Antibiotic susceptibility of clinical isolates of Pseudomonas aeruginosa

Three hundred and twenty two clinical isolates of Pseudomonas aeruginosa collected in Morelia, México, were analyzed for in vitro susceptibility to five antibiotics by agar dilution tests. Antibiotic resistance was shown by 50% of total isolates. Frequencies of resistance were: streptomycin, 47%; gentamicin, 13%; tobramycin, 8%; and carbenicillin, 7%; no amikacin resistance was found. The more common resistance patterns were streptomycin, gentamicin-streptomycin, and tobramycin-gentamicin-streptomycin. Resistance to either tobramycin, gentamicin or carbenicillin was found mainly in pyocin type 10 isolates. The proportion of antibiotic resistant isolates ranged from 37 to 75% in four hospitals, and amounted 24% in three clinical laboratories.     

Pseudomonas aeruginosa in a hydropathic facility: diversity, susceptibility and imipenem resistance mutation

Aims: To detect Pseudomonas aeruginosa in water and treatment equipment biofilms of a thermae hydropathic facility and to study antibiotic susceptibility and genetic diversity. Methods and Results: One hundred and fifty‐four planktonic isolates were obtained from 2220 water samples during 4 years. Seventy‐two biofilm isolates were obtained from 23 samples of inner parts of three inhalation equipments. Antibiotic susceptibility was determined by disc diffusion. All isolates were susceptible to tested antimicrobials, except two biofilm isolates and one planktonic isolate. Twenty‐one resistant mutants were observed (nine from biofilms), mostly with imipenem (IP) resistance (81%), by diminished expression of OprD porin, as it was observed by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS–PAGE). Random amplification polymorphic DNA showed a genetically heterogeneous population that is spread through the entire system and persistent in time. IP resistance mutation ability was spread through the population. Conclusions: The permanent assessment of Ps. aeruginosa is necessary not only in water, as expressed in official programmes, but also in equipments where biofilms are evident. Ps. aeruginosa was more prevalent in biofilm populations and presented higher ability to adapt to antibiotic pressure. Significance and Impact of the Study: Twenty‐one million people use thermae in Europe. Official microbiological quality control programmes only consider water surveillance. Present study proves the need of a review on current official programmes.     

Susceptibility of Pseudomonas aeruginosa Urinary Tract Isolates and Influence of Urinary Tract Conditions on Antibiotic Tolerance

Pseudomonas aeruginosa is an opportunistic human pathogen, which can cause severe urinary tract infections (UTIs). Because of the high intrinsic antibiotic resistance of P. aeruginosa and its ability to develop new resistances during antibiotic treatment, these infections are difficult to eradicate. The antibiotic susceptibility of 32 P. aeruginosa isolates from acute and chronic UTIs were analysed under standardized conditions showing 19% multi-drug resistant strains. Furthermore, the antibiotic tolerance of two P. aeruginosa strains to ciprofloxacin and tobramycin was analysed under urinary tract-relevant conditions which considered nutrient composition, biofilm growth, growth phase, and oxygen concentration. These conditions significantly enhance the antibiotic tolerance of P. aeruginosa up to 6000-fold indicating an adaptation of the bacterium to the specific conditions present in the urinary tract. This reversible phenomenon is possibly due to the increased formation of persister cells and is based on iron limitation in artificial urine. The results suggest that the general high antibiotic resistance of P. aeruginosa urinary tract isolates together with the increasing tolerance of P. aeruginosa grown under urinary tract conditions decrease the efficiency of antibiotic treatment of UTIs.     

Nitrite production by ammonia-oxidizing bacteria mediates chloramine decay and resistance in a mixed-species community

