Interspecies effects in a ceftazidime-treated mixed culture of Pseudomonas aeruginosa, Burkholderia cepacia and Staphylococcus aureus: analysis at the single-species level

Population pharmacokinetics-pharmacodynamics of ceftazidime in neonates and young infants: Dosing optimization for neonatal sepsis

Ceftazidime is a third-generation cephalosporin with high activity against many pathogens. But the ambiguity and diversity of the dosing regimens in neonates and young infants impair access to effective treatment. Thus, we conducted a population pharmacokinetic study of ceftazidime in this vulnerable population and recommended a model-based dosage regimen to optimize sepsis therapy. Totally 146 neonates and young infants (gestational age (GA): 36-43.4 weeks, postnatal age (PNA): 1-81 days, current weight (CW): 900-4500 g) were enrolled based on inclusion and exclusion criteria. Ceftazidime bloods samples (203) were obtained using the opportunistic sampling strategy and determined by the high-performance liquid chromatography. The population pharmacokinetic-pharmacodynamic analysis was conducted by nonlinear mixed effects model (NONMEM). A one-compartment model with first-order elimination best described the pharmacokinetic data. Covariate analysis showed the significance of GA, PNA, and CW on developmental pharmacokinetics. Monte Carlo simulation was performed based on above covariates and minimum inhibitory concentration (MIC). In the newborns with PNA ≤ 3 days (MIC=8 mg/L), the dose regimen was 25 mg/kg twice daily (BID). For the newborns with PNA > 3 days (MIC=16 mg/L), the optimal dose was 30 mg/kg three times daily (TID) for those with GA ≤ 37 weeks and 40 mg/kg TID for those with GA > 37 weeks. Overall, on the basis of the developmental population pharmacokinetic-pharmacodynamic analysis covering the whole range of neonates and young infants, the evidence-based ceftazidime dosage regimens were proposed to optimize neonatal early-onset and late-onset sepsis therapy.

Melting corneal ulcers (keratomalacia) in dogs: A 5-year clinical and microbiological study (2014-2018)

OBJECTIVES

To identify bacterial microorganisms associated with canine keratomalacia, review their antimicrobial sensitivity, and evaluate clinical outcomes compared to results of microbial culture.

METHODS

Retrospective analysis of clinical records of dogs diagnosed with a melting corneal ulcer presented to a referral hospital in Hertfordshire, UK between 2014 and 2018.

RESULTS

One hundred and ten melting corneal ulcers were sampled in 106 dogs. The most common pure bacterial isolate was Pseudomonas aeruginosa (n = 26) followed by β-hemolytic Streptococcus (n = 12). Melting corneal ulcers that cultured coagulase-positive Staphylococcus, coliform bacteria, Pasteurella multocida, Enterococcus, and Streptococcus viridans presented in smaller numbers and were analyzed together (n = 16). Multiple cultures were identified in nine cases (n = 9). Forty-seven cultures yielded no bacterial growth (n = 47). The susceptibility to fluoroquinolones remained high with the exception of β-hemolytic Streptococci. There was no significant difference in the ulcer severity at presentation in regard to the cultured bacteria. Overall, 63 eyes (57%) received surgical grafting in addition to medical treatment. In 14 cases (13%), the progression of corneal melting despite medical ± surgical treatment resulted in enucleation. Fifty-seven percent (8/14) of the enucleated eyes cultured pure Pseudomonas aeruginosa isolates. In contrast, all β-hemolytic Streptococcus-associated ulcers healed.

CONCLUSIONS

The most common bacterial species associated with canine keratomalacia were Pseudomonas aeruginosa and β-hemolytic Streptococcus. Because of the variation in antibacterial sensitivity between these two species, bacterial culture and sensitivity testing should be performed in all dogs presenting with keratomalacia. Melting corneal ulcers associated with pure Pseudomonas infection were significantly more likely to result in globe loss than melting corneal ulcers associated with other cultures.

In Vitro Activity of Fenticonazole against Candida and Bacterial Vaginitis Isolates Determined by Mono- or Dual-Species Testing Assays

We determined the in vitro activity of fenticonazole against 318 vaginitis isolates of Candida and bacterial species and selected 28 isolates for time-kill studies. At concentrations equal to 4× MIC, fenticonazole reached the 99.9% killing endpoint by ∼10 h for Staphylococcus aureus, Streptococcus agalactiae, and Escherichia coli and by ∼17 h for Candida albicans and Candida parapsilosis; and at concentrations equal to 8× MIC, by ∼19 and ∼20 h for Candida glabrata and Candida tropicalis, respectively. At concentrations equal to 2× MIC, fenticonazole required ∼20 h to reach the above endpoint against C. albicans in mixed culture with S. aureus, S. agalactiae, or E. coli versus ∼17 h against C. albicans in pure culture. Supra-MICs are achievable in topically treated patients' vaginal surfaces.

