Expression analysis of drug-resistant gene (blaOXA-51) in carbapenemases producing Acinetobacter baumannii treated with imipenem/sulbactam combination

Drug-resistant Acinetobacter baumannii is a frightening reality. The aim of this study is to examine the expression profiles of blaOXA-51 gene in carbapenemases producing A. baumannii treated with imipenem/sulbactam combination. Carbapenemases producing A. baumannii was identified among clinical isolates of A. baumannii obtained from patients at Shahid Rajaee hospital, Gachsaran, Iran, from January to June 2018. Synergism testing of imipenem/sulbactam on carbapenemases producing A. baumannii was carried out by broth microdilution method. Eventually, the expression of blaOXA-51 gene was carried out to investigate the inhibitory properties of imipenem/sulbactam combination against carbapenemases producing A. baumannii using quantitative real time RT-PCR. Among A. baumannii isolates, 24% were carbapenemases producing A. baumannii. Imipenem/sulbactam combination revealed synergistic and partial synergistic effect for all tested isolates (FIC= 0.313-0.75). Finally, imipenem/sulbactam combination displayed significant down-regulation of blaOXA-51 gene in carbapenemases producing A. baumannii. Imipenem synergizes with sulbactam against carbapenemases producing A. baumannii by targeting of the blaOXA-51 gene.

Evaluación de la elución de sensidiscos para la determinación de susceptibilidad a colistín en bacilos gramnegativos multi-resistentes / Evaluation of sensi-disk elution for colistin susceptibility determination in multidrug resistant gramnegative bacilli

Resumen Utilizando cepas clínicas de bacilos gramnegativos multi-resistentes (MDR), comparamos las CIM obtenidas de la microdilución en caldo, el método de referencia y el método de elución de sensidiscos. Encontramos que, con la excepción de A. baumannii, los resultados fueron muy similares. El método de elución de sensidiscos podría ser una buena alternativa y confiable para la determinación de la resistencia a colistín.

Abstract Using clinical strains of multidrug resistant (MDR) Gram negative bacilli, we compared MICs obtained from both broth microdilution, the reference method, and sensi-disk elution method. We found that, with A. baumannii exception, results were very similar. Sensi-disk elution method could be a good and reliable alternative for colistin resistance determination.

Antimicrobial resistance and antibiotic consumption in Mexican hospitals / Resistencia antimicrobiana y consumo de antibióticos en hospitales mexicanos

Abstract: Objective: To establish the current situation of antimicrobial resistance and antibiotic consumption in Mexican hospitals. Materials and methods: Antimicrobial susceptibility data from blood and urine isolates were collected. Defined daily dose (DDD) of antibiotic consumption/100 occupied beds (OBD) was calculated. Results: Study period: 2016 and 2017. Of 4 382 blood isolates, E. coli and K. pneumoniae were most frequently reported, with antimicrobial resistance >30% for most drugs tested, only for carbapenems and amikacin resistance were <20%. A. baumannii had antimicrobial resistance >20% to all drugs. Resistance to oxacillin in S. aureus was 20%. From 12 151 urine isolates, 90% corresponded to E. coli; resistance to ciprofloxacin, cephalosporins and trimethoprim/sulfamethoxazole was >50%, with good susceptibility to nitrofurantoin, amikacin and carbapenems. Global median antimicrobial consumption was 57.2 DDD/100 OB. Conclusions: This report shows a high antimicrobial resistance level in Gram-negative bacilli and provides an insight into the seriousness of the problem of antibiotic consumption.

Resumen: Objetivo: Establecer la situación actual de la resistencia antimicrobiana y el consumo de antibióticos en hospitales mexicanos. Material y métodos:F Se colectaron datos de susceptibilidad antimicrobiana de aislamientos de sangre y orina. Se calculó la dosis diaria definida (DDD) del consumo de antibióticos/100 estancias. Resultados: Periodo de estudio de 2016 a 2017. De 4 382 aislamientos en sangre, E. coli y K. pneumoniae fueron las más frecuentes, con resistencia >30% a la mayoría de las drogas evaluadas; sólo para carbapenémicos y amikacina la resistencia fue <20%. A. baumannii tuvo resistencia >20% a todos los fármacos. La resistencia a oxacilina en S. aureus fue de 20%. De 12 151 aislamientos en urocultivos, 90% correspondió a E. coli; la resistencia a ciprofloxacina, cefalosporinas y trimetoprima/sulfametoxazol fue >50%, con buena susceptibilidad a nitrofurantoína, amikacina y carbapenémicos. La mediana del consumo global de antibióticos en DDD/100 estancias fue de 57.2. Conclusiones: Este reporte muestra el nivel elevado de resistencia en bacilos Gram-negativos y brinda una perspectiva de la gravedad del problema del consumo de antibióticos.

