Role of efflux pumps in reduced susceptibility to tigecycline in Acinetobacter baumannii

Correlation of RND efflux pump expression and AdeRS mutations in tigecycline-resistant Acinetobacter baumannii from Thai clinical isolates

Tigecycline-resistant Acinetobacter baumannii (TRAB) is increasing in Thailand, complicating antibiotic treatment due to limited antibiotic options. The specific resistance mechanism behind tigecycline resistance is still unclear, necessitating further investigation. We investigated the presence of OXA-type carbapenemases, the antimicrobial susceptibility profile, the inhibitory effect of carbonyl cyanide m-chlorophenylhydrazone (CCCP) on tigecycline susceptibility, the expression levels of RND-type efflux pumps and amino acid substitutions within a two-component regulatory system on 30 Thai clinical isolates. Our investigation revealed that most of (73.3%) TRAB isolates expressed at least one member of the Ade efflux pumps. The adeB was most frequently expressed (63.3%), followed by adeR (50%), adeS (43.3%), adeJ (30%) and adeG (10%). Overexpression of the AdeABC was associated with increased tigecycline minimum inhibitory concentrations (MICs) and amino acid substitutions within the AdeRS. Notably, isolates harbouring simultaneous mutations in these genes exhibited an increase in the transcription level of the adeB. Our findings highlight the significant role of the AdeABC system in tigecycline resistance among Thai clinical TRAB isolates. This is supported by point mutations within the AdeRS and upregulated expression of the adeB. These results provide valuable insights for understanding resistance mechanisms and developing novel therapeutic strategies.

What role do biocontrol agents with Mg2+ play in the fate of antibiotic resistome and pathogenic bacteria in the phyllosphere?

The contamination of the plant phyllosphere with antibiotics and antibiotic resistance genes (ARGs), caused by application of antibiotics, is a significant environmental issue in agricultural management. Alternatively, biocontrol agents are environmentally friendly and have attracted a lot of interest. However, the influence of biocontrol agents on the phyllosphere resistome remains unknown. In this study, we applied biocontrol agents to control the wildfire disease in the Solanaceae crops and investigated their effects on the resistome and the pathogen in the phyllosphere by using metagenomics. A total of 250 ARGs were detected from 15 samples, which showed a variation in distribution across treatments of biocontrol agents (BA), BA with Mg2+ (T1), BA with Mn2+ (T2), and kasugamycin (T3) and nontreated (CK). The results showed that the abundance of ARGs under the treatment of BA-Mg2+ was lower than that in the CK group. The abundance of cphA3 (carbapenem resistance), PME-1 (carbapenem resistance), tcr3 (tetracycline antibiotic resistance), and AAC (3)-VIIIa (aminoglycoside antibiotic resistance) in BA-Mg2+ was significantly higher than that in BA-Mn2+ (P < 0.05). The abundance of cphA3, PME_1, and tcr3 was significantly negatively related to the abundance of the phyllosphere pathogen Pseudomonas syringae (P < 0.05). We also found that the upstream and downstream regions of cphA3 were relatively conserved, in which rpl, rpm, and rps gene families were identified in most sequences (92%). The Ka/Ks of cphA3 was 0 in all observed sequences, indicating that under the action of purifying selection, nonsynonymous substitutions are often gradually eliminated in the population. Overall, this study clarifies the effect of biocontrol agents with Mg2+ on the distribution of the phyllosphere resistome and provides evolutionary insights into the biocontrol process.

IMPORTANCE

Our study applied metagenomics analysis to examine the impact of biocontrol agents (BAs) on the phyllosphere resistome and the pathogen. Irregular use of antibiotics has led to the escalating dissemination of antibiotic resistance genes (ARGs) in the environment. The majority of BA research has focused on the effect of monospecies on the plant disease control process, the role of the compound BA with nutrition elements in the phyllosphere disease, and the resistome is still unknown. We believe BAs are eco-friendly alternatives for antibiotics to combat the transfer of ARGs. Our results revealed that BA-Mg2+ had a lower relative abundance of ARGs compared to the CK group, and the phyllosphere pathogen Pseudomonas syringae was negatively related to three specific ARGs, cphA3, PME-1, and tcr3. These three genes also present different Ka/Ks. We believe that the identification of the distribution and evolution modes of ARGs further elucidates the ecological role and facilitates the development of BAs, which will attract general interest in this field.

Detection of Aminoglycoside Modifying Enzyme (AME) genes in Acinetobacter baumannii isolates and the inhibitory effect of efflux pump activity on drug susceptibility pattern

INTRODUCTION

The development of aminoglycoside modifying enzymes (AMEs) and increased efflux activity are considered important aminoglycosides resistance mechanisms.

AIM

This study is focused on the detection of the AMEs gene and assessing the effect of efflux pump inhibitor on the reversal of A. baumannii drug susceptibility.

METHODOLOGY

Bacterial DNA was amplified using AMEs gene-specific primers. Isolates were also investigated for efflux pump activity using efflux pump inhibitor (EPI) i.e. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and the impact of both mechanisms was analyzed.

RESULTS

Among A. baumannii isolates, 55% isolates (n ​= ​22/40) were identified to have aminoglycoside modifying enzymes genes; ant(3')-I gene (50%, 11/22), aac(6')-Ib gene (45.4%, 10/22), aph(3')-I gene (18.1%, 4/22) and aac(3)-I (9.1%, 2/22). Total 70% isolates have shown MIC alteration in different classes of drugs in response to EPI-CCCP. Such alteration was found in 100% amikacin sensitive and 58.6% amikacin resistant, 93.7% and 57.1% gentamicin sensitive and resistant isolates respectively.

