Antimicrobial susceptibility and mechanisms of resistance to quinolones and beta-lactams in Acinetobacter genospecies 3

Development of Nanoparticle Adaptation Phenomena in Acinetobacter baumannii: Physiological Change and Defense Response

The present work describes the evolution of a resistance phenotype to a multitargeting antimicrobial agent, namely, silver nanoparticles (nanosilver; NAg), in the globally prevalent bacterial pathogen Acinetobacter baumannii. The Gram-negative bacterium has recently been listed as a critical priority pathogen requiring novel treatment options by the World Health Organization. Through prolonged exposure to the important antimicrobial nanoparticle, the bacterium developed mutations in genes that encode the protein subunits of organelle structures that are involved in cell-to-surface attachment as well as in a cell envelope capsular polysaccharide synthesis-related gene. These mutations are potentially correlated with stable physiological changes in the biofilm growth behavior and with an evident protective effect against oxidative stress, most likely as a feature of toxicity defense. We further report a different adaptation response of A. baumannii to the cationic form of silver (Ag+). The bacterium developed a tolerance phenotype to Ag+, which was correlated with an indicative surge in respiratory activity and changes in cell morphology, of which these are reported characteristics of tolerant bacterial populations. The findings regarding adaptation phenomena to NAg highlight the risks of the long-term use of the nanoparticle on a priority pathogen. The findings urge the implementation of strategies to overcome bacterial NAg adaptation, to better elucidate the toxicity mechanisms of the nanoparticle, and preserve the efficacy of the potent alternative antimicrobial agent in this era of antimicrobial resistance. IMPORTANCE Several recent studies have reported on the development of bacterial resistance to broad-spectrum antimicrobial silver nanoparticles (nanosilver; NAg). NAg is currently one of the most important alternative antimicrobial agents. However, no studies have yet established whether Acinetobacter baumannii, a globally prevalent nosocomial pathogen, can develop resistance to the nanoparticle. The study herein describes how a model strain of A. baumannii with no inherent silver resistance determinants developed resistance to NAg, following prolonged exposure. The stable physiological changes are correlated with mutations detected in the bacterium genome. These mutations render the bacterium capable of proliferating at a toxic NAg concentration. It was also found that A. baumannii developed a "slower-to-kill" tolerance trait to Ag+, which highlights the unique antimicrobial activities between the nanoparticulate and the ionic forms of silver. Despite the proven efficacy of NAg, the observation of NAg resistance in A. baumannii emphasises the potential risks of the repeated overuse of this agent on a priority pathogen.

DNA microarray for genotyping antibiotic resistance determinants in Acinetobacter baumannii clinical isolates

In recent decades, Acinetobacter baumannii has emerged as an organism of great concern due to its ability to accumulate antibiotic resistance. In order to improve the diagnosis of resistance determinants in A. baumannii in terms of lead time and accuracy, we developed a microarray that can be used to detect 91 target sequences associated with antibiotic resistance within 4 h from bacterial culture to result. The array was validated with 60 multidrug-resistant strains of A. baumannii in a blinded, prospective study. The results were compared to phenotype results determined by the automated susceptibility testing system VITEK2. Antibiotics considered were piperacillin-tazobactam, ceftazidime, imipenem, meropenem, trimethoprim-sulfamethoxazole, amikacin, gentamicin, tobramycin, ciprofloxacin, and tigecycline. The average positive predictive value, negative predictive value, sensitivity, and specificity were 98, 98, 99, and 94%, respectively. For carbapenemase genes, the array results were compared to singleplex PCR results provided by the German National Reference Center for Gram-Negative Pathogens, and results were in complete concordance. The presented array is able to detect all relevant resistance determinants of A. baumannii in parallel. The short handling time of 4 h from culture to result helps to provide fast results in order to initiate adequate anti-infective therapy for critically ill patients. Another application would be data acquisition for epidemiologic surveillance.

