Unmet needs in pneumonia research: a comprehensive approach by the CAPNETZ study group

Insights into the diagnostic and prognostic value of paraoxonase 1-related variables and inflammatory markers in community-acquired pneumonia

Community-acquired pneumonia (CAP) remains a major health concern, with oxidative stress and inflammation playing key roles in its pathophysiology. This study examines the potential of paraoxonase-1 (PON1)-related variables as biomarkers for diagnosing and managing CAP. A prospective case-control study included 78 patients with community-acquired pneumonia (CAP) who were hospitalized and 80 healthy controls. Serum PON1 concentration (PON1c), PON1 arylesterase (ARE) and paraoxonase (PARX) activities, and inflammatory markers (C-reactive protein, procalcitonin, and C-C motif chemokine ligand 2 were measured and compared with lipid profiles and clinical severity scores. Receiver operating characteristic curve analysis was used to assess their diagnostic and prognostic value. CAP patients exhibited significantly lower ARE and PARX activities but higher PON1c levels than controls, with these changes correlating with increased inflammatory markers and decreased total and high-density lipoprotein cholesterol concentrations. PON1-related variables demonstrated strong diagnostic accuracies (areas under the curve >0.90), outperforming traditional inflammatory markers. However, although these variables provided insights into disease severity and pathophysiology, their prognostic value and ability to differentiate microbial etiologies were limited. These findings suggest that PON1-related variables may serve as promising diagnostic biomarkers for CAP, but their role in prognosis and guiding antimicrobial therapy requires further investigation. Future studies should validate these results in larger and more diverse populations while exploring the mechanistic involvement of PON1 in CAP progression.

Severe community-acquired pneumonia (sCAP): advances in management and future directions

Severe community-acquired pneumonia (sCAP) is a major global health challenge, with high morbidity and mortality, especially among patients requiring intensive care. Despite advancements in antimicrobial therapies and supportive care, sCAP remains a significant threat, particularly for those needing invasive mechanical ventilation or vasopressor support. Recent progress in diagnostics, therapeutics and management strategies offers hope for improved outcomes. Pathogen-specific management is now central to sCAP care, with molecular diagnostics enhancing pathogen detection accuracy and enabling tailored antimicrobial therapy. These tools help combat antimicrobial resistance by reducing unnecessary broad-spectrum antibiotic use.Host immune responses in sCAP vary widely and significantly impact outcomes. Some patients face an overwhelming pathogen burden, while others experience excessive immune responses, such as acute respiratory distress syndrome. This distinction is vital for guiding immunomodulatory therapies, as immunosuppression may benefit hyperinflammatory states but harm those overwhelmed by infection. Corticosteroids, though controversial, show potential benefits in select populations but carry risks like secondary infections and hyperglycaemia, requiring a nuanced approach.Non-invasive respiratory support strategies, such as high-flow nasal oxygen, have transformed care by improving oxygenation and reducing the need for invasive ventilation. However, their efficacy depends on timing, patient tolerance and disease severity, necessitating careful monitoring.Global disparities in sCAP management, particularly in low-income and middle-income countries, highlight the need for region-specific guidelines and scalable solutions. Limited access to advanced diagnostics and critical care resources exacerbates poor outcomes, underscoring the importance of investments in affordable diagnostics, infection control and multidisciplinary training. Emerging technologies, such as artificial intelligence and advanced imaging, promise to revolutionise sCAP management by enabling precision medicine and real-time insights into disease severity. A holistic, multidisciplinary approach integrating these advancements is essential to improving outcomes and advancing personalised care for this life-threatening condition.

Multiorgan sequelae following non-COVID-19 respiratory infections: a review

BACKGROUND

While numerous studies have documented severe and long-term health impacts of COVID-19 infections on various organs, the prolonged multisystemic implications of other acute respiratory infections (ARIs) are poorly understood. This review therefore analyzed currently available studies about these sequelae of ARIs excluding COVID-19.

