Evaluation of phenotypic methods for detection of polymyxin B-resistant bacteria.

Determination of sensitivity to polymyxins has always been a challenge, especially in clinical laboratory routines. This study evaluated two rapid, simple, and inexpensive phenotypic methods to test polymyxin B (PMB) susceptibility in Enterobacterales and non-fermenting Gram-negative bacilli. One hundred isolates were used in the tests. The isolates were collected in three hospitals in southern and southeastern Brazil from 1995 to 2019. We compared broth microdilution (reference method) with the broth disk elution test and modified drop test, using polymyxin B -disk or PMB -powder in 2 concentrations (12 and 16 µg/ml). For the broth disk elution and modified drop test with the concentration of 12 µg/ml, categorical agreement values exceeded 90%. The modified drop test with a concentration of 12 µg/ml and broth disk elution may be excellent for initial screening of polymyxin-resistance in laboratory routines. Moreover, these methods are simple and use inexpensive supplies, and may optimize therapeutic decisions.

Genomic insights of Acinetobacter baumannii ST374 reveal wide and increasing resistome and virulome.

WGS-based surveillance has significantly improved the ability to track global spread and emergence of multidrug-resistant clones of clinically relevant pathogens. In this study, we performed the genomic characterization and comparative analysis of an Acinetobacter baumannii (strain Ac56) belonging to the sequence type ST374, which was isolated for the first time in Brazil, in 1996. Genomic analysis of Ac56 predicted a total of 5373 genes, with 3012 being identical across nine genomes of A. baumannii isolates of ST374 from European, Asian, North and South American countries. GoeBURST analysis grouped ST374 lineages into clonal complex CC3 (international clone IC-III). Resistome analysis of ST374 clone predicted genes associated with resistance to heavy metals and clinically relevant beta-lactams and aminoglycosides antibiotics. In this regard, in two closely related A. baumannii strains, the intrinsic blaADC gene was linked to the insertion sequence ISAba1; including the Ac56 strain, where it has been possibly associated with intermediate susceptibility to meropenem. Other four carbapenem-resistant A. baumannii strains carried the ISAba1/blaOXA-23 gene array, which was associated with the transposon Tn2008 or with Tn2006 in an AbaR4-type resistance island. While most virulence genes were shared for A. baumannii strains of ST374, three isolates from Thailand harbored KL49 capsular loci, previously identified in the hypervirulent A. baumannii LAC-4 strain. Analysis of thirty-four predicted plasmids showed eight major groups, of which GR-6 (LN-1) and GR-2 (LN-2) were prevalent. All strains, including the earliest isolate Ac56 harbored at least one complete prophage, whereas none CRISPR-associated (cas) gene was detected. In summary, genomic data of A. baumannii ST374 reveal a potential of this lineage to become a successful clone.

Pharmacodynamic evaluation of suppression of in vitro resistance in Acinetobacter baumannii strains using polymyxin B-based combination therapy.

The emergence of polymyxin resistance in Gram-negative bacteria infections has motivated the use of combination therapy. This study determined the mutant selection window (MSW) of polymyxin B alone and in combination with meropenem and fosfomycin against A. baumannii strains belonging to clonal lineages I and III. To evaluate the inhibition of in vitro drug resistance, we investigate the MSW-derived pharmacodynamic indices associated with resistance to polymyxin B administrated regimens as monotherapy and combination therapy, such as the percentage of each dosage interval that free plasma concentration was within the MSW (%TMSW) and the percentage of each dosage interval that free plasma concentration exceeded the mutant prevention concentration (%T>MPC). The MSW of polymyxin B varied between 1 and 16 µg/mL for polymyxin B-susceptible strains. The triple combination of polymyxin B with meropenem and fosfomycin inhibited the polymyxin B-resistant subpopulation in meropenem-resistant isolates and polymyxin B plus meropenem as a double combination sufficiently inhibited meropenem-intermediate, and susceptible strains. T>MPC 90% was reached for polymyxin B in these combinations, while %TMSW was 0 against all strains. TMSW for meropenem and fosfomycin were also reduced. Effective antimicrobial combinations significantly reduced MSW. The MSW-derived pharmacodynamic indices can be used for the selection of effective combination regimen to combat the polymyxin B-resistant strain.

