Dynamics of microbiota during mechanical ventilation in aspiration pneumonia

Microbiological etiology of aspiration pneumonia in Japan: Insights from a systematic review and meta-analysis

BACKGROUND

Anaerobic bacteria were historically considered the primary causative agent of aspiration pneumonia. However, recent studies suggest their role may have been overemphasized, and the microbial profile of aspiration pneumonia remains uncertain owing to diagnostic limitations. This study explored its microbiological epidemiology through a systematic review and meta-analysis.

METHODS

We searched for English and Japanese articles published since 1990, evaluating the etiological bacterial species associated with aspiration pneumonia using PubMed and Ichushi-Web databases. The detection frequency (%) of each bacterial species was calculated using Review Manager and analyzed separately for Japan and other countries. Regional differences in detection of bacteria between these countries were also compared.

RESULTS

This study included 21 articles: 14 from Japan and 7 from other countries. The most prevalent bacteria were Streptococcus pneumoniae (11.7 %, 95 % confidence interval [CI] 8.5-15.0 %) and Klebsiella pneumoniae (11.8 %, 95 % CI: 2.5-21.1 %), respectively. Gram-negative bacteria such as K. pneumoniae, Escherichia coli, and Pseudomonas aeruginosa were frequently detected, whereas oral streptococci and anaerobic bacteria were uncommon in both regions. Significant regional differences were observed in the detection frequencies of Staphylococcus aureus, S. pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis.

CONCLUSIONS

This study highlights the bacterial profile of aspiration pneumonia and clarified the current understanding, showing that S. pneumoniae and gram-negative bacteria were frequently detected in aspiration pneumonia, whereas anaerobes and oral streptococci were less commonly identified. However, further investigation is needed to better characterize the bacterial spectrum, as a standardized definition of aspiration pneumonia and the pathogenicity of detected microbes remains uncertain.

The lower airway microbiome in paediatric health and chronic disease

The advent of next generation sequencing has rapidly challenged the paediatric respiratory physician's understanding of lung microbiology and the role of the lung microbiome in host health and disease. In particular, the role of "microbial key players" in paediatric respiratory disease is yet to be fully explained. Accurate profiling of the lung microbiome in children is challenging since the ability to obtain lower airway samples coupled with processing "low-biomass specimens" are both technically difficult. Many studies provide conflicting results. Early microbiota-host relationships may be predictive of the development of chronic respiratory disease but attempts to correlate lower airway microbiota in premature infants and risk of developing bronchopulmonary dysplasia (BPD) have produced mixed results. There are differences in lung microbiota in asthma and cystic fibrosis (CF). The increased abundance of oral taxa in the lungs may (or may not) promote disease processes in asthma and CF. In CF, correlation between microbiota diversity and respiratory decline is commonly observed. When one considers other pathogens beyond the bacterial kingdom, the contribution and interplay of fungi and viruses within the lung microbiome further increase complexity. Similarly, the interaction between microbial communities in different body sites, such as the gut-lung axis, and the influence of environmental factors, including diet, make the co-existence of host and microbes ever more complicated. Future, multi-omics approaches may help uncover novel microbiome-based biomarkers and therapeutic targets in respiratory disease and explain how we can live in harmony with our microbial companions.

Bacteriology of Aspiration Pneumonia: The Lung Microbiome and the Changing Microbial Etiology

Aspiration pneumonia refers to the process of alveolar inflammation induced by the inhalation of oropharyngeal secretions into the lower respiratory tract. Predisposing factors comprise swallowing dysfunction, impaired cough reflex, and degenerative neurological diseases. Accumulating evidence projects a fading contribution of anaerobic bacteria in aspiration pneumonia at the expense of Gram-negative bacilli, with Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa, becoming the predominant organisms recovered from respiratory specimens. Aspiration of oropharyngeal secretions colonized with respiratory pathogens induces a profound disequilibrium of the lung microbiota resulting in a state of dysbiosis. Understanding this complex temporal variability between microbiome-host associations was only made possible with the introduction of metagenomic sequencing. In this narrative review, we summarize existing knowledge and elaborate on the evolving microbiology of aspiration pneumonia including the link between oral microbiome and pulmonary aspiration. We also highlight the progress and challenges in instituting microbiome-targeted strategies for preventing and treating the sequelae of aspiration pneumonia.