MAIN BODY

Multiple pathogens causing ARIs are associated with significant long-lasting impairments across various organ systems. Cardiovascular events occur in 10-35% of patients following ARIs, with an elevated risk persisting for 10 years. The stroke incidence ratio increases significantly after ARIs up to 12.3. Pulmonary sequelae are common, including abnormal lung function in 54%, parenchymal opacification in 51%, lung fibrosis in 33-62%, asthma in 30%, and bronchiectasis in 24% of patients. The risk of developing dementia is increased 2.2-fold. Posttraumatic stress disorder, depression, anxiety, and chronic fatigue occur in 15-43%, 15-36%, 14-62%, and 27-75% of patients, respectively. 28-day mortality from CAP with (versus no) additional cardiovascular event is increased to 36% (versus 10%). Long-term mortality from CAP (versus no CAP) remains elevated for years post-infection, with a 1-year, 5-year, and 7-year mortality rate of 17% (versus 4%), 43% (versus 19%), and 53% (versus 24%), respectively. Patients´ quality of life is significantly reduced, with 17% receiving invalidity pensions and 22% retiring within 4 years of severe ARIs.

CONCLUSION

Non-COVID-19 ARIs are associated with clinically relevant, frequent, and long-term sequelae involving multiple organ systems. Further prospective studies are needed.

Fluorescent antibody-based detection and ultrastructural analysis of Streptococcus pneumoniae in human sputum

BACKGROUND

Pneumococcal pneumonia continues to be a significant global health burden, affecting both children and adults. Traditional diagnostic methods for sputum analysis remain challenging. The objective of this study was twofold: to develop a rapid and easy-to-perform assay for the identification of Streptococcus pneumoniae (Spn) directly in sputum specimens using fluorescence microscopy, and to characterize with high-resolution confocal microscopy the ultrastructure of pneumococci residing in human sputum.

METHODS

We fluorescently labeled antibodies against the pneumococcal capsule (Spn-FLUO). The specificity and sensitivity of Spn-FLUO for detecting Spn was evaluated in vitro and in vivo using mouse models of carriage and disease, human nasopharyngeal specimens, and sputum from patients with pneumococcal pneumonia. Spn was confirmed in the specimens using culture and a species-specific qPCR assay. Spn strains were serotyped by Quellung. Confocal microscopy and Imaris software analysis were utilized to resolve the ultrastructure of pneumococci in human sputum.

RESULTS

Compared with cultures and qPCR, Spn-FLUO demonstrated high sensitivity (78-96%) in nasopharyngeal samples from mice and humans. The limit of detection (LOD) in nasopharyngeal samples was ≥ 1.6 × 10⁴ GenEq/ml. The specificity in human nasopharyngeal specimens was 100%. In lung specimens from mice infected with pneumococci, Spn-FLUO reached 100% sensitivity with a LOD of ≥ 1.39 × 10⁴ GenEq/ml. In human sputum, the sensitivity for detecting Spn was 92.7% with a LOD of 3.6 × 10³ GenEq/ml. Ultrastructural studies revealed that pneumococci are expectorated as large aggregates with a median size of 1336 μm².

CONCLUSIONS

Spn-FLUO is a rapid and sensitive assay for detecting Spn in human sputum within 30 min, encompassing a range of both vaccine and non-vaccine serotypes associated with pneumococcal pneumonia. The study highlights that most pneumococci form aggregates in human sputum.

The impact of oral anticoagulants on mortality from pneumonia: a propensity score matching analysis

OBJECTIVE

Pneumonia continues to be one of the leading causes of death. During the COVID pandemic, pre-existing anticoagulant therapy with direct oral anticoagulants (DOACs) appeared to be beneficial. The present study aimed to investigate the impact of pre-existing DOAC therapy on mortality from community-acquired, non-COVID pneumonia.

METHODS

The study utilized data from the eICU Collaborative Research Database, a comprehensive, multi-institutional critical care database. We included all adult patients with community-acquired pneumonia, selecting for patients with a primary admission diagnosis of pneumonia or pulmonary sepsis who were admitted to the ICU <24 h after admission to the hospital. To adjust for confounders, we performed propensity score matching, matching patients receiving DOACs to an equivalent cohort of patients not receiving DOAC therapy. Our primary outcome was overall survival. Secondary outcomes included all-cause in-hospital mortality, all-cause in-ICU mortality, intubation within 24 h following ICU admission, incidence of acute kidney injury and renal replacement therapy, vasopressor administration, and mechanical ventilation days.

RESULTS

Our final matched cohort included 198 DOAC patients matched to 594 patients without DOAC therapy. Survival was significantly higher in DOAC patients with a hazard ratio of 0.56 [95% CI = 0.36-088]. Both all-cause in-unit mortality (6.1% [95% CI = 2.7-9.4%] vs. 13.3% [95% CI = 10.6-16.0%], p = 0.008) and all-cause in-hospital mortality (11.6% [95% CI = 7.2-16.1%] vs. 19.7% [95% CI = 16.5-22.9%], p = 0.013) were significantly lower in patients receiving DOACs.

