Genome analysis of third-generation cephalosporin-resistant Escherichia coli and Salmonella species recovered from healthy and diseased food-producing animals in Europe

The animal reservoir of Enterobacterales producing Extended-Spectrum-β-Lactamases (ESBL) and plasmid-borne cephalosporinases (pAmpC) is a global concern. Using genome data, we analyzed a population of Escherichia coli and Salmonella species resistant to third-generation cephalosporins (3GC-R) recovered from healthy food animals (HA) and diseased food animals (DA) across Europe. Among the isolates collected from HA (n = 4,498) and DA (n = 833) in up to twelve European countries, 62 (1.4%) and 45 (5.4%) were 3GC-R, respectively. The genomes of these 3GC-R 107 isolates were sequenced to identify blaESBL and blaAmpC, sequence types (STs), virulence-associated genes, and Salmonella serovars. We also assessed their population structure using core genome multilocus sequence typing. The 78 3GC-R Escherichia coli originated from poultry (n = 27), swine (n = 26), and cattle (n = 25). Almost all (n = 77; 98.7%) harbored at least one blaESBL or blaAmpC, with blaCTX-M-1 predominating. We identified 51 STs, with ST10 and ST101 being the most frequent. The population of 3GC-R E. coli was polyclonal. The 29 3GC-R Salmonella spp. were mostly retrieved from healthy broiler (96.5%). blaCMY-2 dominated in this population. We found two clusters of CMY-2-producing Salmonella spp. in Germany: one with 15 isolates of S. Heidelberg isolates and another with six S. Minnesota, all of them with blaCMY-2. Our results confirm the low prevalence of 3GC-R E. coli and Salmonella spp. in HA and DA. blaCTX-M-1 was dominating in a highly diverse population of E. coli. 3GC-R E.coli isolated from HA and DA were genetically unrelated, with high clonal diversity suggesting multiple origins of contamination. This contrasted with the clonal population of 3GC-R Salmonella spp. in which blaCMY-2 dominated through two dominant serovars in this collection.

Antimicrobial susceptibility among respiratory tract pathogens isolated from diseased cattle and pigs from different parts of Europe

AIMS

To survey antibiotic susceptibility of bacteria causing cattle and pig respiratory infections in 10 European countries.

METHODS AND RESULTS

Non-replicate nasopharyngeal/nasal or lung swabs were collected from animals with acute respiratory signs during 2015-2016. Pasteurella multocida, Mannheimia haemolytica, Histophilus somni from cattle (n = 281), and P. multocida, Actinobacillus pleuropneumoniae, Glaesserella parasuis, Bordetella bronchiseptica, and Streptococcus suis from pigs (n = 593) were isolated. MICs were assessed following CLSI standards and interpreted using veterinary breakpoints where available. Histophilus somni isolates were fully antibiotic susceptible. Bovine P. multocida and M. haemolytica were susceptible to all antibiotics, except tetracycline (11.6%-17.6% resistance). Low macrolide and spectinomycin resistance was observed for P. multocida and M. haemolytica (1.3%-8.8%). Similar susceptibility was observed in pigs, where breakpoints are available. Resistance in P. multocida, A. pleuropneumoniae, and S. suis to ceftiofur, enrofloxacin, and florfenicol was absent or <5%. Tetracycline resistance varied from 10.6% to 21.3%, but was 82.4% in S. suis. Overall multidrug-resistance was low. Antibiotic resistance in 2015-2016 remained similar as in 2009-2012.

CONCLUSIONS

Low antibiotic resistance was observed among respiratory tract pathogens, except for tetracycline.

European-wide antimicrobial resistance monitoring in commensal Escherichia coli isolated from healthy food animals between 2004 and 2018

OBJECTIVES

To describe the susceptibility of Escherichia coli to medically important antibiotics, collected over four periods (2004-2006, 2008-2009, 2013-2014, 2017-2018), from food-producing animals at slaughter.

METHODS

Intestinal contents from cattle, pigs and broilers were randomly sampled (5-6 countries/host; ≥4 abattoirs/country; one sample/animal/farm) for isolation of Escherichia coli; antimicrobial susceptibilities were centrally determined by CLSI agar dilution. Clinical breakpoints (CLSI) and epidemiological cut-off values (EUCAST) were applied for data interpretation.

RESULTS

In total, 10 613 E. coli strains were recovered. In broilers, resistance percentages were the lowest (P ≤ 0.01) in the latest time period. A significant decrease in MDR over time was also observed for broilers and a tendency for a decrease for pigs. Resistance to meropenem and tigecycline was absent, and resistance to azithromycin was 0.2%-2.0%. Also, low resistance to third-generation cephalosporins (1.1%-7.4%) was detected in broilers. Resistance to colistin varied between 0.1%-4.8%. E. coli from broilers showed high resistance to ciprofloxacin (7.3%-23.3%), whereas for cattle and pigs this was 0.2%-2.5%. Low/moderate resistance to chloramphenicol (9.3%-21.3%) and gentamicin (0.9%-7.0%) was observed in pigs and broilers. The highest resistance was noted for ampicillin (32.7%-65.3%), tetracycline (41.3%-67.5%), trimethoprim (32.0%-35.7%) and trimethoprim/sulfamethoxazole (27.5%-49.7%) from pigs and broilers, with marked country differences. MDR peaked in pigs and broilers with 24 and 26 phenotypes, with 21.9%-26.2% and 18.7%-34.1% resistance, respectively.

CONCLUSIONS

In this pan-EU survey antibiotic susceptibility of commensal E. coli varied largely between antibiotics, animal species and countries. Resistance to critically important antibiotics for human medicine was absent or low, except for ciprofloxacin in broilers and ampicillin in pigs and broilers.