Pyomelanin biosynthetic pathway in pigment-producer strains from the pandemic Acinetobacter baumannii IC-5

BACKGROUND Acinetobacter baumannii outbreaks have been associated with pandemic International Clones (ICs), but the virulence factors involved with their pathogenicity are sparsely understood. Pigment production has been linked with bacterial pathogenicity, however, this phenotype is rarely observed in A. baumannii. OBJECTIVES This study aimed to characterise the reddish-brown pigment produced by A. baumannii strains, and to determine its biosynthetic pathway by genomic approaches. METHODS Pigment characterisation and antimicrobial susceptibility were conducted by phenotypic tests. The clonal relationship was obtained by pulsed field gel electrophoresis (PFGE) and multi-locus sequence typing (MLST). The genome of an A. baumannii was obtained for characterisation of genes involved with pigment production. FINDINGS The pyomelanin was the pigment produced by A. baumannii. Strains were extensively drug resistant and belonged to the IC-5/ST79. The pyomelanin biosynthetic pathway was determined and presented a particular architecture concerning the peripheral (tyrB, phhB and hpd) and central (hmgB, hmgC and hmgR) metabolic pathway genes. The identification of a distant HmgA homologue, probably without dioxygenase activity, could explain pyomelanin production. Virulence determinants involved with adherence (csuA/BABCDE and a T5bSS-carrying genomic island), and iron uptake (basABCDEFGHIJ, bauABCDEF and barAB) were characterised. MAIN CONCLUSION There is a biosynthetic pathway compatible with the pyomelanin production observed in persistent A. baumannii IC-5 strains.

Investigation of carbapenemases and aminoglycoside modifying enzymes of Acinetobacter baumannii isolates recovered from patients admitted to intensive care units in a tertiary-care hospital in Brazil

Abstract INTRODUCTION: Acinetobacter baumannii are opportunistic bacteria, highly capable of acquiring antimicrobial resistance through the production of carbapenemases and aminoglycoside modifying enzymes (AMEs). METHODS: Carbapenemase and AME genes were investigated in A. baumannii recovered from inpatients of a Brazilian hospital. RESULTS: The key genes found were bla OXA-51-like, the association ISAba1- bla OXA-23-like, and the AME genes aph(3´)-VI, aac(6´)-Ib, aac(3)-Ia, and aph(3´)-Ia. Different clusters spread through the institution wards. CONCLUSIONS: The dissemination of bla OXA-23-like and AME-carrying A. baumannii through the hospital highlights the need for improved preventive measures to reduce the spread of infection.

Perfil fenotípico de resistência à colistina e tigeciclina em um hospital público no Brasil / Phenotypical profile of colistin and tigecycline resistance in a public hospital in Brazil / Perfil fenotípico de resistencia a la colistina y tigeciclina en un hospital público de Brasil