Multidrug- and Extensively Drug-Resistant Acinetobacter baumannii in a Tertiary Hospital from Brazil: The Importance of Carbapenemase Encoding Genes and Epidemic Clonal Complexes in a 10-Year Study.

This study aimed to characterize the main mechanisms of acquired antimicrobial resistance of 103 multidrug-resistant Acinetobacter baumannii isolated from bloodstream from 2006 to 2016 from a hospital in Londrina, Brazil. All 103 isolates were identified as A. baumannii by amplification of the blaOXA-51-like and rpoB genes. Mortality was observed in the majority (81.6%) of the patients. High non-susceptibility rates (100.0-10.7%) were obtained for the evaluated antimicrobials, including colistin, polymyxin B, and tigecycline, and most isolates were classified as extensively drug-resistant (78.6%). Carbapenemase production was observed in 92.2% of the isolates. All carbapenem-resistant isolates showed a carbapenem-hydrolyzing class D ß-lactamase being either blaOXA-23-like (97.9%) or blaOXA-143-like (2.1%). None of the isolates had the genes blaOXA-24-like, blaOXA-58-like, blaOXA-48, blaKPC, blaNDM, blaSPM-1, blaSIM-1, blaVIM, blaIMP, blaGIM, blaGES, mcr-1, qnrA, qnrB, qnrC, qnrS, and qnrVc. As a genetic context of the blaOXA-23-like gene, Tn2006 was predominated (86.0%), and Tn2008 was less frequent (12.9%). Isolates harboring the blaOXA-143-like gene showed the blaOXA-253-like variant. A polyclonal profile was observed among the A. baumannii isolates. The presence of the international clonal complexes CC113/79, CC109/1, CC110/25, and CC103/15 was detected, with prevalence of CC113/79 (38.8%). This study provides essential information to understand the antimicrobial resistance patterns of A. baumannii and can be used to strengthen infection control measures in our hospital. Also, the study reinforces the urgent need to develop stewardship programs to avoid the spread and potential outbreaks by this pathogen.

Pharmacodynamic Effects of Sulbactam/Meropenem/Polymyxin-B Combination Against Extremely Drug Resistant Acinetobacter baumannii Using Checkerboard Information.

Aim: The aims of the study are to evaluate the activity of sulbactam, meropenem, and polymyxin B alone and in combination against six isolates of extremely drug resistant Acinetobacter baumannii and to determine dosing regimens that achieve a sufficient joint probability of target attainment (PTA) based on combination antimicrobial pharmacodynamics. Materials and Methods: The combinations were evaluated by the checkerboard method and were considered synergistic when the fractional inhibitory concentration index (FICI) ≤0.5. Pharmacodynamic analyses were carried out by evaluating dosing regimens that achieve ≥90% joint PTA at the percentage of time over a 24-h period wherein the free drug concentration is above the minimum inhibitory concentration (%fT> MIC) of 40% and 60% for meropenem and sulbactam, respectively, and 20 for the ratio of the area under the free drug concentration-time curve over MIC (fAUC/MIC) for polymyxin B. Results: For both polymyxin B-resistant and susceptible isolates, the addition of sulbactam in combination with meropenem and subinhibitory concentration of polymyxin B showed important synergistic activity (five isolates; FICI ≤0.281); the recommended dosing regimens were 2/4 g meropenem/sulbactam q8 hours and 0.5 mg/kg polymyxin B q12 hours. Conclusion: This in vitro study showed that sulbactam can significantly improve the action of meropenem and polymyxin B in OXA-producing A. baumannii isolates, especially when there are no new treatment options available for infections caused by these microorganisms.

