Impact of residual and therapeutic doses of ciprofloxacin in the human-flora-associated mice model

Activation of class 1 integron integrase is promoted in the intestinal environment

Class 1 integrons are widespread genetic elements playing a major role in the dissemination of antibiotic resistance. They allow bacteria to capture, express and exchange antibiotic resistance genes embedded within gene cassettes. Acquisition of gene cassettes is catalysed by the class 1 integron integrase, a site-specific recombinase playing a key role in the integron system. In in vitro planktonic culture, expression of intI1 is controlled by the SOS response, a regulatory network which mediates the repair of DNA damage caused by a wide range of bacterial stress, including antibiotics. However, in vitro experimental conditions are far from the real lifestyle of bacteria in natural environments such as the intestinal tract which is known to be a reservoir of integrons. In this study, we developed an in vivo model of intestinal colonization in gnotobiotic mice and used a recombination assay and quantitative real-time PCR, to investigate the induction of the SOS response and expression and activity of the class 1 integron integrase, IntI1. We found that the basal activity of IntI1 was higher in vivo than in vitro. In addition, we demonstrated that administration of a subinhibitory concentration of ciprofloxacin rapidly induced both the SOS response and intI1 expression that was correlated with an increase of the activity of IntI1. Our findings show that the gut is an environment in which the class 1 integron integrase is induced and active, and they highlight the potential role of integrons in the acquisition and/or expression of resistance genes in the gut, particularly during antibiotic therapy.

Class 1 integrons are genetic systems allowing bacteria to acquire antibiotic resistance genes through the recombination activity of the IntI1 integrase. These genetic platforms are involved in the spread of antibiotic resistance among bacteria. So far, most of the studies for understanding the mechanistic of integrons have been performed in vitro, experimental conditions which are far from the lifestyle of bacteria in natural environments such as the gut. We developed a new in vivo model using gnotobiotic mice and we showed that in the gut, the basal activity of IntI1 is significantly higher than in in vitro conditions. In addition, we showed that a subinhibitory concentration of ciprofloxacin rapidly triggers the SOS response, leading to increased activity of IntI1 in the mouse gut. Our results demonstrate that the intestinal environment promotes the acquisition/expression of antibiotic resistance genes through the integron system and that this phenomenon can be enhanced by antibiotic therapy.

Bugs as Drugs: Understanding the Linkage between Gut Microbiota and Cancer Treatment Microbiome in Cancer Therapy

BACKGROUND

The commensal microbiota is known to regulate host physiology. Dysbiosis or compromised Resilience in the microbial ecology is related to the impending risk of cancer. A potential link between cancer and microbiota is indicated by a lot of evidence.

OBJECTIVE

The current review explores in detail the various links leading to and /or facilitating oncogenesis, providing sound reasoning or a basis for its utilization as potential therapeutic targets. The present review emphasizes the existing knowledge of the microbiome in cancer and further elaborates on the factors like genetic modifications, effects of dietary components, and environmental agents that are considered to assess the direct and indirect effect of microbes in the process of oncogenesis and on the host's health. Strategies modulating the microbiome and novel biotherapeutics are also discussed. Pharmacomicrobiomics is one such niche accounting for the interplay between the microbiome, xenobiotic, and host responses is also looked upon.

METHODS

The literature search strategy for this review was conducted by following the methodology of the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA). The method includes the collection of data from different search engines like PubMed, ScienceDirect, SciFinder etc. to get coverage of relevant literature for accumulating appropriate information regarding microbiome, cancer, and their linkages.

RESULTS

These considerations are made to expand the existing literature on the role of gut microbiota on the host's health, the interaction between host and microbiota, and the reciprocal relationship between the microbiome and modified neoplastic cells.

CONCLUSION

Potential therapeutic implications of cancer microbiomes that are yet unexplored and have rich therapeutic dividends improving human health are discussed in detail in this review.