Justificativa e objetivos: Infecções Relacionadas à Assistência à Saúde (IRAS) causadas por bacilos Gram negativos multirresistentes (BGN-MDR) são consideradas um problema de saúde pública e um impacto nas taxas de mortalidade nas Unidades de Terapia Intensiva (UTI). O objetivo deste estudo foi verificar o perfil fenotípico de resistência à colistina e à tigeciclina, consideradas como último recurso terapêutico aos BGN-MDR. Métodos: Os dados foram coletados nas fichas de busca ativa do serviço de controle de infecções e prontuários médicos de pacientes internados em duas UTIs de um hospital público de Joinville, entre janeiro de 2016 e junho de 2017. Resultados: Ocorreram 256 IRAS por BGN, acometendo principalmente o gênero masculino (62%), com mediana de idade de 65 anos. Entre os BGN, 37% expressaram MDR; sendo as espécies mais frequentes: Klebsiella pneumoniae e (47%), Acinetobacter baumannii (23%) e Stenotrophomonas maltophilia (11%). A resistência de BGN-MDR à colistina e tigeciclina foi de 5% e de 12%, respectivamente; 5% dos isolados foram resistentes aos dois antibióticos. A taxa de óbito entre os pacientes com IRAS por BGN-MDR resistentes à colistina foi mais alta (60%) que aquelas à tigeciclina (45%). Conclusão: K. pneumoniae e A. baumannii produtores de carbapenemases, resistentes a colistina e tigeciclina prevaleceram entre os BGN-MDR, e estiveram associadas a maioria dos óbitos. Essas observações, junto com o alto uso de carbapenêmicos na terapia empírica, mostra a necessidade do uso racional de antimicrobianos.(AU)

Background and objectives: Healthcare-associated Infections (HAIs) caused by multidrug-resistant Gram-negative bacilli (GNB-MDR) are considered a public health problem and have an impact on mortality rates in Intensive Care Units (ICU). The aim of this study was to verify the phenotypic profile of resistance to colistin and tigecycline, considered as the last antimicrobial choice to treat BGNMDR infections. Methods: Data were collected on the active search records of the infection control service and medical records of patients admitted to two ICUs at a public hospital in Joinville between January 2016 and June 2017. Results: There were 256 HAIs caused by GNB, mainly affecting males (62%), with a median age of 65 years. Among GNBs, 37% expressed MDR; the most frequent species were: Klebsiella pneumoniae (47%), Acinetobacter baumannii (23%) and Stenotrophomonas maltophilia (11%). The resistance of GNB-MDR to colistin and tigecycline was 5% and 12%, respectively; 5% of the isolates were resistant to both antibiotics. The death rate among patients with HAIs caused by colistin-resistant GNB-MDR was higher (60%) than those to tigecycline (45%). Conclusion: Carbapenemase-producing K. pneumoniae and A. baumannii, resistant to colistin and tigecycline, prevailed among GNB-MDRs, and were associated with most deaths. These observations, coupled with the high use of carbapenems in empirical therapy, show the need for rational use of antimicrobials.(AU)

Justificación y objetivos: Las Infección nosocomial (IHs) causadas por bacilos Gram negativos multirresistentes (BGN-MDR) se consideran un problema de salud pública y un impacto en las tasas de mortalidad en las Unidades de Terapia Intensiva (UTI). El objetivo de este estudio fue verificar el perfil fenotípico de resistencia a la colistina ya la tigeciclina, consideradas como último recurso terapéutico a los BGN-MDR. Métodos: Los datos fueron recolectados en las fichas de búsqueda activa del servicio de control de infecciones y prontuarios médicos de pacientes internados en dos UTIs de un hospital público de Joinville, entre enero de 2016 y junio de 2017. Resultados: Ocurrieron 256 IHs por BGN, que afectan principalmente al género masculino (62%), con mediana de edad de 65 años. Entre los BGN, el 37% expresó MDR; siendo las especies más frecuentes: Klebsiella pneumoniae (47%), Acinetobacter baumannii (23%) y Stenotrophomonas maltophilia (11%). La resistencia de BGN-MDR a la colistina y tigeciclina fue del 5% y del 12%, respectivamente; 5% de los aislados fueron resistentes a los dos antibióticos. La tasa de muerte entre los pacientes con IH causadas por los BGN-MDR resistentes la colistina fue más alta (60%) que aquellas a tigeciclina (45%). Conclusión: K. pneumoniae y A. baumannii productoras de carbapenemases, resistentes la colistina y la tigeciclina, fueron más frecuentes entre los BGN-MDR y su asociación estuvo presente en la mayoría de las muertes. Estas observaciones, junto con el alto uso de carbapenems en la terapia empírica, muestran la necesidad de un uso racional de los antimicrobianos.(AU)

Epidemiología molecular de aislados de Acinetobacter baumannii resistentes a carbapenems en Argentina / Molecular epidemiology of carbapenem-resistant isolates of Acinetobacter baumannii in Argentina