CONCLUSION

The presence of aminoglycosides modifying enzymes was frequent among aminoglycosides resistant A. baumannii isolates and the coexistence of efflux pumps activity also plays an important role to increase drug resistance.

REPOSITORIES

Genbank and their accession numbers are MT903331[aac(3)-I], MT903332 MT903333 [ant(3')-I], MT903334, MT903335 [aph(3')-I)] and MT903336, MT940242 [ aac(6')-Ib].

Efflux-Related Carbapenem Resistance in Acinetobacter baumannii Is Associated with Two-Component Regulatory Efflux Systems' Alteration and Insertion of ΔAbaR25-Type Island Fragment

The efflux pumps, beside the class D carbapenem-hydrolysing enzymes (CHLDs), are being increasingly investigated as a mechanism of carbapenem resistance in Acinetobacter baumannii. This study investigates the contribution of efflux mechanism to carbapenem resistance in 61 acquired bla_CHDL-genes-carrying A. baumannii clinical strains isolated in Warsaw, Poland. Studies were conducted using phenotypic (susceptibility testing to carbapenems ± efflux pump inhibitors (EPIs)) and molecular (determining expression levels of efflux operon with regulatory-gene and whole genome sequencing (WGS)) methods. EPIs reduced carbapenem resistance of 14/61 isolates. Upregulation (5-67-fold) of adeB was observed together with mutations in the sequences of AdeRS local and of BaeS global regulators in all 15 selected isolates. Long-read WGS of isolate no. AB96 revealed the presence of AbaR25 resistance island and its two disrupted elements: the first contained a duplicate ISAba1-bla_OXA-23, and the second was located between adeR and adeA in the efflux operon. This insert was flanked by two copies of ISAba1, and one of them provides a strong promoter for adeABC, elevating the adeB expression levels. Our study for the first time reports the involvement of the insertion of the ΔAbaR25-type resistance island fragment with ISAba1 element upstream the efflux operon in the carbapenem resistance of A. baumannii.

Plastic wastes and surface antibiotic resistance genes pollution in mangrove environments

Mangroves are located at the intersection of land and sea and are also heavily affected by plastic wastes. Biofilms of plastic wastes in mangroves are reservoirs for antibiotic resistance genes (ARGs). In this study, plastic wastes and ARG pollution were investigated from three typical mangrove areas in Zhanjiang, South China. Transparent was the dominant colors of plastic wastes in three mangroves. Fragment and film shape accounted for 57.73-88.23% of plastic waste samples in mangroves. In addition, 39.50% of plastic wastes in protected area mangroves are PS. The metagenomic results shows that the 175 ARGs were found on plastic wastes of the three mangroves, the abundance accounting for 91.11% of the total ARGs. The abundance of Vibrio accounted for 2.31% of the total bacteria genera in aquaculture pond area mangrove. Correlation analysis shows that a microbe can carry multiple ARGs that may improve resistance to antibiotics. Microbes are the potential hosts of most ARGs, suggesting that ARGs can be transmitted by microbes. Because the mangroves are closely related to human activities and the high abundance of ARGs on plastic increases the ecological risks, people should improve plastic waste management and prevent the spread of ARGs by reducing plastic pollution.

The role of efflux pumps ın antıbıotıc resıstance of gram negatıve rods

Antibiotic resistance is an important public health problem today, causing increased morbidity and mortality. Resistance to antibiotics in bacteria can develop by various mechanisms such as a change in the target site of the drug, a change in the outer membrane permeability, enzymatic defusing of the drug and efflux of the antimicrobial compound. Some bacteria have the potential to develop resistance to more than one drug by using several mechanisms together. One of the important resistance mechanisms of bacteria is active efflux pumps (EPs). EPs are pump proteins found in all cell types, located in the cell membrane. They are responsible for the excretion of various intracellular and extracellular substances (antibiotics, etc.) out of the cell. There is much research on various antimicrobials that cause antibiotic resistance in Gram negative rods, but studies on EPs are relatively few. Due to the concern that antibiotics will be insufficient in the treatment of diseases, a good understanding of EPs and the discovery of new EP inhibitors will shed light on the future of humanity. In this review, the structure of bacterial EPs in Gram negative bacteria, the role of EPs in multidrug resistance, the importance of EP inhibitors in the fight against antibiotic resistance and the phenotypic and genotypic detection methods of EPs are discussed.

Efflux Pumps and Different Genetic Contexts of tet(X4) Contribute to High Tigecycline Resistance in Escherichia fergusonii from Pigs

Tigecycline is a last-resort antibiotic for the treatment of infections caused by multidrug-resistant bacteria. The emergence of plasmid-mediated tigecycline resistance genes is posing a serious threat to food safety and human health and has attracted worldwide attention. In this study, we characterized six tigecycline-resistant Escherichia fergusonii strains from porcine nasal swab samples collected from 50 swine farms in China. All the E. fergusonii isolates were highly resistant to tigecycline with minimal inhibitory concentration (MIC) values of 16-32 mg/L, and all contained the tet(X4) gene. In addition, 13-19 multiple resistance genes were identified in these isolates, revealed by whole-genome sequencing analysis. The tet(X4) gene was identified as being located in two different genetic structures, hp-abh-tet(X4)-ISCR2 in five isolates and hp-abh-tet(X4)-ΔISCR2-ISEc57-IS26 in one isolate. The role of efflux pumps in tigecycline resistance was evaluated by using inhibitor carbonyl cyanide 3-chlorophenylhydrazone (CCCP). The MIC values of tigecycline showed a 2- to 4-fold reduction in the presence of CCCP, indicating the involvement of active efflux pumps in tigecycline resistance in E. fergusonii. The tet(X4) gene was found to be transferable to Escherichia coli J53 by conjugation and resulted in the acquisition of tigcycline resistances in the transconjugants. Whole-genome multilocus sequence typing (wgMLST) and phylogenetic analysis showed a close relationship of five isolates originating from different pig farms, suggesting the transmission of tet(X4)-positive E. fergusonii between farms. In conclusion, our findings suggest that E. fergusonii strains in pigs are reservoirs of a transferable tet(X4) gene and provide insights into the tigecycline resistance mechanism as well as the diversity and complexity of the genetic context of tet(X4) in E. fergusonii.