As water distribution centres increasingly switch to using chloramine to disinfect drinking water, it is of paramount importance to determine the interactions of chloramine with potential biological contaminants, such as bacterial biofilms, that are found in these systems. For example, ammonia-oxidizing bacteria (AOB) are known to accelerate the decay of chloramine in drinking water systems, but it is also known that organic compounds can increase the chloramine demand. This study expanded upon our previously published model to compare the decay of chloramine in response to alginate, Pseudomonas aeruginosa, Nitrosomonas europaea and a mixed-species nitrifying culture, exploring the contributions of microbial by-products, heterotrophic bacteria and AOBs to chloramine decay. Furthermore, the contribution of AOBs to biofilm stability during chloramination was investigated. The results demonstrate that the biofilm matrix or extracellular polymeric substances (EPS), represented by alginate in these experiments, as well as high concentrations of dead or inactive cells, can drive chloramine decay rather than any specific biochemical activity of P. aeruginosa cells. Alginate was shown to reduce chloramine concentrations in a dose-dependent manner at an average rate of 0.003 mg l⁻¹ h⁻¹ per mg l⁻¹ of alginate. Additionally, metabolically active AOBs mediated the decay of chloramine, which protected members of mixed-species biofilms from chloramine-mediated disinfection. Under these conditions, nitrite produced by AOBs directly reacted with chloramine to drive its decay. In contrast, biofilms of mixed-species communities that were dominated by heterotrophic bacteria due to either the absence of ammonia, or the addition of nitrification inhibitors and glucose, were highly sensitive to chloramine. These results suggest that mixed-species biofilms are protected by a combination of biofilm matrix-mediated inactivation of chloramine as well as the conversion of ammonia to nitrite through the activity of AOBs present in the community.     

The extracellular matrix protects Pseudomonas aeruginosa biofilms by limiting the penetration of tobramycin

Biofilm cells are less susceptible to antimicrobials than their planktonic counterparts. While this phenomenon is multifactorial, the ability of the matrix to reduce antibiotic penetration into the biofilm is thought to be of limited importance studies suggest that antibiotics move fairly rapidly through biofilms. In this study, we monitored the transport of two clinically relevant antibiotics, tobramycin and ciprofloxacin, into non‐mucoid Pseudomonas aeruginosa biofilms. To our surprise, we found that the positively charged antibiotic tobramycin is sequestered to the biofilm periphery, while the neutral antibiotic ciprofloxacin readily penetrated. We provide evidence that tobramycin in the biofilm periphery both stimulated a localized stress response and killed bacteria in these regions but not in the underlying biofilm. Although it is unclear which matrix component binds tobramycin, its penetration was increased by the addition of cations in a dose‐dependent manner, which led to increased biofilm death. These data suggest that ionic interactions of tobramycin with the biofilm matrix limit its penetration. We propose that tobramycin sequestration at the biofilm periphery is an important mechanism in protecting metabolically active cells that lie just below the zone of sequestration.     

Determination of the Efflux Pump-Mediated Resistance Prevalence in Pseudomonas aeruginosa, Using an Efflux Pump Inhibitor

In Gram negative bacteria, fluoroquinolone resistance is acquired by target mutations in topoisomerase genes or by reducing the permeation of drugs due to the increase in expression of endogenous multidrug efflux pumps that expel structurally unrelated antimicrobial agents. An ongoing challenge is searching for new inhibitory substances in order to block efflux pumps and restore the antibiotic drugs susceptibility. In this research, we sought to investigate the interplay between ciprofloxacin and an efflux pump inhibitor (EPI), phenyl alanine arginyl β-naphtylamide (PAβN), to determine the prevalence of efflux pump overexpression in clinical isolates of Pseudomonas aeruginosa. Ciprofloxacin was tested at different concentrations (256–0.25 μg/ml) with a fixed concentration of PAβN (50 μg/ml). The isolates susceptibility profiles were analyzed by disc diffusion and agar dilution methods using 10 antibiotic discs and 4 powders. It was found that in the presence of PAβN, resistance to ciprofloxacin was inhibited obviously and MIC values were decreased. The comparison between subgroups of P. aeruginosa isolates with different resistance profiles indicates that efflux pump overexpression (EPO) is present in 35% of ciprofloxacin resistant isolates with no cross resistance and in variable frequencies among isolates showing cross resistance to other tested antibiotics: gentamicin (31%), ceftazidime (29%), and imipenem (18%). Altogether, these results imply that PAβN maybe effective to restore the fluoroquinolone drugs susceptibility in clinical treatment procedures. Results also show that increased use of a fluoroquinolone drug such as ciprofloxacin can affect the susceptibility of P. aeruginosa to other different antipseudomonal agents.     

Resistance to Antimicrobial Agents in Lactobacilli Isolated from Caper Fermentations

A collection of lactobacilli comprising species of Lactobacillus plantarum (43 isolates), Lactobacillus brevis (9 isolates) and Lactobacillus fermentum (6 isolates) obtained from spontaneous fermentations of capers (the fruits of Capparis spinosa) were investigated for resistance to antimicrobial agents. All isolates were resistant to vancomycin and teicoplanin (MIC > 16 μg/ml). Resistance to ciprofloxacin (MIC > 2 μg/ml) was detected in all isolates of L. brevis and L. fermentum as well as in most isolates of L. plantarum, whilst resistance to levofloxacin showed a much lower incidence. Among L. plantarum and L. brevis isolates, low levels of resistance to tetracycline and/or nitrofurantoin were detected. Higher resistance levels were also detected in some isolates. Resistance to penicillin and rifampicin were also detected among L. plantarum isolates. All isolates were sensitive to ampicillin, erythromycin, chloramphenicol, gentamicin, streptomycin, and quinupristin/dalfopristin.     

