargatz
,
D. A.
,
P. J.
Fedorka-Cray
,
S. R.
Ladely
,
C. A.
Kopral
,
K.
Ferris
, and
M. L.
Headrick
.
2003
.
Prevalence and antimicrobial susceptibility of Salmonella spp. isolates from U.S. cattle in feedlots in 1999 and 2000
.
J. Appl. Microbiol
.
95
:
753
761
.
16.
Dargatz
,
D. A.
,
R. A.
Strohmeyer
,
P. S.
Morley
,
D. R.
Hyatt
, and
M. D.
Salman
.
2005
.
Characterization of Escherichia coli and Salmonella enterica from cattle feed ingredients
.
Foodborne Pathog. Dis
.
2
:
341
347
.
17.
European Centre for Disease Prevention and Control
.
2014
.
The European Union summary report on trends and sources of zoonoses, zoonotic agents and food-borne outbreaks in 2012
.
EFSA J
.
12
:
3547
.
18.
Ge
,
B.
,
P. C.
LaFon
,
P. J.
Carter
,
S. D.
McDermott
,
J.
Abbott
,
A.
Glenn
,
S. L.
Ayers
,
S. L.
Friedman
,
J. C.
Paige
,
D. D.
Wagner
,
S.
Zhao
,
P. F.
McDermott
, and
M. A.
Rasmussen
.
2013
.
Retrospective analysis of Salmonella, Campylobacter, Escherichia coli, and Enterococcus in animal feed ingredients
.
Foodborne Pathog. Dis
.
10
:
684
691
.
19.
Gopinath
,
S.
,
S.
Carden
, and
D.
Monack
.
2012
.
Shedding light on Salmonella carriers
.
Trends Microbiol
.
20
:
320
327
.
20.
Gupta
,
A.
,
J.
Fontana
,
C.
Crowe
,
B.
Bolstorff
,
A.
Stout
,
S. V.
Duyne
,
M. P.
Hoekstra
,
J. M.
Whichard
, and
T. J.
Barrett
.
2003
.
Emergence of multidrug-resistant Salmonella enterica serotype Newport infections resistant to expanded-spectrum cephalosporins in the United States
.
J. Infect. Dis
.
188
:
1707
1716
.
21.
Gustafson
,
R. H.
, and
R. E.
Bowen
.
1997
.
Antibiotic use in animal agriculture
.
J. Appl. Microbiol
.
83
:
531
541
.
22.
Houlihan
,
A. J.
, and
J. B.
Russell
.
2003
.
The susceptibility of ionophore-resistant Clostridium aminophilum F to other antibiotics
.
J. Antimicrob. Chemother
.
52
:
623
628
.
23.
Hsieh
,
Y.-C.
,
K.-M.
Lee
,
T. L.
Poole
,
M.
Runyon
,
B. D.
Jones
, and
T. J.
Herrman
.
2014
.
Salmonella spp. detection in animal feeds from 2007–2011
.
Int. J. Regul. Sci
.
2
:
14
27
.
24.
Kado
,
C. I.
, and
S. T.
Liu
.
1981
.
Rapid procedure for detection and isolation of large and small plasmids
.
J. Bacteriol
.
145
:
1365
1373
.
25.
Li
,
X.
,
L. A.
Bethune
,
Y.
Jia
,
R. A.
Lovell
,
T. A.
Proescholdt
,
S. A.
Benz
,
T. C.
Schell
,
G.
Kaplan
, and
D. G
McChesney
.
2012
.
Surveillance of Salmonella prevalence in animal feeds and characterization of the Salmonella isolates by serotyping and antimicrobial susceptibility
.
Foodborne Pathog. Dis
.
9
:
692
698
.
26.
Liebana
,
E.
,
A.
Carattoli
,
T. M.
Coque
,
H.
Hasman
,
A.-P.
Magiorakos
,
D.
Mevius
,
L.
Peixe
,
L.
Poirel
,
G.
Schuepbach-Regula
,
K.
Torneke
,
J.
Torren-Edo
,
C.
Torres
, and
J.
Threlfall
.
2013
.
Public health risks of enterobacterial isolates producing extended-spectrum β-lactamases or AmpC β-lactamases in food and food-producing animals: an EU perspective of epidemiology, analytical methods, risk factors, and control options
.
Clin. Infect. Dis
.
56
:
1030
1037
.
27.
Liebert
,
C. A.
,
R. M.
Hall
, and
A. O.
Summers
.
1999
.
Transposon Tn21, flagship of the floating genome
.
Microbiol. Mol. Biol. Rev
.
63
:
507
522
.
28.
Martinez
,
N.
,
M. C.
Mendoza
,
B.
Guerra
,
M. A.
Gonzalez-Hevia
, and
M. R.
Rodicio
.
2005
.
Genetic basis of antimicrobial drug resistance in clinical isolates of Salmonella enterica serotype Hadar from a Spanish region
.