A non-targeted LC-HRMS approach for detecting exposure to illegal veterinary treatments: The case of cephalosporins in commercial laying Hens

Cephalosporins are of particular importance in human medicine and should be reserved for second-line curative treatment in the veterinary field to avoid any emerging antimicrobial resistance. Due to misuse of ceftiofur in the poultry sector in France, it is now recommended to completely stop using cephalosporins in this sector. Methods currently used for the control of veterinary practices are mostly based on liquid chromatography coupled to mass spectrometry in a targeted mode, including parent compounds and any major metabolites. The aim of the present study was to evaluate the relevance of untargeted metabolomic approaches to highlight a possible exposure of laying hens to cephalosporins using a predictive model including selected treatment biomarkers. An experimentation carried out on living animals involved the administration of cefquinome and ceftiofur. Three biological matrices-droppings, eggs and liver-were investigated. Metabolites were extracted and analysed by liquid chromatography coupled to high resolution mass spectrometry in a full scan mode. Metabolites impacted by the treatment were selected by using univariate and multivariate statistical analyses. Predictive models built from the potential biomarkers selected in the "droppings" matrix were validated and able to classify "treated" and "control" hens. PLS-DA and logistic regression models were compared and both models gave satisfactory results in terms of prediction. Results were of less interest for other matrices in which only biomarkers of exposure to cefquinome were detected.

Exposure to Antibiotics Affects Saponin Immersion-Induced Immune Stimulation and Shift in Microbial Composition in Zebrafish Larvae

In the last decades, pollution of the environment by large scale use of antibiotics in agriculture and human medicine have led to increased antimicrobial resistance in both the environment and the host animal microbiome. Disturbances in the host microbiome can result in impaired immunity and reduced resilience of aquaculture species. Here, we investigated whether environmentally measured levels of the commonly used antibiotics ciprofloxacin and oxytetracycline influences the host microbiome and susceptibility toward saponin-induced immune stimulation in larval zebrafish. Firstly, neutrophil and macrophage reporter zebrafish larvae were exposed to different concentrations of soy saponin by immersion. A dose-dependent increase in neutrophil presence in the intestinal area was observed together with increased expression of immune genes il1b, tnfa, il22 and mmp9. To investigate the effect of antibiotics, larval zebrafish were immersed in ciprofloxacin or oxytetracycline in the presence or absence of a low dose of saponin. In vivo imaging revealed that antibiotic treatment did not reduce the number of neutrophils that were recruited to the intestinal area upon saponin exposure, although it did tend to lower pro-inflammatory cytokine levels. Microbial sequencing of whole larvae revealed that exposure to a low dose of saponin already shifted the microbial composition. The combination of oxytetracycline and saponin significantly increased α-diversity compared to the controls. In conclusion, the current study provides evidence that the combination of low levels of antibiotics with low levels of anti-nutritional factors (saponin) can induce inflammatory phenotypes and can modify the microbiota, which might lead to altered disease susceptibility.

Therapeutic administration of enrofloxacin in mice does not select for fluoroquinolone resistance inCampylobacter jejuni

Enrofloxacin is registered for therapeutic use in beef cattle to treat bovine respiratory disease in Canada. A murine model was used to experimentally examine the impact of therapeutic administration of enrofloxacin on fluoroquinolone resistance development in Campylobacter jejuni. Administration of enrofloxacin to mice via subcutaneous injection or per os routes resulted in equivalent levels of bioactive enrofloxacin within the intestine, but bioactivity was short-lived (<48 h after cessation). Enrofloxacin administration did not affect densities of total bacteria, Firmicutes, or Bacteroidetes in digesta and had modest impacts on densities of Enterobacteriaceae. All mice inoculated with C. jejuni NCTC 11168 became persistently colonized by the bacterium. Enrofloxacin reduced C. jejuni cell densities within the cecal and colonic digesta for all treatments, and densities shed in feces as a function of antibiotic duration. None of the C. jejuni isolates recovered from mice after administration of enrofloxacin (n = 260) developed resistance to ciprofloxacin regardless of method or duration of administration. Furthermore, only modest shifts in the minimum inhibitory concentration of the isolates by treatment were noted. The study findings indicate that the risk posed by short-term subcutaneous administration of enrofloxacin for the development of fluoroquinolone resistance in mammals is low.

