Activity of vancomycin combined with linezolid against clinical vancomycin-resistant Enterococcus strains

INTRODUCTION

Because multi-drug-resistant Gram-positive bacteria have been isolated frequently worldwide and are difficult to treat, alternative treatment choices are required. Combination antibiotherapies have a distinct advantage over monotherapies in terms of their broad spectrum and synergistic effect. In the present study, it was aimed to investigate the in vitro activity of vancomycin combined with linezolid against clinical vancomycin-resistant enterococci (VRE) strains with high-level aminoglycoside resistance.

MATERIAL AND METHODS

A total of 30 randomly selected clinical VRE strains were studied. Susceptibility to agents tested was investigated using broth microdilution assay. The inoculum of strain was adjusted to approximately 5 × 10⁵ CFU/ml in the wells. The results were interpreted in accordance with Clinical and Laboratory Standards Institute guidelines. In vitro activities of antibiotics in combination were assessed using the broth microcheckerboard technique. The fractional inhibitory concentration indexes (FICIs) were interpreted as follows: synergism, FICI ≤ 0.5; additive/indifference, FICI ≤ 0.5 - ≤ 4; antagonism, FICI > 4.

RESULTS

All strains were resistant to vancomycin and susceptible to linezolid. The MIC_50,90 and MIC_range values of antimicrobials were 512, 512, and 512-1024 μg/ml for vancomycin; 2, 2, and 2-4 μg/ml for linezolid. The rate of synergy was found to be 46.6% (14/30) for linezolid combined with vancomycin. No antagonism was observed.

CONCLUSIONS

The results of the study suggest that this combination may contribute to the treatment of VRE infections for their synergistic effect and because no antagonism was observed.

Characteristics of High-Level Aminoglycoside-Resistant Enterococcus faecalis Isolated from Bulk Tank Milk in Korea

Enterococci, which are considered environmental mastitis-causing pathogens, have easily acquired aminoglycoside-resistant genes that encode various aminoglycoside-modifying enzymes (AME). Therefore, this study was conducted to compare the distribution of high-level aminoglycoside-resistant (HLAR) and multidrug-resistant (MDR) Enterococcus faecalis (E. faecalis) bacteria isolated from bulk tank milk in four dairy companies in Korea. Moreover, it analyzed the characteristics of their antimicrobial resistance genes and virulence factors. Among the 301 E. faecalis bacteria studied, 185 (61.5%) showed HLAR with no significant differences among the dairy companies. Furthermore, 129 (69.7%) of the 185 HLAR E. faecalis showed MDR without significant differences among companies. In contrast, HLAR E. faecalis from companies A, B, and C were significantly higher in resistance to the four classes than those in company D, which had the highest MDR ability against the three antimicrobial classes (p < 0.05). In addition, in the distribution of AME genes, 72 (38.9%) and 36 (19.5%) of the isolates carried both aac(6')Ie-aph(2″)-la and ant(6)-Ia genes, and the ant (6)-Ia gene alone, respectively, with significant differences among the companies (p < 0.05). In the distribution of virulence genes, the ace (99.5%), efa A (98.9%), and cad 1 (98.4%) genes were significantly prevalent (p < 0.05). Thus, our results support that an advanced management program by companies is required to minimize the dissemination of antimicrobial resistance and virulence factors.

High-Level Aminoglycoside Resistance in Human Clinical Klebsiella pneumoniae Complex Isolates and Characteristics of armA-Carrying IncHI5 Plasmids

Aminoglycosides are important options for treating life-threatening infections. However, high levels of aminoglycoside resistance (HLAR) among Klebsiella pneumoniae isolates have been observed to be increasing frequently. In this study, a total of 292 isolates of the K. pneumoniae complex from a teaching hospital in China were analyzed. Among these isolates, the percentage of HLAR strains was 13.7% (40/292), and 15 aminoglycoside resistance genes were identified among the HLAR strains, with rmtB being the most dominant resistance gene (70%, 28/40). We also described an armA-carrying Klebsiella variicola strain KP2757 that exhibited a high-level resistance to all aminoglycosides tested. Whole-genome sequencing of KP2757 demonstrated that the strain contained one chromosome and three plasmids, with all the aminoglycoside resistance genes (including two copies of armA and six AME genes) being located on a conjugative plasmid, p2757-346, belonging to type IncHI5. Comparative genomic analysis of eight IncHI5 plasmids showed that six of them carried two copies of the intact armA gene in the complete or truncated Tn1548 transposon. To the best of our knowledge, for the first time, we observed that two copies of armA together with six AME genes coexisted on the same plasmid in a strain of K. variicola with HLAR. Comparative genomic analysis of eight armA-carrying IncHI5 plasmids isolated from humans and sediment was performed, suggesting the potential for dissemination of these plasmids among bacteria from different sources. These results demonstrated the necessity of monitoring the prevalence of IncHI5 plasmids to restrict their worldwide dissemination.

