Evidence for a disseminated erythromycin resistance determinant mediated by Tn917-like sequences among group D streptococci isolated from pigs, chickens, and humans.

A total of 199 streptococci isolated from feces of healthy chickens, pigs, and beef cattle and 26 human clinical isolates were tested for resistance to kanamycin, streptomycin, tetracycline, erythromycin, and lincomycin. Of 66 isolates resistant to these antibiotics, 12 transferred one or more resistance traits by conjugation in broth. Erythromycin resistance (Emr) was transferred from 10 of the 12 successful donors. AvaI digests of plasmids isolated from Emr transconjugants derived from two human, two chicken, and one pig isolate contained three fragments similar in size to those produced from Tn3871, an Emr transposon. The three fragments from each of the five digests on Southern blots hybridized to radiolabeled Tn3871. Plasmid DNA from a transconjugant derived from a second pig isolate contained two of the three Tn3871-associated AvaI fragments. One of the AvaI fragments from each of the six plasmids hybridized with a radiolabeled probe containing a cloned AvaI fragment from Tn3871 that contained the Emr determinant. Transposition of the Emr trait was demonstrated for the plasmids derived from one human and one pig isolate. We concluded that extensive DNA homology existed between plasmids from streptococcal strains obtained from two human patients, two chickens, and two pigs and the Emr transposon Tn3871, which is very similar or identical to the well-characterized Emr transposon Tn917. The detection of Tn3871-like sequences in streptococcal isolates from Arkansas, Illinois, North Carolina, and Washington, D.C. indicates wide dissemination of Emr mediated by the same or closely related transposons.

Incidence of salmonellae in fecal samples of production swine and swine at slaughter plants in the United States in 1978.

Nine swine slaughter plants and 19 swine production units were randomly selected for sampling from the six highest swine-producing states representing a total of 64% of the United States swine production. Three composites of 10 fresh swine fecal samples were obtained from each slaughter plant, representing three different farm sources of swine. Two composite fecal samples were collected from two different production pens from each production unit. Samples were analyzed for salmonellae. Isolated salmonellae were biochemically and serologically identified and tested for antibiotic susceptibility and resistance transfer ability. Salmonellae were recovered from swine at seven of the nine slaughter plants and 16 of the 27 composites of slaughter swine. Of the 19 production units, 3 had swine shedding salmonellae. Resistances found included streptomycin, tetracycline, and sulfadiazine. Of the 52 total isolates tested, 71% had some level of antibiotic resistance. Only 3 of 37 resistant isolated could transfer resistance under the conditions used.

Effect of penicillin and virginiamycin on drug resistance in lactose-fermenting enteric flora.

Three groups of beagle dogs were fed either a control diet, a diet containing virginiamycin (55 microgram/g of diet), or a diet containing penicillin (110 microgram/g of diet). The proportions of lactose-fermenting organisms in their feces that were resistant to ampicillin, dihydrostreptomycin, tetracycline, or chloramphenicol were measured by a comparative plate-counting procedure. Both antibiotic-supplemented diets resulted in an increase (P < 0.001) in the occurrence of ampicillin, dihydrostreptomycin, and tetracycline resistances during the time of their administration. The occurrence of these resistances was greater (P < 0.001) in the group receiving penicillin than in the group receiving virginiamycin. In addition to the above resistances, a greater (P < 0.001) occurrence of resistance to a sulfonamide (sulfamethoxypyridazine) due to treatment was found by susceptibility testing of isolates. Representative isolates were able to transfer their resistance to a strain of Escherichia coli K-12.

Effect of feeding chlortetracycline on the reservoir of Salmonella typhimurium in experimentally infected swine.