A comprehensive evaluation of risk factors for mortality, infection and colonization associated with CRGNB in adult solid organ transplant recipients: a systematic review and meta-analysis

BACKGROUND

The burden of carbapenem-resistant gram-negative bacteria (CRGNB) among solid organ transplant (SOT) recipients has not been systematically explored. Here, we discern the risk factors associated with CRGNB infection and colonization in SOT recipients.

METHODS

This study included observational studies conducted among CRGNB-infected SOT patients, which reported risk factors associated with mortality, infection or colonization. Relevant records will be searched in PubMed, Embase and Web of Science for the period from the time of database construction to 1 March 2023.

RESULTS

A total of 23 studies with 13,511 participants were included, enabling the assessment of 27 potential risk factors. The pooled prevalence of 1-year mortality among SOT recipients with CRGNB was 44.5%. Prolonged mechanical ventilation, combined transplantation, reoperation and pre-transplantation CRGNB colonization are salient contributors to the occurrence of CRGNB infections in SOT recipients. Renal replacement therapy, post-LT CRGNB colonization, pre-LT liver disease and model for end-stage liver disease score increased the risk of infection. Re-transplantation, carbapenem use before transplantation and ureteral stent utilization increaesd risk of CRGNB colonization.

CONCLUSION

Our study demonstrated that SOT recipients with CRGNB infections had a higher mortality risk. Invasive procedure may be the main factor contribute to CRGNB infection.

Microbial Composition of the Laryngotracheal Region: A Systematic Review

INTRODUCTION

Microbiome research has predominantly focused on the oral cavity and oropharynx's role in disease, while the upper airway, specifically the larynx and trachea, has been relatively overlooked. Examining the microbial communities in these regions can shed light on how dysbiosis influences diseases and their management. This review evaluates laryngotracheal microbial compositions in both healthy and diseased patients.

METHODS

We conducted a systematic review in EMBASE, MEDLINE, and Cochrane Central databases, yielding 1383 studies in the initial search. Inclusion criteria involved participants aged over 18 years and the use of next-generation 16s ribosomal sequencing methods.

RESULTS

We included 10 studies-seven focused on larynx sequencing and four on trachea sequencing (one investigated both sites). In a healthy larynx, diverse species such as Streptococcus, Cloacibacterium, Prevotella, and Helicobacter were found. Benign laryngeal diseases exhibited reduced microbial diversity, mainly dominated by Streptococcus. Subglottic stenosis patients showed diminished diversity in both idiopathic and iatrogenic scars. Laryngeal squamous cell carcinoma displayed increased diversity, primarily featuring Fusobacterium. Among non-respiratory-compromised surgery patients, the tracheal microbiome was more diverse in diabetics and those later developing lower respiratory infections. Pneumonia patients exhibited an abundance of Prevotella and Streptococcus, linked to an increased 28-day survival rate, while Streptococcus and Haemophilus abundance correlated with successful extubation.

CONCLUSIONS

The laryngotracheal region hosts a unique microbial community influenced by both benign and malignant conditions. Many lesions remain unexplored, underscoring the need for future studies encompassing diverse laryngotracheal conditions. Clinical trials assessing microbiome modifications may unveil novel therapeutic avenues.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:4167-4175, 2024.

Exploring the complex relationship between the lung microbiome and ventilator-associated pneumonia

INTRODUCTION

Understanding the presence and function of a diverse lung microbiome in acute lung infections, particularly ventilator-associated pneumonia (VAP), is still limited, evidencing significant gaps in our knowledge.

AREAS COVERED

In this comprehensive narrative review, we aim to elucidate the contribution of the respiratory microbiome in the development of VAP by examining the current knowledge on the interactions among microorganisms. By exploring these intricate connections, we endeavor to enhance our understanding of the disease's pathophysiology and pave the way for novel ideas and interventions in studying the respiratory tract microbiome.