Cationic amphiphilic alpha-helical peptides for the treatment of carbapenem-resistant Acinetobacter baumannii infection

The emergence of multidrug-resistant Gram-negative bacteria, in particular Acinetobacter baumannii and Pseudomonas aeruginosa, is a critical clinical problem worldwide. Antimicrobial peptides (AMPs) have received increasing attention due to their ability to overcome multidrug-resistant microbes. We recently reported that cysteine-functionalized alpha-helical peptides LLKKLLKKC and CLLKKLLKKC effectively eradicated Gram-negative bacteria in vitro. In this study, the antibacterial properties of these peptides against carbapenem-resistant clinical isolates of A. baumannii were studied both in vitro and in vivo. The minimum inhibitory concentrations (MICs) of the peptides against 20 clinical isolates of carbapenem-resistant A. baumannii were determined in comparison with imipenem. The results showed that the A. baumannii isolates were more susceptible to (LLKK)(2)C than to C(LLKK)(2)C in vitro, and 90% of the 20 tested strains had an MIC of lower than or equal to 36.8 and 63.1 μmol/L, respectively. However, the bactericidal effect of C(LLKK)(2)C was much faster than that of (LLKK)(2)C. Furthermore, these peptides also showed excellent potency in mouse models of peritonitis and pneumonia infections caused by carbapenem-resistant A. baumannii. Importantly, both peptides had a high therapeutic index (>25), but caused no significant adverse effects on the liver and kidney functions and the balance of electrolytes in the blood. These peptides can be a promising alternative treatment modality to traditional antibiotics for nosocomial bacterial infections caused by multidrug-resistant Gram-negative bacteria, especially carbapenem-resistant A. baumannii.

Clinical impact and molecular basis of antimicrobial resistance in non-baumannii Acinetobacter

Species of Acinetobacter other than Acinetobacter baumannii are involved in nosocomial infections. Acinetobacter lwoffii, Acinetobacter genomospecies 3 and Acinetobacter genomospecies 13TU are found in community- and nosocomial-acquired infections as well as in neonatal intensive care units. The non-baumannii Acinetobacter are normally highly susceptible to ciprofloxacin, ampicillin/sulbactam, gentamicin and tigecycline. Carbepenems show good activity although resistant isolates have been reported. Resistance to β-lactams other than carbapenems is associated with overexpression of chromosomal cephalosporinases and extended-spectrum β-lactamase acquisition, whereas resistance to carbapenems involves acquisition of carbapenemases. Quinolone resistance is related to gyrA and/or parC mutations but overexpresion of efflux proteins also plays an important role. With the development of novel and more accurate typing methodologies, an increase in infections caused by non-baumannii Acinetobacter might be observed in the future.

Genotypic and phenotypic characterization of the Acinetobacter calcoaceticus-Acinetobacter baumannii complex with the proposal of Acinetobacter pittii sp. nov. (formerly Acinetobacter genomic species 3) and Acinetobacter nosocomialis sp. nov. (formerly Acinetobacter genomic species 13TU)

Acinetobacter genomic species (gen. sp.) 3 and gen. sp. 13TU are increasingly recognized as clinically important taxa within the Acinetobacter calcoaceticus-Acinetobacter baumannii (ACB) complex. To define the taxonomic position of these genomic species, we investigated 80 strains representing the known diversity of the ACB complex. All strains were characterized by AFLP analysis, amplified rDNA restriction analysis and nutritional or physiological testing, while selected strains were studied by 16S rRNA and rpoB gene sequence analysis, multilocus sequence analysis and whole-genome comparison. Results supported the genomic distinctness and monophyly of the individual species of the ACB complex. Despite the high phenotypic similarity among these species, some degree of differentiation between them could be made on the basis of growth at different temperatures and of assimilation of malonate, l-tartrate levulinate or citraconate. Considering the medical relevance of gen. sp. 3 and gen. sp. 13TU, we propose the formal names Acinetobacter pittii sp. nov. and Acinetobacter nosocomialis sp. nov. for these taxa, respectively. The type strain of A. pittii sp. nov. is LMG 1035(T) (=CIP 70.29(T)) and that of A. nosocomialis sp. nov. is LMG 10619(T) (=CCM 7791(T)).

Influence of genospecies of Acinetobacter baumannii complex on clinical outcomes of patients with acinetobacter bacteremia

BACKGROUND

acinetobacter baumannii complex infections are increasing in prevalence and are associated with a high mortality. Biochemical classification tests cannot differentiate A. baumannii (genospecies 2) from other genospecies. Genospecies typing offers a potential tool to determine whether there are major differences in pathogenicity among the genospecies.

