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Biomedical subjects

A H Linton

Publications and source records attributed to A H Linton.

At least 19 recordsLinked to original sources
PubMed1993

Effects of low concentrations of ampicillin in feed on the intestinal Escherichia coli of chicks.

Ampicillin in low concentrations (1.7 and 5 g t-1) was incorporated in the feed given to 1-d-old chicks for 2 weeks. This was sufficient to select, in the intestinal contents, resistant Escherichia coli strains for which the minimum inhibitory concentration of ampicillin was > 100 micrograms ml-1. Different clones of E. coli were identified by their biotype, pattern of resistance to antibacterial agents and plasmid profile (designated P-P types). The experiment was repeated twice and the average proportion of ampicillin-resistant P-P types among 72 isolates of E. coli from chicks given feed containing 0, 1.7 and 5 g ampicillin t-1 were 10%, 31% and 46% respectively.

Ampicillin↗
PubMed1988

Control of salmonella infections in broiler chickens by the acid treatment of their feed.

In three experiments a solution of formic acid (Bioadd) was added to feed 'naturally' contaminated with salmonellas. In two of them no salmonella infections were demonstrated in broiler chickens given feed containing 0.6 per cent (w/w) of the formic acid solution for seven weeks and in the third the infection rate was reduced considerably. In four other experiments the feed was contaminated artificially with between 1 and 47 Salmonella kedougou/g and a second product (BPO12), consisting of a mixture of formic acid and propionic acid, was evaluated. When it was added at 0.5 to 0.68 per cent (w/w) only one of 30 groups of 10 to 12 birds became infected with S kedougou by two weeks of age, compared with 22 of 27 control groups. The treatment of the feed with BPO12 one week before the addition of the salmonellas prevented the establishment of infection in chicks given the treated feed, indicating that acid treatment will provide protection against subsequent recontamination of the feed with the organisms. The isolation rate of S kedougou from artificially contaminated feed treated with BPO12 was reduced only slightly but the feed was apparently 'safe' for the birds which consumed it.

Animal Feed↗
PubMed1988

Monitoring for the development of antimicrobial resistance during the use of olaquindox as a feed additive on commercial pig farms.

Since 1982, when olaquindox was introduced as a pig-feed additive in the UK, about 12 commercial farms in Suffolk have been monitored annually to check for the possible emergence of resistance to olaquindox and chloramphenicol among the coliform flora of the pigs and their environment. In spite of the sampling variability and the impossibility of controlling the use of feed additives and management on the farms, the overall results obtained were consistent and, it is suggested, the method is widely applicable. A steady, albeit low, increasing incidence and level of resistance to olaquindox was recorded (1982-1984) on farms using it and, to a lesser degree, on neighbouring farms that did not. No significant increase in the level of chloramphenicol resistance was observed. Genetical studies on a selection of olaquindox-resistant isolates suggested that the genes determining resistance were likely to be borne on the chromosome.

Animal Feed↗
PubMed1988

The effect of growth-promoting antibiotics on the faecal enterococci of healthy young chickens.

Small groups of chickens were given feed containing either avoparcin, nitrovin, virginiamycin or zinc bacitracin from the day of their purchase as day-olds. Differences between the birds receiving growth promoters and the untreated controls were observed during the last third of the 23 d survey period. The enterococcal population of the 'dosed' birds contained a greater proportion of Enterococcus faecium than did that of the control birds while the converse was true for Ent. gallinarum. This apparent selection of Ent. faecium by the growth-promoting antibiotics had an influence on the incidence of resistance to therapeutic antibiotics among the enterococcal population as a whole. This was because this species was generally more resistant than Ent. gallinarum to cephalothin, the MLS antibiotics (erythromycin, lincomycin and tylosin) and tetracycline.

Animal Feed↗
PubMed1988

Olaquindox resistance in the coliform flora of pigs and their environment: an ecological study.

Faecal samples were taken weekly from a chosen pen on each of four commercial pig farms, two of which used olaquindox as a feed additive. Coliform bacteria were isolated from the samples and the incidence and level of resistance to olaquindox and to four therapeutic antibiotics determined. The coliforms isolated were biotyped to follow the emergence of drug-resistant sub-populations. The results showed that the coliform flora was complex and that a turnover of biotypes was associated with changes in the occupancy of the pen and, possibly, diet. This turnover led to large fluctuations in both incidence and level of resistance to olaquindox and the antibiotics. Olaquindox resistance was not specifically linked with resistance to any therapeutic antibiotic, and the possible origin of olaquindox-resistant strains is discussed in relation to the biotypes.

Ampicillin↗
PubMed1988

Plasmids in the environment.