Microb. Drug Resist
.
11
:
185
193
.
29.
Mulvey
,
M. R.
,
E.
Susky
,
M.
McCracken
,
D. W.
Morck
, and
R. R.
Read
.
2009
.
Similar cefoxitin-resistance plasmids circulating in Escherichia coli from human and animal sources
.
Vet. Microbiol
.
134
:
279
287
.
30.
Pires
,
S. M.
,
A. R.
Vieira
,
T.
Hald
, and
D.
Cole
.
2014
.
Source attribution of human salmonellosis: an overview of methods and estimates
.
Foodborne Pathog. Dis
.
11
:
667
676
.
31.
Poole
,
T. L.
,
T. S.
Edrington
,
D. M.
Brichta-Harhay
,
A.
Carattoli
,
R. C.
Anderson
, and
D. J.
Nisbet
.
2009
.
Conjugative transferability of the A/C plasmids from Salmonella enterica isolates that possess or lack blaCMY in the A/C plasmid backbone
.
Foodborne Pathog. Dis
.
6
:
1185
1194
.
32.
Poole
,
T. L.
,
J. L.
McReynolds
,
T. S.
Edrington
,
J. A.
Byrd
,
T. R.
Callaway
, and
D. J.
Nisbet
.
2006
.
Effect of flavophospholipol on conjugation frequency between Escherichia coli donor and recipient pairs in vitro and in the chicken gastrointestinal tract
.
J. Antimicrob. Chemother
.
58
:
359
366
.
33.
Sapkota
,
A. R.
,
L. Y.
Lefferts
,
S.
McKenzie
, and
P.
Walker
.
2007
.
What do we feed to food-production animals? A review of animal feed ingredients and their potential impacts on human health
.
Environ. Health Perspect
.
115
:
663
670
.
34.
Scallan
,
E.
,
R. M.
Hoekstra
,
F. J.
Angulo
,
R. V.
Tauxe
,
M.-A.
Widdowson
,
S. L.
Roy
,
J. L.
Jones
, and
P. M.
Griffin
.
2011
.
Foodborne illness acquired in the United States—major pathogens
.
Emerg. Infect. Dis
.
17
:
7
15
.
35.
Torres
,
G. J.
,
F. J.
Piquer
,
L.
Algarra
,
C.
de Frutos
, and
O. J.
Sobrino
.
2011
.
The prevalence of Salmonella enterica in Spanish feed mills and potential feed-related risk factors for contamination
.
Prev. Vet. Med
.
98
:
81
87
.
36.
U.S. Food and Drug Administration, Center for Veterinary Medicine
.
2004
.
2004 Retail meat report, National Antimicrobial Resistance Monitoring System
. .
37.
U.S. Food and Drug Administration, Center for Veterinary Medicine
.
2011
.
Summary report on antimicrobials sold or distributed for use in food-producing animals
. .
38.
Whyte
,
P.
,
K.
McGill
, and
J. D.
Collins
.
2003
.
A survey of the prevalence of Salmonella and other enteric pathogens in a commercial poultry feed mill
.
J. Food Saf
.
23
:
13
24
.
39.
Wierup
,
M.
, and
T.
Kristoffersen
.
2014
.
Prevention of Salmonella contamination of finished soybean meal used for animal feed by a Norwegian production plant despite frequent Salmonella contamination of raw soy beans, 1994–2012
.
Acta Vet. Scand
.
56
:
1
9
.
40.
Winokur
,
P. L.
,
A.
Brueggemann
,
D. L.
DeSalvo
,
L.
Hoffmann
,
M. D.
Apley
,
E. K.
Uhlenhopp
,
M. A.
Pfaller
, and
G. V.
Doern
.
2000
.
Animal and human multidrug-resistant, cephalosporin-resistant Salmonella isolates expressing a plasmid-mediated CMY-2 AmpC β-lactamase
.
Antimicrob. Agents Chemother
.
44
:
2777
2783
.
41.
Zhao
,
S.
,
K.
Blickenstaff
,
A.
Glenn
,
S. L.
Ayers
,
S. L.
Friedman
,
J. W.
Abbott
, and
P. F.
McDermott
.
2009
.
β-Lactam resistance in Salmonella strains isolated from retail meats in the United States by the National Antimicrobial Resistance Monitoring System between 2002 and 2006
.
Appl. Environ. Microbiol
.
75
:
7624
7630
.
42.
Zhao
,
S.
,
S.
Qaiyumi
,
S.
Friedman
,
R.
Singh
,
S. L.
Foley
,
D. G.
White
,
P. F.
McDermott
,
T.
Donkar
,
C.
Bolin
,
S.
Munro
,
E. J.
Baron
, and
R. D.
Walker
.
2003
.
Characterization of Salmonella enterica serotype Newport isolated from humans and food animals
.
J. Clin. Microbiol
.
41
:
5366
5371
.