Phenotypic Characterization and Heterogeneity among Modern Clinical Isolates of Acinetobacter baumannii

Acinetobacter baumannii is an opportunistic pathogenic bacterium prioritized by WHO and CDC because of its increasing antibiotic resistance. Heterogeneity among strains represents the hallmark of A. baumannii bacteria. We wondered to what extent extensively used strains, so-called reference strains, reflect the dynamic nature and intrinsic heterogeneity of these bacteria. We analyzed multiple phenotypic traits of 43 nonredundant, modern, and multidrug-resistant, extensively drug-resistant, and pandrug-resistant clinical isolates and broadly used strains of A. baumannii. Comparison of these isolates at the genetic and phenotypic levels confirmed a high degree of heterogeneity. Importantly, we observed that a significant portion of modern clinical isolates strongly differs from several historically established strains in the light of colony morphology, cellular density, capsule production, natural transformability, and in vivo virulence. The significant differences between modern clinical isolates of A. baumannii and established strains could hamper the study of A. baumannii, especially concerning its virulence and resistance mechanisms. Hence, we propose a variable collection of modern clinical isolates that are characterized at the genetic and phenotypic levels, covering a wide range of the phenotypic spectrum, with six different macrocolony type groups, from avirulent to hypervirulent phenotypes, and with naturally noncapsulated to hypermucoid strains, with intermediate phenotypes as well. Strain-specific mechanistic observations remain interesting per se, and established "reference" strains have undoubtedly been shown to be very useful to study basic mechanisms of A. baumannii biology. However, any study based on a specific strain of A. baumannii should be compared to modern and clinically relevant isolates. IMPORTANCE Acinetobacter baumannii is a bacterium prioritized by the CDC and WHO because of its increasing antibiotic resistance, leading to treatment failures. The hallmark of this pathogen is the high heterogeneity observed among isolates, due to a very dynamic genome. In this context, we tested if a subset of broadly used isolates, considered "reference" strains, was reflecting the genetic and phenotypic diversity found among currently circulating clinical isolates. We observed that the so-called reference strains do not cover the whole diversity of the modern clinical isolates. While formerly established strains successfully generated a strong base of knowledge in the A. baumannii field and beyond, our study shows that a rational choice of strain, related to a specific biological question, should be taken into consideration. Any data obtained with historically established strains should also be compared to modern and clinically relevant isolates, especially concerning drug screening, resistance, and virulence contexts.

Acinetobacter baumannii: Pathogenesis, virulence factors, novel therapeutic options and mechanisms of resistance to antimicrobial agents with emphasis on tigecycline

WHAT IS KNOWN AND OBJECTIVE

Acinetobacter baumannii is one of the most important nosocomial pathogens with the ability to cause infections such as meningitis, pneumonia, urinary tract, septicaemia and wound infections. A wide range of virulence factors are responsible for pathogenesis and high mortality of A. baumannii including outer membrane proteins, lipopolysaccharide, capsule, phospholipase, nutrient- acquisition systems, efflux pumps, protein secretion systems, quarom sensing and biofilm production. These virulence factors contribute in pathogen survival in stressful conditions and antimicrobial resistance.

COMMENT

According to the World Health Organization (WHO), A. baumannii is one of the most resistant pathogens of ESKAPE group (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, A. baumannii, Pseudomonas aeruginosa and Enterobacter spp.). In recent years, resistance to a wide range of antibiotics in A. baumannii has significantly increased and the high emergence of extensively drug resistant (XDR) isolates is challenging. Among therapeutic antibiotics, resistance to tigecycline as a last resort antibiotic has become a global concern. Several mechanisms are involved in tigecycline resistance, the most important of which is RND (Resistance-Nodulation-Division) family efflux pumps overexpression. The development of new therapeutic strategies to confront A. baumannii infections has been very promising in recent years.

WHAT IS NEW AND CONCLUSION

In the present review we highlight microbiological and virulence traits in A. baumannii and peruse the tigecycline resistance mechanisms and novel therapeutic options. Among the novel therapeutic strategies we focus on combination therapy, drug repurposing, novel antibiotics, bacteriophage therapy, antimicrobial peptides (AMPs), human monoclonal antibodies (Hu-mAbs), nanoparticles and gene editing.

The frequency of efflux pump genes expression in Acinetobacter baumannii isolates from pulmonary secretions