Bacterial plasmids existed in bacteria before the antibiotic era but their presence was brought into prominence by the use of antibiotics which selected for antibiotic resistant strains. Subsequently, the range of genes carried on plasmids was shown to extend far beyond those coding for antibiotic resistance. Any consideration of plasmids in the environment, therefore, must include all plasmids whether or not they are genetically linked with antibiotic resistance. Antibiotic resistant bacteria may be found in the environment either by contamination with excreta from man and animals in which the strains were selected, or by their selection within the environment by antibiotics synthesized in situ or reaching the environment in an undegraded form in sewage from man and animals, or from industry. Other agents, also contaminating the environment, exert a selective pressure such as heavy metals in industrial effluents which select for metal resistance. This paper reviews the incidences and role of plasmids in various habitats including natural waters, soil, pastures, farm wastes, and human sewage from both hospitalised and other populations. Aspects of plasmid ecology, their biological role, and the transmissibility of genetic material between bacteria within the environment are considered. Two recent studies in Bristol, UK, are reported. The first was a genetic study on Escherichia coli isolates from calf slurry. Various DNA probes were used to determine the extent of gene exchange between the various serotypes within the natural environment. The second was a preliminary study to determine the stability of a recombinant plasmid, in a wild strain of Escherichia coli of pig origin, after its release into a semi-contained farm situation. It is now recognized that plasmids are widely distributed in bacterial populations in terrestrial and aquatic environments. Many have been detected by their carriage of genes coding for antibiotic or heavy metal resistance. Others, mainly cryptic in nature, have been demonstrated by plasmid profile studies on isolates from various habitats. Plasmids were shown to be present in a relatively few bacteria deposited in culture collections prior to the antibiotic era. Subsequently, the increased prevalence of R plasmids in bacteria in most ecosystems were due mainly to the selective pressure imposed by the use of antibiotics. This pressure may have been exerted either in the environment in which the strains were found or elsewhere, the environment subsequently being contaminated by antibiotic resistant bacteria.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Enterobacteriaceae associated with animals in health and disease.

Members of at least eight of the 14 general of the Enterobacteriaceae are found associated with animals. A large number of these are present as members of the microbiota of the gut of many animals without causing harmful effects. Indeed, their presence as part of the stable flora contributes to the defences of the animal by excluding enteric pathogens by competitive exclusion which contributes to the phenomenon of [colonization resistance' (van der Waaij 1983). some of these strains can cause disease if they contaminate a normally 'sterile' site such as the body cavities. In contrast, particular strains within each bacterial species are overt pathogens and are regularly associated with disease. Nevertheless, even these may colonize the animal without producing clinical illness. In this paper E. coli and Salmonella spp. have been used to illustrate the various aspect of host-parasite relationships.

Animals↗
PubMed1987

The effect of ampicillin and tylosin on the faecal enterococci of healthy young chickens.

Enterococcal isolates from young chickens were differentiated into one of three species, namely Enterococcus faecalis, Ent. faecium and Ent. gallinarum. The proportion of each species among the enterococcal population changed with time in birds not dosed with antibiotics. This pattern of change was modified in birds dosed with either tylosin or ampicillin even though ampicillin did not select for ampicillin resistance among the enterococcal population. A gradual increase in tylosin resistance was recorded with time among the enterococci of the 'undosed' control birds. This was associated with an increase in the proportion of Ent. faecium, a species commonly resistant to tylosin, among the enterococci of the birds as they grew older.

Ampicillin↗
PubMed1986

The influence of dietary protein and antimicrobial feed additives on salmonella carriage by broiler chickens.

Salmonella carriage in broiler chickens was not apparently influenced by the protein concentration of the ration, pelleting or by the coccidiostat monensin. The inclusion of penicillin in the diet was associated with an increase in salmonella shedding, particularly in the first half of the rearing period, but did not influence the lactobacillary count in the crop or the pH of the contents of the crop, gizzard and caecum. Furazolidone medication (150 mg/kg food) for the first 10 days had no effect on salmonella carriage at the time of slaughter (seven weeks) and studies into the influence of growth promoting antibiotics (avoparcin, bacitracin and virginiamycin) on salmonella shedding were inconclusive. Circumstantial evidence indicated that the food was the most probable source of infection for the birds studied in these experiments.

Animals↗
PubMed1986

Speciation, serotyping, antimicrobial sensitivity and plasmid content of Proteeae from the environment of calf-rearing units in South West England.