CONCLUSION

This study demonstrates a positive association between the pre-existing intake of direct oral anticoagulants and the survival of community acquired pneumonia. Future prospective studies should evaluate supportive therapy with DOACs.

Proteomic, metabolomic and lipidomic profiles in community acquired pneumonia for differentiating viral and bacterial infections

Community-acquired pneumonia (CAP) has a significant impact on public health, especially in light of the recent SARS-CoV-2 pandemic. To enhance disease characterization and improve understanding of the underlying mechanisms, a comprehensive analysis of the plasma lipidome, metabolome and proteome was conducted in patients with viral and bacterial CAP infections, including those induced by SARS-CoV-2. Lipidomic, metabolomic and proteomic profiling were conducted on plasma samples of 69 patients suffering either from viral or bacterial CAP. Lipid and metabolite analyses were LC-MS-based, while proteomic analyses were performed using multiple panels of the Olink platform. Statistical methods, machine learning and pathway analyses were conducted investigating differences between the infection types. Through comparison of the bacterial and viral pathogen groups, distinct signatures were observed in the plasma profiles. Notably, linoleic acid-derived inflammation signaling metabolites (EpOME and DiHOME) were increased in viral CAP compared to bacterial CAP. Similarly, proteins involved in cellular immune response and apoptosis (LAG-3 and TRAIL) showed elevated levels in viral CAP, while bacterial CAP exhibited notable elevation in pattern-recognizing receptors (CLEC4D and EN-RAGE). Additionally, within the lipidomic profile at baseline, several lipids displayed notable differences between viral and bacterial pneumonia, including bile acids (GCA, TCA, TCDCA), various tri- and diglycerides (TGs and DGs), and several phosphatidylcholines (PCs). These findings hold promise for facilitating the differential diagnosis of viral and bacterial pulmonary infections based on the systemic lipidome, metabolome and proteome, enabling timely treatment decisions. Additionally, they highlight potential targets for drug research, advancing therapeutic interventions in CAP. By providing valuable insights into the molecular characterization of CAP, this study contributes to the improvement of understanding the disease and, ultimately, the development of effective treatment strategies.

A Review on Risk Factors, Traditional Diagnostic Techniques, and Biomarkers for Pneumonia Prognostication and Management in Diabetic Patients

People of all ages can contract pneumonia, and it can cause mild to severe disease and even death. In addition to being a major cause of death for elderly people and those with prior medical conditions such as diabetes, it isthe world's biggest infectious cause of death for children. Diabetes mellitus is a metabolic condition with a high glucose level and is a leading cause of lower limb amputation, heart attacks, strokes, blindness, and renal failure. Hyperglycemia is known to impair neutrophil activity, damage antioxidant status, and weaken the humoral immune system. Therefore, diabetic patients are more susceptible to pneumonia than people without diabetes and linked fatalities. The absence of quick, precise, simple, and affordable ways to identify the etiologic agents of community-acquired pneumonia has made diagnostic studies' usefulness contentious. Improvements in biological markers and molecular testing techniques have significantly increased the ability to diagnose pneumonia and other related respiratory infections. Identifying the risk factors for developing severe pneumonia and early testing in diabetic patients might lead to a significant decrease in the mortality of diabetic patients with pneumonia. In this regard, various risk factors, traditional testing techniques, and pathomechanisms are discussed in this review. Further, biomarkers and next-generation sequencing are briefly summarized. Finding biomarkers with the ability to distinguish between bacterial and viral pneumonia could be crucial because identifying the precise pathogen would stop the unnecessary use of antibiotics and effectively save the patient's life.

Microbial aetiology of community-acquired pneumonia in hospitalised adults: A prospective study utilising comprehensive molecular testing

OBJECTIVES

This study aimed to describe the microbial aetiology of community-acquired pneumonia (CAP) in adults admitted to a tertiary care hospital and assess the impact of syndromic polymerase chain reaction (PCR) panels on pathogen detection.

METHODS

Conducted at Haukeland University Hospital, Norway, from September 2020 to April 2023, this prospective study enrolled adults with suspected CAP. We analysed lower respiratory tract samples using both standard-of-care tests and the BIOFIRE® FILMARRAY® Pneumonia Plus Panel (FAP plus). The added value of FAP Plus in enhancing the detection of clinically relevant pathogens, alongside standard-of-care diagnostics, was assessed.