Se estudiaron 100 aislados consecutivos y no epidemiológicamente relacionados de Acinetobacter baumannii resistentes a los carbapenems, recuperados entre enero y agosto de 2016 de muestras clínicas en 11 hospitales de 10 provincias de la Argentina, ubicadas en distintas regiones del país. Los genes que codifican las carbapenemasas de Ambler clase D y clase B se investigaron mediante la técnica de PCR utilizando cebadores específicos. Todos los aislados se agruparon mediante las técnicas de 3-locus sequence typing y la secuenciación del gen blaOXA-51-like. El gen blaOXA-23 se recuperó en todos los aislados estudiados. La población de A. baumannii resistente a carbapenems en Argentina estuvo asociada, principalmente, con ST1 (45%), ST25 (34%) y ST79 (15%). ST25 se recuperó en todas las regiones estudiadas y no se detectó CC2.

One hundred sequential, epidemiologically unrelated carbapenem-resistant- Acinetobacter baumannii isolates from 11 hospitals in 10 Argentine provinces were collected between January and August 2016. Genes coding for Ambler class D and B carbapenemases were investigated by PCR using specific primers. All isolates were typed using the 3-locus sequence typing and b/aOXA-51-like sequence-based typing techniques. The blaOXA-23 gene was recovered in all isolates studied. The population of carbapenem-resistant- A. baumannii in Argentina was principally associated with ST1 (45%), ST25 (34%) and ST79 (15%). ST25 was recovered in all the regions studied and CC2 was not detected.

Origanum vulgare L. essential oil inhibits the growth of carbapenem-resistant gram-negative bacteria

Abstract INTRODUCTION: Plant products are sources for drug development against multidrug resistant bacteria. METHODS The antimicrobial activity of Origanum vulgare L. essential oil (OVeo) against carbapenem-resistant strains was assessed by disk-diffusion, microdilution (REMA-Resazurin Microtiter Assay), and time kill assays. RESULTS Carbapenemase production was confirmed for all strains. OVeo exhibited a minimum inhibitory concentration of 0.059% v/v for Klebsiella pneumoniae and Serratia marcescens, and of 0.015 % v/v for Acinetobacter baumannii. A decrease in cell count was observed after a 4 h treatment. CONCLUSIONS OVeo antimicrobial effect was rapid and consistent, making it a candidate for developing alternative therapeutic options against carbapenem-resistant strains.

Plasmid-mediated mcr-1 gene in acinetobacter baumannii and pseudomonas aeruginosa: first report from pakistan

Abstract INTRODUCTION: The increased use of colistin against infections caused by Acinetobacter baumannii and Pseudomonas aeruginosa has resulted in colistin resistance. The purpose of this study was to detect plasmid-mediated mcr-1 gene in colistin-resistant A. baumannii and P. aeruginosa isolates. METHODS: A total of 146 clinical isolates of A. baumannii (n = 62) and P. aeruginosa (n = 84) were collected from the four largest tertiary care hospitals in Peshawar, Pakistan. All bacterial isolates were phenotypically screened for multidrug resistance using the Kirby-Baur disc diffusion method. The minimum inhibitory concentration (MIC) of colistin in all isolates was phenotypically performed using dilution methods. mcr-1 gene was detected through polymerase chain reaction and the nucleotide sequence of amplicon was determined using Sanger sequencing. RESULTS: Approximately 96.7% A. baumannii and 83.3% P. aeruginosa isolates were resistant to multiple antibiotics. Colistin resistance was found in 9.6% (6/62) of A. baumannii and 11.9% (10/84) of P. aeruginosa isolates. Among 16 colistin resistant isolates, the mcr-1 gene was detected in one A. baumannii (1.61% of total isolates; 16.6% of colistin resistant isolates) and one P. aeruginosa strain (1.19% of total isolates; 10% of colistin resistant isolates). Nucleotide BLAST showed 98-99% sequence similarity to sequences of the mcr-1 gene in GenBank. CONCLUSIONS: Our study reports, for the first time, the emergence of plasmid-mediated mcr-1-encoded colistin resistance in multidrug resistant strains of A. baumannii and P. aeruginosa. Further large scales studies are recommended to investigate the prevalence of this mode of resistance in these highly pathogenic bacteria.