Development of a flow cytometric assay to assess the bacterial count in boar semen

The aim of the study was to develop a new flow cytometric assay for the determination of the bacterial count in commercially processed boar semen. In total 224 fresh boar semen samples collected at an AI-station were analyzed. The number of total viable counts (TVC) was determined by using flow cytometry after staining with SYBR Green I and Propidium Iodide (PI). In the first part of the study 111 fresh boar semen samples were spiked with pure cultures of defined numbers of bacteria commonly detected in boar ejaculates and analyzed by flow cytometry. In the second part, 113 fresh semen samples were assessed on the day of collection through flow cytometry and the Most Probable Number (MPN) method, as the standard bacteriological method. The first part of the study showed a strong correlation between the detected and expected numbers (r = 0.96; P < 0.001), while in the second part of the study the TVC determined by flow cytometry and by the MPN method correlated only moderately (r = 0.28; P < 0.01; median MPN: 24,000 ± MAD 21,600 bacteria/mL; median flow cytometry: 24,426 ± MAD 15,610 bacteria/mL). In summary flow cytometry is a fast alternative to the classical culture technique to determine highly contaminated boar ejaculates. The developed flow cytometric protocol enables one to enumerate the viable bacteria within fresh boar ejaculates without requiring numerous treatment steps, and thus offering the possibility of an on-line use in AI-centers.

Community Composition Determines Activity of Antibiotics against Multispecies Biofilms

In young cystic fibrosis (CF) patients, Staphylococcus aureus is typically the most prevalent organism, while in adults, Pseudomonas aeruginosa is the major pathogen. More recently, it was observed that also Streptococcus anginosus plays an important role in exacerbations of respiratory symptoms. These species are often coisolated from CF lungs, yet little is known about whether antibiotic killing of one species is influenced by the presence of others. In the present study, we compared the activities of various antibiotics against S. anginosus, S. aureus, and P. aeruginosa when grown in monospecies biofilms with the activity observed in a multispecies biofilm. Our results show that differences in antibiotic activity against species grown in mono- and multispecies biofilms are species and antibiotic dependent. Fewer S. anginosus cells are killed by antibiotics that interfere with cell wall synthesis (amoxicillin plus sulbactam, cefepime, imipenem, meropenem, and vancomycin) in the presence of S. aureus and P. aeruginosa, while for ciprofloxacin, levofloxacin, and tobramycin, no difference was observed. In addition, we observed that the cell-free supernatant of S. aureus, but not that of P. aeruginosa biofilms, also caused this decrease in killing. Overall, S. aureus was more affected by antibiotic treatment in a multispecies biofilm, while for P. aeruginosa, no differences were observed between growth in mono- or multispecies biofilms. The results of the present study suggest that it is important to take the community composition into account when evaluating the effect of antimicrobial treatments against certain species in mixed biofilms.

Tobramycin-Treated Pseudomonas aeruginosa PA14 Enhances Streptococcus constellatus 7155 Biofilm Formation in a Cystic Fibrosis Model System

Cystic fibrosis (CF) is a human genetic disorder which results in a lung environment that is highly conducive to chronic microbial infection. Over the past decade, deep-sequencing studies have demonstrated that the CF lung can harbor a highly diverse polymicrobial community. We expanded our existing in vitro model of Pseudomonas aeruginosa biofilm formation on CF-derived airway cells to include this broader set of CF airway colonizers to investigate their contributions to CF lung disease, particularly as they relate to the antibiotic response of the population. Using this system, we identified an interspecies interaction between P. aeruginosa, a bacterium associated with declining lung function and worsening disease, and Streptococcus constellatus, a bacterium correlated with the onset of pulmonary exacerbations in CF patients. The growth rate and cytotoxicity of S. constellatus 7155 and P. aeruginosa PA14 were unchanged when grown together as mixed biofilms in the absence of antibiotics. However, the addition of tobramycin, the frontline maintenance therapy antibiotic for individuals with CF, to a mixed biofilm of S. constellatus 7155 and P. aeruginosa PA14 resulted in enhanced S. constellatus biofilm formation. Through a candidate genetic approach, we showed that P. aeruginosa rhamnolipids were reduced upon tobramycin exposure, allowing for S. constellatus 7155 biofilm enhancement, and monorhamnolipids were sufficient to reduce S. constellatus 7155 biofilm viability in the absence of tobramycin. While the findings presented here are specific to a biofilm of S. constellatus 7155 and P. aeruginosa PA14, they highlight the potential of polymicrobial interactions to impact antibiotic tolerance in unanticipated ways.

