Antimicrobial susceptibilities and occurrence of resistance genes in bovine Helcococcus ovis isolates

The aim of the study was to investigate antimicrobial susceptibilities of bovine Helcococcus ovis isolates and to detect genes encoding for H. ovis erythromycin and tetracycline resistance. Twenty-nine isolates were included and the minimal inhibitory concentrations (MICs) of seven antimicrobials were determined using test conditions as given in an approved CLSI guideline for the pyridoxal-dependent Abiotrophia spp. and Granulicatella spp. Furthermore, the macrolide resistance phenotype was examined by the erythromycin-clindamycin double-disk test (D-zone test). Erythromycin MICs of ≥ 8 μg/ml were found in three (10%) isolates which also presented the macrolide, lincosamide, and streptogramin B (MLS(B)) resistance phenotype, either constitutive or inducible. The erm(B) gene was detected in one of these isolates. Increased tetracycline MICs (≥ 8 μg/ml) were obtained for 24 (83%) isolates, mostly associated with the tet(M) gene alone (n=21) or both the tet(L) and tet(M) genes (n=2). The MICs determined for penicillin, ampicillin, amoxicillin-clavulanic acid, and cephalothin did not indicate resistance to these antimicrobials. The study suggests that resistance to MLS(B) antimicrobials and tetracycline is frequent in H. ovis. Moreover, this is the first report about occurrence of the resistance genes erm(B), tet(L), and tet(M) in the Helcococcus genus.     

饲喂不同抗生素对肉牛废弃粪便中四环素耐药基因数量及持久性的影响

通过对肉牛饲喂低于治疗水平的不同种类抗生素,研究其对肉牛废弃粪便微生物群落中四环素耐药基因数量与持久性的影响。试验肉牛被分成不同抗生素处理组,即氯四环素组、氯四环素和磺胺甲嘧啶组及对照组。将每个围栏中所有动物的新鲜粪便混匀为一份混合样品作为模式样品(每个处理组3份),分别露天放置,在第7、14、28、42、56、70、84、98、112、126和175 天时采样并提取DNA,利用Real-time PCR方法测定四环素耐药基因tet(B),tet(C),tet(L),tet(M),tet(W)及16S rRNA的浓度。结果显示,16S rRNA的浓度在不同处理间相似,在56 d内均有增加的趋势(P＜0.05);总体上看,四环素组的耐药基因初始浓度较高(P＜0.05);所有处理组的tet(B)和tet(C)浓度到56 d时均增长了1~2个对数级,到175 d时又降低到初始水平,而tet(M)与tet(W)的浓度与其他耐药基因相比较高。因此,四环素耐药基因可以在废弃粪便中持续存在超过175 d,而某些基因的最初数量可能会导致错估其后的变化,其浓度的暂时改变并不能归因于微生物群落数量的变化。     

Antimicrobial resistance and resistance genes in Staphylococcus aureus strains from rabbits

Fifty-six Staphylococcus aureus isolates recovered between 1998 and 2003 from 31 rabbit farms with and without problems of chronic staphylococcosis, were screened for resistance to enrofloxacin, erythromycin, gentamicin, lincomycin, neomycin, penicillin and tetracyclines using the agar dilution test. For penicillin, a disk diffusion test was also performed. The detection of tetP(B), tet(K), tet(L), tet(M), tet(O), tet(T), tet(W), erm(A), erm(B), erm(C) and mec(A) genes was done via a PCR assay. Four isolates showed resistance to erythromycin and lincomycin. These isolates were positive for the erm(C) gene in the PCR. Eleven strains were resistant to tetracyclines and all harboured the tet(K) gene. In the agar dilution test, five isolates showed resistance to penicillin, whereas in the disk diffusion test 12 isolates showed resistance. None of these 12 resistant isolates carried the mec(A) gene. Only one strain showed resistance to gentamicin, and all strains were susceptible to enrofloxacin and neomycin. This study demonstrates that resistance to antimicrobial agents in S. aureus isolates originating from rabbits is relatively rare compared to resistance in S. aureus isolates originating from other animals and humans.     