Multidrug-resistant Acinetobacter baumannii clones persist on hospital inanimate surfaces

ABSTRACT Acinetobacter baumannii is one of the most frequent Gram-negative opportunistic pathogens associated with hospital-acquired infection worldwide. We briefly describe A. baumannii isolates that were recovered from surrounding ICU bed surfaces, exhibiting multidrug resistance phenotype and belonging to some widely spread clonal complexes of clinical A. baumannii isolates.

¿Se debe revisar la determinación de la CIM de colistín en Acinetobacter baumannii? / The colistin MIC determination in Acinetobacter baumannii, Should it be reviewed?

Currently, there is a controversy in how to determine the minimal inhibitory concentration (MIC) of colistin against Acinetobacter baumannii. We compared three methods, concluding that the addition of Tween-80 (0.002%) to Müller-Hinton broth in the microdilution method could improve MIC determination and it could reduce false resistance.

Carbapenem-resistant Acinetobacter baumannii contamination in an intensive care unit

Abstract INTRODUCTION: Acinetobacter baumannii is a major pathogen causing infections in intensive care units (ICUs). In this study, we aimed to evaluate the presence of A. baumannii in an ICU environment and gloves from ICU workers and to characterize the antimicrobial resistance of the isolates in comparison with those isolated from ICU patients at the same hospital. METHODS: ICU samples were collected from March to November 2010. Isolates biochemically characterized as Acinetobacter calcoaceticus-Acinetobacter baumannii complex were evaluated by PCR targeting the 16S rDNA and bla OXA-51 genes. Antimicrobial susceptibility was determined using the disk diffusion method, and carbapenem-resistant isolates were also evaluated for the minimum inhibitory concentration of imipenem using broth microdilution. The presence of the bla OXA-23 gene was evaluated in isolates with reduced susceptibility to carbapenems. RESULTS: A. baumannii was detected in 9.5% (84) of the 886 samples collected from the ICU environment, including from furniture, medical devices, and gloves, with bed rails being the most contaminated location (23.8%; 20/84). Multidrug-resistant (MDR) A. baumannii was found in 98.8% (83/84) of non-clinical and 97.8% (45/46) of clinical isolates. Reduced susceptibility to carbapenems was detected in 83.3% (70/84) of non-clinical and 80.4% (37/46) of clinical isolates. All isolates resistant to carbapenems harbored bla OXA-23. CONCLUSIONS: We found a strong similarity between the antimicrobial susceptibility profiles of non-clinical and clinical A. baumannii isolates. Such data highlight the ICU environment as a potential origin for the persistence of MDR A. baumannii, and hence the ICU may be a source of hospital-acquired infections caused by this microorganism.

Mobile genetic elements related to carbapenem resistance in Acinetobacter baumannii

Abstract Acinetobacter baumannii is widely recognized as an important pathogen associated with nosocomial infections. The treatment of these infections is often difficult due to the acquisition of resistance genes. A. baumannii presents a high genetic plasticity which allows the accumulation of these resistance determinants leading to multidrug resistance. It is highlighted the importance of the horizontal transfer of resistance genes, through mobile genetic elements and its relationship with increased incidence of multidrug resistant A. baumannii in hospitals. Considering that resistance to carbapenems is very important from the clinical and epidemiological point of view, the aim of this article is to present an overview of the current knowledge about genetic elements related to carbapenem resistance in A. baumannii such as integrons, transposons, resistance islands and insertion sequences.

Clinical and microbiological characteristics of OXA-23- and OXA-143-producing Acinetobacter baumannii in ICU patients at a teaching hospital, Brazil