Acinetobacter baumannii is an important opportunistic pathogen, and the cause of nosocomial infections worldwide in recent decades. Efflux pumps are considered as the important causes of multidrug resistance of A. baumannii. The aim of this study was to determine the frequency of efflux pump genes, and evaluate the antibiotic effect of Tigecycline on the expression of adeB gene in isolates of multidrug-resistant. A. baumannii. 70 isolates of A. baumannii were collected and confirmed by biochemical and molecular tests. Antibiotic resistance (Ciprofloxacin, Trimethoprim-sulfamethoxazole, and Tigecycline) was performed based on the minimum inhibitory concentration (MIC) method. Then, the effect of Carbonyl cyanide m-chlorophenyl hydrazone inhibitor (CCCP) on isolates was investigated and the frequency of adeB, adeG, adeJ and abeM genes were examined by PCR for isolates with reduced in MIC titer. Also, the antibiotic effect of Tigecycline on adeB gene expression in A. baumannii isolates was analyzed by Real-Time PCR. The antibiotic resistance for Ciprofloxacin, Trimethoprim-sulfamethoxazole, and Tigecycline was 97.1%, 95.8% and 37.2%, respectively. Following CCCP inhibitor use, the MIC titer had a decrease in MIC titer containing CCCP inhibitor was 64.3% for Ciprofloxacin, 51.5% for Trimethoprim-sulfamethoxazole and 50% for Tigecycline. The frequencies of genes associated with adeB, adeG, adeJ and abeM efflux pump were 100%, 92.8%, 86% and 98.5%, respectively. Real-Time PCR results showed a correlation between the antibiotic effects of Tigecycline on adeB gene expression. The antibiotic resistance of the isolates was relatively high. The isolates were resistant to Ciprofloxacin and Trimethoprim-sulfamethoxazole antibiotics, while more sensitive to Tigecycline. Also, efflux pump genes, which are the antibiotic resistance factors of A. baumannii, are frequently high in the isolates but it seems that isolates use other effluxe pumps than RND family to exit tigecycline.

The frequencies of genes associated with adeB, adeG, adeJ and abeM efflux pump were 100%, 92.8%, 86% and 98.5%, respectively.

Real-Time PCR results showed a correlation between the antibiotic effects of Tigecycline on adeB gene expression. The antibiotic resistance of the isolates was relatively high.

Also, efflux pump genes, which are the antibiotic resistance factors of A. baumannii, are frequently high in the isolates but it seems that isolates use other effluxe pumps than RND family to exit tigecycline.

Coexistence of blaNDM-1, blaOXA-51, blaOXA-23 and armA in conjunction with novel mutations detected in RND efflux pump regulators in tigecycline resistant clinical isolates of Acinetobacter baumannii

This study has investigated a total of 51 A. baumannii isolates for the prevalence of resistant determinants in tigecycline susceptible and non-susceptible clinical isolates of A. baumannii. Antimicrobial susceptibility testing revealed 74% of isolates were tigecycline resistant. Mutations in RND-efflux pump regulatory genes and the expression of efflux pump genes were measured in tigecycline resistant isolates. There was a strong co-relation between the blaNDM-1 and armA wherein majority of the isolates that are positive for blaNDM-1 have also harbored armA. Compared with TSAB (tigecycline susceptible A. baumannii), TNAB (tigecycline non-susceptible A. baumannii) isolates show increased distribution of blaNDM-1 (p = 0.048), blaIMP-1 (p<0.0001) and blaOXA-51 (p = 0.0029) carbapenemase genes. The variants of RND-efflux pump regulatory genes due to amino-acid mutations in adeS (F12S, K84E, W61R, N268H and Q299R) and adeL (G21R and Q262R) were identified in tigecycline resistant isolates as well as ISAba1 mediated disruption of adeN were observed causing overexpression of adeIJK efflux pump. Additionally, mutations in adeRS were also associated with increased expression of adeABC efflux pump. Besides, TNAB isolates showed significantly (p<0.0001) higher ability of biofilm formation as compared to TSAB isolates. The tigecycline resistance due to mutations in contemporary A. baumannii isolates having a higher ability to form biofilm may pose therapeutic difficulties.

The effect of the efflux pump inhibitor Carbonyl Cyanide m-Chlorophenylhydrazone (CCCP) on the susceptibility to imipenem and cefepime in clinical strains of Acinetobacter baumannii

Introduction

In the last years the rapid expansion of multidrug-resistant A. baumannii strains have become a major health problem. Efflux pumps are a group of transport proteins that contribute to the development of antibiotic resistance. The aim of this study was to evaluate the effect of the efflux pump inhibitor carbonyl cyanide 3-chlorophenylhydrazone (CCCP) on the antimicrobial action of imipenem and cefepime on clinical strains of A. baumannii.

Materials and methods

A total of 49 non-duplicate clinical samples were collected during January through December of 2018 from patients hospitalized in the Hospital Regional Docente de Cajamarca. Of the 49 samples obtained, the confirmatory identification of A. baumannii was performed on 47 samples by molecular methods. The amplification of the blaOXA-51-like gene was carried out by polymerase chain reaction (PCR). The determination of the minimum inhibitory concentration (MIC) was calculated using the microdilution method in culture broth. The susceptibility to both antibiotics (cefepime and imipenem) was evaluated in the presence and absence of the inhibitor carbonyl cyanide 3-chlorophenylhydrazone (CCCP).

Results

A total of 47 strains of A. baumannii were isolated: 97.87% (46/47) were resistant to Imipenem, 2.13% (1/47) of them were classified as intermediate and none of these strains were susceptible. On the other hand, 51.06% (24/47) of isolates were resistant to cefepime; 19.15% (9/47) intermediate and 29.79% (14/47) susceptible. We considered a significant difference in antibiotic susceptibility if the MIC changed at least 4 dilutions, after the addition of the inhibitor. In the case of CCCP in addition to imipenem, 2.1% (1/47) had a significant change of 4 or more reductions in MIC, 59.6% (28/47) achieved a change equal or less than 3 dilutions and 17.0% (8/47) did not have any change. In the case of CCCP with cefepime the percentage of strains with the significant change of MIC was 8.5% (4/47). On the other hand, 53.2% (24/47) presented a reduction equal or less than 3 dilutions and 12.8% (6/47) did not show changes.

Conclusion

In conclusion, our results demonstrate that the use of CCCP may improve the antibiotic effect of imipenem and cefepime on clinical strains of A. baumannii. The relevance of this study is that it provides evidence that this efflux pump inhibitor may be an alternative treatment against multidrug-resistant A. baumannii.

