Nonculture Laboratory Methods for the Diagnosis of Infectious Endocarditis

A Dual-Pathogen Mitral Valve Endocarditis Caused by Coxiella burnetii and Streptococcus gordonii-Which Came First?

Infective endocarditis (IE) is still a life-threatening disease with high morbidity and mortality. While usually caused by a single bacterium, poly-microbial infective endocarditis (IE) is rare. Here, we report a (blood-culture-negative) dual pathogen mitral valve IE caused by Coxiella burnetii and Streptococcus gordonii: A 53-year-old woman was presented to an internal medicine department with abdominal pain for further evaluation. Within the diagnostic work up, transthoracic echocardiography (TTE) revealed an irregularly shaped echogenic mass (5 × 13 mm) adherent to the edge of the posterior mitral valve leaflet and protruding into the left atrium. As infected endocarditis was suspected, blood cultures were initially obtained, but they remained negative. Chronic Q fever infection was diagnosed using serologic testing. After the occurrence of cerebral thromboembolic events, the patient was admitted for mitral valve surgery. Intraoperatively, a massively destructed mitral valve with adhering vegetations was noted. Examination of the mitral valve by broad-range bacterial polymerase chain reaction (PCR) and amplicon sequencing confirmed Coxiella burnetii infection and yielded Streptococcus gordonii as the second pathogen. Based on the detailed diagnosis, appropriate antibiotic therapy of both pathogens was initiated, and the patient could be discharged uneventfully on the 11th postoperative day after a successful minimal-invasive mitral valve replacement.

Immunohistochemical diagnosis of human infectious diseases: a review

BACKGROUND

Immunohistochemistry (IHC) using monoclonal and polyclonal antibodies is a useful diagnostic method for detecting pathogen antigens in fixed tissues, complementing the direct diagnosis of infectious diseases by PCR and culture on fresh tissues. It was first implemented in a seminal publication by Albert Coons in 1941.

MAIN BODY

Of 14,198 publications retrieved from the PubMed, Google, Google Scholar and Science Direct databases up to December 2021, 230 were selected for a review of IHC techniques, protocols and results. The methodological evolutions of IHC and its application to the diagnosis of infectious diseases, more specifically lice-borne diseases, sexually transmitted diseases and skin infections, were critically examined. A total of 59 different pathogens have been detected once in 22 different tissues and organs; and yet non-cultured, fastidious and intracellular pathogens accounted for the vast majority of pathogens detected by IHC. Auto-IHC, incorporating patient serum as the primary antibody, applied to diseased heart valves surgically collected from blood culture-negative endocarditis patients, detected unidentified Gram-positive cocci and microorganisms which were subsequently identified as Coxiella burnetii, Bartonella quintana, Bartonella henselae and Tropheryma whipplei. The application of IHC to ancient tissues dated between the ends of the Ptolemaic period to over 70 years ago, have also contributed to paleomicrobiology diagnoses.

CONCLUSION

IHC plays an important role in diagnostic of infectious diseases in tissue samples. Paleo-auto-IHC derived from auto-IHC, is under development for detecting non-identified pathogens from ancient specimens.

A novel molecular microbiologic technique for the rapid diagnosis of microbial invasion of the amniotic cavity and intra-amniotic infection in preterm labor with intact membranes

PROBLEM

The diagnosis of microbial invasion of the amniotic cavity (MIAC) has been traditionally performed using traditional cultivation techniques, which require growth of microorganisms in the laboratory. Shortcomings of culture methods include the time required (days) for identification of microorganisms, and that many microbes involved in the genesis of human diseases are difficult to culture. A novel technique combines broad-range real-time polymerase chain reaction with electrospray ionization time-of-flight mass spectrometry (PCR/ESI-MS) to identify and quantify genomic material from bacteria and viruses.

METHOD OF STUDY

AF samples obtained by transabdominal amniocentesis from 142 women with preterm labor and intact membranes (PTL) were analyzed using cultivation techniques (aerobic, anaerobic, and genital mycoplasmas) as well as PCR/ESI-MS. The prevalence and relative magnitude of intra-amniotic inflammation [AF interleukin 6 (IL-6) concentration ≥ 2.6 ng/mL], acute histologic chorioamnionitis, spontaneous preterm delivery, and perinatal mortality were examined.