Genomic Diversity and Virulence Potential of ESBL- and AmpC-β-Lactamase-Producing Escherichia coli Strains From Healthy Food Animals Across Europe

The role of livestock animals as a putative source of ESBL/pAmpC E. coli for humans is a central issue of research. In a large-scale pan-European surveillance, 2,993 commensal Escherichia spp. isolates were recovered from randomly collected fecal samples of healthy cattle, pigs and chickens in various abattoirs. One-hundred Escherichia spp. isolates (0.5% from cattle, 1.3% pigs, 8.0% chickens) fulfilled the criteria for cefotaxime and ceftazidime non-wildtype (EUCAST). In silico screening of WGS data of 99 isolates (98 E. coli and 1 E. fergusonii) revealed bla_SHV–12 (32.3%), bla_CTX–M–1 (24.2%), and bla_CMY–2 (22.2%) as predominant ESBL/pAmpC types. Other types were bla_SHV–2 (1.0%), bla_{CTX–M–2/–14/–15} (1.0/6.1/1.0%), and bla_TEM–52 (5.1%). Six isolates revealed AmpC-promoter mutations (position −42 (C > T) and one carried mcr-1. The majority (91.3%) of ESBL/pAmpC genes were located on plasmids. SHV-12 was mainly (50%) encoded on IncI1α plasmids (pST-3/-26/-95), followed by IncX3 (12.5%) and IncK2 (3.1%). The bla_TEM–52 genes were located on IncI1α-pST-36 (60%) and IncX1 plasmids (20%). The dominant plasmid lineage among CTX-M-1 isolates was IncI1α (pST-3/-295/-317) (87.5%), followed by IncN-pST-1 (8.3%). CMY-2 was mostly identified on IncI1α (pST-12/-2) (54.5%) and IncK2 (31.8%) plasmids. Several plasmids revealed high similarity to published plasmids from human and animal Enterobacteriaceae. The isolates were assigned to phylogroups A/C (34.7/7.1%), B1 (27.6%), B2 (3.1%), D/F (9.2/10.2%), E (5.1%), and to E. clades (3.0%). With 51 known and 2 novel MLST types, a wide variety of STs was found, including STs previously observed in human isolates (ST10/38/117/131/648). ESBL/AmpC types or STs were rarely correlated with the geographic origin of the isolates or animal species. Virulence gene typing identified extraintestinal pathogenic E. coli (ExPEC; 2.0%), avian pathogenic E. coli (APEC; 51.5%), and atypical enteropathogenic E. coli (EPEC; 6.1%). In conclusion, the high diversity of STs and phylogenetic groups provides hardly any hint for clonal spread of single lineages but hints toward the dissemination of cephalosporin resistance genes in livestock via distinct, globally successful plasmid lineages. Even though a number of isolates could not be assigned to a distinct pathotype, our finding of combined multidrug-resistance and virulence in this facultative pathogen should be considered an additional threat to public health.

Minimal inhibitory concentration of seven antimicrobials to Mycoplasma gallisepticum and Mycoplasma synoviae isolates from six European countries

Mycoplasma gallisepticum and Mycoplasma synoviae are bacterial pathogens that cause disease in poultry, adversely affecting their health and welfare, and are a financial burden on producers. This manuscript describes the results of the MycoPath project that is the first international antimicrobial susceptibility programme for mycoplasma pathogens isolated from poultry. Improved comparative analysis of minimal inhibitory concentration (MIC) results from participating countries was facilitated by using one laboratory determining all MICs. Chicken and turkey isolates were obtained from France, Germany, Great Britain, Hungary, Italy and Spain during 2014-2016. One isolate per farm was retained. The MIC of seven antimicrobial agents was determined using a broth microdilution method, with Friis Medium (M. gallisepticum) or Modified Chanock's Medium (M. synoviae). Of the 222 isolates recovered, 82 were M. gallisepticum and 130 were M. synoviae. M. gallisepticum MIC_50/90 values were 0.12/0.5, 2/8, 0.5/4, 0.12/>64, 0.008/0.062, 0.008/32, 0.062/4 mg/l for doxycycline, enrofloxacin, oxytetracycline, spiramycin, tiamulin, tilmicosin and tylosin, respectively. For M. synoviae, the values were 0.5/1, 8/16, 0.5/1, 0.5/8, 0.25/0.5, 0.062/2 and 0.062/16 mg/l respectively. A bimodal MIC distribution for the fluoroquinolone (enrofloxacin) and the macrolides (spiramycin, tilmicosin and tylosin) indicate that both species have sub-populations that are less susceptible in vitro to those antimicrobials. Some differences in susceptibilities were observed according to host species, Mycoplasma species, and country of origin. This study provides a baseline of novel data for future monitoring of antimicrobial resistance in poultry Mycoplasma species. Additionally, this information will facilitate the selection of the antimicrobial agents most likely to be effective, thus ensuring their minimal use with targeted and correct therapeutic treatments.Highlights First large-scale pan-European collection of representative Mg and Ms isolates.MIC values assessed in central laboratory for Mg and Ms from chickens and turkeys.Range of MIC values for 82 Mg and 130 Ms isolates to seven licenced antibiotics shown.Data can be used to help determine Mg and Ms veterinary-specific breakpoints.

Antimicrobial susceptibility monitoring of Mycoplasma hyopneumoniae isolated from seven European countries during 2015-2016

Mycoplasma hyopneumoniae is the causative agent of porcine enzootic pneumonia, a chronic respiratory disease, causing significant economic losses. Results from the 2015-2016 MycoPath pan-European antimicrobial susceptibility monitoring survey of M. hyopneumoniae are presented. In total, 147 M. hyopneumoniae porcine isolates from Belgium, France, Germany, Great Britain, Hungary, Italy, and Spain were tested. One isolate per farm was retained from pigs that had not been recently treated with antimicrobial agents. The minimal inhibitory concentration (MIC) of 13 antimicrobial agents was determined in a central laboratory using a broth microdilution method, with Friis Medium, incubated at 35 ± 1 °C for 5-12 days. M. hyopneumoniae NCTC 10110 was used as Quality Control. MIC₅₀/MIC₉₀ (mg/L) values were: enrofloxacin 0.06/1; marbofloxacin 0.06/2; spiramycin 0.06/0.25; tulathromycin ≤0.001/0.004; gamithromycin 0.06/0.5; tylosin 0.016/0.06; tilmicosin 0.06/0.5; florfenicol 0.5/1; doxycycline 0.25/1; oxytetracycline 0.25/2; lincomycin 0.06/0.25; tiamulin 0.016/0.06 and valnemulin ≤0.001/0.004. Compared with the data from 2010 to 2012 MycoPath study (50 isolates), MIC_50/90 results were similar and the majority were within ± two dilution steps, except for the MIC₅₀ of oxytetracycline which is more than two dilution steps higher in the present study. Between-country comparisons show some differences in the MIC values for the fluoroquinolones, tulathromycin and tylosin, but the limited sample size per country precludes performing meaningful country comparisons for several countries. Standardized laboratory methods and interpretive criteria for MIC testing of veterinary mycoplasmas are clearly needed; there are currently no clinical breakpoints available to facilitate data interpretation and correlation of MICs with in vivo efficacy.