EXPERT OPINION

The conventional perception of lungs as sterile is deprecated since it is currently recognized the existence of a diverse microbial community within them. However, despite extensive research on the role of the respiratory microbiome in healthy lungs, respiratory chronic diseases and acute lung infections such as pneumonia are not fully understood. It is crucial to investigate further the relationship between the pathophysiology of VAP and the pulmonary microbiome, elucidating the mechanisms underlying the interactions between the microbiome, host immune response and mechanical ventilation for the development of VAP.

The upper and lower respiratory tract microbiome in severe aspiration pneumonia

Uncertainty persists whether anaerobic bacteria represent important pathogens in aspiration pneumonia. In a nested case-control study of mechanically ventilated patients classified as macro-aspiration pneumonia (MAsP, n = 56), non-macro-aspiration pneumonia (NonMAsP, n = 91), and uninfected controls (n = 11), we profiled upper (URT) and lower respiratory tract (LRT) microbiota with bacterial 16S rRNA gene sequencing, measured plasma host-response biomarkers, analyzed bacterial communities by diversity and oxygen requirements, and performed unsupervised clustering with Dirichlet Multinomial Models (DMM). MAsP and NonMAsP patients had indistinguishable microbiota profiles by alpha diversity and oxygen requirements with similar host-response profiles and 60-day survival. Unsupervised DMM clusters revealed distinct bacterial clusters in the URT and LRT, with low-diversity clusters enriched for facultative anaerobes and typical pathogens, associated with higher plasma levels of SPD and sCD14 and worse 60-day survival. The predictive inter-patient variability in these bacterial profiles highlights the importance of microbiome study in patient sub-phenotyping and precision medicine approaches for severe pneumonia.

Profiles of oral microbiome associated with nasogastric tube feeding

Background

Dysbiosis of oral microbiome causes chronic diseases including dental caries and periodontitis, which frequently affect older patient populations. Severely disabled individuals with impaired swallowing functions may require nutritional supply via nasogastric (NG) tubes, further impacting their oral condition and possibly microbial composition. However, little is known about the effect of NG tube on oral microbes and its potential ramification.

Methods

By using 16S rRNA amplicon sequencing, we characterized the tongue microbiome of 27 patients fed with NG tubes and 26 others fed orally.

Results

The microbial compositions of NG-tube and oral-feeding patients were substantially different, with more Gram-negative aerobes enriched in the presence of NG tube. Specifically, NG-tube patients presented more opportunistic pathogens like Pseudomonas and Corynebacterium associated with pneumonia and lower levels of commensal Streptococcus and Veillonella. Co-occurrence analysis further showed an inverse relationship between commensal and pathogenic species.

Conclusion

We present a systematic, high-throughput profiling of oral microbiome with regard to long-term NG tube feeding among the older patient population.

Differences in the microbiota of oral rinse, lesion, and normal site samples from patients with mucosal abnormalities on the tongue

The oral microbiota associated with mucosal diseases, including oral squamous cell carcinoma and oral potentially malignant disorders, have been extensively analyzed at the phylum and genus levels. However, the details of the oral microbiota remain unclear at the species and operational taxonomic unit (OTU) levels. We aimed to determine differences in the microbiota of oral rinse, lesion and normal site swab samples of patients with mucosal abnormalities on the tongues. Oral samples were obtained from 10 patients with oral mucosal abnormalities. Alpha and beta diversity at the OTU and genus levels of the microbiota samples were analyzed using OTUs clustered with 99.6% similarity based on 16S rRNA gene sequences obtained using the Sanger method. At the OTU level, the microbiota of the lesions were the least diverse but were different from those of the normal site and oral rinse samples. The OTUs corresponding to Streptococcus infantis and Haemophilus parainfluenzae were suggested to contribute to the differences between the microbiota of the lesions and normal sites. At the genus level, no significant differences between these microbiota were observed. In conclusion, strict OTU-level microbiota analysis might be able to discriminate lesions from normal sites of patients with mucosal abnormalities.