Swine were fed either a diet containing 110 mg of chlortetracycline (CTC) per kg (100 g/ton) or a control diet and were inoculated orally with Salmonella typhimurium that was either susceptible or resistant to CTC. The quantity, duration, and prevalence of fecal elimination of S. typhimurium, as well as the effect of CTC on the transmission of S. typhimurium from infected to uninfected swine, were determined. When animals were infected with CTC-resistant S. typhimurium, CTC increased the quantity (P < 0.05), duration (P < 0.05), and prevalence (P < 0.01) of fecal shedding, the transmission from infected to uninfected swine, and the recovery of the infecting organism at necropsy. When animals were infected with CTC-susceptible S. typhimurium, CTC reduced the quantity (between 7 and 10 days postinfection) (P < 0.01), duration (P < 0.05), and prevalence (P < 0.05) of fecal shedding, the transmission from infected to uninfected swine, and the recovery of the infecting organism at necropsy. Resistance to tetracycline was transferred in vivo to 4 and 6% of the susceptible infecting S. typhimurium recovered from the untreated and treated groups, respectively. The increased reservoir of S. typhimurium and the transfer of resistance to susceptible S. typhimurium have implications for both animal and public health.

Effect of low concentrations of dihydrostreptomycin on drug resistance in enteric bacteria.

Beagle dogs were fed a diet containing 0, 2, or 10 mug of dihydrostreptomycin (DSM) per g of feed. The 2-mug/g level was selected to represent a residue level of the antibiotic. In both treatment groups, medicated feed resulted in a shift from a predominantly streptomycin (SM)-susceptible coliform fecal population to an SM-resistant population. The proportion of resistant organisms was significant (P < 0.01) for both treatment groups. A definitive response did not occur with animals maintained on DSM-free diets. An increase in the prevalence of DSM-resistant organisms was observed after 15 days of DSM-supplemented feeding and persisted during the posttreatment phase of the study. The predominant pattern of resistance was SM-sulfamethoxypyridazine. Fifty-nine percent of SM-resistant strains transferred resistant determinants by conjugation to Escherichia coli K-12 recipients.

Efficacy of dichlorvos administered orally in single and repeated doses for removal of canine whipworms.

A single dose of dichlorvos (2,2-dichlorovinyl dimethyl phosphate) was administered orally at a dosage of 30 mg/kg to 19 dogs naturally infected with Trichuris vulpis and to 13 dogs experimentally infected with T vulpis. Based on the presence or absence of whipworm eggs in the feces, 10 to 14 days after treatment, dogs were either killed and the number of remaining worms counted, or the dogs were given a 2nd treatment. After the initial treatment, 18 dogs had a mean of 0.5 worms (SD +/- 0.5) remaining, and 14 dogs had a mean of 55 worms (SD +/- 85) remaining. After these 14 dogs were given a 2nd treatment, the number of womrs remaining decreased to a mean of 14 (SD +/- 28). Although a high degree of efficacy was attained in 56% of the dogs after the 1st dose, the data suggested that additional treatment may often be necessary.

Factors affecting serum oxytetracycline levels in beef calves.

Fifteen Aberdeen Angus steers, 295-364 kg, were dosed with either 4.4 or 11 mg of oxytetracycline hydrochloride/kg im. The antimicrobial activity of the serum was determined periodically, and the resulting data were treated statistically to determine the sources of variation. Variance in serum levels of oxytetracycline activity was attributed to dose, time of bleeding, order of dosing, animal, and assay. The total variance component was proportionately greater for the 11-mg/kg dose than for the 4.4-mg/kg dose. Animal variance increased with the higher dose level of oxytetracycline. The influence of dose on serum level was tested by applying a t test to the mean serum levels and their standard deviations at each bleeding time. The 4.4- and 11-mg/kg serum levels were significantly different (p less than 0.01) at all bleeding times. The 4.4-mg/kg serum levels mutliplied by 2.5 were not significantly different (p less than 0.05) from the 11-mg/kg serum levels at all bleeding times.

Persistence of transferable drug resistance in the lactose-fermenting enteric flora of swine following antimicrobial feeding.