Determination of the pharmacokinetic-pharmacodynamic cut-off values of marbofloxacin in horses to support the establishment of a clinical breakpoint for antimicrobial susceptibility testing

BACKGROUND

Marbofloxacin (MBX), a fluoroquinolone (FQ), is considered as a critical antibiotic requiring antimicrobial susceptibility testing (AST) for prudent use. No clinical breakpoint (CBP) currently exists to interpret the results of such tests in horses.

OBJECTIVES

To compute PK/PD cut-offs (PK/PD_CO ) that is one of the three minimum inhibitory concentrations (MICs) considered establishing a CBP for antimicrobial susceptibility test interpretation.

STUDY DESIGN

A meta-analysis conducted by combining five sets of previously published pharmacokinetic data, obtained in clinical and nonclinical settings.

METHODS

Horses (n = 131) received MBX intravenously at doses of either 2 or 10 mg/kg BW. They were richly sampled (five or six samples per horse). A population model was built to generate a virtual population of 5000 MBX disposition curves by Monte Carlo simulations (MCS) over 24 hours. The selected PK/PD index was the ratio of Area Under the free plasma concentration-time Curve divided by the MIC (fAUC/MIC). The PK/PD_CO , which is the highest MIC for which 90% of horses can achieve an a priori selected critical value for the numerical value of the PK/PD index, was established for Gram-positive and Gram-negative bacteria for a dose of 2 mg/kg.

RESULTS

The PK/PD_CO of MBX in horses was 0.125 mg/L for Gram-positive pathogens and 0.0625 mg/L for Gram-negative pathogens. MBX MICs determined by broth microdilution for 54 Escherichia coli and 189 Streptococcus equi isolates are reported.

MAIN LIMITATION

No clinical data are taken into account in the determination of a PK/PD_co .

CONCLUSION

The computed PK/PD_co predicts that MBX may be efficacious in horses to treat infections associated with Enterobacteriaceae but unlikely to those involving Staphylococcus aureus or Streptococcus equi.

Antimicrobial susceptibility monitoring of canine and feline skin and ear pathogens isolated from European veterinary clinics: results of the ComPath Surveillance programme

BACKGROUND

The ComPath project is a pan-European programme dedicated to the monitoring of antimicrobial susceptibility of canine and feline pathogens using standardized methods and centralized minimal inhibitory concentration (MIC) determination.

OBJECTIVES

To report antimicrobial susceptibilities of major pathogens isolated from nontreated animals with acute clinical signs of skin, wound or ear infections in 2013-2014.

METHODS AND MATERIALS

MICs were determined by agar dilution for commonly used drugs and interpreted using Clinical and Laboratory Standards Institute (CLSI) breakpoints, if available.

RESULTS

Of 1,676 isolates recovered, the main species isolated from dogs were Staphylococcus pseudintermedius, followed by Streptococcus spp., Pseudomonas aeruginosa and Escherichia coli. In cats, Pasteurella multocida, coagulase-negative staphylococci (CoNS) and Staphylococcus aureus were isolated most frequently. Resistance rates observed for S. pseudintermedius were <26.7% for penicillin, clindamycin and chloramphenicol, and ≤11.5% for ampicillin, amoxicillin/clavulanate, cefalexin, cefovecin, gentamicin and fluoroquinolones. For S. aureus, resistance rates ranged up to 90.9% for β-lactams, and were 19.7% for clindamycin, 27% for fluoroquinolones and 0.0-6.1% for other drugs. The mecA gene was confirmed by PCR in 10.6% of S. pseudintermedius, 11.6% of CoNS and 31.4% of S. aureus isolates. In streptococci/enterococci, resistance to penicillin, ampicillin and chloramphenicol ranged from 0.0% to 11.3%, whereas fluoroquinolone resistance ranged from 0.0% to 8.5%. For E. coli, resistance ranged from 13.8 to 15.9% for fluoroquinolones and from 86.2% to 100.0% for β-lactams. Low rates of resistance (0.0-6.3%) were observed in P. multocida, and for P. aeruginosa resistance to gentamicin was 10.3%.

CONCLUSION

Overall, antimicrobial resistance of cutaneous/otic pathogens isolated from dogs and cats was low (1-10%) to moderate (10-20%). For several pathogens, the paucity of CLSI recommended breakpoints for veterinary use is a bottleneck.

Antimicrobial susceptibility of nine udder pathogens recovered from bovine clinical mastitis milk in Europe 2015-2016: VetPath results

VetPath is an ongoing pan-European antimicrobial susceptibility monitoring programme collecting pathogens from diseased cattle, pigs and poultry not recently treated with antibiotics. Non-duplicate isolates (n = 1244) were obtained from cows with acute clinical mastitis in eight countries during 2015-2016 for centrally antimicrobial susceptibility testing according CLSI standards. Among Escherichia coli (n = 225), resistance was high to ampicillin and tetracycline, moderate to kanamycin and low to amoxicillin/clavulanic acid and cefazolin. The MIC_50/90 of danofloxacin, enrofloxacin and marbofloxacin were 0.03 and 0.06 μg/mL. For Klebsiella spp. (n = 70), similar results were noted, except for ampicillin and kanamycin. We detected 3.7 % (11/295) Enterobacteriaceae isolates carrying an ESBL/AmpC gene. Staphylococcus aureus (n = 247) and coagulase-negative staphylococci (CoNS; n = 189) isolates were susceptible to most antimicrobials tested except to penicillin (25.1 and 29.1 % resistance). Two S. aureus and thirteen CoNS isolates harboured mecA gene. Streptococcus uberis isolates (n = 208) were susceptible to β-lactam antibiotics (87.1-94.7 % susceptibility), 23.9 % were resistant to erythromycin and 37.5 % to tetracycline. Resistance to pirlimycin was moderate. For Streptococcus dysgalactiae (n = 132) the latter figures were 10.6 and 43.2 %; pirlimycin resistance was low. MIC values for Streptococcus agalactiae, Trueperella pyogenes and Corynebacterium spp. were generally low. This current VetPath study shows that mastitis pathogens were susceptible to most antimicrobials with exceptions of staphylococci against penicillin and streptococci against erythromycin or tetracycline. For most antimicrobials, the percentage resistance and MIC_50/90 values among the major pathogens were comparable to that of the preceeding VetPath surveys. This work highlights the high need to set additional clinical breakpoints for antimicrobials frequently used to treat mastitis.

