In vitro and in vivo synergy of levofloxacin or amikacin both in combination with ceftazidime against clinical isolates of Pseudomonas aeruginosa

Hetero-antagonism of avibactam and sulbactam with cefiderocol in carbapenem-resistant Acinetobacter spp

Cefiderocol, a siderophore-cephalosporine conjugate antibiotic, shows promise as a therapeutic option for carbapenem-resistant (CR) Acinetobacter infections. While resistance has already been reported in A. baumannii, combination therapies with avibactam or sulbactam reduce MICs of cefiderocol, extending its efficacy. However, careful consideration is necessary when using these combinations. In our experiments, exposure of A. baumannii and A. lwoffii to cefiderocol and sulbactam or avibactam led to the selection of cefiderocol-resistant strains. Three of those were subjected to whole genome sequencing and transcriptomic analysis. The strains all possessed synonymous and non-synonymous substitutions and short deletions. The most significant mutations affected efflux pumps, transcriptional regulators, and iron homeostasis genes. Transcriptomics showed significant alterations in expression levels of outer membrane proteins, iron homeostasis, and β-lactamases, suggesting adaptive responses to selective pressure. This study underscores the importance of carefully assessing drug synergies, as they may inadvertently foster the selection of resistant variants and complicate the management of CR Acinetobacter infections.IMPORTANCEThe emergence of carbapenem-resistant Acinetobacter strains as a serious global health threat underscores the urgent need for effective treatment options. Although few drugs show promise against CR Acinetobacter infections, resistance to both drugs has been reported. In this study, the molecular characterization of spontaneous cefiderocol-resistant variants, a CR A. baumannii strain with antagonism to sulbactam, and an A. lwoffii strain with antagonism to avibactam, provides valuable insights into the mechanisms of resistance to cefiderocol. Some mechanisms observed are associated with mutations affecting efflux pumps, regulators, and iron homeostasis genes. These findings highlight the importance of understanding resistance mechanisms to optimize treatment options. They also emphasize the importance of early evaluation of drug synergies to address the challenges of antimicrobial resistance in Acinetobacter infections.

Rapid and Simple Morphological Assay for Determination of Susceptibility/Resistance to Combined Ciprofloxacin and Ampicillin, Independently, in Escherichia coli

Current antibiograms cannot discern the particular effect of a specific antibiotic when the bacteria are incubated with a mixture of antibiotics. To prove that this task is achievable, Escherichia coli strains were treated with ciprofloxacin for 45 min, immobilized on a slide and stained with SYBR Gold. In susceptible strains, the nucleoid relative surface started to decrease near the MIC, being progressively condensed as the dose increased. The shrinkage level correlated with the DNA fragmentation degree. Ciprofloxacin-resistant bacilli showed no change. Additionally, E. coli strains were incubated with ampicillin for 45 min and processed similarly. The ampicillin-susceptible strain revealed intercellular DNA fragments that increased with dose, unlike the resistant strain. Co-incubation with both antibiotics revealed that ampicillin did not modify the nucleoid condensation effect of ciprofloxacin, whereas the quinolone partially decreased the background of DNA fragments induced by ampicillin. Sixty clinical isolates, with different combinations of susceptibility-resistance to each antibiotic, were co-incubated with the EUCAST breakpoints of susceptibility of ciprofloxacin and ampicillin. The morphological assay correctly categorized all the strains for each antibiotic in 60 min, demonstrating the feasible independent evaluation of a mixture of quinolone and beta-lactam. The rapid phenotypic assay may shorten the incubation times and necessary microbial mass currently required for evaluation.

Meropenem potentiation of aminoglycoside activity against Pseudomonas aeruginosa: involvement of the MexXY-OprM multidrug efflux system

Objectives

To assess the ability of meropenem to potentiate aminoglycoside (AG) activity against laboratory and AG-resistant cystic fibrosis (CF) isolates of Pseudomonas aeruginosa and to elucidate its mechanism of action.