Pharmacodynamic Attainment of the Synergism of Meropenem and Fosfomycin Combination against Pseudomonas aeruginosa Producing Metallo-ß-Lactamase.

Fosfomycin combined with other antimicrobials has shown good efficacy against multidrug-resistant (MDR) bacteria in both in vitro and clinical studies; however, the activity of fosfomycin combined with other antimicrobials against metallo-ß-lactamase (MBL)-producing Pseudomonas aeruginosa strains has not been tested. The objective of this study was to determine the synergism and optimal intravenous dosing regimens of fosfomycin with meropenem against MDR and MBL-producing P. aeruginosa strains. The MICs of both antimicrobials were determined by the checkerboard method and analyzed by two synergism tests with 19 clones of P. aeruginosa isolates, 10 of which were MBL producers. A pharmacodynamic (PD) analysis was performed for meropenem (administered at 1 g every 8 h [q8h], 1.5 g every 6 h [q6h], and 2 g q8h) and fosfomycin (administered at 4 g q8h, 4 g q6h, 6 g q8h, and 8 g q8h) regimens with a dose reduction for renal impairment by determining the probability of target attainment (PTA) for target PD indices of meropenem (the percentage of the time in a 24-h duration at which the free drug concentration remains above the MIC [fT>MIC], ≥40%) and fosfomycin (the ratio of the area under the free drug concentration-versus-time curve over 24 h and the MIC [fAUC/MIC], ≥40.8). The combination reduced the MIC50 and MIC90 by 8-fold. Seven (44%) isolates with MICs in the intermediate or resistant ranges became sensitive to meropenem. For the MBL-producing isolates, the combination resulted in 40% of isolates becoming sensitive to meropenem. The meropenem regimens reached a PTA of ≥90% (MIC = 4 µg/ml) in 6 (32%) isolates when they were used as monotherapy and 13 (68%) isolates when they were combined with fosfomycin. None of the fosfomycin monotherapy regimens reached the PTA of ≥90% (MIC = 16 µg/ml). When combined with meropenem, the fosfomycin regimens reached the PTA of ≥90% in 14 (74%) isolates. The increase in pharmacodynamic activities resulting from the synergistic action of meropenem with fosfomycin demonstrates the potential relevance of this combination to fight infections caused by MDR and MBL-producing P. aeruginosa strains.

Synergistic activity of polymyxin B combined with vancomycin against carbapenem-resistant and polymyxin-resistant Acinetobacter baumannii: first in vitro study.

PURPOSE: The effect of a combination of polymyxin B (PMB) and vancomycin (VAN) was assessed against six Acinetobacter baumannii clinical isolates belonging to six different clusters (three PMB-susceptible and three PMB-resistant). METHODOLOGY: The synergistic effect of the PMB-VAN combination was determined with the checkerboard, time-kill, disk-diffusion and M.I.C.Evaluator assays. PMB-resistance was investigated with mcr-1 gene amplification and a mutant frequency assay. RESULTS: In the checkerboard assay, all PMB-resistant isolates showed a synergistic effect. The time-kill assay demonstrated that the PMB-VAN combination had a bactericidal effect at 24 h against isolates with a high mutant rate for PMB, suggesting that this combination may block the hypermutation of some isolates. No antagonism was detected. All PMB-resistant isolates also showed synergism in the disk-diffusion test, and a significant decrease in VAN MICs in the M.I.C.Evaluator assay. CONCLUSION: Our findings indicate that the PMB-VAN combination has a synergistic effect on A. baumannii, especially against PMB-resistant isolates.

Temporal evolution of Acinetobacter baumannii ST107 clone: conversion of blaOXA-143 into blaOXA-231 coupled with mobilization of ISAba1 upstream occAB1.