Genome analysis of enterobacteriaceae with non-wild type susceptibility to third-generation cephalosporins recovered from diseased dogs and cats in Europe

BACKGROUND

Extended-spectrum-β-lactamases (ESBL) and plasmid-mediated cephalosporinases (pAmpC)-producing Enterobacteriaceae isolates are now reported worldwide in humans, animals, and in the environment. We identified the determinants of resistance to β-lactams and associated resistance genes as well as phylogenetic diversity of 53 ESBL- or pAmpC-producing Enterobacteriaceae isolated from dogs and cats in Europe.

MATERIALS/METHODS

Of a collection of 842 Enterobacteriaceae isolates that were recovered in 2013 and 2014 from 842 diseased and untreated dogs and cats, for 242 ampicillin or amoxicillin resistant isolates (MIC ≥ 16 mg/L), cefotaxime (CTX) and ceftazidime (CAZ) MICs were determined. Isolates with CTX and/or CAZ MIC ≥ 1 mg/L (n = 63) were selected, and their genomes were fully sequenced using Illumina Technology. Genomic data were explored to identify the resistance determinants, the plasmid incompatibility groups, and the sequence types (STs). Plasmid location of bla_ESBL and bla_AmpC was evaluated for all isolates based on the co-localization of resistance and plasmid incompatibility group genes on the same contig. Phylogenetic trees were constructed using core-genome MLST.

RESULTS

Of the 63 sequenced isolates, 53 isolates harbored a bla_ESBL or bla_AmpC gene. Ten CTX and/or CAZ non-wild type isolates had neither bla_ESBL nor bla_AmpC. Among the 63 isolates, 44 (69.8 %) were Escherichia coli, 11 (17.5 %) were Klebsiella pneumoniae, and 8 (12.7 %) were Proteus mirabilis. Fifty-one (80.9 %) isolates originated from dogs and 12 (19.1 %) from cats. Isolates were sampled from urinary tract (n = 36), skin and soft tissue (n = 22) and respiratory tract infections (n = 5). Thirty-two isolates (32/53, 60.4 %) carried bla_ESBL genes, including bla_CTX-M-15 (n = 12), bla_CTX-M-14 (n = 6), bla_CTX-M-1 (n = 5), bla_CTX-M-2 (n = 3), bla_CTX-M-27 (n = 3), bla_SHV-28 (n = 4), bla_SHV-12 (n = 2), and bla_VEB-6 (n = 1). Four isolates of K. pneumoniae had both bla_CTX-M-15 and bla_SHV-28. Twenty-one isolates (21/53, 39.6 %) carried genes encoding pAmpC, including bla_CMY-2 (n = 19) and bla_DHA-1 (n = 2). Thirteen E. coli isolates harbored both bla_ESBL or bla_AmpC genes and plasmids of incompatibility groups IncIB (9/13), IncI1 (8/13), and IncFII (6/13). In addition to the reduced susceptibility to CTX and/or CAZ, reduced susceptibility or evidence of acquired resistance to at least one other relevant class of antibiotics was observed for all 63 isolates. E. COLI: isolates clustered in 23 STs, including B2 virulent clones from humans such as ST131 (n = 5), K. pneumoniae isolates mostly clustered in 3 STs: ST11 (n = 4), ST307 (n = 3), and ST16 (n = 2). Phylogenetic analysis identified the spread of E. coli ST131 bla_CTX-M-27, and of K. pneumoniae ST307 harboring bla_CTX-M-15 and bla_SHV-28 or ST11 bla_CTX-M-15.

CONCLUSIONS

We report here a 6.3 % prevalence of ESBL/pAmpC producing Enterobacteriaceae in diseased dogs and cats. This EU survey confirms that dogs and cats can be infected with epidemic multidrug resistant clones that may also spread in humans.

New antimicrobial susceptibility data from monitoring of Mycoplasma bovis isolated in Europe

Mycoplasma bovis is an important respiratory pathogen of cattle across Europe and is included in the MycoPath pan-European antimicrobial susceptibility monitoring programme. M. bovis strains (232) were isolated from cattle, not recently treated with antimicrobials, at diverse geographical locations in France, Great Britain, Hungary, Italy and Spain during 2014 to 2016. Only one isolate per farm and per outbreak was retained. For each isolate, the MICs of ten antimicrobials were determined in a central laboratory using a broth microdilution method with modified Eaton's medium and incubation at 35 °C ± 1 °C for 24 ± 6 h. MIC₅₀/MIC₉₀ (mg/L) values for the 232 strains were: danofloxacin 0.25/1; enrofloxacin 0.5/8; marbofloxacin 1/4; gamithromycin >64/>64; spiramycin 8/16; tilmicosin >64/>64; tulathromycin >64/>64; tylosin 64/>64; florfenicol 4/8; oxytetracycline 8/32. Minor between-country differences in the MIC₉₀ values were observed for the fluoroquinolones, spiramycin and oxytetracycline, whilst the MIC values for the other compounds were similar. Spain and Italy had the higher MIC₉₀ values for the fluoroquinolones. Compared with the 2010-2012 study (156 isolates) results are similar, with an overall MIC₅₀ increase of at most one doubling dilution for enrofloxacin, spiramycin, tylosin, florfenicol and oxytetracycline. In contrast, the MIC₉₀ value for oxytetracycline decreased from >64 to 32 mg/L. Standardized laboratory methods and interpretive criteria for MIC testing of veterinary mycoplasmas are clearly needed; there are currently no clinical breakpoints available to facilitate data interpretation and correlation of MICs with in vivo efficacy.

Comparison of in vitro static and dynamic assays to evaluate the efficacy of an antimicrobial drug combination against Staphylococcus aureus

An easily implementable strategy to reduce treatment failures in severe bacterial infections is to combine already available antibiotics. However, most in vitro combination assays are performed by exposing standard bacterial inocula to constant concentrations of antibiotics over less than 24h, which can be poorly representative of clinical situations. The aim of this study was to assess the ability of static and dynamic in vitro Time-Kill Studies (TKS) to identify the potential benefits of an antibiotic combination (here, amikacin and vancomycin) on two different inoculum sizes of two S. aureus strains. In the static TKS (sTKS), performed by exposing both strains over 24h to constant antibiotic concentrations, the activity of the two drugs combined was not significantly different the better drug used alone. However, the dynamic TKS (dTKS) performed over 5 days by exposing one strain to fluctuating concentrations representative of those observed in patients showed that, with the large inoculum, the activities of the drugs, used alone or in combination, significantly differed over time. Vancomycin did not kill bacteria, amikacin led to bacterial regrowth whereas the combination progressively decreased the bacterial load. Thus, dTKS revealed an enhanced effect of the combination on a large inoculum not observed in sTKS. The discrepancy between the sTKS and dTKS results highlights that the assessment of the efficacy of a combination for severe infections associated with a high bacterial load could be demanding. These situations probably require the implementation of dynamic assays over the entire expected treatment duration rather than the sole static assays performed with steady drug concentrations over 24h.