RESULTS

(i) The prevalence of MIAC in patients with PTL was 7% using standard cultivation techniques and 12% using PCR/ESI-MS; (ii) seven of ten patients with positive AF culture also had positive PCR/ESI-MS [≥17 genome equivalents per PCR reaction well (GE/well)]; (iii) patients with positive PCR/ESI-MS (≥17 GE/well) and negative AF cultures had significantly higher rates of intra-amniotic inflammation and acute histologic chorioamnionitis, a shorter interval to delivery [median (interquartile range-IQR)], and offspring at higher risk of perinatal mortality, than women with both tests negative [90% (9/10) versus 32% (39/122) OR: 5.6; 95% CI: 1.4-22; (P < 0.001); 70% (7/10) versus 35% (39/112); (P = 0.04); 1 (IQR: <1-2) days versus 25 (IQR: 5-51) days; (P = 0.002), respectively]; (iv) there were no significant differences in these outcomes between patients with positive PCR/ESI-MS (≥17 GE/well) who had negative AF cultures and those with positive AF cultures; and (v) PCR/ESI-MS detected genomic material from viruses in two patients (1.4%).

CONCLUSION

(i) Rapid diagnosis of intra-amniotic infection is possible using PCR/ESI-MS; (ii) the combined use of biomarkers of inflammation and PCR/ESI-MS allows for the identification of specific bacteria and viruses in women with preterm labor and intra-amniotic infection; and (iii) this approach may allow for administration of timely and specific interventions to reduce morbidity attributed to infection-induced preterm birth.

DNA persistence and relapses questions on the treatment strategies of Enterococcus infections of prosthetic valves

We used amplification of the 16S rRNA gene followed by sequencing to evaluate the persistence of bacterial DNA in explanted heart valve tissue as part of the routine work of a clinical microbiology laboratory, and we analyzed the role of this persistence in the relapses observed in our center. We enrolled 286 patients treated for infective endocarditis (IE) who had valve replacement surgery and were diagnosed according to the modified Duke's criteria described by Li et al. from a total of 579 IE cases treated in our center. The patients were grouped based on the infecting bacteria, and we considered the 4 most common bacterial genus associated with IE separately (144 were caused by Streptococcus spp., 52 by Enterococcus spp., 58 by Staphylococcus aureus and 32 by coagulase-negative Staphylococcus). Based on our cohort, the risk of relapse in patients with enterococcal prosthetic valve infections treated with antibiotics alone was 11%. Bacterial DNA is cleared over time, but this might be a very slow process, especially with Enterococcus spp. Based on a comprehensive review of the literature performed on Medline, most reports still advise combined treatment with penicillin and an aminoglycoside for as long as 4-6 weeks, but there has been no consensus for the treatment of enterococcal infection of prostheses in IE patients.

Development and assessment of a new early scoring system using non-specific clinical signs and biological results to identify children and adult patients with a high probability of infective endocarditis on admission

Objectives

The aim of this study was to assess whether non-specific clinical signs or biological results can identify patients with a high probability of infective endocarditis (IE) to improve outcome.

Patients and methods

All patients tested for IE were included in a cohort and classified according to the modified Duke criteria. Patients with rejected endocarditis served as controls. Univariate and multivariate analyses were performed, and a score was calculated by adding 1 when a variable independently associated with IE (excluding major Duke criteria) was present and 0 when the variable was absent. A second score for patients with prior valvular damage (PVD) was also used. Scores were evaluated using the ROC curve method.

Results

IE was diagnosed in 402 of 2039 participants (19.7%). By multivariate analysis, PVD, fever, emboli, stroke, splenomegaly, finger clubbing, leucocytosis and erythrocyte sediment rate >50 were independently associated with IE. The rate of IE increased significantly from 4% (10/254) for a score of 0 to 83% (10/12) for a score of 6 in all patients, and from 9.5% (23/241) to 100% (10/10) in patients with PVD. The area under the ROC curve was 0.75 for the first score and 0.7 for the second. In a prospective study of 117 patients with suspicion of IE, the proportion of confirmed IE was 19% and the area under the ROC curve was 0.72.

Conclusions

This simple score can be used to identify patients with a high probability of IE, in the emergency room or on admission, to speed up diagnosis, or to initiate empirical antimicrobial therapy without replacing the modified Duke criteria.

Epidemiology of infective endocarditis in Tunisia: a 10-year multicenter retrospective study

BACKGROUND

Since the first description of infective endocarditis, the profile of the disease has evolved continuously with stable incidence. However, epidemiological features are different in developing countries compared with western countries.