Efflux Pump Activity and Mutations Driving Multidrug Resistance in Acinetobacter baumannii at a Tertiary Hospital in Pretoria, South Africa

Acinetobacter baumannii (A. baumannii) has developed several resistance mechanisms. The bacteria have been reported as origin of multiple outbreaks. This study aims to investigate the use of efflux pumps and quinolone resistance-associated genotypic mutations as mechanisms of resistance in A. baumannii isolates at a tertiary hospital. A total number of 103 A. baumannii isolates were investigated after identification and antimicrobial susceptibility testing by VITEK2 followed by PCR amplification of bla OXA-51 . Conventional PCR amplification of the AdeABC efflux pump (adeB, adeS, and adeR) and quinolone (parC and gyrA) resistance genes were performed, followed by quantitative real-time PCR of AdeABC efflux pump genes. Phenotypic evaluation of efflux pump expression was performed by determining the difference between the MIC of tigecycline before and after exposure to an efflux pump inhibitor. The Sanger sequencing method was used to sequence the parC and gyrA amplicons. A phylogenetic tree was drawn using MEGA 4.0 to evaluate evolutionary relatedness of the strains. All the collected isolates were bla OXA-51 -positive. High resistance to almost all the tested antibiotics was observed. Efflux pump was found in 75% of isolates as a mechanism of resistance. The study detected parC gene mutation in 60% and gyrA gene mutation in 85%, while 37% of isolates had mutations on both genes. A minimal evolutionary distance between the isolates was reported. The use of the AdeABC efflux pump system as an active mechanism of resistance combined with point mutation mainly in gyrA was shown to contribute to broaden the resistance spectrum of A. baumannii isolates.

Antibiogram Profile of Acinetobacterbaumannii Recovered from Selected Freshwater Resources in the Eastern Cape Province, South Africa

Acinetobacter species have been found in a variety of environments, including soil, food, plants, hospital environments and water. Acinetobacter baumannii is an opportunistic and emerging waterborne pathogen. It has been implicated in several nosocomial infections that demonstrate resistance to commonly administered antibiotics. We investigated phenotypic antibiotic resistance (PAR) and relevant antibiotic resistance genes (ARGs) in A. baumannii isolated from three freshwater resources in the Eastern Cape Province, South Africa; A. baumannii (410) was confirmed by the recA and gyrB genes of 844 suspected Acinetobacter species in the water samples. The PAR of the confirmed isolates was assessed using a panel of 11 antibiotics by the disc diffusion method, while ARGs were investigated in isolates exhibiting PAR. The A. baumannii isolates were resistant to piperacillin-tazobactam (11.2%), ceftazidime (12%), cefotaxime (18.8%), cefepime (8.8%), imipenem (2.7%), meropenem (4.15%), amikacin (2.4%), gentamicin (8.8%), tetracycline (16.8%), ciprofloxacin (11%) and trimethoprim/sulfamethoxazole (20.5%). For multidrug resistance (MDR), two isolates were resistant to all antibiotics and 28 isolates were resistant to imipenem and meropenem. Moreover, β-lactamases bla_TEM (64.4%) and bla_OXA-51 (28.70%) as well as sulphonamides sul1 (37.1%) and sul2 (49.4%) were common ARGs. Overall, PAR and ARGs had positive correlations (r) in all rivers. Detection of MDR-A. baumannii in freshwater resources could be linked to possible wastewater discharge from the nearby animal farms, indicating potential implications for public health.

Overexpression of the adeB Efflux Pump Gene in Tigecycline-Resistant Acinetobacter baumannii Clinical Isolates and Its Inhibition by (+)Usnic Acid as an Adjuvant

Acinetobacter species are among the most life-threatening Gram-negative bacilli, causing hospital-acquired infections, and they are associated with high morbidity and mortality. They show multidrug resistance that acts via various mechanisms. In Acinetobacter baumannii, efflux pump-mediated resistance to many antimicrobial compounds, including tigecycline, has been widely reported. Natural compounds have been used for their various pharmacological properties, including anti-efflux pump activity. The present study aimed to evaluate the efflux pump-mediated resistance mechanism of Acinetobacter baumannii and the effect of (+)Usnic acid as an efflux pump inhibitor with tigecycline. For detecting the efflux pump activity of tigecycline-resistant Acinetobacter baumannii isolates, microbroth dilution method and real-time quantitative reverse transcription–polymerase chain reaction was used. (+)Usnic acid was added to tigecycline and tested by the checkerboard method to evaluate its efficacy as an efflux pump inhibitor. qRT-PCR analysis was carried out to show the downregulation of the efflux pump in the isolates. Out of 42 tigecycline-resistant Acinetobacter baumannii isolates, 19 showed efflux pump activity. All 19 strains expressed the adeB gene. (+)Usnic acid as an adjuvant showed better efficacy in lowering the minimum inhibitory concentration compared with the conventional efflux pump inhibitor, carbonyl cyanide phenylhydrazone.

Growth of Stenotrophomonas maltophilia and expression of Sme efflux pumps encoding genes in the presence of supernatants from amoebal and bacterial co-cultures: towards the role of amoebal secondary metabolites

Resistance-Nodulation-Division (RND) efflux pumps are relevant determinants of Stenotrophomonas maltophilia multidrug resistance as they can extrude a broad range of antibiotics and compounds involved in virulence and physiological functions. S. maltophilia, an environmental bacterium, was shown to be associated with amoebae and able to multiply inside them. To explore whether S. maltophilia RND efflux pumps play a role when interacting with amoebae, we evaluated the effect of amoebal culture and co-culture supernatants on the growth of S. maltophilia and the expression of sme efflux pump genes. Acanthamoeba castellanii and Willaertia magna were used as amoebal models and strain S. maltophilia BurE1 as bacterial one. Our data showed that both bacterial growth and sme gene expression were not modified by amoebal culture supernatants. On the contrary, co-culture supernatants negatively impacted the growth of BurE1 and induced the expression of three out of eight efflux pump genes, i.e. smeE, smeN and smeZ. Finally, we evidenced the production of A. castellanii secondary metabolites, putatively belonging to the diterpene family, in the amoebal supernatant and in the co-culture supernatant of A. castellanii and BurE1. Whether these compounds act directly as substrates of the efflux pumps and/or inducers of the sme genes need further investigations.