RESULTS

Of the 3238 patients screened, 640 met the inclusion criteria, with 384 confirmed to have CAP at discharge. In these patients, pathogens with proven or probable clinical significance were identified in 312 (81.3%) patients. Haemophilus influenzae was the most prevalent pathogen, found in 118 patients (30.7%), followed by SARS-CoV-2 in 74 (19.3%), and Streptococcus pneumoniae in 64 (16.7%). Respiratory viruses were detected in 186 (48.4%) patients. The use of FAP plus improved the pathogen detection rate from 62.8% with standard-of-care methods to 81.3%.

CONCLUSIONS

Pathogens were identified in 81% of CAP patients, with Haemophilus influenzae and respiratory viruses being the most frequently detected pathogens. The addition of the FAP plus panel, markedly improved pathogen detection rates compared to standard-of-care diagnostics alone.

Efficacy and safety of vitamin C supplementation in the treatment of community-acquired pneumonia: a systematic review and meta-analysis with trial sequential analysis

Community-acquired pneumonia (CAP) poses a significant global health challenge, prompting exploration of innovative treatments. This systematic review and meta-analysis aimed to evaluate the efficacy and safety of vitamin C supplementation in adults undergoing treatment for CAP. A comprehensive search of the MEDLINE, Embase, CINAHL, the Cochrane Central Register of Controlled Trials, and Clinical Trials.gov databases from inception to 17 November 2023 identified six randomized-controlled-trials (RCTs) meeting inclusion criteria. The primary outcome analysis revealed a non-significant trend towards reduced overall mortality in the vitamin C group compared to controls (RR 0.51; 95% CI 0.24 to 1.09; p = 0.052; I2 = 0; p = 0.65). Sensitivity analysis, excluding corona-virus-disease 2019 (COVID-19) studies and considering the route of vitamin C administration, confirmed this trend. Secondary outcomes, including hospital length-of-stay (LOS), intensive-care-unit (ICU) LOS, and mechanical ventilation, exhibited mixed results. Notably, heterogeneity and publication bias were observed in hospital LOS analysis, necessitating cautious interpretation. Adverse effects were minimal, with isolated incidents of nausea, vomiting, hypotension, and tachycardia reported. This meta-analysis suggests potential benefits of vitamin C supplementation in CAP treatment. However, inconclusive findings and methodological limitations warrants cautious interpretation, emphasising the urgency for high-quality trials to elucidate the true impact of vitamin C supplementation in CAP management.

Role of vitamin C in treatment of community-acquired pneumonia in adult patients requiring hospitalisation: a systematic review protocol

INTRODUCTION

Community-acquired pneumonia (CAP) is a leading cause of hospitalisation and is associated with a high mortality. Vitamin C is a powerful antioxidant and has been used in treatment of infections; however, its role as an adjunctive treatment in CAP is unclear. This review aims to assess the efficacy and safety of vitamin C in adults who require hospitalisation for CAP.

METHODS AND ANALYSES

Searches will be conducted from inception to November 2023 on Ovid MEDLINE Daily and MEDLINE, Embase CINAHL, the Cochrane Central Register of Controlled Trials, Scopus, Web of Science and ClinicalTrials.gov databases with the aid of a medical librarian. We will include data from randomised controlled trials reporting vitamin C supplementation in patients with CAP requiring hospitalisation. Two independent reviewers will select studies, extract data and will assess the risk of bias by use of the Risk of Bias tool. The overall certainty of evidence will be assessed by use of the Grading of Recommendations Assessment, Development and Evaluation framework. Random-effects meta-analyses will be conducted, and effect measures will be reported as relative risks with 95% CIs.

ETHICS AND DISSEMINATION

No previous ethical approval is required for this review. The findings of this review will be submitted to a scientific journal and presented at an international medical conference.

PROSPERO REGISTRATION NUMBER

483860.

Longitudinal metabolite profiling of Streptococcus pneumoniae-associated community-acquired pneumonia

INTRODUCTION

Longitudinal biomarkers in patients with community-acquired pneumonia (CAP) may help in monitoring of disease progression and treatment response. The metabolic host response could be a potential source of such biomarkers since it closely associates with the current health status of the patient.

OBJECTIVES

In this study we performed longitudinal metabolite profiling in patients with CAP for a comprehensive range of metabolites to identify potential host response biomarkers.