Trends in antimicrobial susceptibility in relation to antimicrobial usage and presence of resistance genes in Staphylococcus hyicus isolated from exudative epidermitis in pigs

From 1996 to 2001 a total of 467 Staphylococcus hyicus isolates from exudative epidermitis (EE) in pigs in Denmark were examined for susceptibility to 13 different antimicrobial agents. The presence of selected genes encoding macrolide (erm(A), erm(B) and erm(C)), penicillin (blaZ), streptogramin (vat, vga, vga(B), vat(B), vat(D) and vat(E)), streptomycin (aadE) and tetracycline resistance (tet(K), tet(L), tet(M) and tet(O)) were determined in selected isolates.The occurrence of erythromycin resistance increased from 33% in 1996 to a maximum of 62% in 1997 and decreased to 26% in 2001. Resistance to sulphametazole increased from 17% in 1996 to 30% in 1998 but has since decreased to 4% in 2001. Resistance to trimethoprim increased to 51% in 1997 and decreased to 21% in 2001. Resistance to tetracycline (21-31%) remained relatively constant during 1996-2000, but increased to 47% in 2001. Resistance to penicillin (54-75%) streptomycin (33-53%) and tetracycline (21-47%) remained relatively constant over the time investigated.All 48 penicillin resistant isolates examined contained the blaZ gene and 40 (85%) of the streptomycin resistant isolates the aadE gene. It was not possible to detect any streptogramin resistance gene in four streptogramin resistant isolates. Of the 55 erythromycin resistant isolates examined, five contained erm(A), 13 erm(B), 35 erm(C) and two both erm(A) and erm(C). The presence of erm(B) was confirmed by hybridization to plasmid profiles in all 13 PCR-positive isolates. Of 52 tetracycline resistant isolates examined, two contained tet(L), 38 tet(K) and 12 both tet(K) and tet(L).     

Effects of feeding wet corn distillers grains with solubles with or without monensin and tylosin on the prevalence and antimicrobial susceptibilities of fecal foodborne pathogenic and commensal bacteria in feedlot cattle

Distillers grains, a coproduct of ethanol production from cereal grains, are composed principally of the bran, protein, and germ fractions and are commonly supplemented in ruminant diets. The objective of this study was to assess the effect of feeding wet distillers grains with solubles (WDGS) and monensin and tylosin on the prevalence and antimicrobial susceptibilities of fecal foodborne and commensal bacteria in feedlot cattle. Cattle were fed 0 or 25% WDGS in steam-flaked corn-based diets with the addition of no antimicrobials, monensin, or monensin and tylosin. Fecal samples were collected from each animal (n = 370) on d 122 and 136 of the 150-d finishing period and cultured for Escherichia coli O157. Fecal samples were also pooled by pen (n = 54) and cultured for E. coli O157, Salmonella, commensal E. coli, and Enterococcus species. Antimicrobial resistance was assessed by determining antimicrobial susceptibilities of pen bacterial isolates and quantifying antimicrobial resistance genes in fecal samples by real-time PCR. Individual animal prevalence of E. coli O157 in feces collected from cattle fed WDGS was greater (P < 0.001) compared with cattle not fed WDGS on d 122 but not on d 136. There were no treatment effects on the prevalence of E. coli O157 or Salmonella spp. in pooled fecal samples. Antimicrobial susceptibility results showed Enterococcus isolates from cattle fed monensin or monensin and tylosin had greater levels of resistance toward macrolides (P = 0.01). There was no effect of diet or antimicrobials on concentrations of 2 antimicrobial resistance genes, ermB or tetM, in fecal samples. Results from this study indicate that WDGS may have an effect on the prevalence of E. coli O157 and the concentration of selected antimicrobial resistance genes, but does not appear to affect antimicrobial susceptibility patterns in Enterococcus and generic E. coli isolates.     