Differential Activity of the Combination of Vancomycin and Amikacin on Planktonic vs. Biofilm-Growing Staphylococcus aureus Bacteria in a Hollow Fiber Infection Model

Combining currently available antibiotics to optimize their use is a promising strategy to reduce treatment failures against biofilm-associated infections. Nevertheless, most assays of such combinations have been performed in vitro on planktonic bacteria exposed to constant concentrations of antibiotics over only 24 h and the synergistic effects obtained under these conditions do not necessarily predict the behavior of chronic clinical infections associated with biofilms. To improve the predictivity of in vitro combination assays for bacterial biofilms, we first adapted a previously described Hollow-fiber (HF) infection model by allowing a Staphylococcus aureus biofilm to form before drug exposure. We then mimicked different concentration profiles of amikacin and vancomycin, similar to the free plasma concentration profiles that would be observed in patients treated daily over 5 days. We assessed the ability of the two drugs, alone or in combination, to reduce planktonic and biofilm-embedded bacterial populations, and to prevent the selection of resistance within these populations. Although neither amikacin nor vancomycin exhibited any bactericidal activity on S. aureus in monotherapy, the combination had a synergistic effect and significantly reduced the planktonic bacterial population by -3.0 to -6.0 log₁₀ CFU/mL. In parallel, no obvious advantage of the combination, as compared to amikacin alone, was demonstrated on biofilm-embedded bacteria for which the addition of vancomycin to amikacin only conferred a further maximum reduction of 0.3 log₁₀ CFU/mL. No resistance to vancomycin was ever found whereas a few bacteria less-susceptible to amikacin were systematically detected before treatment. These resistant bacteria, which were rapidly amplified by exposure to amikacin alone, could be maintained at a low level in the biofilm population and even suppressed in the planktonic population by adding vancomycin. In conclusion, by adapting the HF model, we were able to demonstrate the different bactericidal activities of the vancomycin and amikacin combination on planktonic and biofilm-embedded bacterial populations, suggesting that, for biofilm-associated infections, the efficacy of this combination would not be much greater than with amikacin monotherapy. However, adding vancomycin could reduce possible resistance to amikacin and provide a relevant strategy to prevent the selection of antibiotic-resistant bacteria during treatments.

Antimicrobial susceptibility of enterococci recovered from healthy cattle, pigs and chickens in nine EU countries (EASSA Study) to critically important antibiotics

The European Antimicrobial Susceptibility Surveillance in Animals (EASSA) program collects zoonotic and commensal bacteria from food-producing animals at slaughter and tracks their susceptibility to medically important antibiotics. Results of commensal enterococci species (2013-2014) are presented here. Intestinal content from cattle, pigs and chickens were randomly sampled (5-6 countries/host; ≥4 abattoirs/country; 1 sample/animal/farm) for isolation of enterococci, MICs of 9 antibiotics were assessed by CLSI agar dilution in a central laboratory. Clinical breakpoints (CLSI) and epidemiological cut-off values (EUCAST) were applied for data interpretation. In total 960 Enterococcus faecium and 779 Enterococcus faecalis strains were recovered. Seven porcine E. faecium/faecalis strains of Spanish origin were resistant to linezolid. One avian E. faecalis and one porcine E. faecium strain were non-wild type (MICs 8 mg/L) to daptomycin. Clinical vancomycin resistance was absent; 2 poultry E. faecium and 1 bovine E. faecalis strains were non-wild type, all with MICs of 8 mg/L. None of the strains tested were clinically resistant to tigecycline. Little clinical resistance to ampicillin or gentamicin was observed. Clinical resistance of E. faecium to quinupristin/dalfopristin was slightly higher (2.2-12.0%) but 61.9-83.2% of the strains were classified as non-wild type. Very high percentages resistance to tetracycline (67.4-78.3%) and to erythromycin (27.1-57.0%) were noted for both E. faecium and E. faecalis in pigs and chickens compared to cattle (5.2-30.4 and 9.0-10.4%, respectively). Similar non-wild type results were observed for E. hirae (n = 557), E. durans (n = 218) and E. casseliflavus (n = 55) including percentage non-wild type for daptomycin, linezolid, tigecycline being absent and for vancomycin low. For these species percentage non-wild type to erythromycin was lower as compared to E. faecalis/faecium. This pan-EU survey shows high variability in antibiotic susceptibility of commensal enterococci from healthy food animals. Clinical resistance to critically important antibiotics for human medicine was absent or low, except for erythromycin.

Multidrug-Resistant Escherichia coli in Bovine Animals, Europe

Of 150 Escherichia coli strains we cultured from specimens taken from cattle in Europe, 3 had elevated MICs against colistin. We assessed all 3 strains for the presence of the plasmid-mediated mcr-1 gene and identified 1 isolate as mcr-1–positive and co-resistant to β-lactam, florfenicol, and fluoroquinolone antimicrobial compounds.

Monitoring of antimicrobial susceptibility of udder pathogens recovered from cases of clinical mastitis in dairy cows across Europe: VetPath results

VetPath is an ongoing pan-European antimicrobial susceptibility monitoring programme collecting pathogens from diseased cattle, pigs and poultry not recently treated with antibiotics. Non-duplicate milk samples were collected from cows with acute clinical mastitis in nine countries and 934 isolates were obtained during 2009-2012 for subsequent antimicrobial susceptibility testing in a central laboratory. CLSI broth microdilution methodology was used, and where available, MICs were interpreted using CLSI approved veterinary-specific (ceftiofur) otherwise human clinical breakpoints. Among Escherichia coli (n=207) and Klebsiella spp., (n=87), resistance was moderate to tetracycline and high to cephapirin (E. coli only) whereas resistance to other β-lactam antibiotics was very low (ceftiofur) to low (amoxicillin/clavulanic acid, cephalexin, cephalonium). The MIC₉₀ of enrofloxacin and marbofloxacin was 0.03 and 0.06μg/mL respectively (E. coli) with 0.5% strains displaying higher MICs. Staphylococcus aureus (n=192) and coagulase-negative staphylococci (CNS; n=165) strains were susceptible to most antibiotics tested except to penicillin (25.0 and 29.1% resistance), respectively. Three S. aureus and seven CNS strains were oxacillin-resistant and harboured mecA. Streptococcus uberis strains (n=188) were susceptible to the β-lactam antibiotics although 35.6% were penicillin intermediately susceptible, and 20.2% were resistant to erythromycin, 36.7% to tetracycline. For Streptococcus dysgalactiae (n=95) the latter figures were 13.7 and 56.8%, respectively. For most antibiotics, the percentage resistance among E. coli, S. aureus and S. uberis was comparable to that of the VetPath 2002-2006 survey. This current, expanded VetPath study shows that mastitis pathogens were susceptible to most antibiotics with exceptions of staphylococci tested against penicillin and streptococci against erythromycin or tetracycline. This work highlights the high need to set additional clinical breakpoints for antibiotics frequently used to treat mastitis.