METHODS

Previously collected serum samples from CAP patients with confirmed Streptococcus pneumoniae infection (n = 25) were used. Samples were collected at multiple time points, up to 30 days after admission. A wide range of metabolites was measured, including amines, acylcarnitines, organic acids, and lipids. The associations between metabolites and C-reactive protein (CRP), procalcitonin, CURB disease severity score at admission, and total length of stay were evaluated.

RESULTS

Distinct longitudinal profiles of metabolite profiles were identified, including cholesteryl esters, diacyl-phosphatidylethanolamine, diacylglycerols, lysophosphatidylcholines, sphingomyelin, and triglycerides. Positive correlations were found between CRP and phosphatidylcholine (34:1) (cor = 0.63) and negative correlations were found for CRP and nine lysophosphocholines (cor = - 0.57 to - 0.74). The CURB disease severity score was negatively associated with six metabolites, including acylcarnitines (tau = - 0.64 to - 0.58). Negative correlations were found between the length of stay and six triglycerides (TGs), especially TGs (60:3) and (58:2) (cor = - 0.63 and - 0.61).

CONCLUSION

The identified metabolites may provide insight into biological mechanisms underlying disease severity and may be of interest for exploration as potential treatment response monitoring biomarker.

Imaging in pulmonary infections of immunocompetent adult patients

Pneumonia is a clinical syndrome characterised by fever, cough and alveolar infiltration of purulent fluid, caused by infection with a microbial pathogen. It can be caused by infections with bacteria, viruses or fungi, but a causative organism is identified in less than half of cases. The most common type of pneumonia is community-acquired pneumonia, which is caused by infections acquired outside the hospital. Current guidelines for pneumonia diagnosis require imaging to confirm the clinical suspicion of pneumonia. Thus, imaging plays an important role in both the diagnosis and management of pneumonia, with each modality having specific advantages and limitations. Chest radiographs are commonly used but have limitations in terms of sensitivity and specificity. Lung ultrasound shows high sensitivity and specificity. Computed tomography scans offer higher diagnostic accuracy but involve higher radiation doses. Radiological patterns, including lobar, lobular and interstitial pneumonia, provide valuable insights into causative pathogens and treatment decisions. Understanding these radiological patterns is crucial for accurate diagnosis. In this review, we will summarise the most important aspects pertaining to the role of imaging in pneumonia and will highlight the imaging characteristics of the most common causative organisms.

Diabetes Status, c-Reactive Protein, and Insulin Resistance in Community-Acquired Pneumonia-A Prospective Cohort Study

C-reactive protein (CRP) is commonly used to guide community-acquired pneumonia (CAP) treatment. A positive association between admission glucose and CRP levels has been observed in patients with CAP. The associations between prediabetes, unknown diabetes, acute-on-chronic hyperglycaemia, and CRP levels, and between admission CRP levels and insulin resistance (IR) in CAP, remain unexplored. This study investigated the associations firstly between chronic, acute, and acute-on-chronic hyperglycaemia and CRP levels, and secondly between admission CRP levels and IR in CAP. In a prospective cohort study of adults with CAP, the associations between chronic, acute, and acute-on-chronic hyperglycaemia (admission glucose minus HbA1c-derived average glucose) and CRP levels until admission day 3 were modelled with repeated-measures linear mixed models. IR was estimated with the homeostasis model assessment of IR (HOMA-IR). The association between admission CRP levels and HOMA-IR was modelled with linear regression. In 540 patients, no association between chronic, acute, or acute-on-chronic hyperglycaemia and CRP levels was found. In 266 patients, every 50 mg/L increase in admission CRP was associated with a 7% (95% CI 1-14%) higher HOMA-IR. In conclusion, our findings imply that hyperglycaemia does not influence CRP levels in patients with CAP, although admission CRP levels were positively associated with IR.

The pharmacokinetic evaluation of omadacycline (Oral Only Dosing Regimen) for the treatment of Community-Acquired Bacterial Pneumonia (CABP)

INTRODUCTION

Omadacycline is a new analog of the tetracycline class active against atypical bacteria, as well as against staphylococci, including methicillin-resistant strains, and Streptococcus pneumoniae.

AREAS COVERED

This review has summarized the available clinical evidence on the use of oral omadacycline in the treatment of community-acquired pneumonia (CAP) and described the mechanism of action, pharmacokinetic/pharmacodynamic (PK/PD) parameters in healthy and special populations and the latest research on omadacycline.