Green Tea Polyphenols Modulate Colonic Microbiota Diversity and Lipid Metabolism in High-Fat Diet Treated HFA Mice

There is an increasing interest in the effect of dietary polyphenols on the intestinal microbiota and the possible associations between this effect and the development of obesity. However, limited information is available on how these polyphenols affect the gut microbiota and lipid metabolism. The co-action of a high-fat diet (HFD) and tea polyphenol (TP) on gut microbiota and lipid metabolism using a human flora-associated (HFA) C57BL/6J mice model is studied. TP reduced serum total cholesterol, triglyceride, low density lipoprotein, glucose (GLU) and insulin (INS) levels of HFD mice in a dose-dependent manner (P < 0.01). TP also significantly increased acetic acid and butyric acid levels in HFA mice. 16S rRNA V3 region Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis (PCR-DGGE) profiles showed that HFD mice had significantly reduced microbial diversity. This reduction could be alleviated by TP, with a significant increase in the richness and diversity of colonic microbiota in the high-fat diet with 0.2% TP (TPM) and high-fat diet with 0.05% TP (TPL) groups (P < 0.05). 454 pyrosequencing analysis showed that the HFD group had a significant increase in the Bacteroidetes to Firmicutes (F/B) ratio (P < 0.001), which could effectively be reversed by TP. The results showed that the changes in composition and diversity of colonic microbiota by TP administration suppressed the host body weight and blood lipid increase in high-fat diet HFA mice.

PRACTICAL APPLICATION

A high fat diet significantly impacted gut microbiota composition and lipid metabolism in human flora-associated mice, which were largely ameliorated by tea polyphenol (TP). Therefore, TPs may be effectively used in controlling or treating obesity, hyperlipidemia and other related metabolic diseases.

An update discussion on the current assessment of the safety of veterinary antimicrobial drug residues in food with regard to their impact on the human intestinal microbiome

The human gastrointestinal tract ecosystem consists of complex and diverse microbial communities that have now been collectively termed the intestinal microbiome. Recent scientific breakthroughs and research endeavours have increased our understanding of the important role the intestinal microbiome plays in human health and disease. The use of antimicrobial new animal drugs in food-producing animals may result in the presence of low levels of drug residues in edible foodstuffs. There is concern that antimicrobial new animal drugs in or on animal-derived food products at residue-level concentrations could disrupt the colonization barrier and/or modify the antimicrobial resistance profile of human intestinal bacteria. Therapeutic doses of antimicrobial drugs have been shown to promote shifts in the intestinal microbiome, and these disruptions promote the emergence of antimicrobial-resistant bacteria. To assess the effects of antimicrobial new animal drug residues in food on human intestinal bacteria, many national regulatory agencies and international committees follow a harmonized process, VICH GL36(R), which was issued by a trilateral organization of the European Union, the USA, and Japan called the International Cooperation on Harmonization of Technical Requirements for Veterinary Medicinal Products (VICH). The guidance describes a general approach currently used by national regulatory agencies and international committees to assess the effects of antimicrobial new animal drug residues in animal-derived food on human intestinal bacteria. The purpose of this review is to provide an overview of this current approach as part of the antimicrobial new animal drug approval process in participating countries, give insights on the microbiological endpoints used in this safety evaluation, and discuss the availability of new information. Copyright © 2016 John Wiley & Sons, Ltd.

Impact of enrofloxacin and florfenicol therapy on the spread of OqxAB gene and intestinal microbiota in chickens

Horizontal transfer of plasmid-encoded multidrug-resistant determinants is a major health problem and has attracted much public attention. We studied the dissemination of the efflux pump gene oqxAB located on transferable plasmid pHXY0908 between Salmonella Typhimurium and Escherichia coli in the gut of chickens. After an inoculation with Salmonella Typhimurium harboring oqxAB-bearing plasmid pHXY0908, chickens were treated with enrofloxacin and florfenicol. Inoculated, but non-treated chickens were included as a control group. Our results revealed that commensal E. coli isolates from the gut of chickens acquired the oqxAB-bearing plasmid in both treated and non-treated groups. Additionally, in the florfenicol treatment group, the average isolation rate of oqxAB-positive E. coli was significantly higher than that in the non-treated group. PFGE analysis showed that oqxAB-positive E. coli strains belonged to different patterns with one predominating. Moreover, multilocus sequence typing analysis revealed that E. coli ST533 was closely associated with the spread of oqxAB gene. qPCR analysis indicated that antibiotic administration provided selective advantages for sustaining a significantly high level of oqxAB gene from the DNA extracted from the feces. There was also a fluctuation in the intestinal microbiota with antibiotic therapy. In conclusion, the present study indicates that the oqxAB gene could be readily spread within the intestinal microflora. This could be enhanced by administrated with clinical doses of florfenicol and enrofloxacin, resulting in the enlargement of resistance gene reservoirs. In addition, ST533 E. coli isolates were found to contribute to transfer of the oqxAB gene.