Tetracycline resistance mediated by tet efflux pumps in clinical isolates of Acinetobacter baumannii

Acinetobacter baumannii is one of the most frequent nosocomial pathogen capable of acquiring resistance to different antimicrobials. The aim of this study was to investigate the activity of tetracycline, doxycycline and minocycline, the prevalence of tet(A) and tet(B) determinants, and the role of efflux pump in tetracycline resistance among the A. baumannii clinical isolates. Susceptibility of 98 A. baumannii isolates to tetracyclines was evaluated by disk diffusion method. The presence of active efflux pump was investigated by determination of the minimum inhibitory concentration (MIC) of tetracycline using the carbonyl cyanide 3-chlorophenylhydrazone (CCCP). Polymerase chain reaction (PCR) was performed to investigate the presence of tet(A) and tet(B) determinants in tetracycline-resistant isolates. The rate of resistance to tetracycline, doxycycline and minocycline was 47.95%, 0%, and 30.61%, respectively. Among the 47 tetracycline-resistant isolates, 29.79% were originated from burned patients and showed MIC ranging from 128-256 μg/mL with both MIC 50 and MIC90 values of 256 μg/mL, while 70.21% were from ventilator-associated pneumonia (VAP) patients and had MIC values ranging from 32-1024 μg/mL, with MIC50 and MIC90 of 512 μg/mL and 1024 μg/mL, respectively. The tet(B) gene was found in 61.7% of tetracycline-resistant isolates, while none of the isolates carried the tet(A) gene. CCCP led to 2-128-fold reduction in tetracycline MIC of the tested isolates. The results showed that doxycycline and minocycline are promising agents for the treatment of A. baumannii infections. This study has also revealed the role of efflux activity in the resistance to tetracycline of A. baumannii isolates. The emergence of resistance to these agents is likely due to the spread of clones presenting with a higher prevalence of resistance determinants.

Synergistic Effect of Tazobactam on Amikacin MIC in Acinetobacter baumannii Isolated from Burn Patients in Tehran, Iran

BACKGROUND

Burn is still an important global public health challenge. Wound colonization of antibiotic resistant bacteria such as Acinetobacter baumannii can lead to high morbidity and mortality in burn patients. The aim of this study was to evaluate the inhibitory effect of tazobactam on efflux pump, which can cause aminoglycoside resistant in A. baumannii isolated from burn patients.

METHODS

In this study, 47 aminoglycoside resistant A. baumannii spp. were obtained from burn patients, admitted to the Shahid Motahari Burns Hospital in Tehran, Iran, during June-August 2018. The inhibitory effect of tazobactam against adeB such as efflux pump was evaluated by Minimum Inhibitory Concentration (MIC) determination of amikacin alone and in combination with tazobactam. Fractional Inhibitory Concentration index (FIC) was used to determine the efficacy of tazobactam/ amikacin combination. Further, semi-quantitative Real- Time PCR was performed to quantify the expression rates of the adeB gene before and after addition of tazobactam/amikacin.

RESULTS

The MIC values were significantly reduced when a combined amikacin and tazobactam was utilized. The most common interaction observed was synergistic (78.2%), followed by.additive effects (21.8%), as per FIC results. The adeB mRNA expression levels were found to be downregulated in 60.7% of isolates treated with tazobactam.

CONCLUSION

Tazobactam can have impact on resistance to aminoglycoside by inhibiting efflux pump. Thus, the combination of tazobactam with amikacin can be used as an alternative treatment approach in multidrug resistant A. baumannii infections.

Evaluating the antimicrobial resistance patterns and molecular frequency of bla oxa-48 and bla GES-2 genes in Pseudomonas aeruginosa and Acinetobacter baumannii strains isolated from burn wound infection in Tehran, Iran

The aim of this study is to evaluate the antimicrobial resistance patterns and molecular frequency of bla_GES-2 and bla_oxa-48 genes in Pseudomonas aeruginosa and Acinetobacter baumannii strains isolated from burn wound infection in Tehran, Iran. In this study, 50 isolates of A. baumannii and 48 isolates of P. aeruginosa were collected from the Burn Unit of Shahid Motahari Hospital at Tehran, Iran. Antibiotic susceptibility tests of all isolates were carried out using the disc diffusion method, and the production of extended-spectrum β-lactamases (ESBLs) in isolates was surveyed by the double disc synergy method and based on CLSI (2019 AST M100) criteria. Finally, the frequency of bla_GES-2 and bla_oxa-48 genes was surveyed by PCR. Antibiotic susceptibility tests showed that 48/48 (100%) of P. aeruginosa isolates and 49/50 (98%) of A. baumannii isolates were resistant to ceftriaxone and cefotaxime, respectively. Ceftazidime exhibited the lowest (26/48; 54.1%) resistance rates against P. aeruginosa isolates. The production of ESBLs was seen in 8/48 (16.6%) and 3/50 (6%) of P. aeruginosa and A. baumannii isolates, respectively. On the basis of conventional PCR and sequencing, the frequencies of the bla_GES-2 gene among P. aeruginosa and A. baumannii was 87.5% and 58%, respectively. Moreover, bla_oxa-48 gene was detected in 70.83% and 92% of P. aeruginosa and A. baumannii isolates, respectively. Results suggest that antibiotic-resistant A. baumannii and P. aeruginosa strains isolated from burn patients are frequently found; therefore, it is absolutely necessary to implement continuous screening and follow-up programmes for detecting antimicrobial resistance.