METHODS

adult patients with A. baumannii complex bacteremia in intensive care units were prospectively observed from January 2007 through July 2009. A. baumannii complex was identified by biochemical methods and the Phoenix bacterial identification system. Genospecies were identified by 16S-23S ribosomal RNA intergenic-spacer sequencing.

RESULTS

among the 135 patients with A. baumannii complex bacteremia, 87 (64.4%) had isolates that belonged to genospecies 2, 36 (26.7%) had isolates that belonged to genospecies 13TU, and 12 (8.9%) had isolates that belonged to genospecies 3. Patients with A. baumannii (genospecies 2) bacteremia were more likely to have pneumonia than were patients with bacteremia due to genospecies 13TU (63.2 % vs 27.8%; P =.001), whereas patients with bacteremia due to genospecies 13TU were more likely to have primary bacteremia (69.4% vs 20.7%; P <.001). Genospecies 2 was less susceptible to antibiotics than were other genospecies. It was associated with a higher rate of mortality than was genospecies 13TU (58.6% vs 16.7%; P < .001). On multivariate analysis, genospecies 2 was an independent predictor of mortality (odds ratio, 5.46; 95% confidence interval, 2.00-14.91; P = .001).

CONCLUSIONS

genospecies 2 of the A. baumannii complex was associated with greater resistance to antibiotics and higher mortality among bacteremic patients, compared with other genospecies, especially genospecies 13TU. These findings emphasize the need to focus on genospecies to better understand the pathogenesis and epidemiology of infections caused by the A. baumannii complex.

Clinical features of infections and colonization by Acinetobacter genospecies 3

Two hundred twenty-one isolates of Acinetobacter baumannii and 15 of Acinetobacter genospecies 3 (AG3) were consecutively collected in a 30-day period during the nationwide project GEIH-Ab2000. Nosocomial acquisition (P=0.01), intensive care unit admission (P=0.02), and antibiotic pressure (P = 0.03) were observed to be lower in the AG3 group. AG3 isolates were more frequently implied in wound infections (P=0.05), while A. baumannii tended to be recovered from respiratory samples (P=0.08). To our knowledge, this is the first report analyzing the clinical differences among Acinetobacter genospecies, with our findings suggesting that clinical features of AG3 may not be equivalent to those traditionally described for A. baumannii.

OXA-24 carbapenemase gene flanked by XerC/XerD-like recombination sites in different plasmids from different Acinetobacter species isolated during a nosocomial outbreak

A clinical strain of Acinetobacter calcoaceticus resistant to carbapenems was isolated from a blood culture sample from an inpatient in a hospital in Madrid (Spain) during a large outbreak of infection (affecting more than 300 inpatients), caused by a multidrug-resistant Acinetobacter baumannii clone. The carbapenem resistance in both the A. calcoaceticus and A. baumannii clones was due to a bla(OXA-24) gene harbored in different plasmids. The plasmids were fully sequenced, revealing the presence of site-specific recombination binding sites putatively involved in mobilization of the bla(OXA-24) gene. Comparison of plasmids contained in the two strains revealed possible horizontal transmission of resistance genes between the Acinetobacter species.

Antimicrobial susceptibility and mechanisms of resistance in Shigella and Salmonella isolates from children under five years of age with diarrhea in rural Mozambique

The antimicrobial susceptibility and mechanisms of resistance of 109 Shigella and 40 Salmonella isolates from children with diarrhea in southern Mozambique were assessed. The susceptibility to seven antimicrobial agents was tested by disk diffusion, and mechanisms of resistance were searched by PCR or colorimetric method. A high proportion of Shigella isolates were resistant to chloramphenicol (Chl) (52%), ampicillin (Amp) (56%), tetracycline (Tet) (66%), and trimethoprim-sulfamethoxazole (Sxt) (84%). Sixty-five percent of the isolates were multidrug resistant. Shigella flexneri isolates were more resistant than those of Shigella sonnei to Amp (66% versus 0.0%, P < 0.001) and Chl (61% versus 0.0%, P < 0.001), whereas S. sonnei isolates presented higher resistance to Tet than S. flexneri isolates (93% versus 64%, P = 0.02). Resistance among Salmonella isolates was as follows: Tet and Chl, 15% each; Sxt, 18%; and Amp, 25%. Only 3% of Salmonella isolates were resistant to nalidixic acid (Nal), and none to ciprofloxacin or ceftriaxone (Cro). Among Salmonella isolates, multiresistance was found in 23%. Among Shigella isolates, antibiotic resistance was related mainly to the presence of oxa-1-like beta-lactamases for Amp, dfrA1 genes for Sxt, tetB genes for Tet, and Chl acetyltransferase (CAT) activity for Chl. Among Salmonella isolates, resistance was conferred by tem-like beta-lactamases for Amp, floR genes and CAT activity for Chl, tetA genes for Tet, and dfrA1 genes for Sxt. Our data show that Shigella isolates are resistant mostly to the most available, inexpensive antibiotics by various molecular mechanisms but remain susceptible to ciprofloxacin, Cro, and Nal, which is the first line for empirical treatment of shigellosis in the country.