Comparative diversity analysis of gut microbiota in two different human flora-associated mouse strains

The Kunming (KM) mouse is a closed colony mouse strain widely used in Chinese pharmacology, toxicology, and microbiology research laboratories. However, few studies have examined human flora-associated (HFA) microbial communities in KM mice. In this study, HFA models were built from germ-free KM and C57BL/6J mouse strains, and gut microbial diversity was analyzed by denaturing gradient gel electrophoresis (DGGE) and DNA sequencing. We found that the two strains of HFA mice were significantly different based on the UPGMA dendrogram and the Richness index, but dice similarity coefficients of mouse replicates were not significantly different between HFA-KM and HFA-C57BL/6J. Most of the dominant phyla of human gut microflora could be transferred into the guts of the two mouse strains. However, the predominant genus that formed in HFA-KM was Clostridium sp. and that in HFA-C57BL/6J was Blautia sp. These results imply that genotypes difference between the two mice strains is a critical factor in shaping the intestinal microflora. However, genetic differences of individuals within KM mouse populations failed to lead to individual difference in microflora. Successful generation of HFA-KM mice will facilitate studies examining how diet affects gut microbial structure, and will enable comparative studies for uncovering genetic factors that shape gut microbial communities.

Gut microbiota composition modifies fecal metabolic profiles in mice

The gut microbiome is known to be extensively involved in human health and disease. In order to reveal the metabolic relationship between host and microbiome, we monitored recovery of the gut microbiota composition and fecal profiles of mice after gentamicin and/or ceftriaxone treatments. This was performed by employing (1)H nuclear magnetic resonance (NMR)-based metabonomics and denaturing gradient gel electrophoresis (DGGE) fingerprint of gut microbiota. The common features of fecal metabolites postantibiotic treatment include decreased levels of short chain fatty acids (SCFAs), amino acids and primary bile acids and increased oligosaccharides, d-pinitol, choline and secondary bile acids (deoxycholic acid). This suggests suppressed bacterial fermentation, protein degradation and enhanced gut microbial modification of bile acids. Barnesiella, Prevotella, and Alistipes levels were shown to decrease as a result of the antibiotic treatment, whereas levels of Bacteroides, Enterococcus and Erysipelotrichaceae incertae sedis, and Mycoplasma increased after gentamicin and ceftriaxone treatment. In addition, there was a strong correlation between fecal profiles and levels of Bacteroides, Barnesiella, Alistipes and Prevotella. The integration of metabonomics and gut microbiota profiling provides important information on the changes of gut microbiota and their impact on fecal profiles during the recovery after antibiotic treatment. The correlation between gut microbiota and fecal metabolites provides important information on the function of bacteria, which in turn could be important in optimizing therapeutic strategies, and developing potential microbiota-based disease preventions and therapeutic interventions.

Correlation between fecal concentrations of ciprofloxacin and fecal counts of resistant Enterobacteriaceae in piglets treated with ciprofloxacin: toward new means to control the spread of resistance?

We assessed in a piglet model the relationship between fecal ciprofloxacin concentrations and ciprofloxacin-resistant Enterobacteriaceae counts. Twenty-nine piglets were orally treated with placebo or with 1.5 or 15 mg ciprofloxacin/kg of body weight/day from day 1 (D1) to D5. Areas under the curve (AUC) of concentrations increased sharply with dose and correlated positively with AUC of resistant bacteria log counts between D1 and D9. Removing residual colonic quinolones could help to control the emergence of resistance in fecal flora.

Peroral ciprofloxacin therapy impairs the generation of a protective immune response in a mouse model for Salmonella enterica serovar Typhimurium diarrhea, while parenteral ceftriaxone therapy does not