Nine carbapenem-resistant Acinetobacter baumannii isolates carrying blaOXA-231 and an ISAba1 upstream occAB1 were evaluated. They were clonally related and belonged to ST107. An OXA-143-producing A. baumannii ST107 strain (Ac-148) that did not possess ISAba1 upstream occAB1 was included in the analysis. Reduction in the expression of occAB1 and a 4-fold increase of carbapenem MICs were observed for all isolates, except for the Ac-148 strain, probably due to the presence of ISAba1 upstream occAB1 but in the same transcriptional orientation. We reported an A. baumannii ST107 clone carrying blaOXA-143 that acquired a mutation resulting into blaOXA-231 and mobilized ISAba1 upstream occAB1.

Prevalence and antimicrobial susceptibility profile of Streptococcus agalactiae in pregnant women seen at the University Hospital of Londrina, Paraná, Brazil / Prevalência e sensibilidade aos antimicrobianos de Streptococcus agalactiae em gestantes atendidas no Hospital Universitário de Londrina, Paraná, Brazil

A retrospective study of pregnant women seen at the University Hospital of Londrina, Paraná, Brazil was performed to determine the prevalence of Group B Streptococcus (GBS) vaginal-rectal colonization, and the GBS susceptibility for antimicrobials used in intrapartum antibiotic prophylaxis. A vaginal-rectal swab was collected from 2,901 women between 35 and 37 weeks of gestation. Of these, 527 (18.2%) had a positive culture for GBS, and 0.4%, 10.2% and 10% of the isolates were resistant to penicillin, erythromycin and clindamycin, respectively. These results highlight the importance of continuous surveillance of GBS colonization in pregnant women for preventing GBS infections in neonates.

Um estudo retrospectivo foi realizado com gestantes atendidas no Hospital Universitário de Londrina, Paraná, Brasil para determinar a prevalência de colonização vaginal-retal por estreptococos do Grupo B (EGB) e o perfil de sensibilidade de EGB aos antimicrobianos utilizados para a antibioticoterapia profilática intraparto. Swabs vaginais-retais foram coletados de 2.901 mulheres entre a 35ª e 37ª semana de gestação. Destes, 527 (18,2%) apresentaram cultura positiva para EGB, e 0,4%, 10,2% e 10% dos isolados foram resistentes à penicilina, eritromicina e clindamicina, respectivamente. Estes resultados destacam a importância de vigilância contínua da colonização por EGB em gestantes para a prevenção de infecções em neonatos por EGB.

Bacteremia causada por Staphylococcus aureus: Uma análise de quinze anos da sensibilidade a antimicrobianos em um hospital terciário do Brasil / Bacteremia caused by Staphylococcus aureus: a fifteen-year analysis of antimicrobial susceptibility in a tertiary hospital in Brazil / Bacteremia causada por Staphylococcus aureus: Un análisis de quince años de la sensibilidad a los antimicrobianos en un hospital terciario de Brasil

Justificativa e Objetivos: Infecções da corrente sanguínea por Staphylococcus aureus constituem uma das principais causas de morbidade e mortalidade em todo mundo. O tratamento de infecções por S. aureus é complexo, em parte, devido à elevada prevalência de resistência aos antimicrobianos. Compreender a epidemiologia e os padrões de resistência deste microrganismo é um ponto crítico para a prescrição empírica adequada de antimicrobianos. Assim, este estudo teve por objetivo avaliar a evolução de resistência antimicrobiana de S. aureus em um período de quinze anos. Métodos: Foram analisados os perfis de sensibilidade para os antimicrobianos ciprofloxacina (5µg); clindamicina (2µg); eritromicina (15µg); gentamicina (10µg); oxacilina (30µg); penicilina (10U); rifampicina (5µg); sulfametoxazol-trimetoprima (23.75/1.25µg) e tetraciclina (30µg) em 720 isolados de S. aureus provenientes de hemoculturas em um hospital terciário do sul do Brasil. Os valores de sensibilidade adotados foram aqueles contidos no CLSI, 2017. Os dados foram obtidos do Sistema de Informação AGTA Healthcare, módulo LABHOS®. Resultados: A frequência média de S. aureus resistente a meticilina foi de 43,74%. Com exceção de penicilina, ocorreu variação significativa da resistência para todos os antimicrobianos no período avaliado (ρ<0,001). Ciprofloxacina (51,14%), eritromicina (44,99%) e clindamicina (39,85%) apresentaram os maiores índices de resistência com tendência de aumento. Surpreendentemente, gentamicina (4%) e sulfametoxazol-trimetoprima (4%) apresentaram queda significativa nos percentuais de resistência. Para vancomicina, do ano 2010 a 2015, observou-se um aumento das concentrações inibitórias mínimas. Conclusão: Embora o índice de resistência tenha aumentado nos quinze anos para a maioria dos antimicrobianos, para sulfametoxazol-trimetoprima e gentamicina ocorreu redução significativa. Este estudo evidenciou, ainda, a emergência do fenótipo S. aureus com resistência intermediária a vancomicina.(AU)