Effects of the antimicrobial growth promoter tylosin on subclinical infection of pigs with Salmonella enterica serotype Typhimurium

OBJECTIVE: To determine whether feeding tylosin, an antimicrobial growth promoter, to pigs was associated with increased risk of infection with and excretion of Salmonella enterica serotype Typhimurium. ANIMALS: 17 healthy pigs. PROCEDURE: A commercial pelleted dry feed was given in 2 feeding trials. In trial A, 11 pigs were given feed with tylosin, 11 pigs were given feed without tylosin, and 11 pigs were given feed with tylosin before and feed without tylosin after inoculation with S Typhimurium. In trial B, 44 pigs were given feed that contained tylosin, and 44 pigs were given feed without tylosin. Three weeks after the start of each trial, pigs were orally inoculated with approximately 5 x 10(6) colony-forming units of S Typhimurium. Feces were examined for S Typhimurium, using semiquantitative microbiologic techniques before and for 5 or 6 weeks after inoculation. Serum antibody titers against S enterica were measured by use of ELISA. RESULTS: None of the pigs developed clinical signs of salmonellosis. However, after inoculation, S Typhimurium was isolated from feces of most pigs, and all but 2 pigs developed serum antibodies against S enterica. Significant differences were not detected between experimental and control groups in either trial. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicate that tylosin fed as an antimicrobial growth promoter to pigs may not be an important factor in promoting infection with or excretion of S enterica serotype Typhimurium.     

Antimicrobial susceptibility patterns of Salmonella isolates from cattle in feedlots

OBJECTIVE: To evaluate the antimicrobial susceptibility patterns of Salmonella isolates from feedlot cattle. DESIGN: Cross-sectional study. SAMPLE POPULATION: 263 Salmonella isolates. PROCEDURES: Fecal samples were collected from the floor of 2 pens in each of 100 feedlots. Two hundred eighty Salmonella isolates were recovered after bacteriologic culture from 38 pens. Of these, 263 isolates were available for antimicrobial susceptibility testing to 16 antimicrobials, using microbroth dilution breakpoint plates. RESULTS: Less than 5% of isolates were resistant to any of the antimicrobials tested, with the exception of sulfamethoxazole (15; 5.7%) and tetracycline (61; 23.2%). Most isolates (197; 74.9%) were susceptible to all antimicrobials tested, whereas 18 (6.8%) were resistant to 2 or more antimicrobials. The percentage of isolates with resistance to any antimicrobial varied by serotype. The percentage of isolates resistant to various antimicrobials was not related to concurrent use of antimicrobials in the feed. CONCLUSIONS AND CLINICAL RELEVANCE: With the exception of tetracycline and sulfamethoxazole, resistance of Salmonella isolates to any of the antimicrobials was uncommon. Most isolates were susceptible to all antimicrobials tested. Antimicrobial resistance was not related to the presence of antimicrobials in the ration being fed at the time of sample collection.     

Assessing the effect of a single dose florfenicol treatment in feedlot cattle on the antimicrobial resistance patterns in faecal Escherichia coli

The objective of this clinical trial was to examine the effect of a single dose of florfenicol on antimicrobial resistance patterns in faecal E. coli of feedlot steers. Steers (n = 370), were purchased from two sources and housed in outdoor concrete floored pens. Two cattle from each pen (n = 42 pens, 84 cattle) were randomly selected for faecal sampling at study day 1, 14, 28, and 42. One sampled animal from each of 21 pens was randomly selected to receive a single 39.6 mg/kg dose of florfenicol subcutaneously at study day 11. Ten lactose positive colonies were isolated from faecal swabs and tested for antimicrobial resistance to 11 antimicrobials using the disk diffusion method. Zones of inhibition were grouped using cluster analysis and clusters were ordered by increasing multiple resistance. A cumulative logistic regression model using generalized estimating equations was used to assess factors associated with increasing levels of multiple resistance. Immediately post-treatment, all isolates obtained from treated cattle belonged to multiple resistant clusters containing chloramphenicol resistance. Though less pronounced in later sampling, resistance to chloramphenicol and other antimicrobials persisted. Antimicrobial treatment, sampling time and animal source, as well as interactions between these variables, were important predictors of the odds of E. coli belonging to a more resistant cluster. A very clear but transitory shift to increasingly multiple resistant faecal E. coli in response to florfenicol treatment was observed. There was no indication of horizontal transfer of resistant E. coli between steers. Level of resistance was influenced by complex interaction of animal source and previous management.     