Clinical practices underlie COVID-19 patient respiratory microbiome composition and its interactions with the host

Understanding the pathology of COVID-19 is a global research priority. Early evidence suggests that the respiratory microbiome may be playing a role in disease progression, yet current studies report contradictory results. Here, we examine potential confounders in COVID-19 respiratory microbiome studies by analyzing the upper (n = 58) and lower (n = 35) respiratory tract microbiome in well-phenotyped COVID-19 patients and controls combining microbiome sequencing, viral load determination, and immunoprofiling. We find that time in the intensive care unit and type of oxygen support, as well as associated treatments such as antibiotic usage, explain the most variation within the upper respiratory tract microbiome, while SARS-CoV-2 viral load has a reduced impact. Specifically, mechanical ventilation is linked to altered community structure and significant shifts in oral taxa previously associated with COVID-19. Single-cell transcriptomics of the lower respiratory tract of COVID-19 patients identifies specific oral bacteria in physical association with proinflammatory immune cells, which show higher levels of inflammatory markers. Overall, our findings suggest confounders are driving contradictory results in current COVID-19 microbiome studies and careful attention needs to be paid to ICU stay and type of oxygen support, as bacteria favored in these conditions may contribute to the inflammatory phenotypes observed in severe COVID-19 patients.

Refinement of microbiota analysis of specimens from patients with respiratory infections using next-generation sequencing

Next-generation sequencing (NGS) technologies have been applied in bacterial flora analysis. However, there is no standardized protocol, and the optimal clustering threshold for estimating bacterial species in respiratory infection specimens is unknown. This study was conducted to investigate the optimal threshold for clustering 16S ribosomal RNA gene sequences into operational taxonomic units (OTUs) by comparing the results of NGS technology with those of the Sanger method, which has a higher accuracy of sequence per single read than NGS technology. This study included 45 patients with pneumonia with aspiration risks and 35 patients with lung abscess. Compared to Sanger method, the concordance rates of NGS technology (clustered at 100%, 99%, and 97% homology) with the predominant phylotype were 78.8%, 71.3%, and 65.0%, respectively. With respect to the specimens dominated by the Streptococcus mitis group, containing several important causative agents of pneumonia, Bray Curtis dissimilarity revealed that the OTUs obtained at 100% clustering threshold (versus those obtained at 99% and 97% thresholds; medians of 0.35, 0.69, and 0.71, respectively) were more similar to those obtained by the Sanger method, with statistical significance (p < 0.05). Clustering with 100% sequence identity is necessary when analyzing the microbiota of respiratory infections using NGS technology.

Factors influencing lower respiratory tract infection in older patients after general anesthesia

Objective

Pulmonary complication is common in older patients after surgery. We analyzed risk factors of lower respiratory tract infection after general anesthesia among older patients.

Methods

In this retrospective investigation, we included older patients who underwent surgery with general anesthesia. Logistic regression analyses were performed to determine risk factors of lower respiratory tract infection.

Results

A total 418 postoperative patients with general anesthesia were included; the incidence of lower respiratory tract infection was 9.33%. Ten cases were caused by gram-positive bacteria, 26 cases by gram-negative bacteria, and 2 cases by fungus. We found significant differences in age, smoking, diabetes, oral/nasal tracheal intubation, and surgery duration. Logistic regression analysis indicated that age ≥70 years (odds ratio [OR] 2.028, 95% confidence interval [CI] 1.115–3.646), smoking (OR 2.314, 95% CI 1.073–4.229), diabetes (OR 2.185, 95% CI 1.166–4.435), nasotracheal intubation (OR 3.528, 95% CI 1.104–5.074), and duration of surgery ≥180 minutes (OR 1.334, 95% CI 1.015–1.923) were independent risk factors of lower respiratory tract infections.

Conclusions

Older patients undergoing general anesthesia after tracheal intubation have a high risk of lower respiratory tract infections. Clinical interventions should be provided to prevent pulmonary infections in patients with relevant risk factors.