Prevalence of High-Level Aminoglycoside Resistance and Genes Encoding Aminoglycoside-Modifying Enzymes in Enterococcus faecalis and Enterococcus faecium Isolated in a University Hospital in Tokyo

High-level aminoglycoside resistance (HLAR) limits treatment options for invasive enterococcal infections. We examined the prevalence of HLAR, carriage of genes encoding aminoglycoside-modifying enzymes, and production of β-lactamase using the disk diffusion method, polymerase chain reaction, and a nitrocefin-based test, respectively, in Enterococcus faecalis and Enterococcus faecium isolated from patients at a university hospital in Tokyo in 2010. Of the 100 E. faecalis isolates analyzed, 30 isolates had high-level resistance (HLR) to gentamicin, and 22 isolates had HLR to streptomycin. Of the 40 E. faecium isolates analyzed, 9 isolates had HLR to gentamicin, and 9 isolates had HLR to streptomycin. Of the 39 gentamicin-HLR enterococcal isolates, 24 isolates were non-HLR to streptomycin. All 39 isolates with HLR to gentamicin as well as 19 of 101 without HLR carried aac(6')-Ie-aph(2'')-Ia. Carriage of ant(6')-Ia was confirmed in 25 of 31 streptomycin-HLR isolates. Production of β-lactamase was documented in none of the E. faecalis and E. faecium isolates. Whole-genome sequencing analysis revealed that all but one E. faecalis isolate that carried aac(6')-Ie-aph(2'')-Ia and ant(6')-Ia belonged to sequence type (ST) 4 (n = 8), ST16 (n = 4), or ST179 (n = 9). Nevertheless, most of the pairs of isolates had > 10 single-nucleotide polymorphisms even among the isolates of the same ST.

Study of vancomycin and high-level aminoglycoside-resistant Enterococcus species and evaluation of a rapid spot test for enterococci from Central Referral Hospital, Sikkim, India

BACKGROUND:

Enterococcus is an important pathogen, and with its emergence of resistance to multiple antimicrobials, the management of infection is becoming increasingly difficult.

AIM:

The aim of the study is to determine the prevalence, antibiotic resistance, and risk factors associated with enterococcal infection or colonization.

MATERIALS AND METHODS:

In this prospective study, samples from inpatients were screened for resistant enterococci. Antibiotic susceptibility testing was performed using the disc diffusion method and minimum inhibitory concentration by the agar dilution method. A modification of a test tube method of sodium chloride-esculin hydrolysis to a spot test was evaluated for its rapidity and reliability in the presumptive diagnosis of enterococci.

STATISTICAL ANALYSIS USED:

Fisher's exact test was used for continuous (Student's t-test) and categorical variables. Multivariate analysis was performed with logistic regression using IBM SPSS 20.0 software (Armonk, NY, USA).

RESULTS:

Enterococcus species were isolated from 182 samples: Enterococcus faecalis (68.7%), Enterococcus faecium (20.9%), Enterococcus gallinarum (6%), and Enterococcus durans (4.4%). Maximum resistance was to ciprofloxacin (59.3%) and least to linezolid (0.5%). The isolation rate of vancomycin-resistant enterococci (VRE) was 13.7%; 30.2% and 20.9% were of high-level gentamicin and streptomycin, respectively. All 182 Enterococcus species gave positive results within 30–60 min by the rapid spot test.