A survey was undertaken of the occurrence, serotype, antimicrobial sensitivity and plasmid content of members of the tribe Proteeae in the environment of two calf-rearing units in the county of Avon in South West England. Examples of the following species were found: Proteus mirabilis, Prot. vulgaris, Prot. vulgaris Biogroup 2, Morganella morganii, Providencia stuartii, Prov. alcalifaciens and Prov. rettgeri. A wide range of serotypes was found, many having been previously reported from nosocomial isolates. A total of 15% of isolates carried plasmids; six pairs of isolates were identified which had identical serotypes but different patterns of plasmid carriage. The antimicrobial sensitivity of the isolates was generally similar to isolates of Proteeae from humans. Although no truly aminoglycoside-resistant isolates were found, some isolates of Prov. stuartii and Prov. rettgeri had MIC's higher than the other isolates to gentamicin and netilmicin, suggesting the presence of low levels of the enzyme AAC 2'. The study demonstrates that there is a considerable diversity of species and types of Proteeae associated with calves and their environment. It seems likely that a potential cause of colonization of the human gut by Proteeae is the consumption of meat.

Animals↗
PubMed1986

Do plasmids influence the survival of bacteria?

Bacterial plasmids may contribute to the fitness of a particular bacterial clone to survive in a specific environment. In many cases, however, the determinants that confer the advantage are unknown. The advantages to be gained from plasmid carriage appear to outweigh potential disadvantages imposed by an increased metabolic demand.

Bacteria↗
PubMed1986

Preliminary observations on the influence of antibiotics on the ecology of Escherichia coli and the enterococci in the faecal flora of healthy young chickens.

Two separate replicated experiments are described. In the first the influence of oxytetracycline on the Escherichia coli flora was assessed, while in the second the effect of tylosin on the enterococci was investigated. In both experiments the administration of the antibiotic was associated with an increase in the average proportion of isolates resistant to the agent although in experiment 1 the agent had no apparent influence on either the total number of E. coli strains, differentiated by biotype and resistance pattern, or the number of 'new' strains per sampling. On the other hand significant differences were noted between the two replicates and also in relation to the age of the birds. In the second experiment fewer enterococcal strains were isolated from the birds dosed with tylosin but since fewer strains on average were isolated from the samples collected prior to dosing it is probable that this difference was due to factors other than the influence of the antibiotic. The enterococci were speciated and the two most commonly isolated were Streptococcus faecalis and S. faecium. The isolation rate of each was expressed as a proportion of the total number of enterococcal isolates per sample and when considered in relation to both dosing and the age of the bird the interaction dosing X age was statistically significant for both species. However, the differences did not appear particularly great and it was possible that the differences were of no practical importance from an ecological point of view.

Animals↗
PubMed1986

Flow of resistance genes in the environment and from animals to man.

Some antibiotic-resistant bacteria occur naturally in the environment but many arise by contamination with human and animal excreta in sewage, slurry and manure. These may be ingested as contaminants of water, animal feeds and cultivated foods or, in the case of animals, by licking their environment. Gene transfer has been demonstrated in voided calf faeces and the resultant transconjugants, subsequently ingested, colonized the calf gut. Identified strains of antibiotic-resistant Escherichia coli have been traced from the gut contents of pigs, calves and chickens to carcasses at slaughter and ultimately shown to colonize the gut of a human volunteer handling and eating the meat. Similarly, antibiotic-resistant strains of Salmonella typhimurium phage type DT204c, have been isolated from calves and humans indicating a possible zoonotic spread. The significance of these observations is discussed.

Animals↗
PubMed1985

Natural subclinical salmonella infection in chickens: a potential model for testing the effects of various procedures on salmonella shedding.

The influence of growth additives on the duration of salmonella shedding has been variously reported. The different conclusions reached were mainly because of the different experimental systems used. In this paper a naturally infected chicken model for evaluating this problem is described. It simulates commercial conditions and proved to be reproducible in 13 groups, each of 125 birds, over a two-year period.

Age Factors↗
PubMed1985

The effects of oxytetracycline on the intestinal Escherichia coli flora of newly weaned pigs.

Four recently weaned pigs were dosed orally with oxytetracycline. This caused a rapid increase in the incidence of tetracycline resistance (TcR) among Escherichia coli isolates from the faecal flora. The isolates were differentiated further on the basis of O-serogroup, biotype and resistance pattern. There was no evidence that the administration of the antibiotic selected for a few TcR clones, but rather a relatively large number of TcR strains were identified during the dosing period. Using selective isolation media a proportion of these strains were demonstrated in the minority faecal Esch. coli flora before dosing, while the remainder were recognized for the first time after dosing commenced. The incidence of TcR among Esch. coli isolates also increased after weaning in other pigs which were not dosed with oxytetracycline or any other antibacterial agent. In a proportion of these animals this increase was associated with the dominance of a TcR enteropathogenic serotype (0149:K 91, K 88a, c) in the faecal Esch. coli flora which was probably ingested in small numbers before weaning. The source of other TcR strains was probably the environment in which each pig was placed after weaning.

Animals↗