Background and Objectives: Bloodstream infections caused by Staphylococcus aureus are a major cause of morbidity and mortality worldwide. Treatment of S. aureus infections is complex, in part, due to the high prevalence of antimicrobial resistance. Understanding the epidemiology and resistance patterns of this microorganism is a critical point for the proper empirical prescription of antimicrobials. Thus, this study aimed to evaluate the evolution of antimicrobial resistance of S. aureus in a period of fifteen years. Methods: Antimicrobial susceptibility profiles was determined for cefoxitin (30µg), penicillin (10 U), erythromycin (15 µg), clindamycin (2 µg), gentamycin (10 µg),ciprofloxacin (5 µg), sulfamethoxazole-trimethoprim (23.75/1.25 µg), rifampicin (5 µg), and tetracycline (30µg) in 720 S. aureus isolated from blood cultures in a tertiary hospital in southern Brazil were analyzed. Sensiblity values was determined according to Clinical Laboratory Standards Institute (CLSI, 2017).The data were obtained from the AGTA Healthcare Information System, LABHOS® module. Results: The mean frequency of methicillin-resistant S. aureus was 43.74%. Except for penicillin, there was a significant variation of resistance for all antimicrobials in the period evaluated (ρ<0.001). Ciprofloxacin (51.14%), erythromycin (44.99%) and clindamycin (39.85%) had the highest rates of resistance, with tendency to increase. Surprisingly, gentamicina (4%) and sulfamethoxazole-trimethoprim (4%) showed a significant percentage decrease in resistance. For vancomycin, from 2010 to 2015, it was observed an increase in minimum inhibitory concentrations. Conclusion: Although the resistance rate increased in the fifteen years for most antimicrobials, for sulfamethoxazole-trimethoprim and gentamicin a significant reduction occurred, indicating a possible clonal change. This study also evidenced the emergence of S. aureus with intermediate resistance to vancomycin phenotype.(AU)