Antibiotic susceptibility of coagulase-negative staphylococci isolated from very low birth weight babies: comprehensive comparisons of bacteria at different stages of biofilm formation

Background

Coagulase-negative staphylococci are major causes of bloodstream infections in very low birth weight babies cared for in Neonatal Intensive Care Units. The virulence of these bacteria is mainly due to their ability to form biofilms on indwelling medical devices. Biofilm-related infections often fail to respond to antibiotic chemotherapy guided by conventional antibiotic susceptibility tests.

Methods

Coagulase-negative staphylococcal blood culture isolates were grown in different phases relevant to biofilm formation: planktonic cells at mid-log phase, planktonic cells at stationary phase, adherent monolayers and mature biofilms and their susceptibilities to conventional antibiotics were assessed. The effects of oxacillin, gentamicin, and vancomycin on preformed biofilms, at the highest achievable serum concentrations were examined. Epifluorescence microscopy and confocal laser scanning microscopy in combination with bacterial viability staining and polysaccharide staining were used to confirm the stimulatory effects of antibiotics on biofilms.

Results

Most coagulase-negative staphylococcal clinical isolates were resistant to penicillin G (100%), gentamicin (83.3%) and oxacillin (91.7%) and susceptible to vancomycin (100%), ciprofloxacin (100%), and rifampicin (79.2%). Bacteria grown as adherent monolayers showed similar susceptibilities to their planktonic counterparts at mid-log phase. Isolates in a biofilm growth mode were more resistant to antibiotics than both planktonic cultures at mid-log phase and adherent monolayers; however they were equally resistant or less resistant than planktonic cells at stationary phase. Moreover, for some cell-wall active antibiotics, concentrations higher than conventional MICs were required to prevent the establishment of planktonic cultures from biofilms. Finally, the biofilm-growth of two S. capitis isolates could be enhanced by oxacillin at the highest achievable serum concentration.

Conclusion

We conclude that the resistance of coagulase-negative staphylococci to multiple antibiotics initially remain similar when the bacteria shift from a planktonic growth mode into an early attached mode, then increase significantly as the adherent mode further develops. Furthermore, preformed biofilms of some CoNS are enhanced by oxacillin in a dose-dependent manner.     

分离自母婴肠道的假小链双歧杆菌的耐药性分析

【背景】由于滥用抗生素导致细菌耐药性日益严重。对于双歧杆菌,人们往往注重其益生功能的挖掘而忽视了对其耐药性的研究,存在一定的安全隐患。【目的】检测母婴肠道中假小链双歧杆菌的耐药性,探究婴儿肠道中假小链双歧杆菌耐药性的来源。【方法】利用微量肉汤稀释法测定48株分离自母婴肠道的假小链双歧杆菌对14种抗生素的耐药性,比较分离自不同家庭母婴肠道中假小链双歧杆菌的耐药性。【结果】48株母婴肠道分离株对四环素、氯霉素、新霉素、环丙沙星100%耐药,对其余10种抗生素耐药率依次为：卡那霉素98%、利福平80%、克林霉素78%、甲氧苄啶63%、红霉素59%、庆大霉素43%、链霉素16%、万古霉素14%、氨苄西林6%、利奈唑胺2%。母婴肠道分离株的耐药性无显著差异,分离自同一家庭母婴肠道的菌株具有相似的耐药表型。【结论】分离自母婴肠道的假小链双歧杆菌对多种抗生素具有耐药性,婴儿肠道中假小链双歧杆菌的耐药性可能是由母亲肠道垂直传递而来。     

High antimicrobial resistance among bacterial isolates of blood stream infections (BSI) in a Nigerian University Teaching Hospital