Nontyphoidal Salmonella (NTS) species cause self-limiting diarrhea and sometimes severe disease. Antibiotic treatment is considered only in severe cases and immune-compromised patients. The beneficial effects of antibiotic therapy and the consequences for adaptive immune responses are not well understood. We used a mouse model for Salmonella diarrhea to assess the effects of per os treatment with ciprofloxacin (15 mg/kg of body weight intragastrically 2 times/day, 5 days) or parenteral ceftriaxone (50 mg/kg intraperitoneally, 5 days), two common drugs used in human patients. The therapeutic and adverse effects were assessed with respect to generation of a protective adaptive immune response, fecal pathogen excretion, and the emergence of nonsymptomatic excreters. In the mouse model, both therapies reduced disease severity and reduced the level of fecal shedding. In line with clinical data, in most animals, a rebound of pathogen gut colonization/fecal shedding was observed 2 to 12 days after the end of the treatment. Yet, levels of pathogen shedding and frequency of appearance of nonsymptomatic excreters did not differ from those for untreated controls. Moreover, mice treated intraperitoneally with ceftriaxone developed an adaptive immunity protecting the mice from enteropathy in wild-type Salmonella enterica serovar Typhimurium challenge infections. In contrast, the mice treated intragastrically with ciprofloxacin were not protected. Thus, antibiotic treatment regimens can disrupt the adaptive immune response, but treatment regimens may be optimized in order to preserve the generation of protective immunity. It might be of interest to determine whether this also pertains to human patients. In this case, the mouse model might be a tool for further mechanistic studies.

Transfer of plasmid-mediated CTX-M-9 from Salmonella enterica serotype Virchow to Enterobacteriaceae in human flora-associated rats treated with cefixime

Food animals are a potential source of CTX-M resistance genes for humans. We evaluated the transfer of the bla(CTX-M-9) gene from an animal strain of Salmonella enterica serotype Virchow to Enterobacteriaceae of the human intestinal flora by using human flora-associated (HFA) rats with and without cefixime treatment. In the absence of antibiotic, no transconjugant enterobacteria were found in the feces of HFA rats. However, the transfer rate was high if Escherichia coli J5 recipient strains were coinoculated orally with Salmonella. S. enterica serotype Virchow persisted in the rat fecal flora both during and after treatment with therapeutic doses of cefixime. The drug did not increase the transfer rate, and E. coli J5 transconjugants were eliminated from the flora before the end of cefixime treatment. No cefixime was recovered in the rat feces. In the presence of recipient strains, the bla(CTX-M-9) resistance gene was transferred from a strain of animal origin to the human intestinal flora, although transconjugant colonization was transient. Antibiotic use enhanced the persistence of donor strains, increasing the resistance gene pool and the risk of its spread.

Risk assessment of ciprofloxacin, flavomycin, olaquindox and colistin sulfate based on microbiological impact on human gut biota

Trace levels of veterinary antibiotics that reside in livestock products may disturb the balance of human intestinal microbiota and impair the colonized barrier function, which is critical to protect against the invasion or overgrowth of exogenous pathogens. We investigated the colonization barrier disruption effect of ciprofloxacin, flavomycin, olaquindox and colistin sulfate by the minimum inhibitory concentration (MIC) assay in pure culture of human gut bacteria and evaluated the no-observed-effect-concentration (NOEC) and acceptable daily intake (ADI) based on the microbiological impact. MICs of the antibiotics were tested for total 100 isolates composed of 10 isolates from each of 10 predominant genera of human faeces that were freshly collected from healthy women at 1x10(5) and 1x10(9) colony-forming units (CFU)/ml. MIC assay was also conducted with 10 ATCC standard bacteria species of human fecal microbiota for the comparison with freshly isolated human fecal mirobiota. The most susceptible bacteria were Escherichia coli for ciprofloxacin and colistin sulfate, Fusobacterium spp. for flavomycin and Eubacterium spp. for olaquindox. MIC values were lower at 1x10(5) than at 1x10(9)CFU/ml. The susceptibility of feacal microbiota freshly isolated from healthy women tended to be similar with those of ATCC standard bactera. NOEC (microg/ml) and ADI (microg/kg BW/day) were evaluated as 0.008 and 0.15 for ciprofloxacin, 0.25 and 1 for flavomycin, 0.125 and 3 for olaquindox and 1.0 and 7 for colistin sulfate, respectively.

Human flora-associated rodents--does the data support the assumptions?