Justificación y objetivos: Infecciones del flujo sanguíneo por Staphylococcus aureus constituyen una de las principales causas de morbilidad y mortalidad en todo el mundo. El tratamiento de las infecciones por S. aureus es complejo, en parte debido a la elevada prevalencia de resistencia a los antimicrobianos. Comprender la epidemiología y los patrones de resistencia de este microorganismo es un punto crítico para la prescripción empírica adecuada de antimicrobianos. Así, este estudio tuvo por objetivo evaluar la evolución de resistencia antimicrobiana de S. aureus en un período de quince años. Métodos: Se analizaron los perfiles de sensibilidad a los antimicrobianos ciprofloxacino (5µg); clindamicina (2µg); eritromicina (15 µg); gentamicina (10µg); oxacilina (30µg); penicilina (10U); rifampicina (5µg); sulfametoxazol-trimetoprima (23.75 / 1.25 µg) y tetraciclina (30µg) de 720 S. aureus aislados de hemocultivos de un hospital terciario del sur de Brasil. Los valores de sensibilidad adoptados fueron aquellos contenidos en el Clinical Laboratory Standards Institute (CLSI, 2017). Los datos fueron obtenidos del Sistema de Información AGTA Healthcare, módulo LABHOS®. Resultados: La frecuencia media de S. aureus resistente a meticilina fue de 43,74%. Con excepción de la penicilina, hubo variación significativa de la resistencia para todos los antimicrobianos en el período evaluado (ρ<0,001). Ciprofloxacino (51,14%), eritromicina (44,99%) y clindamicina (39,85%) presentaron los mayores índices de resistencia con tendencia de aumento. Sorprendentemente, gentamicina (4%) y sulfametoxazol-trimetoprim (4%) presentaron una caída significativa en los porcentajes de resistencia. Para vancomicina del año 2010 a 2015 se puede evidenciar un aumento de las concentraciones inhibitorias mínimas. Conclusiones: Aunque la resistencia a antimicrobianos aumentó en los quince años para la mayoría de los antimicrobianos, para sulfametoxazol-trimetoprim y gentamicina se produjo una reducción significativa, indicando un posible cambio clonal. Este estudio evidenció, además, la emergencia del fenotipo S. aureus con resistencia intermedia a vancomicina.(AU)

Detection of blaVIM-7 in an extensively drug-resistant Pseudomonas aeruginosa isolate belonging to ST1284 in Brazil.

We described for the first time an extensively drug-resistant Pseudomonas aeruginosa isolate belonging to ST1284 carrying a plasmid-mediated blaVIM-7 in Brazil. The blaVIM-7 was harbored by an integron that also carried aacA4 and blaOXA-46. Multiple virulence factors were also detected.

Strategies for the treatment of polymyxin B-resistant Acinetobacter baumannii infections.

In this study, the activity of meropenem (MEM), fosfomycin (FOF) and polymyxin B (PMB), alone and in combination, was analysed. In addition, optimisation of the pharmacodynamic index of MEM and FOF against six isolates of OXA-23-producing Acinetobacter baumannii (including three resistant to PMB) that were not clonally related was assessed. Antimicrobial combinations were evaluated by chequerboard analysis and were considered synergistic when the fractional inhibitory concentration index (FICI) was ≤0.5. Pharmacodynamic analyses of the MEM and FOF dosing schemes were performed by Monte Carlo simulation. The target pharmacodynamic index (%ƒT>MIC) for MEM and FOF was ≥40% and ≥70%, respectively, and a probability of target attainment (PTA) ≥0.9 was considered adequate. Among the PMB-resistant isolates, combinations of PMB+MEM and PMB+FOF+MEM showed the highest synergistic activity (FICI ≤0.125); isolates that were previously PMB-resistant were included in the susceptible category using CLSI interpretive criteria. Pharmacodynamic evaluation found that for a FOF minimum inhibitory concentration (MIC) of ≤16µg/mL, treatment both by bolus dosing and prolonged infusion achieved adequate PTA, whilst for MIC=32µg/mL only infusion achieved adequate PTA. For a MEM MIC of 4µg/mL, only the bolus treatment scheme with 1.5g q6h and the infusion schemes with 1.0g q8h, 1.5g q6h and 2.0g q8h achieved PTA ≥0.9. Results of antimicrobial and pharmacodynamic analyses can assist in treating infections caused by multidrug-resistant A. baumannii. However, in vivo clinical studies are essential to evaluate the true role of these compounds, including intravenous antimicrobial FOF therapy.

Metallo-ß-lactamase-production in meropenem-susceptible Pseudomonas aeruginosa isolates: risk for silent spread.