The antimicrobial resistance profile of 220 bacteria isolated from 1,006 episodes of blood stream infections (BSI) between January 2004 and December 2005 in a University Teaching Hospital, Southwestern Nigeria, were analyzed. Gram positive bacteria constituted 47.3% while Gram negative constituted 52.7%. The most common organisms were Staphylococcus aureus (37.3%), Klebsiella (30%), Pseudomonas (8.2%), Proteus (6.4%), Escherichia coli (5.5%) and coagulase negative staphylococci (4.6%). The cumulative resistance of all the bacteria isolates to ampicillin was 79%, gentamicin 51%, ceftazidime 11% and ciprofloxacin 6%. About 85% of the Gram positive bacteria were resistant to penicillinG, 79% to methicillin and 37% to erythromycin while 74% of the Gram negative bacteria were resistant to cotrimoxazole, 69% to tetracycline and 38% to chloramphenicol. Among the 7 antibiotics tested for each group, 7 patterns of antibiotic resistance were observed for each; 6 were multi-drug pattern with number of antibiotics ranging from 2 to 7. This study demonstrates high antimicrobial resistance among clinical bacterial isolates of BSI to commonly prescribed antibiotics most especially penicillinG, ampicillin, methicillin, cotrimoxazole, tetracycline and gentamicin. Based on the result of this study, it is suggested that the combination of ampicillin and gentamicin normally employed for empirical treatment of BSI in our hospital should be stopped.     

Characterisation of potential virulence markers in <Emphasis Type="Italic">Pseudomonas aeruginosa</Emphasis> isolated from drinking water

Pseudomonas aeruginosa isolates from tap water, mineral water, and artesian well water were investigated for their ability to produce different potential virulence factors or markers such as hemolysins, hemaglutinins, cytotoxins and their ability to adhere to epithelial cells and to abiotic surfaces. The susceptibility to antibiotics, human serum sensitivity and the survival of P. aeruginosa isolates in a chlorinated environment were also examined. Of the 30 isolates tested, 16 possessed the capacity to adhere to abiotic surfaces, and 28 to adhere to epithelial cells; 30 were capable of producing hemolysins, 27 produced cytotoxins, 9 hemagglutinins, and 18 were classified as serum-resistant. For the lowest concentration of chlorine (0.2 mg/l) tested, no killing of biofilm bacteria could be discerned, even after prolonged exposure to the agent. Although all the drinking water isolates were susceptible to aztreonam, cefepime, ceftazidime, ciprofloxacin, imipenem, meropenem, piperacillin-tazobactam, and polymyxin, the P. aeruginosa isolates were resistant to one or more antibiotics. The increasing prevalence of resistance in the isolates from environmental sources may have important therapeutic implications. A notable proportion of the P. aeruginosa isolates from drinking water were able to develop virulence factors, and the incidence of virulence properties was not statistically different among the three sources. A more extensive study of the virulence properties of this bacterium by toxic assays on animals should be explored. Still more interesting would be toxicity assays on immuno-deficient animals with isolates from drinking water in order to better understand the health risk these bacteria may present.     

Mechanisms of antimicrobial resistance and genetic relatedness among enterococci isolated from dogs and cats in the United States

Aims: In this study, mechanisms of antimicrobial resistance and genetic relatedness among resistant enterococci from dogs and cats in the United States were determined. Methods and Results: Enterococci resistant to chloramphenicol, ciprofloxacin, erythromycin, gentamicin, kanamycin, streptomycin, lincomycin, quinupristin/dalfopristin and tetracycline were screened for the presence of 15 antimicrobial resistance genes. Five tetracycline resistance genes [tet(M), tet(O), tet(L), tet(S) and tet(U)] were detected with tet(M) accounting for approx. 60% (130/216) of tetracycline resistance; erm(B) was also widely distributed among 96% (43/45) of the erythromycin‐resistant enterococci. Five aminoglycoside resistance genes were also detected among the kanamycin‐resistant isolates with the majority of isolates (25/36; 69%) containing aph(3′)‐IIIa. The bifunctional aminoglycoside resistance gene, aac(6′)‐Ie‐aph(2″)‐Ia, was detected in gentamicin‐resistant isolates and ant(6)‐Ia in streptomycin‐resistant isolates. The most common gene combination among enterococci from dogs (n = 11) was erm(B), aac(6′)‐Ie‐aph(2″)‐Ia, aph(3′)‐IIIa, tet(M), while tet(O), tet(L) were most common among cats (n = 18). Using pulsed‐field gel electrophoresis (PFGE), isolates clustered according to enterococcal species, source and antimicrobial gene content and indistinguishable patterns were observed for some isolates from dogs and cats. Conclusion: Enterococci from dogs and cats may be a source of antimicrobial resistance genes. Significance and Impact of the Study: Dogs and cats may act as reservoirs of antimicrobial resistance genes that can be transferred from pets to people. Although host‐specific ecovars of enterococcal species have been described, identical PFGE patterns suggest that enterococcal strains may be exchanged between these two animal species.     