There is little direct literature detailing exhaustive bacteriological studies comparing human donor faecal flora, human flora‐associated (HFA) mouse models and conventional rodent faecal flora. While there is a premise that the implanted donor faecal flora from humans is established in the rodent model the evidence is incomplete and indeed for groups such as Bifidobacterium spp. it is lacking. The reviewed bacteriology studies are generally lacking in detail with the exception of one study from which the data have mostly been overlooked when cited by other workers. While there are studies that suggest that the HFA rodent model is more relevant to man than studies with conventional rodents, the hypothesis remains to be proven. This review concludes that the established microbial flora in the HFA rodent model is different to that of donor human faecal flora, and this clearly raises the question as to whether this matters, after all a model is a model and as such models can be useful even should they fail to be a true representation of, in this case, the gastrointestinal tract. What matters is that there is a proper understanding of the limitations of the model as we attempt to unravel the significance of the components of the gastrointestinal flora in health and disease; examples of why such an analysis is important are provided with regard to obesity and nutritional studies. The data do unsurprisingly suggest that diet is an extremely influential variable when interpreting HFA and conventional rodent data. The microbiology data from direct bacteriology and indirect enzyme studies show that the established microbial flora in the HFA rodent model is different to that of donor human faecal flora. The significance of this conclusion remains to be established.

In vitro model of colonization resistance by the enteric microbiota: effects of antimicrobial agents used in food-producing animals

A bioassay was developed to measure the minimum concentration of an antimicrobial drug that disrupts the colonization resistance mediated by model human intestinal microbiota against Salmonella invasion of Caco-2 intestinal cells. The bioassay was used to measure the minimum disruptive concentrations (MDCs) of drugs used in animal agriculture. The MDCs varied from 0.125 microg/ml for some broad-spectrum antimicrobial drugs (e.g., streptomycin) to 16 microg/ml for drugs with limited spectra of antimicrobial activity (e.g., lincomycin). The acceptable daily intake (ADI) residue concentration calculated on the basis of the MDCs were higher for erythromycin, lincomycin, and tylosin than the ADI residue concentrations calculated on the basis of the MICs. The MDC-based ADI values for apramycin, bacitracin, neomycin, novobiocin, penicillin G, streptomycin, tetracycline, and vancomycin were lower than the reported MIC-based ADI values. The effects of antimicrobial drugs at their MDCs on the bacterial composition of the microbiota were observed by denaturing gradient gel electrophoresis of 16S rRNA sequences amplified by PCR. Changes in the population composition of the model colonization resistance microbiota occurred simultaneously with reduced colonization resistance. The results of this study suggest that direct assessment of the effects of antimicrobial drugs on colonization resistance in an in vitro model can be useful in determining ADI values.

Impact of ciprofloxacin in the human-flora-associated (HFA) rat model: Comparison with the HFA mouse model

The ecological impact of different doses of ciprofloxacin was investigated in an experimental germ-free rat model into which human fecal flora was inoculated. Animals received oral doses (gavage) of 0, 0.25, 2.5, and 25 mg/kg body weight (bw) of ciprofloxacin once daily for 5 weeks. All doses of ciprofloxacin significantly reduced aerobic populations. Elimination of Enterobacteriaceae and reduction of bifodibacteria were noticed in the group treated with 25 mg/kg of the antibiotic. The rest of the intestinal flora was not affected. These effects were reversible after the treatment ended. The percentage of resistant enterococci increased in rats treated with 2.5 and 25 mg/kg; however, this increase was not statistically significant. There was a significant (P < 0.05) emergence of ciprofloxacin-resistant Bacteroides fragilis group with 25 mg/kg bw, which is equivalent to a human therapeutic dosage of the antibiotic. The MIC values and the percentage of resistance remained elevated 2 weeks after the end of treatment in this anaerobic population. Although sub-populations of enterococci and Enterobacteriaceae showed decreased susceptibility after ciprofloxacin administration, resistance was not evident. The ability of an exogenous strain of Salmonella to colonize the intestine of animals treated with 25 mg/kg of ciprofloxacin confirmed that the drug disrupted the colonization barrier effect of the indigenous flora at the high dose level tested. No changes in the metabolic parameters occurred during the antibiotic treatment. The results obtained in the HFA rat model were similar to those obtained in our previous study using the HFA mice model where ciprofloxacin at 0.125, 1.25, and 12.5 mg/kg bw induced a decrease of enterococci and Enterobacteriaceae populations. The high dose of ciprofloxacin also induced a decrease in bifidobacteria counts, an increase in levels of resistant B. fragilis group and a significant (P < 0.05) disruption of the colonization resistance of the barrier flora in HFA mice.