Phenotypic and genotypic characterization of antimicrobial resistance in faecal enterococci from wild boars (Sus scrofa)

The objective was to study the prevalence of antimicrobial resistance and the mechanisms implicated in faecal enterococci of wild boars in Portugal. One hundred and thirty-four enterococci (67 E. faecium, 54 E. hirae, 2 E. faecalis, 2 E. durans and 9 Enterococcus spp.) were recovered from 67 wild boars (two isolates/sample), and were further analysed. High percentages of resistance were detected for erythromycin, tetracycline, and ciprofloxacin (48.5%, 44.8%, and 17.9%, respectively), and lower values were observed for high-level-kanamycin, -streptomycin, chloramphenicol, and ampicillin resistance (9%, 6.7%, 4.5%, and 3.7%, respectively). No isolates showed vancomycin or high-level-gentamicin resistance. The erm(B), tet(M), aph(3')-IIIa, and ant(6)-I genes were demonstrated in all erythromycin-, tetracycline-, kanamycin-, and streptomycin-resistant isolates, respectively. Specific genes of Tn916/Tn1545 and Tn5397 transposons were detected in 78% and 47% of our tet(M)-positive enterococci, respectively. The tet(S) and tet(K) genes were detected in one isolate of E. faecium and E. hirae, respectively. Three E. faecium isolates showed quinupristin-dalfopristin resistance and the vat(E) gene was found in all of them showing the erm(B)-vat(E) linkage. Four E. faecium isolates showed ampicillin-resistance and all of them presented seven amino acid substitutions in PBP5 protein (461Q-->K, 470H-->Q, 485M-->A, 496N-->K, 499A-->T, 525E-->D, and 629E-->V), in relation with the reference one; a serine insertion at 466' position was found in three of the isolates. Faecal enterococci from wild boars harbour a variety of antimicrobial resistance mechanisms and could be a reservoir of antimicrobial resistance genes and resistant bacteria that could eventually be transmitted to other animals or even to humans.     

Distribution, characterization and genetic bases of erythromycin resistance in staphylococci and enterococci originating from livestock

The occurrence of staphylococci and enterococci expressing increased resistance to erythromycin (ERY) and, in particular, to macrolide-lincosamide-streptogramin B (MLS(B) ) antibiotics was investigated in dairy cattle, pigs and turkeys. Three hundred rectal (cloacal) swabs of each animal species were examined. A total of 120 and 71 staphylococci and enterococci, respectively, with increased resistance to ERY were identified. These were most frequent in turkeys (42.3% of positive animals), followed by pigs and dairy cattle (6.7% and 6.0% of positive animals, respectively). Similarly, MLS(B) -resistant isolates colonized predominantly turkeys (29.7% of animals), while their occurrence in pigs and dairy cattle was only sporadic (0.8% of animals). At least one of the erm genes encoding for MLS(B) resistance was found in 56.7% and 69.0% of staphylococci and enterococci, respectively. The erm(C) gene prevailed in staphylococci while the erm(B) gene was predominant in enterococci. Macrolide efflux genes msr(A) and msr(C) were also frequent in staphylococci and enterococci, respectively. Macrolide inactivation gene mph(C) occurred mainly in staphylococci. In staphylococci, methicillin resistance was rarely detected (7.5% of isolates), but resistance to telithromycin (ketolides) was frequent in both staphylococci and enterococci (89.2% and 47.9% of isolates, respectively). This study showed that turkeys represent an important source of ERY (MLS(B) )-resistant cocci. In addition, resistance to ketolides was also frequent.     

Effects of dietary zilpaterol hydrochloride on feedlot performance and carcass characteristics of beef steers fed with and without monensin and tylosin