Implementing Precision Antimicrobial Therapy for the Treatment of Bovine Respiratory Disease: Current Limitations and Perspectives

The therapeutic efficacy of an early treatment protocol with an infection-stage adjusted fluoroquinolone regimen was evaluated in a field study on young bulls (YBs) presenting signs of bovine respiratory disease (BRD). A total of 195 YB (Charolais, Limousin, and Rouge-des-Prés breeds) from 6 farms implementing or not prophylactic antimicrobial treatments (PROPHY or absence) were randomly assigned to 1 of 2 experiment groups based on time of detection of BRD and first-line marbofloxacin regimen, early adjusted dose [Early 2 (E2)] or late standard dose [Late 10 (L10)]. Each YB was administered orally a reticulo-rumen bolus, allowing continuous monitoring of ruminal temperature. In the E2 group, YB presenting early signs of BRD, i.e., an increase in ruminal temperature over 40.2°C and persisting more than 12 h, confirmed by a clinical examination showing no or mild signs of BRD, were given 2 mg/kg of marbofloxacin. In the L10 group, YBs presenting moderate or severe signs of BRD at visual inspection, confirmed at clinical examination, were given 10 mg/kg of marbofloxacin. If needed, YBs were given a relapse treatment. The YBs were followed for 30 days. The proportions of first and relapse treatments were calculated, as well as the therapeutic efficacy at day 10. In the E2 group, the first-line treatments’ proportion was significantly higher (P < 0.05), while the relapse treatments’ proportion tended to be higher (P = 0.08), than in the L10 group. Evolution of clinical scores (CSs) of diseased YB was followed for 10 days. In both groups, CS and rectal temperature decreased significantly 24 h after treatment (P < 0.05). Treatment incidences (TI) representing antimicrobial consumption assessed on used daily doses (UDD) were calculated. Antimicrobial consumption of marbofloxacin and relapse treatments were not significantly different between the groups. These values were strongly influenced by the recourse to a prophylactic antimicrobial treatment, accounting for more than 90% of the antimicrobial amount in the herds implementing prophylaxis. The higher number of treatments in the groups treated on the basis of ruminal temperature monitoring, the accuracy of the detection method, and the necessary conditions to implement precision antimicrobial therapy in the field are discussed in this article.

Impact of Low and High Doses of Marbofloxacin on the Selection of Resistant Enterobacteriaceae in the Commensal Gut Flora of Young Cattle: Discussion of Data from 2 Study Populations

In the context of requested decrease of antimicrobial use in veterinary medicine, our objective was to assess the impact of two doses of marbofloxacin administered on young bulls (YBs) and veal calves (VCs) treated for bovine respiratory disease, on the total population of Enterobacteriaceae in gut flora and on the emergence of resistant Enterobacteriaceae. In two independent experiments, 48 YBs from 6 commercial farms and 33 VCs previously colostrum deprived and exposed to cefquinome were randomly assigned to one of the three groups LOW, HIGH, and Control. In LOW and HIGH groups, animals received a single injection of, respectively, 2 and 10 mg/kg marbofloxacin. Feces were sampled before treatment, and at several times after treatment. Total and resistant Enterobacteriaceae enumerating were performed by plating dilutions of fecal samples on MacConkey agar plates that were supplemented or not with quinolone. In YBs, marbofloxacin treatment was associated with a transient decrease in total Enterobacteriaceae count between day (D)1 and D3 after treatment. Total Enterobacteriaceae count returned to baseline between D5 and D7 in all groups. None of the 48 YBs harbored marbofloxacin-resistant Enterobacteriaceae before treatment. After treatment, 1 out of 20 YBs from the Control group and 1 out of 14 YBs from the HIGH group exhibited marbofloxacin-resistant Enterobacteriaceae. In VCs, the rate of fluoroquinolone-resistant Enterobacteriaceae significantly increased after low and high doses of marbofloxacin treatment. However, the effect was similar for the two doses, which was probably related to the high level of resistant Enterobacteriaceae exhibited before treatment. Our results suggest that a single treatment with 2 or 10 mg/kg marbofloxacin exerts a moderate selective pressure on commensal Enterobacteriaceae in YBs and in VCs. A fivefold decrease of marbofloxacin regimen did not affect the selection of resistances among commensal bacteria.

Antimicrobial Susceptibility Monitoring of Bacterial Pathogens Isolated from Urinary Tract Infections in Dogs and Cats Across Europe: ComPath Results

ComPath is a pan-European antimicrobial surveillance program collecting bacterial pathogens from dogs and cats not recently exposed to antimicrobials. We present minimum inhibitory concentration data obtained using Clinical and Laboratory Standards Institute methodology for 616 urinary tract infection (UTI) isolates collected between 2008 and 2010. In both dogs and cats, the most common pathogen was Escherichia coli (59.8% and 46.7%, respectively). Antimicrobial activity against E. coli in dogs and cats was similar with fluoroquinolone and trimethoprim/sulfamethoxazole susceptibility >90%. Ampicillin susceptibility was ∼80%. Staphylococcus intermedius Group isolates from dogs (67/437, 15.3%) had high antimicrobial susceptibility (>90%) toward beta-lactams, fluoroquinolones, and trimethoprim/sulfamethoxazole. Four canine isolates (6%) were oxacillin resistant, and harbored mecA. Proteus mirabilis from dogs (48/437, 11.0%) had high antimicrobial susceptibility (∼90%) to amoxicillin/clavulanic acid, enrofloxacin, and marbofloxacin and slightly lower susceptibility (∼80-85%) to ampicillin and orbifloxacin. Streptococcus canis isolates (35/437, 8.0%) from dogs were all susceptible to ampicillin and amoxicillin/clavulanic acid and >90% susceptible to marbofloxacin. Although resistance was not observed, high intermediate susceptibility was seen for both enrofloxacin (28.6%) and orbifloxacin (85.7%). Overall, antimicrobial in vitro activity appears to be high in UTI pathogens from dogs and cats with low multidrug resistance, although a lack of specific dog and cat breakpoints for important antimicrobials such as cefovecin, cephalexin, and ibafloxacin prevents analysis of susceptibility for these agents.