ABSTRACT Background: Carbapenem-resistant Acinetobacter baumannii (CRAb) is an important cause of nosocomial infections especially in intensive care units. This study aimed to assess clinical aspects and the genetic background of CRAb among ICU patients at a Brazilian teaching hospital. Methods: 56 critically ill patients colonized or infected by CRAb, during ICU stay, were prospectively assessed. Based on imipenem MIC ≥ 4 µg/mL, 28 CRAB strains were screened for the presence of genes encoding metallo-β-lactamases and OXA-type β-lactamases. The blaOXA-type genes were characterized by PCR using primers targeting ISAba-1 or -3. Genetic diversity of blaOXA-positive strains was determined by ERIC-PCR analysis. Results: Patient's mean age (±SD) was 61 (±15.1), and 58.9% were male. Eighty-percent of the patients presented risk factors for CRAb colonization, mainly invasive devices (87.5%) and previous antibiotic therapy (77.6%). Thirty-three patients died during hospital stay (59.0%). Resistance to carbapenems was associated with a high prevalence of blaOXA-23 (51.2%) and/or blaOXA-143 (18.6%) genes. ERIC-PCR genotyping identified 10 clusters among OXA-producing CRAb. Three CRAb strains exhibited additional resistance to polymyxin B (MIC ≥ 4 µg/mL), whereas 10 CRAb strains showed tigecycline MICs > 2 µg/mL. Conclusions: In this study, clonally unrelated OXA-123- and OXA-143-producing A. baumannii strains in ICU patients were strongly correlated to colonization with infected patients being associated with a poor outcome.

The role of the genetic elements bla oxa and IS Aba 1 in the Acinetobacter calcoaceticus-Acinetobacter baumannii complex in carbapenem resistance in the hospital setting

Abstract: INTRODUCTION: Members of the Acinetobacter genus are key pathogens that cause healthcare-associated infections, and they tend to spread and develop new antibiotic resistance mechanisms. Oxacillinases are primarily responsible for resistance to carbapenem antibiotics. Higher rates of carbapenem hydrolysis might be ascribed to insertion sequences, such as the ISAba1 sequence, near bla OXA genes. The present study examined the occurrence of the genetic elements bla OXA and ISAba1 and their relationship with susceptibility to carbapenems in clinical isolates of the Acinetobacter calcoaceticus-Acinetobacter baumannii complex. METHODS: Isolates identified over 6 consecutive years in a general hospital in Joinville, Southern Brazil, were evaluated. The investigation of 5 families of genes encoding oxacillinases and the ISAba1 sequence location relative to bla OXA genes was conducted using polymerase chain reaction. RESULTS: All isolates presented the bla OXA-51-like gene (n = 78), and 91% tested positive for the bla OXA-23-like gene (n = 71). The presence of ISAba1 was exclusively detected in isolates carrying the bla OXA-23-like gene. All isolates in which ISAba1 was found upstream of the bla OXA-23-like gene (n = 69) showed resistance to carbapenems, whereas the only isolate in which ISAba1 was not located near the bla OXA-23-like gene was susceptible to carbapenems. The ISAba1 sequence position of another bla OXA-23-like-positive isolate was inconclusive. The isolates exclusively carrying the bla OXA-51-like gene (n = 7) showed susceptibility to carbapenems. CONCLUSIONS: The presence of the ISAba1 sequence upstream of the bla OXA-23-like gene was strongly associated with carbapenem resistance in isolates of the A. calcoaceticus-A. baumannii complex in the hospital center studied.

Over expression of AdeABC and AcrAB-TolC efflux systems confers tigecycline resistance in clinical isolates of Acinetobacter baumannii and Klebsiella pneumoniae

Abstract: INTRODUCTION: Due to the wide use of tigecycline in the treatment of severe infections caused by multidrug-resistant (MDR) bacteria, clinical resistance to tigecycline has increased in recent years. Here, we investigated the relationship between tigecycline resistance and the expression of efflux pumps. METHODS: Clinical isolates of Acinetobacter baumannii and Klebsiella pneumoniae were consecutively collected from hospitalized patients in three hospitals. The minimum inhibitory concentration (MIC) of tigecycline was determined using the broth microdilution method. Expression levels of efflux pump genes and regulators were examined by quantitative real-time reverse transcription polymerase chain reaction. The correlations between tigecycline MICs and gene expression levels were analyzed. RESULTS: Overall, 1,026 A. baumannii and 725 K. pneumoniae strains were collected. Most strains were isolated from sputum. The tigecycline resistance rate was 13.4% in A. baumannii isolates and 6.5% in K. pneumoniae isolates. Overexpression of AdeABC and AcrAB-TolC efflux systems was observed found in clinical tigecycline-resistant isolates. The tigecycline MIC had a linear relationship with the adeB expression level in A. baumannii isolates, but not with the acrB expression level in K. pneumoniae isolates. There were significant linear trends in the overexpression of ramA as the tigecycline MIC increased in K. pneumoniae isolates. CONCLUSIONS: Tigecycline resistance in A. baumannii and K. pneumoniae was strongly associated with the overexpression of efflux systems. More studies are needed to elucidate whether there are other regulators that affect the expression of adeB in A. baumannii and how ramA affects the expression of acrB in K. pneumoniae.