Perspective on the clone library method for infectious diseases

Recently, culture-independent molecular methods, such as DNA sequencing techniques targeting the 16S-ribosomal RNA (rRNA) gene and/or other housekeeping genes with Sanger method-based technologies, next generation sequencing (NGS), and metagenomic analysis, have been developed for detecting microorganisms in the human body; these can provide information on microbiomes of samples from individuals with or without infectious diseases. Determining the bacterial species is crucial in identifying causative bacteria of upper and lower respiratory tract infections, especially for Streptococcus species, but NGS analysis is often not precise enough to identify bacteria at the species level. This review briefly introduces previous observations of the microbiome of samples from various respiratory and other infections assessed using the clone library method with Sanger sequencing of the 16S-rRNA gene. On analysis of 16S-rRNA gene-sequence data of bronchoalveolar lavage fluid obtained from pneumonia lesions in patients with bacterial pneumonia and lung abscess, anaerobes are often detected in non-elderly patients with pneumonia, and the detection rate of Staphylococcus aureus in patients with hospital-acquired pneumonia is lower than that previously reported. Analysis of pleural effusion samples from patients with pleurisy indicated a more important role of anaerobes than previous believed. The other topics reviewed include microbiomes of nontuberculous mycobacteriosis and lower respiratory tract infections in children with permanent tracheostomy due to neuromuscular disorders, in nasal discharge, in bacterial vaginosis, in the intracystic fluid of postoperative maxillary cyst, and in bacterial conjunctivitis; urine microbiota in urethritis; fecal microbiota; and newly detected infectious organisms in the human respiratory tract.

The respiratory microbiota during and following mechanical ventilation for respiratory infections in children

The lower respiratory tract (LRT) harbours distinct, dynamic low-density microbial communities, established through micro-aspiration from the upper respiratory tract (URT) [1–3]. However, during intubation and mechanical ventilation, the endotracheal tube temporarily alters the anatomical continuity between URT and LRT, and may provide a bridge for airborne microbes and a barrier for micro-aspiration. Shortly after intubation for a severe LRT infection (LRTI) in children, the microbiota of the nasopharynx and LRT were shown to be very similar [4]. However, it remains unknown how the respiratory microbial community develops while the child recovers from the infection under treatment with mechanical ventilation and antibiotics. We therefore analysed respiratory microbiota changes in children participating in our study on acute LRTIs and who were admitted to the paediatric intensive care unit (PICU) for mechanical ventilation [4].

During mechanical ventilation for an LRTI in children, the respiratory microbiota shifted from Haemophilus- and Moraxella-dominated profiles to profiles dominated by antibiotic-resistant Enterobacteriaceae, and Staphylococcus and Streptococcus species.https://bit.ly/3pGfvhQ

Bacterial composition of nasal discharge in children based on highly accurate 16S rRNA gene sequencing analysis

Nasopharyngeal colonization by bacteria is a prerequisite for progression to respiratory disease and an important source of horizontal spread within communities. We aimed to perform quantitative analysis of the bacterial cells and reveal the microbiota of the nasal discharge in children at the species level based on highly accurate 16S rRNA gene sequencing. This study enrolled 40 pediatric patients with rhinorrhea. The bacterial cells in the nasal discharge were counted by epifluorescence microscopic analysis. The microbiota was analyzed by using the 16S rRNA gene clone library sequencing method. We demonstrated that a high abundance (median 2.2 × 10⁷ cells/mL) of bacteria was contained in the nasal discharge of children. Of the 40 samples, 37 (92.5%) were dominated by OTUs corresponding to Haemophilus aegyptius/influenzae, Moraxella catarrhalis/nonliquefaciens, or Streptococcus pneumoniae. These samples showed higher cell abundance and lower alpha diversity than the remaining three samples in which the other bacteria coexisted. In addition, 12 sequences with low homology to type strains were considered as previously unknown bacterial lineages. In conclusion, the nasal discharge of most young children contains a large amount of respiratory pathogens and several unknown bacteria, which could not only cause endogenous infection but also be a source of transmission to others.