Genetic variability among ampC genes from acinetobacter genomic species 3

As a part of a nationwide study in Spain, 15 clinical isolates of Acinetobacter genomic species 3 (AG3) were analyzed. The main objective of the study was to characterize the ampC genes from these isolates and to determine their involvement in beta-lactam resistance in AG3. The 15 AG3 isolates showed different profiles of resistance to ampicillin (range of MICs, 12 to >256 microg/ml). Nucleotide sequencing of the 15 ampC genes yielded 12 new AmpC enzymes (ADC-12 to ADC-23). The 12 AG3 enzymes showed 93.7 to 96.1% amino acid identity with respect to the AmpC enzyme from Acinetobacter baumannii (ADC-1 enzyme). Eight out of fifteen ampC genes were expressed in Escherichia coli cells under the control of a common promoter, and with the exception of one isolate (isolate 65, which showed lower beta-lactam MICs), significant differences in overall beta-lactam MICs for E. coli cells expressing AG3 ampC genes were not revealed. No significant differences in ampC gene expression in AG3 clinical isolates were revealed by reverse transcription-PCR analysis. A detailed analysis of the 12 AmpC protein sequences revealed that amino acid replacements (in comparison with those of ADC-1) occurred mainly in the same positions, although none were located in important functional domains such as the Omega- loop or conserved beta-lactamase motifs. Kinetic experiments performed with three representative AmpC enzymes (ADC-14, -16, and -18) in some cases revealed dramatic changes in K(m) and k(cat) values for beta-lactams. No ISAba1 was detected upstream of the ampC genes. Our results reveal 12 new ampC genes in AG3. The enzymes showed a moderate degree of variability, and they are tentatively named ADC-12 to ADC-23.

Characterization of the carbapenem-hydrolyzing oxacillinase oxa-58 in an Acinetobacter genospecies 3 clinical isolate

Based on imipenem resistance in an Acinetobacter genospecies 3 clinical isolate, we were able to identify, for the first time in this genomic species, a plasmid-encoded bla(OXA-58) gene that was 100% homologous to the same gene in Acinetobacter baumannii.

Risk-factors for the acquisition of imipenem-resistant Acinetobacter baumannii in Spain: a nationwide study

Potential risk-factors for the acquisition of imipenem-resistant Acinetobacter baumannii were investigated in a cohort study in 25 Spanish hospitals. The clonal relationship among isolates was determined by pulsed-field gel electrophoresis (PFGE). In total, A. baumannii was isolated from 203 patients, with imipenem-resistant (MIC(90) 128 mg/L) isolates being obtained from 88 patients (43%), and imipenem-susceptible isolates from 115 patients (57%). A wide clonal distribution was observed among the imipenem-resistant isolates, but spread of the same clone among centres was not demonstrated. The results indicated that imipenem-resistant A. baumannii is a widely distributed nosocomial pathogen in Spain and reaches an alarming frequency in some centres. Independent risk-factors for the acquisition of imipenem-resistant A. baumannii were a hospital size of >500 beds (multivariate OR, 6.5; 95% CI, 1.8--23), previous antimicrobial treatment (multivariate OR, 4.3; 95% CI, 1.6--11), a urinary catheter (multivariate OR, 2.7; 95% CI, 1.1--6.7) and surgery (multivariate OR, 2; 95% CI, 1.07--3.8).