In Vitro Interactions of Antibiotic Combinations of Colistin, Tigecycline, and Doripenem Against Extensively Drug-Resistant and Multidrug-Resistant Acinetobacter baumannii

BACKGROUND: Acinetobacter baumannii infections are difficult to treat owing to the emergence of various antibiotic resistant isolates. Because treatment options are limited for multidrug-resistant (MDR) A. baumannii infection, the discovery of new therapies, including combination therapy, is required. We evaluated the synergistic activity of colistin, doripenem, and tigecycline combinations against extensively drug-resistant (XDR) A. baumannii and MDR A. baumannii. METHODS: Time-kill assays were performed for 41 XDR and 28 MDR clinical isolates of A. baumannii by using colistin, doripenem, and tigecycline combinations. Concentrations representative of clinically achievable levels (colistin 2 microg/mL, doripenem 8 microg/mL) and achievable tissue levels (tigecycline 2 microg/mL) for each antibiotic were used in this study. RESULTS: The colistin-doripenem combination displayed the highest rate of synergy (53.6%) and bactericidal activity (75.4%) in 69 clinical isolates of A. baumannii. Among them, thedoripenem-tigecycline combination showed the lowest rate of synergy (14.5%) and bacteri-cidal activity (24.6%). The doripenem-tigecycline combination showed a higher antagonistic interaction (5.8%) compared with the colistin-tigecycline (1.4%) combination. No antagonism was observed for the colistin-doripenem combination. CONCLUSIONS: The colistin-doripenem combination is supported in vitro by the high rate of synergy and bactericidal activity and lack of antagonistic reaction in XDR and MDR A. baumannii. It seems to be necessary to perform synergy tests to determine the appropri-ate combination therapy considering the antagonistic reaction found in several isolates against the doripenem-tigecycline and colistin-tigecycline combinations. These findings should be further examined in clinical studies.

In Vitro Synergistic Effects of Antimicrobial Combinations on Extensively Drug-Resistant Pseudomonas aeruginosa and Acinetobacter baumannii Isolates

BACKGROUND: Extensively drug-resistant (XDR) Pseudomonas aeruginosa and Acinetobacter baumannii are a threat to hospitalized patients. We evaluated the effects of antimicrobial combinations on XDR P. aeruginosa and A. baumannii isolates. METHODS: P. aeruginosa and A. baumannii isolates, which were resistant to all antibiotics except colistin (CL), were collected from eight hospitals in Korea. Genes encoding metallo-beta-lactamases (MBLs) and OXA carbapenemases were detected by PCR in eight P. aeruginosa and 30 A. baumannii isolates. In vitro synergy of antimicrobial combinations was tested by using the checkerboard method. RESULTS: Minimum inhibitory concentrations of beta-lactams, aminoglycosides, and fluoroquinolones were very high, while that of CL was low for majority of XDR P. aeruginosa and A. baumannii isolates. Antimicrobial combinations including Imipenem (IPM)-CL, ceftazidime (CAZ)-CL, and rifampin (RIF)-CL exerted only additive/indifferent effects on majority of XDR P. aeruginosa isolates. Proportions of XDR A. baumannii isolates that showed synergistic and additive/indifferent inhibition after treatment with antimicrobial combinations used are as follows: IPM-ampicillin-sulbactam (AMS), 17% and 80% isolates, respectively; IPM-rifampin (RIF), 13% and 81% isolates, respectively; IPM-CL, 13% and 87% isolates, respectively; and RIF-COL, 20% and 73% isolates, respectively. Significant proportion (19%) of XDR P. aeruginosa isolates produced MBLs, and majority (82%) of A. baumannii isolates produced either MBLs or OXA-23. CONCLUSIONS: Our results suggest that combinations of IPM-AMS, IPM-RIF, IPM-CL, and RIF-CL are more useful than individual drugs for treating 13-20% of XDR A. baumannii infections.