Studies of two Frankia strains isolated from Trevoa trinervis Miers

The Frankia strains TtI 11 and TtI 12 isolated from T. trinervis Miers were characterized regarding their carbon source utilization, intrinsic antibiotic resistance, infectivity, and effectivity on the original host. Both strains grew on BAP medium supplemented with glucose, maltose, and sucrose, but differed in their ability to use other carbon sources such as propionate, pyruvate, acetate, succinate, citrate, and mannitol. The isolates were sensitive to five of the twelve antibiotics tested at 1 μg mL⁻¹ concentration: chloramphenicol, tobramycin, eritromycin, streptomycin, and rifampicin. They exhibited a variable degree of resistance at 1 μg mL⁻¹ concentraction to penicillin G, 4-fluorouracil, oleandomycin, and lincomycin. Both isolates were able to infect and nodulate the original host plant, and thus represent the first reported infective and effective microsymbionts for T. trinervis Miers, a rhamnaceous actinorhizal host. R O D Dixon Section editor     

Isolation of Tn1546-like elements in vancomycin-resistant Enterococcus faecium isolated from wood frogs: an emerging risk for zoonotic bacterial infections to humans

Aim: Isolation and characterization of vancomycin‐resistant enterococci (VRE), mainly Enterococcus faecium, from the faecal pellet of wood frogs (Rana sylvatica). Methods and Results: The frog VRE isolates were tested for their susceptibility to various antibiotics and were found resistant to ampicillin (Am), chloramphenicol (Cm), erythromycin (Em), gentamicin (Gm), tetracycline (Tc), teicoplanin (Tp) and vancomycin (Vn). The linkage of multiple antibiotic resistances to Em, Tc, Tp and Vn was observed in 84% of resistant Ent. faecium. Inducible antibiotic resistance (MIC ≥ 512 μg ml^?1) to Vn was also detected in these isolates. PCR analysis revealed the presence of vanA in all strains, and none of the strains were positive for vanB, indicating the existence of vanA phenotype. Furthermore, the PCR–RFLP analysis of the frog vanA amplicon with PstI, BamHI and SphI generated identical restriction patterns similar to Tn1546‐like elements found in human VRE isolates. DNA homoduplex analysis also confirmed that vanA from the frog VRE has DNA sequence homology with the vanA of Tn1546‐like elements of human and animal isolates. Blastx analysis of frog vanA sequence showed similarities with protein sequences generated from protein database of Vn‐resistant Ent. faecium, Baccilus circulans, Paenibacillus apiarius and Oerskovia turbata isolates. Horizontal transfer of Vn resistance was not detected in frog isolates as revealed by filter mating conjugal experiment. Conclusions: In summary, our results demonstrated that wood frogs carry Vn‐resistant bacteria, and resistance genes (vanA) are located on Tn1546‐like elements. Significance and Impact of the Study: This study highlights a previously less recognized role of amphibians as sentinels for multidrug‐resistant bacteria and alerts the public health workers for an emerging risk of zoonotic bacterial infections to humans.     

Population diversity in model potable water biofilms receiving chlorine or chloramine residual

Abstract

Most water utilities use chlorine or chloramine to produce potable water. These disinfecting agents react with water to produce residual oxidants within a water distribution system (WDS) to control bacterial growth. While monochloramine is considered more stable than chlorine, little is known about the effect it has on WDS biofilms. Community structure of 10-week old WDS biofilms exposed to disinfectants was assessed after developing model biofilms from unamended distribution water. Four biofilm types were developed on polycarbonate slides within annular reactors while receiving chlorine, chloramine, or inactivated disinfectant residual. Eubacteria were identified through 16S rDNA sequence analysis. The model WDS biofilm exposed to chloramine mainly contained Mycobacterium and Dechloromonas sequences, while a variety of alpha- and additional beta-proteobacteria dominated the 16S rDNA clone libraries in the other three biofilms. Additionally, bacterial clones distantly related to Legionella were found in one of the biofilms receiving water with inactivated chlorine residual. The biofilm reactor receiving chloraminated water required increasing amounts of disinfectant after 2 weeks to maintain chlorine residual. In contrast, free chlorine residual remained steady in the reactor that received chlorinated water. The differences in bacterial populations of potable water biofilms suggest that disinfecting agents can influence biofilm development. These results also suggest that biofilm communities in distribution systems are capable of changing in response to disinfection practices.     