Impact of Timing and Dosage of a Fluoroquinolone Treatment on the Microbiological, Pathological, and Clinical Outcomes of Calves Challenged with Mannheimia haemolytica

The efficacy of an early and low inoculum-adjusted marbofloxacin treatment was evaluated on microbiological and clinical outcomes in calves infected with 4.10⁷ CFU of Mannheimia haemolytica A1. Twenty-two calves were included based on their rectal temperature rise in the 10 h after challenge and allocated in four groups, receiving a single intramuscular injection of saline (CON), 2 mg/kg marbofloxacin 2–4 h after inclusion (early treatment, E2), 2 or 10 mg/kg marbofloxacin 35–39 h after inclusion (late treatments, L2, L10). In CON calves, M. haemolytica DNA loads in bronchoalveolar lavages continuously increased from inclusion to day 4, and were associated with persistent respiratory clinical signs and lung lesions. At times of early and late treatments, M. haemolytica loads ranged within 3.5–4 and 5.5–6 log₁₀ DNA copies/mL, respectively. Early 2 mg/kg marbofloxacin treatment led to rapid and total elimination of bacteria in all calves. The late treatments induced a reduction of bacterial loads, but 3 of 6 L2 and 1 of 6 L10 calves were still positive for M. haemolytica at day 4. Except for CON calves, all animals exhibited clinical improvement within 24 h after treatment. However, early 2 mg/kg treatment was more efficacious to prevent pulmonary lesions, as indicated by the reduction of the extension and severity of gross lesions and by the histopathological scores. These results demonstrated for the first time that a reduced antibiotic regimen given at an early stage of the disease and targeting a low bacterial load could be efficacious in a natural bovine model of pneumonia.

Increase of efflux-mediated resistance in Pseudomonas aeruginosa during antibiotic treatment in patients suffering from nosocomial pneumonia

Increases in antibiotic minimum inhibitory concentrations (MICs) for Pseudomonas aeruginosa during treatment are commonly observed but their relationship to efflux overexpression remains poorly documented. In this study, pairs of first [at time of diagnosis (D0)] and last [during treatment (DL)] P. aeruginosa isolates were obtained from patients treated for suspicion of nosocomial pneumonia. Pair clonality was determined by repetitive extragenic palindromic PCR. Overexpression of mexA and mexX was assessed by real-time PCR, and expression of mexC and mexE was assessed by PCR. Antibiotics received by patients before and during treatment were determined from clinical charts. For D0 isolates, 24% were from patients without antibiotics for 1 month and 64% were negative for mexA/mexX overexpression and mexC/mexE expression. For DL isolates, approximately one-half of the patients had received piperacillin/tazobactam, amikacin, meropenem and/or cefepime, and 17% had received ciprofloxacin (alone or in combination); 38% did not show changes in expression of the four genes, whereas 38% showed increased expression for one gene (mainly mexA or mexX), 19% for two genes (mainly mexA and mexX) and 5% for three or four genes. Isolates overexpressing mexA or mexX had median MICs above EUCAST clinical resistance breakpoints for ciprofloxacin, cefepime and meropenem, or for ciprofloxacin, amikacin, cefepime and meropenem, respectively. mexA or mexX overexpression was statistically significantly associated with patients' exposure to ciprofloxacin and meropenem or cefepime and meropenem, respectively. Overexpression of genes encoding antibiotic transporters in P. aeruginosa during treatment is frequent and is associated with increases in MICs above EUCAST clinical susceptibility breakpoints.

Bacterial Species-Specific Activity of a Fluoroquinolone against Two Closely Related Pasteurellaceae with Similar MICs: Differential In Vitro Inoculum Effects and In Vivo Efficacies

We investigated the antimicrobial activity of a fluoroquinolone against two genetically close bacterial species belonging to the Pasteurellaceae family. Time-kill experiments were used to measure the in vitro activity of marbofloxacin against two strains of Mannheimia haemolytica and Pasteurella multocida with similar MICs. We observed that marbofloxacin was equally potent against 105 CFU/mL inocula M. haemolytica and P. multocida. However, an inoculum effect was observed with P. multocida, meaning that marbofloxacin activity was decreased against a 108 CFU/mL inoculum, whereas no inoculum effect was observed with M. haemolytica. Marbofloxacin activity was also tested in a lung infection model with immunocompromised mice intratracheally infected with 109 CFU of each bacteria. At the same dose, the clinical and bacteriological outcomes were much better for mice infected with M. haemolytica than for those infected with P. multocida. Moreover, bacteriological eradication was obtained with a lower marbofloxacin dose for mice infected with M. haemolytica. Our results suggest that the differential in vivo marbofloxacin efficacy observed with the two bacterial species of similar MIC could be explained by a differential inoculum effect. Consequently, MICs determined on 105 CFU inocula were not predictive of the differences in antibiotic efficacies against high bacterial inocula of closely related bacterial strains. These results could stimulate further investigations on bacterial species-specific antibiotic doses in a clinical setting.

Characterization of ESBL- and AmpC-Producing Enterobacteriaceae from Diseased Companion Animals in Europe

The study aimed to characterize beta-lactam resistance mechanisms of Enterobacteriaceae isolates recovered from diseased dogs and cats between 2008 and 2010 in a European surveillance program (ComPath I) for the antibiotic susceptibility of bacterial pathogens. A total of 608 non-duplicated Enterobacteriaceae isolates were obtained prior antibiotic treatment from diseased dogs (n=464) and cats (n=144). Among the 608 Enterobacteriaceae isolates, 22 presented a minimal inhibitory concentration against cefotaxime above EUCAST breakpoints of susceptibility. All the 22 isolates remained susceptible to carbapenems. Ten isolates were confirmed as extended-spectrum-beta-lactamase (ESBL) producers by PCR-sequencing of bla coding genes including 9 blaCTX-M (CTX-M-1, 14, 15, 32,…) and 1 blaTEM-52 and 12 were AmpC-producing isolates (10 plasmidic CMY-2 group and 2 isolates overexpressing their chromosomal AmpC). ESBLs and plasmid-mediated AmpC (pAmpC)-producing isolates were mainly recovered from dogs (n=17) suffering from urinary tract infections (n=13) and originated from eight different countries. ESBL-bearing plasmids were mostly associated with IncFII incompatibility groups while CMY-2 was predominantly associated with plasmid of the IncI1 group. ESBL/pAmpC-producing Escherichia coli belonged to phylogroup A (n=5), B2 (n=4), and D (n=5). Multilocus sequence typing analysis revealed that among three CTX-M-15-producing E. coli, two belong to sequence type (ST) 131 and one to ST405. The presence of CTX-M-15 including on IncFII plasmids in E. coli ST131-B2 has also been described in isolates of human origin. This suggests the possibility of exchanges of these isolates from humans to companion animals or vice-versa.