Methods

AG resistance gene deletions were engineered into P. aeruginosa laboratory and CF isolates using standard gene replacement technology. Susceptibility to AGs ± meropenem (at ½ MIC) was assessed using a serial 2-fold dilution assay. mexXY expression and MexXY-OprM efflux activity were quantified using quantitative PCR and an ethidium bromide accumulation assay, respectively.

Results

A screen for agents that rendered WT P. aeruginosa susceptible to a sub-MIC concentration of the AG paromomycin identified the carbapenem meropenem, which potentiated several additional AGs. Meropenem potentiation of AG activity was largely lost in a mutant lacking the MexXY-OprM multidrug efflux system, an indication that it was targeting this efflux system in enhancing P. aeruginosa susceptibility to AGs. Meropenem failed to block AG induction of mexXY expression or MexXY-OprM efflux activity, suggesting that it may be interfering with some MexXY-dependent process linked to AG susceptibility. Meropenem potentiated AG activity versus AG-resistant CF isolates, enhancing susceptibility to at least one AG in all isolates and susceptibility to all tested AGs in 50% of the isolates. Notably, meropenem potentiation of AG activity was linked to MexXY in some but not all CF isolates in which this was examined.

Conclusions

Meropenem potentiates AG activity against laboratory and CF strains of P. aeruginosa, both dependent on and independent of MexXY, highlighting the complexity of AG resistance in this organism.

An update on technical, interpretative and clinical relevance of antimicrobial synergy testing methodologies

Testing for antimicrobial interactions has gained popularity in the last decade due to the increasing prevalence of drug-resistant organisms and limited options for the treatment of these infections. In vitro combination testing provides information, on which two or more antimicrobials can be combined for a good clinical outcome. Amongst the various in vitro methods of drug interactions, time-kill assay (TKA), checkerboard (CB) assay and E-test-based methods are most commonly used. Comparative performance of these methods reveals the TKA as the most promising method to detect synergistic combinations followed by CB assay and E-test. Various combinations of antimicrobials have been tested to demonstrate synergistic activity. Promising results were obtained for the combinations of meropenem plus colistin and rifampicin plus colistin against Acinetobacter baumannii, colistin plus carbapenem and carbapenem plus fluoroquinolones against Pseudomonas aeruginosa and colistin/polymyxin B plus rifampicin/meropenem against Klebsiella pneumoniae. Antagonism was detected in only few instances. The presence of synergy or antagonism with a combination seems to correlate with minimum inhibitory concentration of the agent and molecular mechanism involved in the resistance. Further studies need to be conducted to assess the utility of in vitro testing to predict clinical outcome and direct therapy for drug-resistant organisms.

Potentiation of Aminoglycoside Activity in Pseudomonas aeruginosa by Targeting the AmgRS Envelope Stress-Responsive Two-Component System

A screen for agents that potentiated the activity of paromomycin (PAR), a 4,5-linked aminoglycoside (AG), against wild-type Pseudomonas aeruginosa identified the RNA polymerase inhibitor rifampin (RIF). RIF potentiated additional 4,5-linked AGs, such as neomycin and ribostamycin, but not the clinically important 4,6-linked AGs amikacin and gentamicin. Potentiation was absent in a mutant lacking the AmgRS envelope stress response two-component system (TCS), which protects the organism from AG-generated membrane-damaging aberrant polypeptides and, thus, promotes AG resistance, an indication that RIF was acting via this TCS in potentiating 4,5-linked AG activity. Potentiation was also absent in a RIF-resistant RNA polymerase mutant, consistent with its potentiation of AG activity being dependent on RNA polymerase perturbation. PAR-inducible expression of the AmgRS-dependent genes htpX and yccA was reduced by RIF, suggesting that AG activation of this TCS was compromised by this agent. Still, RIF did not compromise the membrane-protective activity of AmgRS, an indication that it impacted some other function of this TCS. RIF potentiated the activities of 4,5-linked AGs against several AG-resistant clinical isolates, in two cases also potentiating the activity of the 4,6-linked AGs. These cases were, in one instance, explained by an observed AmgRS-dependent expression of the MexXY multidrug efflux system, which accommodates a range of AGs, with RIF targeting of AmgRS undermining mexXY expression and its promotion of resistance to 4,5- and 4,6-linked AGs. Given this link between AmgRS, MexXY expression, and pan-AG resistance in P. aeruginosa, RIF might be a useful adjuvant in the AG treatment of P. aeruginosa infections.