Confronting Tigecycline-Resistant Acinetobacter baumannii via Immunization Against Conserved Resistance Determinants

Antimicrobial-resistant (AMR) bacterial infections, including those caused by Acinetobacter baumannii, have emerged as a clinical crisis worldwide. Immunization with AMR determinants has been suggested as a novel approach to combat AMR bacteria, but has not been validated. The present study targeted tigecycline (TGC) resistance determinants in A. baumannii to test the feasibility of this approach. Using bioinformatic tools, four candidates, AdeA, AdeI, AdeK, and TolC, belonging to the resistance-nodulation-division (RND) efflux pump were identified as highly conserved and exposed antigens from 15 A. baumannii genomes. Antisera generated from recombinant proteins showed the capability to reserve Hoechst 33342, a substrate of the efflux pump, in bacterial cells. The rTolC antisera had the highest complement-dependent killing and opsonophagocytosis effect compared to the sera from phosphate-buffered saline immunized mice. Among the antisera, anti-rAdeK-specific antisera decreased the minimal inhibitory concentration of TGC in 26.7% of the tested isolates. Immunization with rAdeK significantly potentiated TGC efficacy in treating TGC-resistant A. baumannii pneumonia in the murine model. The bacterial load (7.5 × 10⁵ vs. 3.8 × 10⁷, p < 0.01) and neutrophil infiltration in the peri-bronchial vasculature region of immunized mice was significantly lower compared to the PBS-immunized mice when TGC was administrated concomitantly. Collectively, these results suggest that active immunization against resistance determinants might be a feasible approach to combat multidrug-resistant pathogens in high risk population.

L-lysine potentiates aminoglycosides against Acinetobacter baumannii via regulation of proton motive force and antibiotics uptake

Acinetobacter baumannii, a Gram-negative opportunistic pathogen, is a leading cause of hospital- and community-acquired infections. Acinetobacter baumannii can rapidly acquire diverse resistance mechanisms and undergo genetic modifications that confer resistance and persistence to all currently used clinical antibiotics. In this study, we found exogenous L-lysine sensitizes Acinetobacter baumannii, other Gram-negative bacteria (Escherichia coli and Klebsiella pneumoniae) and a Gram-positive bacterium (Mycobacterium smegmatis) to aminoglycosides. Importantly, the combination of L-lysine with aminoglycosides killed clinically isolated multidrug-resistant Acinetobacter baumannii and persister cells. The exogenous L-lysine can increase proton motive force via transmembrane chemical gradient, resulting in aminoglycoside acumination that further accounts for reactive oxygen species production. The combination of L-lysine and antibiotics highlights a promising strategy against bacterial infection.

Alterations in AdeS and AdeR regulatory proteins in 1-(1-naphthylmethyl)-piperazine responsive colistin resistance of Acinetobacter baumannii

Colistin resistant Acinetobacter baumannii strains are of great concern worldwide. However, the role of efflux pumps in colistin resistance needs to be elucidated. We investigated the changes in colistin MICs of 29 colistin resistant A. baumannii isolates in response to resistance-nodulation-division (RND)-type efflux pump inhibitor (EPI) and the alterations in AdeR and AdeS two-component regulatory proteins previously associated with the overproduction of AdeAB. The EPI, 1-(1-naphthylmethyl)-piperazine (NMP), led to significant reductions in colistin MICs. At least one of the following amino acid substitutions was found in AdeS proteins from 18 of the isolates: L172P, A94V, V27I, V32I, G186V, and G164A. Besides, A136V and V120I alterations were identified in AdeR from five isolates. Therefore, EPI-responsive colistin resistance in our isolates is most likely due to the action of an RND-type efflux system. The underlying mechanism of resistance might be the result of certain AdeRS alterations, leading to AdeAB overexpression.

How Phages Overcome the Challenges of Drug Resistant Bacteria in Clinical Infections

Nowadays the most important problem in the treatment of bacterial infections is the appearance of MDR (multidrug-resistant), XDR (extensively drug-resistant) and PDR (pan drug-resistant) bacteria and the scarce prospects of producing new antibiotics. There is renewed interest in revisiting the use of bacteriophage to treat bacterial infections. The practice of phage therapy, the application of phages to treat bacterial infections, has been around for approximately a century. Phage therapy relies on using lytic bacteriophages and purified phage lytic proteins for treatment and lysis of bacteria at the site of infection. Current research indicates that phage therapy has the potential to be used as an alternative to antibiotic treatments. It is noteworthy that, whether phages are used on their own or combined with antibiotics, phages are still a promising agent to replace antibiotics. So, this review focuses on an understanding of challenges of MDR, XDR, and PDR bacteria and phages mechanism for treating bacterial infections and the most recent studies on potential phages, cocktails of phages, and enzymes of lytic phages in fighting these resistant bacteria.

Association Between Biofilm Formation, Structure, and the Expression Levels of Genes Related to biofilm formation and Biofilm-Specific Resistance of Acinetobacter baumannii Strains Isolated from Burn Infection in Ahvaz, Iran

Background

The ability of biofilm formation is an effective way for Acinetobacter baumannii survival from stressed conditions. This present study was aimed to evaluate the association between biofilm formation, structure, the expression levels of genes related to biofilm formation and biofilm-specific resistance of A. baumannii strains isolated from burn infections in Ahvaz, Iran.