EXPERT OPINION

The available clinical evidence on oral omadacycline for the treatment of CAP shows that its properties provide reliable empirical coverage for pathogens such as Haemophilus influenzae, Moraxella catarrhalis, and species of Legionella, Chlamydia, and Mycoplasma. Omadacycline is also active against methicillin-resistant Staphylococcus aureus (MRSA); penicillin-resistant and multidrug-resistant Streptococcus pneumoniae, Streptococcus pyogenes, and Streptococcus agalactiae; and vancomycin-resistant Enterococcus spp. A dose of 450 mg orally once daily is recommended, followed by a maintenance dose of 300 mg orally once daily. Importantly, omadacycline does not require dose adjustment for patients based on BMI, age, gender, or renal or hepatic impairment.

Identification and quantification of nine compounds in Fangwen Jiuwei decoction by liquid chromatography-mass spectrometry

Fangwen Jiuwei Decoction is a traditional Chinese medicine preparation for the treatment of pneumonia developed by Shenzhen Bao'an Chinese Medicine Hospital, which shows remarkable clinical responses. Qualitative and quantitative analyses of the main active compounds are crucial for the quality control of traditional Chinese medicine prescription in clinical application. In this study, we identified nine active compounds essential for the pharmacological effects of Fangwen Jiuwei Decoction based on the analysis of the Network Pharmacology and relevant literature. Moreover, these compounds can interact with several crucial drug targets in pneumonia based on molecular docking. We applied high-performance liquid chromatography-tandem mass spectrometry method was established these nine active ingredients' qualitative and quantitative detections. The possible cleavage pathways of nine active components were determined based on secondary ions mass spectrometry. The results of high-performance liquid chromatography-tandem mass spectrometry were further validated, which show a satisfactory correlation coefficient (r > 0.99), recovery rate (≥93.31%), repeatability rate (≤5.62%), stability (≤7.95%), intra-day precision (≤6.68%), and inter-day precision (≤9.78%). The limit of detection was as low as 0.01 ng/ml. In this study, we established a high-performance liquid chromatography-tandem mass spectrometry method to qualitatively and quantitatively analyze the chemical components in the Fangwen Jiuwei Decoction extract.

Prevention of Scarring of Lung Parenchyma in COVID-19 Patients through Glucocorticoid Treatment

Objective: The objective of this research was to determine the impact of glucocorticoid treatment on preventing scarring of lung parenchyma in COVID-19 patients by considering their health behavior. Methods: A sample of 65 Turkish patients who had pneumonia and were hospitalized between March/December 2020 were included in this research. The data for this research was collected after the consent of the hospital. The structural equation model approach was used in data analysis and empirical findings. Results: The research identified that the patients with appropriate health behavior were satisfied with their clinical treatment of scarring of lung parenchyma by the method of glucocorticoid treatment. The research also identified that the patient's health behavior was a significant indicator to improve their perception of the clinical treatment. Conclusion: This study has reliable theoretical implications that are significantly important in the literature because of the nature and uniqueness of the findings. However, this research also has some practical implications related to the patient's lungs mostly infected by COVID-19. Furthermore, the findings of this research can be generalized in a significant way because the respondents of this research belonged to a diverse population.

Atypical Pathogens in Adult Community-Acquired Pneumonia and Implications for Empiric Antibiotic Treatment: A Narrative Review

Atypical pathogens are intracellular bacteria causing community-acquired pneumonia (CAP) in a significant minority of patients. Legionella spp., Chlamydia pneumoniae and psittaci, Mycoplasma pneumoniae, and Coxiella burnetii are commonly included in this category. M. pneumoniae is present in 5-8% of CAP, being the second most frequent pathogen after Streptococcus pneumoniae. Legionella pneumophila is found in 3-5% of inpatients. Chlamydia spp. and Coxiella burnetii are present in less than 1% of patients. Legionella longbeachae is relatively frequent in New Zealand and Australia and might also be present in other parts of the world. Uncertainty remains on the prevalence of atypical pathogens, due to limitations in diagnostic means and methodological issues in epidemiological studies. Despite differences between CAP caused by typical and atypical pathogens, the clinical presentation alone does not allow accurate discrimination. Hence, antibiotics active against atypical pathogens (macrolides, tetracyclines and fluoroquinolones) should be included in the empiric antibiotic treatment of all patients with severe CAP. For patients with milder disease, evidence is lacking and recommendations differ between guidelines. Use of clinical prediction rules to identify patients most likely to be infected with atypical pathogens, and strategies of narrowing the antibiotic spectrum according to initial microbiologic investigations, should be the focus of future investigations.