In vivo efficacy of the combination of ciprofloxacin and cefotaxime against Vibrio vulnificus sepsis

Objectives

The invivo efficacy of a cefotaxime-ciprofloxacin combination against Vibrio vulnificus and the effects on rtxA1 expression of commonly used antibiotics are unknown.

Methods

In vitro time-kill studies were performed to evaluate synergism. Female BALB/c mice were injected subcutaneously with 1×10⁷ or 1×10⁸ cfu of V. vulnificus. Antibiotic therapy was initiated at 2 h after inoculation in the following four therapy groups: cefotaxime; ciprofloxacin; cefotaxime-plus-ciprofloxacin; and cefotaxime-plus-minocycline. The cytotoxicity of V. vulnificus for HeLa cells was measured using the lactate dehydrogenase assay; rtxA1 transcription was measured in a transcriptional reporter strain using a β-galactosidase assay.

Results

In vitro time-kill assays exhibited synergism between cefotaxime and ciprofloxacin. In the animal experiments, the 96-h survival rate for the cefotaxime-plus-ciprofloxacin group (85%; 17/20) was significantly higher than that of the cefotaxime-plus-minocycline (35%; 7/20) and cefotaxime alone (0%; 0/20) groups (P<0.05 for both). Bacterial counts in the liver and spleen were significantly lower in the cefotaxime-plus-ciprofloxacin group 24 and 48 h after treatment, relative to the other groups. At sub-inhibitory concentrations, ciprofloxacin inhibited more effectively rtxA1 transcription and mammalian cell cytotoxicity than either minocycline or cefotaxime (P<0.05 for both).

Conclusions

Ciprofloxacin is more effective at reducing rtxA1 transcription and subsequent cytotoxicity than either minocycline or cefotaxime, and the combination of ciprofloxacin and cefotaxime was more effective in clearing V. vulnificus invivo than previously used regimens. These data suggest that the combination of ciprofloxacin and cefotaxime is an effective option for the treatment of V. vulnificus sepsis in humans.

Susceptibility of Stenotrophomonas maltophilia clinical strains in China to antimicrobial combinations

We aimed to investigate the activity levels of several combinations of antimicrobials against Stenotrophomonas maltophilia. In this study, the antimicrobial susceptibility of S. maltophilia clinical isolates was determined, and the synergistic activity of three pairs of antimicrobial combinations was evaluated by the fractional inhibitory concentration index (FICI). The antimicrobial susceptibility in vitro against 83 S. maltophilia strains was greater for minocycline (80·7%) than for trimethoprim-sulfamethoxazole (51·8%), and levofloxacin (50·6%). The rate of resistance was highest for ticarcillin-clavulanate and ceftazidime (63·8%) and resistance to trimethoprim-sulfamethoxazole (TMP-SMX) was 48·2%. All three combinations were tested against susceptible isolates. Two of the combinations, TMP-SMX+ceftazidime and levofloxacin+ceftazidime were more effective than the combination of TMP-SMX+levofloxacin. We recommend acquiring more clinical data in order to explore combination therapy, which is a promising treatment of S. maltophilia infections.