A feedlot experiment was conducted under commercial conditions in the Texas Panhandle using 3,757 feedlot steers (average of 94 steers/pen) to evaluate the effects of feeding zilpaterol hydrochloride with or without monensin and tylosin on feedlot performance and carcass characteristics. The experiment was conducted using a randomized complete block design. Treatments were arranged as a 2 (no zilpaterol vs. zilpaterol) x 2 (monensin and tylosin withdrawn vs. monensin and tylosin fed during the final 35 d on feed) factorial. Steers were fed for a total of 161 to 167 d, and treatments were administered during the final 35 d that cattle were on feed. When included in the diet, zilpaterol, monensin, and tylosin were supplemented at 8.3, 33.1, and 12.2 mg/kg (DM basis), respectively. Zilpaterol was included in the diet for 30 d at the end of the finishing period and withdrawn from the diet for the last 5 or 6 d cattle were on feed. Cattle were harvested and carcass data collected. There were no zilpaterol x monensin/tylosin interactions (P >or= 0.12) for ADG or G:F. Feeding zilpaterol increased ADG (P < 0.001) by 0.20 kg and G:F (P < 0.001) by 0.029 kg/kg during the last 35 d on feed. Likewise, when feedlot variables were measured throughout the entire 161- to 167-d feeding trial, ADG (3.4%; P < 0.001) and G:F (3.9%; P < 0.001) were increased. Feeding zilpaterol increased (P < 0.001) dressing percent and HCW and decreased (P < 0.001) total liver abscess rate compared with controls. In addition, zilpaterol increased (P < 0.001) LM area by an average of 8.0 cm(2). There was a zilpaterol x monensin/tylosin interaction (P = 0.03) for marbling score. Zilpaterol decreased (P < 0.001) marbling score regardless of monensin and tylosin treatment, although withdrawal of monensin and tylosin for 35 d decreased marbling to a greater extent (31 vs. 17 degrees). Zilpaterol decreased (i.e., improved; P < 0.001) calculated yield grade regardless of monensin and tylosin treatment, but feeding zilpaterol in combination with the withdrawal of monensin and tylosin for 35 d decreased calculated yield grade to a greater extent (0.49 vs. 0.29) compared with the zilpaterol, monensin, and tylosin combination treatment (zilpaterol x monensin/tylosin interaction, P = 0.03). Results suggest that monensin and tylosin can be withdrawn from the diet during the zilpaterol feeding period (final 35 d on feed) with minimal effect on animal performance, although feeding zilpaterol in combination with monensin and tylosin seemed to moderate effects on carcass quality.     

Proportion of corn silage in diets of feedlot steers fed to achieve stepwise increases in growth.

Two experiments were conducted to investigate the effects of proportion of dietary corn silage during periods of feed restriction on performance of steers. In Exp. 1, Simmental x Angus steer calves (n = 107; initial BW = 273 +/- 3.8 kg) were allotted to 12 pens with eight or nine steers/pen and four pens/treatment. Periods of growth were 273 to 366 kg BW (Period 1), 367 to 501 kg BW (Period 2), and 502 to 564 kg BW (Period 3). In two of the dietary regimens, steers were given ad libitum access to feed throughout the experiment and were fed either a 15% corn silage diet in each period or an 85, 50, and 15% corn silage diet in Periods 1, 2, and 3; respectively. In the third feeding regimen, a programmed intake feeding regimen was used. Steers were fed a 15% corn silage diet in each period. However, feed intake was restricted to achieve a predicted gain of 1.13 kg/d in Period 1 and 1.36 kg/d in Period 2, and feed was offered for ad libitum consumption in Period 3. For the entire experiment, ADG was similar (P = 0.41) among treatments and feed efficiency was lower (P < 0.10) for steers in the corn silage regimen than for steers in the programmed intake and ad libitum regimens. In Exp. 2, Simmental x Angus steer calves (n = 106; initial BW = 233 +/- 2 kg) were allotted by BW to 12 pens (three pens/treatment) and fed in three periods similar to those described in Exp. 1. Four feeding regimens were investigated: 1) AL; steers were offered a 15% corn silage diet for ad libitum consumption in all three periods; 2) PI; DMI was programmed to achieve gains as described in Exp. 1; 3) CS-HLL; programmed intake as described above except diets contained 85, 15, and 15% corn silage in Periods 1, 2, and 3, respectively; and 4) CS-HIL; same feeding regimens as CS-HLL, except diets contained 85, 50, and 15% corn silage in Periods 1, 2, and 3, respectively. Steers were given ad libitum access to feed in Period 3. Overall ADG was lower (P < 0.05) for steers in the CS-HLL and CS-HIL feeding regimens than for steers in the AL and PI regimens; feed efficiency was greatest for steers in the PI regimen. Few effects of feeding regimen on carcass characteristics were observed.     