Methods

In this study, we assessed the antibiotic susceptibilities, ERIC-PCR typing, capacity of biofilm formation and biofilm structure of 64 A. baumannii isolates collected from burn infections. The distribution and the expression levels of genes involved in the biofilm formation including bap, ompA, abaI, pgaA and csuE were assessed by PCR and real-time PCR, respectively.

Results

We classified A. baumannii isolates in 14 clonal types of ERIC-PCR. Most A. baumannii isolates were resistant to all antibiotics tested except to tigecycline and colistin and had the biofilm formation capability but with different capacities. There was a significant inverse relationship between resistance to antibiotic agents and biofilm formation. The biofilm matrix of 50 strains consisted of polysaccharides together with DNA or proteins. The genes involved in the biofilm formation were detected in both biofilm-forming and non-biofilm forming; however, the expression levels of these genes were higher in biofilm producers compared with non-producers.

Conclusion

The biofilm cells exhibited dramatically decreased susceptibility to antibiotic agents; hence, they have great significance for public health. Therefore, the determination of antibiotic susceptibilities in biofilm and planktonic mode, molecular typing, and capacity of biofilm formation in clinical setting is essential.

Structural optimization of natural product nordihydroguaretic acid to discover novel analogues as AcrB inhibitors

Drug efflux pumps confer multidrug resistance to dangerous bacterial pathogens which makes these proteins promising drug targets. Herein, we present initial chemical optimization and structure-activity relationship (SAR) data around a previously described efflux pump inhibitor, nordihydroguaretic acid (NDGA). Four series of novel NDGA analogues that target Escherichia coli AcrB were designed, synthesized and evaluated for their ability to potentiate the activity of antibiotics, to inhibit AcrB-mediated substrate efflux and reduce off-target activity. Nine novel structures were identified that increased the efficacy of a panel of antibiotics, inhibited drug efflux and reduced permeabilization of the bacterial outer and inner membranes. Among them, WA7, WB11 and WD6 possessing broad-spectrum antimicrobial sensitization activity were identified as NDGA analogues with favorable properties as potential AcrB inhibitors, demonstrating moderate improvement in potency as compared to NDGA. In particular, WD6 was the most broadly active analogue improving the activity of all four classes of antibacterials tested.

Evaluation of Nano-curcumin effects on expression levels of virulence genes and biofilm production of multidrug-resistant Pseudomonas aeruginosa isolated from burn wound infection in Tehran, Iran

Background

P. aeruginosa is considered as one of the most important pathogens, and high antibiotic resistance to P. aeruginosa has become an alarming concern. This study attempts to further improve curcumin solubility and stability by producing the involved nanoparticle and investigate the effect of this nanoparticle on those virulence genes of P. aeruginosa in pathogenicity and biofilm formation.

Methods

In this study, the curcumin nanoparticles were synthesized and characterized, and the antibacterial and antibiofilm effects of Nano-curcumin and curcumin were investigated by microdilution broth and microtiter plate, respectively. In addition, cytotoxic effect of Nano-curcumin on human epithelial cell lines (A549) was determined. The effects of Nano-curcumin on P. aeruginosa virulence genes, mexD, mexB, and mexT (efflux pumps), lecA (adhesion), nfxB (negative regulator of MexCD-OprJ), and rsmZ (biofilm formation) were determined using real-time quantitative PCR.

Results

Synthesized Nano-curcumins were soluble in water, which inhibited the growth of multidrug-resistant (MDR) P. aeruginosa at 128 µg/mL, whereas it was inhibited at 256 µg/mL for soluble curcumin in DMSO. Sub-inhibitory concentrations of Nano-curcumin reduced biofilm formation and, at 64 μg/mL, disrupted 58% of the established bacterial biofilms. In addition, curcumin nanoparticle downregulated the transcription of virulence genes except nfxB and exerted no cytotoxic effect on human epithelial cell lines (A549).

Conclusions

Results suggest that Nano-curcumin could be potentially used to reduce P. aeruginosa virulence and biofilm. However, in vivo studies with respect to an animal model are necessary to validate these results.

Evaluating the antimicrobial resistance patterns among major bacterial pathogens isolated from clinical specimens taken from patients in Mofid Children's Hospital, Tehran, Iran: 2013-2018

BACKGROUND

This study evaluates the epidemiology and antimicrobial resistance profile of Gram-negative bacteria (GNB) and Gram-positive bacteria (GPB) isolated from clinical specimens in children admitted to Mofid Children's Hospital.

METHODS

This was a retrospective study of the patients' clinical specimens collected from January 2013 until the end of December 2018. All specimens were evaluated to determine the presence of infection-causing agents using a BACTEC 9120 blood culture. Isolation and identification of bacterial strains were performed using conventional biochemical tests. Antibiotic resistance was determined using Kirby-Bauer disk diffusion and broth microdilution methods. Results were interpreted according to CLSI and EUCAST.

RESULTS

A total of 1130 different pathogenic bacteria were detected from 14,690 different clinical specimens and the overall detection rate was 7.7% (1130/14,690). Among bacterial pathogen isolated from clinical specimens, 55% (n=622) were GNB and 45% (n=508) were GPB. The predominant GNB isolates were Pseudomonas aeruginosa, Klebsiella spp., Acinetobacter baumannii, Escherichia coli, Enterobacter spp., Citrobacter spp., respectively. Among GPB, CoNS was the most frequent and Enterococcus spp. was found to have low levels of resistance to linezolid. In GNB, most A. baumannii and P. aeruginosa were ceftriaxone resistant. P. aeruginosa was found to have low levels of resistance to levofloxacin and ciprofloxacin.

CONCLUSIONS

Our findings revealed that the resistance rate among GNB and GPB associated with different infections in children is very high. These results suggest a constant screening and follow-up programs for the detection of antibiotic resistance, and it also suggests to develop antimicrobial stewardship programs in Tehran, Iran.