IMPORTANCE

Deep-sequencing studies have demonstrated that the CF lung can harbor a diverse polymicrobial community. By recapitulating the polymicrobial communities observed in the CF lung and identifying mechanisms of interspecies interactions, we have the potential to select the best therapy for a given bacterial community and reveal potential opportunities for novel therapeutic interventions. Using an in vitro model of bacterial infection on CF airway cells, we tested how a particular polymicrobial community grows, damages human cells, and responds to antibiotics in single and mixed infections. We describe here the mechanism of an interspecies interaction between two pathogens in the CF lung, P. aeruginosa and S. constellatus, which is potentiated by a commonly prescribed antibiotic, tobramycin.

A microfluidic approach to study the effect of bacterial interactions on antimicrobial susceptibility in polymicrobial cultures

An easy-to-use, lab-on-a-chip platform to rapidly quantify the efficacy of antibiotics to treat polymicrobial infections.

Species-specific viability analysis of Pseudomonas aeruginosa, Burkholderia cepacia and Staphylococcus aureus in mixed culture by flow cytometry

BACKGROUND

Bacterial species coexist commonly in mixed communities, for instance those occurring in microbial infections of humans. Interspecies effects contribute to alterations in composition of communities with respect to species and thus, to the course and severity of infection. Therefore, knowledge concerning growth and viability of single species in medically-relevant mixed communities is of high interest to resolve complexity of interspecies dynamics and to support development of treatment strategies. In this study, a flow cytometric method was established to assess the species-specific viability in defined three-species mixed cultures. The method enables the characterization of viability of Pseudomonas aeruginosa, Burkholderia cepacia and Staphylococcus aureus, which are relevant to lung infections of Cystic Fibrosis (CF) patients. The method combines fluorescence detection by antibody and lectin labeling with viability fluorescence staining using SYBRGreen I and propidium iodide. In addition, species-specific cell enumeration analysis using quantitative terminal restriction fragment length polymorphisms (qT-RFLP) was used to monitor the growth dynamics. Finally, to investigate the impact of substrate availability on growth and viability, concentrations of main substrates and metabolites released were determined.

RESULTS

For each species, the time course of growth and viability during mixed culture cultivations was obtained by using qT-RFLP analysis in combination with flow cytometry. Comparison between mixed and pure cultures revealed for every species differences in growth properties, e.g. enhanced growth of P. aeruginosa in mixed culture. Differences were also observed for B. cepacia and S. aureus in the time course of viability, e.g. an early and drastic reduction of viability of S. aureus in mixed culture. Overall, P. aeruginosa clearly dominated the mixed culture with regard to obtained cell concentrations.

CONCLUSIONS

In combination with qT-RFLP analysis, the methods enabled monitoring of species-specific cell concentrations and viability during co-cultivation of theses strains. Experimental findings suggest that the predominance of P. aeruginosa over B. cepacia and S. aureus in mixed culture under the chosen cultivation conditions is promoted by more efficient substrate consumption of P. aeruginosa, and antagonistic interspecies effects induced by P. aeruginosa.

A flow cytometric method for viability assessment of Staphylococcus aureus and Burkholderia cepacia in mixed culture

Mixed bacterial communities are commonly encountered in microbial infections of humans. Knowledge on the composition of species and viability of each species in these communities allows for a detailed description of the complexity of interspecies dynamics and contributes to the assessment of the severity of infections. Several assays exist for quantification of specific species in mixed communities, including analysis of quantitative terminal restriction fragment length polymorphisms. While this method allows for species-specific cell enumeration, it cannot provide viability data. In this study, flow cytometry was applied to assess the viability of Staphylococcus aureus and Burkholderia cepacia in mixed culture by membrane integrity analysis using SYBR® Green I and propidium iodide staining. Both bacteria are relevant to pulmonary infections of cystic fibrosis patients. Fluorescence staining was optimized separately for each species in pure culture due to differences between species in cell wall structure and metabolic capabilities. To determine viability of species in mixed culture, a protocol was established as a compromise between optimum conditions determined before for pure cultures. This protocol allowed the detection of viable and dead cells of both species, exhibiting an intact and a permeabilized membrane, respectively. To discriminate between S. aureus and B. cepacia, the protocol was combined with Gram-specific fluorescent staining using wheat germ agglutinin. The established three-color staining method was successfully tested for viability determination of S. aureus and B. cepacia in mixed culture cultivations. In addition, growth of both species was monitored by quantitative terminal restriction fragment length polymorphisms. The obtained data revealed alterations in viability during cultivations for different growth phases and suggest interspecies effects in mixed culture. Overall, this method allows for rapid simultaneous Gram-differentiation and viability assessment of bacterial mixed cultures and is therefore suitable for the analysis of dynamics of mixed communities of medical, environmental, and biotechnological relevance.