禽源沙门菌对四环素的耐药性及耐药基因调查分析

为了更好地了解禽源沙门菌对四环素的耐药性及耐药基因分布,从不同来源的家禽样品中分离沙门菌,调查其对四环素的耐药性以及耐药菌株中8种四环素耐药基因[tet(A)、tet(B)、tet(C)、tet(W)、tet(M)、tet(D)、tet(K)和tet(L)]的携带情况.结果表明,18.8%的沙门菌分离株对四环素耐药,健康成鸡分离株对四环素的耐药性明显高于雏鸡、死胚或病禽分离株;四环素耐药株中tet(A)、tet(B)和tet(M)基因的携带比例分别为73.1%、11.5%和3.8%,说明沙门菌对四环素的耐药机制以tet(A)和tet(B)基因介导的主动外排为主.本研究首次在对四环素耐药的沙门菌中检测到tet(M)基因,说明tet(M)基因在沙门菌对四环素的耐药性方面也具有潜在的作用.     

Susceptibility of Arcanobacterium pyogenes from different sources to tetracycline, macrolide and lincosamide antimicrobial agents

Chlortetracycline, oxytetracycline, and the macrolide, tylosin, are extensively used for growth promotion and disease prophylaxis in the cattle and swine industries in the US. Arcanobacterium pyogenes, a common inhabitant of the mucosal surfaces of cattle and swine, is also a pathogen associated with a variety of infections in these animals. A broth microdilution technique was used to determine the antimicrobial susceptibility of 48 A. pyogenes isolates to macrolides, lincosamides and tetracyclines. The MIC50 and MIC90 for chlortetracycline were 0.12 and 8 mg/l, respectively. Similarly, the MIC50 and MIC90 for oxytetracycline were 0.25 and 8 mg/l, while the MIC50 and MIC90 for tetracycline were 0.25 and 16 mg/l, respectively. The MIC50 and the MIC90 were < or = 0.06 and >64 mg/l, respectively, for erythromycin, tylosin and clindamycin. This resistance pattern indicated that some of these A. pyogenes isolates may carry an MLS(B) resistance determinant. A. pyogenes isolates (12.5%) were resistant to erythromycin, and this percentage doubled when MICs were performed following induction with erythromycin. Of the 48 A. pyogenes isolates, 25 and 41.7% were resistant to MLS(B) antimicrobial agents and the tetracycline derivatives, respectively. MLS(B) resistance was present in 22.2 and 35.3% of A. pyogenes isolates of bovine (n=27) or porcine (n=17) origin. In contrast, 70.6% of porcine isolates were resistant to the tetracyclines, compared with 25.9% of bovine isolates. These data suggest that a large proportion of A. pyogenes field isolates may be resistant to these commonly used antimicrobial agents.     

Changes in antimicrobial susceptibility in a population of Escherichia coli isolated from feedlot cattle administered ceftiofur crystalline-free acid

OBJECTIVE: To determine effects of administration of ceftiofur crystalline-free acid (CCFA) on antimicrobial susceptibility of Escherichia coli in feedlot cattle. ANIMALS: 61 feedlot steers. PROCEDURES: A cohort study was conducted. Steers were housed in pens (5 pens with 10 steers and 1 pen with 11 steers). Five steers in each pen were administered CCFA, and 5 served as control steers (1 pen had 6 control steers). The CCFA administration included a single-dose regimen (6.6 mg/kg, SC, on day 0), two-thirds-dose regimen (4.4 mg/kg, SC, on day 0), and 3-dose regimen (6.6 mg/kg, SC, on days 0, 6, and 13). Fecal samples were collected on days 0, 2, 6, 9, 13, 16, 20, and 28. Fecal samples were collected immediately before CCFA administration. Minimum inhibitory concentrations of 15 antimicrobials were determined for 3 E coli isolates/fecal sample. Escherichia coli were enumerated by use of direct-plating techniques. RESULTS: Resistance to 1 or more antimicrobials was detected in 986 of 1,441 (68.4%) isolates recovered. Administration of CCFA was associated with a transient increase in the population of ceftiofur-resistant isolates. Susceptibility returned to day 0 values (ie, samples collected immediately before CCFA administration) approximately 2 weeks after completion of CCFA administration. Agreement between ceftiofur resistance and co-resistance to ampicillin, chloramphenicol, streptomycin, sulfisoxazole, and tetracycline was almost perfect (kappa 0.97). We did not detect variation in susceptibility of E coli recovered from commingled control steers. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of CCFA provided selection pressure that favored transient expansion of multiple-resistant variants.     