Six groups of swine (85 animals) were fed a combination of antimicrobial drugs (sulfamethazine 100 g/ton, chlortetracycline 100 g/ton and penicillin 50 g/ton). After two weeks the antimicronial drugs were removed from the diet of two groups (28 animals). These swine were compared to four groups fed the medicated diet to determine the effect of duration of treatment and degree of animal isolation on the persistence of resistance in lactose-fermenting enteric organisms. The degree of resistance to penicillin, oxytetracycline, dihydrostreptomycin and neomycin as determined by minimum inhibitory concentrations and the incidence of resistant organisms were examined during and after antibiotic feedings. Ninety-two percent or greater of all isolates tested during and after treatment had minimum inhibitory concentrations for oxytetracycline of greater than 100 mug/ml. Thirty-two weeks after cessation of dietary antibiotic, resistance to oxytetracycline and dihydrostreptomycin remained at 100% and 89% respectively. Variation in degree of contact between swine receiving medicated feed and those receiving nonmedicated feed was not sufficient to reduce the incidence of resistance to oxytetracycline or dihydrostreptomycin in all animals. Factors influencing persistence of resistant enteric organisms are discussed. Addition of the antimicrobials to the ration resulted in significantly greater weight gains for treated animals than for the controls but did not alter feed conversion.

Increased survival in calves of Escherichia coli K-12 carrying an Ent plasmid.

Escherichia coli K-12 strains with and without an Ent plasmid were fed to calves, and the survival of each was monitored by viable bacterial counts of the feces. The E. coli K-12 strain carrying the Ent plasmid survived in the calves at significantly higher levels and for a longer period of time than the E. coli F(-) stain.

Animal model for determining the no-effect level of an antimicrobial drug on drug resistance in the lactose-fermenting enteric flora.

Mature beagles were fed a ground-meal diet containing 0, 2, or 10 mug of oxytetracycline per g for 44 days. The 10-mug/g diet resulted in a shift from a predominantly drug-susceptible population of enteric lactose-fermenting organisms to a multiply antibiotic-resistant population which peaked at 78% resistant organisms. Since a shift to drug-resistant organisms did not occur in the group fed 2 mug/g, the level of oxytetracycline that results in increased incidence of antibiotic resistance lies between 2 and 10 mug/g in this dog model. Rats and hamsters fed diets containing oxytetracycline (10 mug/g or greater) or dihydrostreptomycin (10 mug/g), and provided suspensions of drug-susceptible Escherichia coli, did not develop a population of antibiotic-resistant organisms.

Histologic changes in the seminiferous tubules after vasectomy.

PIP: 4 different studies on the effect of vasectomy on the testis in canines and humans are reported. Testicular biopsies were performed at certain intervals and the results in both humans and dogs were nearly identical. It was found that spermatogenesis 2 to 3 weeks after vasectomy remained unchanged with accumulation of spermatozoa in the tubules. Between 3 and 6 weeks, progressive spermatogenic arrest with few spermatozoa and decreased spermatids were observed. Between 100 and 300 days, occasional mature sperm were found in the tubules indicating a return of spermatogenesis. Meiotic studies showed this to be an arrest in early prophase. It is theorized that spermatogenesis may be sensitive to pressure changes in the tubular system.^ieng

Effect of racephenicol on antibiotic resistance of lactose-fermenting enteric bacteria in chickens.

The incidence of lactose-fermenting enteric organisms resistant to chloramphenicol increased from less than 0.1% prior to treatment to more than 90% in two groups of chickens fed racephenicol-supplemented feed at 50 or 140 g/ton for 10 days, whereas controls remained unchanged. A concurrent increase in the incidence of organisms resistant to dihydrostreptomycin, oxytetracycline, ampicillin, and sulfonamides also occurred. Minimum inhibitory concentrations for chloramphenicol increased from <10 mug/ml to >300 mug/ml in most Escherichia coli isolates in response to treatment. The observed resistance was transferable to E. coli and Salmonella.