CONCLUSIONS:

Overall, high-level aminoglycoside resistance (HLAR) was observed more than glycopeptide resistance. Surveillance strategies need to be upgraded and implemented in order to prevent the emergence and further spread of not only VRE but also HLAR enterococci in the hospital. The spot test gave reliable and rapid results in presumptive identification of enterococci.

High Prevalence of the aac(6')-Ie-aph(2'')-Ia Gene in Hospital Isolates of Enterococcus faecalis Co-Resistant to Gentamicin and Penicillin

Objectives: This study aimed to characterize the molecular mechanism of resistance to gentamicin among penicillin-resistant, ampicillin-susceptible Enterococcus faecalis (PRASEF) isolates by investigating the presence of the aac(6')-Ie-aph(2'')-Ia gene. The co-resistance to antimicrobials of other classes was also evaluated. Results: Among the 151 isolates evaluated, 70 were PRASEF and 81 were penicillin-susceptible and ampicillin-susceptible E. faecalis (PSASEF). No β-lactamase producing isolate was detected. Eighty-three (55.0%) and 35 (23.2%) out of the 151 E. faecalis isolates showed high-level gentamicin resistance (HLGR) and high-level streptomycin resistance (HLSR) phenotypes. However, a significantly higher rate of PRASEF (88.6%) showed HLGR phenotype in comparison with PSASEF (23.5%) (p < 0.01). Conversely, a significantly lower rate of PRASEF (14.3%) showing HLSR was observed in comparison with PSASEF (30.9%) (p = 0.02). The prevalence of isolates displaying multidrug resistance (MDR) phenotype was significantly higher (p < 0.01) in the group of PRASEF (81.4%) than in PSASEF (18.6%). The majority of PSASEF (61.9%) and PRASEF (90.3%) isolates showing HLGR phenotype was harboring the aac(6')-Ie-aph(2'')-Ia gene, which encodes a bifunctional enzyme that inactivates all aminoglycosides except streptomycin. Conclusion: The aac(6')-Ie-aph(2'')-Ia gene was prevalent among the Brazilian PRASEF isolates that usually exhibit co-resistance to gentamicin and to multiple other drugs.

Rising prevalence of antimicrobial resistance in urinary tract infections during pregnancy: necessity for exploring newer treatment options

BACKGROUND

Urinary tract infections (UTI) are one of the most common medical complications of pregnancy. The emergence of drug resistance and particularly the Extended-spectrum beta-lactamase production by Escherichia coli and methicillin resistance in Staphylococci, limits the choice of antimicrobials.

MATERIALS AND METHODS

Patients in different stages of pregnancy with or without symptoms of urinary tract infection attending the antenatal clinic of obstetrics and gynaecology were screened for significant bacteriuria, by standard loop method on 5% sheep blood agar and teepol lactose agar. Isolates were identified by using standard biochemical tests and antimicrobial susceptibility testing was done using Kirby Bauer disc diffusion method.

RESULTS

A total of 4290 (51.2%) urine samples from pregnant females showed growth on culture. Prevalence of asymptomatic bacteriuria 3210 (74.8%) was higher than symptomatic UTI 1080 (25.2%). Escherichia coli was the most common pathogen accounting for 1800 (41.9%) of the urinary isolates. Among the gram-positive cocci, coagulase negative species of Staphylococci 270 (6.4%) were the most common pathogen. Significantly high resistance was shown by the gram negative bacilli as well as gram positive cocci to the β-lactam group of antimicrobials, flouroquinolones and aminoglycosides. Most alarming was the presence of ESBL in 846 (47%) isolates of Escherichia coli and 344 (36.9%) isolates of Klebsiella pneumoniae, along with the presence of methicillin resistance in 41% of Staphylococcus species and high-level aminoglycoside resistance in 45(30%) isolates of Enterococcus species. Glycopeptides and carbepenems were the only group of drugs to which all the strains of gram positive cocci and gram negative bacilli were uniformly sensitive, respectively.

CONCLUSIONS

Regular screening should be done for the presence of symptomatic or asymptomatic bacteriuria in pregnancy and specific guidelines should be issued for testing antimicrobial susceptibility with safe drugs in pregnant women so that these can be used for the treatment. For empirical treatment cefoperazone-sulbactum can be recommended, which is a safe drug, covering both gram positive and gram negative organisms and with a good sensitivity.