Antimicrobial usage and resistance in beef production

Antimicrobials are critical to contemporary high-intensity beef production. Many different antimicrobials are approved for beef cattle, and are used judiciously for animal welfare, and controversially, to promote growth and feed efficiency. Antimicrobial administration provides a powerful selective pressure that acts on the microbial community, selecting for resistance gene determinants and antimicrobial-resistant bacteria resident in the bovine flora. The bovine microbiota includes many harmless bacteria, but also opportunistic pathogens that may acquire and propagate resistance genes within the microbial community via horizontal gene transfer. Antimicrobial-resistant bovine pathogens can also complicate the prevention and treatment of infectious diseases in beef feedlots,threatening the efficiency of the beef production system. Likewise, the transmission of antimicrobial resistance genes to bovine-associated human pathogens is a potential public health concern. This review outlines current antimicrobial use practices pertaining to beef production, and explores the frequency of antimicrobial resistance in major bovine pathogens. The effect of antimicrobials on the composition of the bovine microbiota is examined, as are the effects on the beef production resistome. Antimicrobial resistance is further explored within the context of the wider beef production continuum, with emphasis on antimicrobial resistance genes in the food chain, and risk to the human population.     

Effects of live yeast on differential genetic and functional attributes of rumen microbiota in beef cattle

Several studies have evaluated the effects of live yeast supplementation on rumen microbial population; however,its effect on differential microbial genes and their functional potential has not been described. Thus, this study applied shotgun metagenomic sequencing to evaluate the effects of live yeast supplementation on genetic and functional potential of the rumen microbiota in beef cattle. Eight rumen-cannulated Holstein steers were randomly assigned to two treatments in a cross-over design with two 25-day experimental periods and a 10-day wash-out between the two periods. The steers were housed in individual pens and fed 50% concentrate-mix and 50% red clover/orchard hay ad libitum. Treatments were(1) control(CON; basal diet without additive) and(2) yeast(YEA;basal diet plus 15 g/d of live yeast product). Rumen fluid samples were collected at 3, 6, and 9 h after feeding on the last d of each period. Sequencing was done on an Illumina Hi Seq 2500 platform. Dietary yeast supplementation increased the relative abundance of carbohydrate-fermenting bacteria(such as Ruminococcus albus, R.champanellensis, R. bromii, and R. obeum) and lactate-utilizing bacteria(such as Megasphaera elsdenii, Desulfovibrio desulfuricans, and D. vulgaris). A total of 154 differentially abundant genes(DEGs) were obtained(false discovery rate 0.01). Kyoto Encyclopedia of Genes and Genomes(KEGG) annotation analysis of the DEGs revealed that 10 pathways, including amino sugar and nucleotide sugar metabolism, oxidative phosphorylation, lipopolysaccharide biosynthesis, pantothenate and coenzyme A biosynthesis, glutathione metabolism, beta-alanine metabolism,polyketide sugar unit biosynthesis, protein export, ribosome, and bacterial secretory system, were enriched in steers fed YEA. Annotation analysis of the DEGs in the carbohydrate-active enzymes(CAZy) database revealed that the abundance of genes coding for enzymes belonging to glycoside hydrolases, glycosyltransferases, and carbohydrate binding modules were enriched in steers fed YEA. These results confirm the effectiveness of a live S. cerevisiae product for improving rumen function in beef steers by increasing the abundance of cellulolytic bacteria, lactic acid-utilizing bacteria, and carbohydrate-active enzymes in the rumen.     