Species distribution,virulence factors and antimicrobial resistance of Acinetobacter spp. isolates from dogs and cats: a preliminary study

In this study, the prevalence of virulence factors and antimicrobial resistance among 67 Acinetobacter spp. isolates, consisting of 21 Acinetobacter baumannii and 46 non‐baumannii Acinetobacter obtained from companion animals, was investigated. PCR analysis showed that the most prevalent virulence gene was afa/draBC (29.9%), followed by papC (22.4%) and cvaC (20.9%). Antimicrobial susceptibility testing revealed that resistance to gentamicin (14.9%) and ciprofloxacin (11.9%) was relatively prevalent. Five gentamicin‐ and/or ciprofloxacin‐resistant A. baumannii strains were assigned to ST25, ST149, ST164, ST203 and ST1198. All ciprofloxacin‐resistant isolates harbored point mutations in gyrA and/or parC. To the best of our knowledge, this is the first report of preliminary monitoring of animal‐origin Acinetobacter spp. in Japan.

     

Evaluation of the In Vitro Activity of Tobramycin as Compared with That of Gentamicin Sulfate

The in vitro activity of tobramycin was quantitatively compared with that of gentamicin sulfate against 195 bacterial isolates from clinical material. Tobramycin was found to be twice as active as gentamicin against isolates of Pseudomonas aeruginosa. Conversely, gentamicin proved fourfold more active than tobramycin against isolates of Serratia marcescens. Both drugs were of comparable activity against isolates of Staphylococcus aureus and the majority of the enterobacterial isolates other than S. marcescens. On the basis of the obtained data, the following criteria are proposed for the interpretation of diffusion susceptibility tests with 10-μg discs of gentamicin and tobramycin. Enterobacteriaceae and isolates of S. aureus are designated as susceptible to gentamicin and tobramycin if the zones of inhibition measure 15 mm or more in diameter; zones of 14 mm or less are indicative of resistance. Pseudomonadaceae are interpreted as sensitive to tobramycin and gentamicin if the inhibition zones measure at least 15 and 12 mm in diameter, respectively.     

Selecting for development of fluoroquinolone resistance in a Campylobacter jejuni strain 81116 in chickens using various enrofloxacin treatment protocols

Aims: To determine the effect of various enrofloxacin dose regimes on the colonization and selection of resistance in Campylobacter jejuni strain 81116P in experimentally colonized chickens. Methods and Results: Two experiments were undertaken, in which 14‐day‐old chickens were colonized with 1 × 10⁷–1 × 10⁹ CFU g^?1Camp. jejuni strain 81116P and then treated with enrofloxacin at 12–500 ppm in drinking water for various times. Caecal colonization levels were determined at various time‐points after start‐of‐treatment, and the susceptibility of recovered isolates to ciprofloxacin was monitored. Resistance was indicated by growth on agar containing 4 μg ml^?1 ciprofloxacin, MICs of 16 μg ml^?1 and the Thr86Ile mutation in gyrA. Enrofloxacin at doses of 12–250 ppm reduced Camp. jejuni colonization over the first 48–72 h after start‐of‐treatment. The degree of reduction in colonization was dose, but not treatment time, dependent. In all cases, maximal colonization was re‐established within 4–6 days. Fluoroquinolone‐resistant organisms were recoverable within 48 h of start‐of‐treatment; after a further 24 h all recovered isolates were resistant. In contrast, a dose of 500 ppm enrofloxacin reduced colonization to undetectable levels within 48 h, and the treated birds remained Campylobacter negative throughout the remaining experimental period. By high pressure liquid chromatography, for all doses, the maximum concentrations of enrofloxacin and ciprofloxacin in the caecal contents were detected at the point of treatment completion. Thereafter, levels declined to undetectable by 7 days post‐treatment withdrawal. Conclusions: In a model using chickens maximally colonized with Camp. jejuni 81116P, treatment with enrofloxacin, at doses of 12–250 ppm in drinking water, enables the selection, and clonal expansion, of fluoroquinolone‐resistant organisms. However, this is preventable by treatment with 500 ppm of enrofloxacin. Significance and impact of the study: Treatment of chickens with enrofloxacin selects for resistance in Camp. jejuni in highly pre‐colonized birds. However, a dose of 500 ppm enrofloxacin prevented the selection of resistant campylobacters.