Antimicrobial susceptibility monitoring of mastitis pathogens isolated from acute cases of clinical mastitis in dairy cows across Europe: VetPath results

VetPath is an ongoing pan-European antimicrobial susceptibility monitoring programme collecting pathogens from diseased cattle, pigs and poultry not recently treated with antibiotics. Non-replicate milk samples were collected from cows with acute clinical mastitis in eight countries. Escherichia coli, Staphylococcus aureus and Streptococcus uberis were isolated by standardised methods. Antimicrobial susceptibility was determined in a central laboratory by CLSI broth microdilution methodology; results were interpreted using clinical breakpoints where available. Among E. coli (n=280), resistance to tetracycline (14.3%) and cefapirin (11.1%) were most common. Resistance to other β-lactam antibiotics was absent (ceftiofur) or very low (cefalexin, amoxicillin/clavulanic acid). The MIC90 of enrofloxacin and marbofloxacin was 0.03 and 0.06μg/mL, respectively, with 0.7% of strains displaying a deviating high MIC. Staphylococcus aureus (n=250) were susceptible to most antibiotics tested, although 36.0% were resistant to penicillin G. For other β-lactam antibiotics where a CLSI breakpoint was available, no resistance was detected. Tetracycline resistance was low (5.2%). Streptococcus uberis (n=282) were susceptible to all β-lactam antibiotics, although 29.8% were intermediately susceptible to penicillin G; 18.8% of strains were resistant to erythromycin and 28.7% to tetracycline. This European study shows that bacteria associated with acute clinical mastitis are susceptible to most antibiotics with the exception of penicillin G against S. aureus, and erythromycin and tetracycline against S. uberis. The results of this study should serve as a reference baseline. This work also highlights the urgent need to set additional clinical breakpoints for antibiotics frequently used to treat mastitis.

Involvement of the MexXY-OprM efflux system in emergence of cefepime resistance in clinical strains of Pseudomonas aeruginosa

Cefepime (FEP) and ceftazidime (CAZ) are potent beta-lactam antibiotics with similar MICs (1 to 2 mug/ml) for wild-type strains of Pseudomonas aeruginosa. However, recent epidemiological studies have highlighted the occurrence of isolates more resistant to FEP than to CAZ (FEPr/CAZs profile). We thus investigated the mechanisms conferring such a phenotype in 38 clonally unrelated strains collected in two French teaching hospitals. Most of the bacteria (n=32; 84%) appeared to stably overexpress the mexY gene, which codes for the RND transporter of the multidrug efflux system MexXY-OprM. MexXY up-regulation was the sole FEP resistance mechanism identified (n=12) or was associated with increased levels of pump MexAB-OprM (n=5) or MexJK (n=2), synthesis of secondary beta-lactamase PSE-1 (n=10), derepression of cephalosporinase AmpC (n=1), coexpression of both OXA-35 and MexJK (n=1), or production of both PSE-1 and MexAB-OprM (n=1). Down-regulation of the mexXY operon in seven selected strains by the plasmid-borne repressor gene mexZ decreased FEP resistance from two- to eightfold, thereby demonstrating the significant contribution of MexXY-OprM to the FEPr/CAZs phenotype. The six isolates of this series that exhibited wild-type levels of the mexY gene were found to produce beta-lactamase PSE-1 (n=1), OXA-35 (n=4), or both PSE-1 and OXA-35 (n=1). Altogether, these data provide evidence that MexXY-OprM plays a major role in the development of FEP resistance among clinical strains of P. aeruginosa.

Induction of the MexXY efflux pump in Pseudomonas aeruginosa is dependent on drug-ribosome interaction

MexXY is an inducible efflux system that contributes to the natural resistance of Pseudomonas aeruginosa to antibiotics. Experiments involving real-time PCR after reverse transcription in reference strain PAO1 showed concentration-dependent induction of gene mexY by various ribosome inhibitors (e.g., chloramphenicol, tetracycline, macrolides, and aminoglycosides) but not by antibiotics acting on other cellular targets (e.g., beta-lactams, fluoroquinolones). Confirming a functional link between the efflux system and the translational machinery, ribosome protection by plasmid-encoded proteins TetO and ErmBP increased the resistance of a DeltamexAB-oprM mutant of PAO1 to tetracycline and erythromycin, respectively, as well as the concentrations of both drugs required to induce mexY. Furthermore, spontaneous mutations resulting in specific resistance to dihydrostreptomycin or spectinomycin also raised the minimal drug concentration for mexXY induction in strain PAO1. While strongly upregulated in a PAO1 mutant defective in gene mexZ (which codes for a putative repressor of operon mexXY), gene mexY remained inducible by agents such as tetracycline, chloramphenicol, and spectinomycin, suggesting additional regulatory loci for mexXY. Altogether, these data demonstrate physiological interplays between MexXY and the ribosome and are suggestive of an alternative function for MexXY beyond antibiotic efflux.

MexXY-OprM efflux pump is necessary for a adaptive resistance of Pseudomonas aeruginosa to aminoglycosides

Exposure of Pseudomonas aeruginosa to aminoglycosides frequently selects for recalcitrant subpopulations exhibiting an unstable, "adaptive" resistance to these antibiotics. In this study, we investigated the implication in the phenomenon of MexXY-OprM, an active efflux system known to export aminoglycosides in P. aeruginosa. Immunoblotting experiments demonstrated that the transporter MexY, but not the outer membrane pore OprM, was overproduced during the post-drug exposure adaptation period in wild-type strain PAO1. Furthermore, MexY production was dependent upon the degree of bacterial exposure to gentamicin (drug concentration). In contrast to parental strain PAO1, mutants defective in MexXY or in OprM were unable to develop adaptive resistance. Altogether, these results indicate that the resistance process requires the rapid production of MexXY and the interaction of these proteins with the constitutively produced component OprM.