The aim of this study was to characterize two metallo-ß-lactamases (MBLs)-producing Pseudomonas aeruginosa clinical isolates showing meropenem susceptibility. Antimicrobial susceptibility was assessed by automated testing and Clinical and Laboratory Standards Institute agar dilution method. MBL production was investigated by phenotypic tests. Molecular typing was determined by pulsed field gel electrophoresis (PFGE). MBL-encoding genes, as well as their genetic context, were identified by polymerase chain reaction (PCR) and sequencing. The location of blaIMP-16 was determined by plasmid electrophoresis, Southern blot and hybridization. Transcriptional levels of blaIMP-16, mexB, mexD, mexF, mexY, ampC and oprD were determined by semi-quantitative real time PCR. The P. aeruginosa isolates studied, Pa30 and Pa43, showed imipenem and meropenem susceptibility by automated testing. Agar dilution assays confirmed meropenem susceptibility whereas both isolates showed low level of imipenem resistance. Pa30 and Pa43 were phenotypically detected as MBL producers. PFGE revealed their clonal relatedness. blaIMP-16 was identified in both isolates, carried as a single cassette in a class 1 integron that was embedded in a plasmid of about 60-Kb. Pa30 and Pa43 overexpressed MexAB-OprM, MexCD-OprJ and MexXY-OprM efflux systems and showed basal transcriptional levels of ampC and oprD. MBL-producing P. aeruginosa that are not resistant to meropenem may represent a risk for therapeutic failure and act as silent reservoirs of MBL-encoding genes.

Metallo-β-lactamase-production in meropenem-susceptible Pseudomonas aeruginosa isolates: risk for silent spread

The aim of this study was to characterize two metallo-β-lactamases (MBLs)-producing Pseudomonas aeruginosa clinical isolates showing meropenem susceptibility. Antimicrobial susceptibility was assessed by automated testing and Clinical and Laboratory Standards Institute agar dilution method. MBL production was investigated by phenotypic tests. Molecular typing was determined by pulsed field gel electrophoresis (PFGE). MBL-encoding genes, as well as their genetic context, were identified by polymerase chain reaction (PCR) and sequencing. The location of blaIMP-16 was determined by plasmid electrophoresis, Southern blot and hybridization. Transcriptional levels of blaIMP-16, mexB, mexD, mexF, mexY, ampC and oprD were determined by semi-quantitative real time PCR. The P. aeruginosa isolates studied, Pa30 and Pa43, showed imipenem and meropenem susceptibility by automated testing. Agar dilution assays confirmed meropenem susceptibility whereas both isolates showed low level of imipenem resistance. Pa30 and Pa43 were phenotypically detected as MBL producers. PFGE revealed their clonal relatedness. blaIMP-16 was identified in both isolates, carried as a single cassette in a class 1 integron that was embedded in a plasmid of about 60-Kb. Pa30 and Pa43 overexpressed MexAB-OprM, MexCD-OprJ and MexXY-OprM efflux systems and showed basal transcriptional levels of ampC and oprD. MBL-producing P. aeruginosa that are not resistant to meropenem may represent a risk for therapeutic failure and act as silent reservoirs of MBL-encoding genes.

In vitro activity of polymyxins in combination with ß-lactams against clinical strains of Pseudomonas aeruginosa.

The emergence of multidrug-resistant (MDR) strains has made it difficult to treat infections caused by Pseudomonas aeruginosa. In order to develop new alternative therapies for the treatment of MDR P. aeruginosa infections, the antimicrobial activities of different antibiotic combinations have been studied in vitro and in vivo. In this study, the in vitro antimicrobial activities of six different combinations of polymyxins and ß-lactams against 34 clinical isolates of P. aeruginosa were evaluated. For the combinations tested by the checkerboard method, an indifferent effect was observed for all strains. However, 27 strains (19 MDR) showed reductions in their minimal inhibitory concentration (MIC) for at least one of the antibiotics in the combinations evaluated. Combination with polymyxins resulted in reductions of the ß-lactam MICs, with a change in the resistance category to susceptible in eight MDR strains. These results from the in vitro evaluation suggest that combinations of polymyxins and ß-lactams may significantly reduce the MICs of the antibiotics tested. These combinations require further evaluation for use in medical practice.