Treatment ofPseudomonas aeruginosainfection in critically ill patients

Critically ill patients are on the increase in the present clinical setting. Aging of our population and increasingly aggressive medical and therapeutic interventions, including implanted foreign bodies, organ transplantation and advances in the chemotherapy of malignant diseases, have created a cohort of particularly vulnerable patients. Pseudomonas aeruginosa is one of the leading gram-negative organisms associated with nosocomial infections. This organism is frequently feared because it causes severe hospital-acquired infections, especially in immunocompromised hosts, and is often antibiotic resistant, complicating the choice of therapy. The epidemiology, microbiology, mechanisms of resistance and currently available and future treatment options for the most relevant infections caused by P. aeruginosa are reviewed.

Beta-lactam and fluoroquinolone combination antibiotic therapy for bacteremia caused by gram-negative bacilli

The role of combination antibiotic therapy with a beta-lactam and a fluoroquinolone for bacteremia caused by gram-negative bacilli, to our knowledge, has not been previously described. Much of the previous study of combination therapy has included beta-lactams and aminoglycosides. We conducted a large retrospective cohort study to evaluate 28-day all-cause mortality in patients with monomicrobial bacteremia due to aerobic gram-negative bacilli who received either a combination of beta-lactams and fluoroquinolones or beta-lactam monotherapy. We enrolled adult patients admitted to Mayo Clinic hospitals from 1 January 2001 to 31 October 2006 in the study. After stratification of patients by Pitt bacteremia scores, we used Cox regression models to estimate the hazard ratios (HR) for 28-day all-cause mortality after adjusting for the propensity to receive combination therapy. We identified 398 and 304 unique patients with bacteremia caused by gram-negative bacilli who received single and combination antibiotic therapy, respectively. In less severely ill patients with Pitt bacteremia scores of <4, combination therapy was associated with lower 28-day mortality than single therapy (4.2% [9 of 214] versus 8.8% [28 of 319]; adjusted HR, 0.44; 95% confidence interval [CI], 0.20 to 0.98; P = 0.044). In critically ill patients with Pitt bacteremia scores of >or=4, there was no difference in 28-day mortality between combination and single therapy (25.6% [23 of 90] versus 27.8% [22 of 79]; adjusted HR, 0.87; 95% CI, 0.47 to 1.62; P = 0.660). These findings were consistent for 14-day all-cause mortality. In this large cohort, we found for the first time that combination therapy with beta-lactams and fluoroquinolones was associated with a reduction in 28-day all-cause mortality among less severely ill patients with bacteremia caused by gram-negative bacilli.

[Pseudomonas aeruginosa: combined treatment vs. monotherapy]

Pseudomonas aeruginosa is a pathogen commonly encountered in clinical practice in critically ill patients. It is a serious cause of infection, associated with a high rate of morbidity and mortality. Inappropriate antimicrobial therapy and delay in starting effective antimicrobial therapy is associated with worse prognostic. This microorganism is clinically indistinguishable from others forms of gram-negative bacterial infection. The rate of multidrug-resistant P. aeruginosa has increased in the last years. For these reasons, patients with Pseudomonas infection might receive empirical antibiotics that are inactive against Pseudomonas, especially before antibiotic susceptibility results become available. It remains controversial whether combination therapy, given empirically or as definitive treatment, for suspected Pseudomonas aeruginosa infections is justifiable. In the present article, we aimed to review recent studies that have evaluated the impact of combination therapy on Pseudomonas infections outcome and we exhibit our point of view in this subject. It seems justifiable to start combination therapy with two antipseudomonal agents in patients with risk for Pseudomonas infection during the first 3-5 days, until having microbiological results. This combination therapy must be changed to monotherapy on the basis on the specific susceptibility pattern of the initial isolate. In cases without microbiological diagnosis and poor outcome, combination therapy will be maintained and other causes of infection will be studied. Multicentre prospective randomized trials in critically ill patients are needed to determine which antimicrobials combinations improve outcome in Pseudomonas infections.