Antimicrobial use on 24 beef farms in Ontario

Understanding risk factors for antimicrobial resistance requires knowledge of antimicrobial selection pressure. The objectives of this research were to develop methodology for collecting quantitative antimicrobial use information from beef producers in Ontario, to document the types and quantities of antimicrobials reported (for a minimum of 12 mo), and to compare 2 metrics for injectable use reporting. Twenty-four volunteer beef producers were asked to complete a questionnaire, document drug use in a treatment diary, and retain empty medication containers. For injectable antimicrobials, producers recorded approximately 60% of the total use in the treatment diaries; oxytetracycline, penicillin, macrolides, florfenicol, and spectinomycin were used in the greatest quantities. Based on estimated weights of active ingredients (calculated according to number of animals exposed, duration, and average dose per day) the antimicrobials most commonly used in feed were monensin, tylosin, lasalocid, and tetracyclines. The antimicrobials most commonly used in water were lincomycin-spectinomycin, chlortetracycline, and oxytetracycline. Based on estimated weights and measured quantities, < 1% of antimicrobials used were in the Canadian category of highest importance to human medicine. A comparison of animal daily dosages to kilograms of active ingredient demonstrated that the relative ranking of use of antimicrobials varied with the chosen metric, and that further investigation into the best measure in relation to antimicrobial resistance is warranted.     

Feedlot performance and tissue residues of cattle consuming diets containing aflatoxins

A comparative slaughter feeding trial was conducted using crossbred Hereford-Angus steers. Cottonseed meal known to be contaminated with aflatoxins was fortified with long-grain rice cultures of Aspergillus flavus to produce complete rations that contained 0, 60, 300 and 600 ppb aflatoxin B1 (AFB1). Forty steers, weighing approximately 250 kg were randomly assigned to four equal treatment groups. Animals were housed in individual pens and fed the assigned contaminated ration ad libitum for 155 d. On d 156 all animals were placed on the control ration for 14 d. Animals were weighed and blood samples collected every month for clinical chemistry analysis. Liver, muscle and fat biopsies were collected at 6-wk intervals. These samples were stored in liquid N2 prior to analysis for aflatoxin M1 and B1 content. Separate liver samples were taken for histopathological examination. Aflatoxin was withdrawn from the diet 2 wk prior to slaughter. After 1 wk of withdrawal liver, muscle and fat samples were collected and assayed for AFB1 and aflatoxin M1 (AFM1). Liver samples also were taken for histopathological examination. Growth rates and feed intakes in the 60 and 300 ppb aflatoxin treatments were not significantly different from controls. The 600-ppb treatment group had a decrease in feed intake and rate of gain. Rations containing 60 and 300 ppb of aflatoxin had no significant influence on blood components and enzyme patterns. The 600-ppb treatment caused a slight, but consistent, increase in serum glutamic oxaloacetic transaminase and alkaline phosphatase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Performance of feedlot steers fed diets containing laidlomycin propionate or monensin plus tylosin, and effects of laidlomycin propionate concentration on intake patterns and ruminal fermentation in beef steers during adaptation to a high-concentrate diet.

Two hundred eighty-eight beef steers (British x Continental x Brahman) were fed a 90% concentrate diet containing either no ionophore (control), laidlomycin propionate at either 6 or 12 mg/kg of dietary DM, or monensin plus tylosin (31 and 12 mg/kg of DM, respectively). Neither of the two levels of laidlomycin propionate nor monensin plus tylosin affected (P greater than .10) ADG or feed:gain ratio. Monensin plus tylosin reduced (P less than .01) daily DMI for the 161-d trial period compared with the other three treatments. Laidlomycin propionate at 6 mg/kg increased (P less than .05) DMI relative to the control, laidlomycin propionate at 12 mg/kg, and monensin plus tylosin diets during the 2nd wk of the trial and from d 57 to 84. Treatments did not affect carcass measurements. In a second experiment, 12 ruminally cannulated steers were fed diets containing no ionophore or laidlomycin propionate at either 6 or 12 mg/kg of DM. Samples were obtained for two consecutive days while the dietary concentrate level was 75%, after which the diet was switched abruptly to 90% concentrate, and samples were collected on several days during a 21-d period. The rate at which steers consumed their daily allotment of feed was not altered markedly by laidlomycin propionate. Likewise, laidlomycin propionate did not affect total ruminal VFA concentrations or proportions. Ruminal concentrations of D-lactate were reduced (P less than .10) by 6 but not by 12 mg/kg of laidlomycin propionate.(ABSTRACT TRUNCATED AT 250 WORDS)