OBJECTIVE

To describe epidemiological, microbiological and outcome characteristics of infective endocarditis in Tunisia.

PATIENTS AND METHODS

This was a descriptive multicenter retrospective study of inpatients treated for infective endocarditis from 1991 to 2000. Charts of patients with possible or definite infective endocarditis according to the Duke criteria were included in the study.

RESULTS

Four hundred and forty episodes of infective endocarditis among 435 patients (242 males, 193 females; mean (SD) age=32.4 (16.8) years, range 1-78 years) were reviewed. The most common predisposing heart disease was rheumatic valvular disease (45.2%). Infective endocarditis occurred on prosthetic valves in 17.3% of cases. Causative microorganisms were identified in 50.2% of cases: streptococci (17.3%), enterococci (3.9%), staphylococci (17.9%), and other pathogens (11.1%). Blood cultures were negative in 53.6% and no microorganism was identified in 49.8%. Early valve surgery was performed in 51.2% of patients. The in-hospital mortality was 20.6%.

CONCLUSION

Infective endocarditis is still frequently associated with rheumatic disease among young adults in Tunisia, with a high frequency of negative blood cultures and high in-hospital mortality, given that the population affected is relatively young.

Identification of bacteria in formalin-fixed, paraffin-embedded heart valve tissue via 16S rRNA gene nucleotide sequencing

We applied 16S rRNA gene sequencing to identify bacterial species present in formalin-fixed, paraffin-embedded heart valve tissue. In 40% (12/30) of the cases, we were able to identify the bacterium to the species-genus level. For more recent cases (<or=4 years), the success rate was significantly improved, to 70% (P<0.001).

Quantitative Histological Examination of Bioprosthetic Heart Valves

BACKGROUND

The histological features that characterize infective endocarditis in bioprosthetic valves are not accurately defined. Moreover, bioprosthetic valves may have a noninfective, degenerative evolution associated with calcifications, vegetation-like lesions, and inflammatory infiltrates. Such histological findings may be misdiagnosed as infective endocarditis.

METHODS

Pathologic analysis of inflamed bioprosthetic valve tissues was conducted retrospectively for 21 patients who underwent surgical removal of a bioprosthetic valve because of suspected infective endocarditis and for 67 patients who underwent surgical removal of a bioprosthetic valve because of noninfective dysfunction. To better define the histological criteria for infective endocarditis, we used quantitative image analysis to compare these 2 groups of patients with respect to vegetations, calcifications, and patterns of inflammation.

RESULTS

Histologically, infective endocarditis in patients with bioprostheses was characterized by demonstration of microorganisms, vegetations, and neutrophil-rich, inflammatory infiltrates. Valve tissue specimens from patients whose bioprosthetic valves were removed because of noninfective complications showed, in 30% of cases, inflammatory infiltrates mainly composed of macrophages and lymphocytes. Inflammatory adherent thrombi that can occur to the surface of noninfective degenerative bioprostheses are differentiated because their vegetations have macrophage-rich content. A neutrophil surface area with a cutoff value of > or =1.5% of the total valve tissue surface area is highly specific (94%) for infective endocarditis.

CONCLUSIONS

When no microorganisms are detected and vegetations are not found in bioprosthetic valve tissues during the histological examination, a neutrophil-rich inflammation might better define the term "active endocarditis" in the Duke criteria and would allow differentiation between infective endocarditis and inflammatory, noninfective valve processes in patients with bioprosthetic valves.

Contribution of systematic serological testing in diagnosis of infective endocarditis

Despite progress with diagnostic criteria, the type and timing of laboratory tests used to diagnose infective endocarditis (IE) have not been standardized. This is especially true with serological testing. Patients with suspected IE were evaluated by a standard diagnostic protocol. This protocol mandated an evaluation of the patients according to the modified Duke criteria and used a battery of laboratory investigations, including three sets of blood cultures and systematic serological testing for Coxiella burnetii, Bartonella spp., Aspergillus spp., Legionella pneumophila, and rheumatoid factor. In addition, cardiac valvular materials obtained at surgery were subjected to a comprehensive diagnostic evaluation, including PCR aimed at documenting the presence of fastidious organisms. The study included 1,998 suspected cases of IE seen over a 9-year period from April 1994 to December 2004 in Marseilles, France. They were evaluated prospectively. A total of 427 (21.4%) patients were diagnosed as having definite endocarditis. Possible endocarditis was diagnosed in 261 (13%) cases. The etiologic diagnosis was established in 397 (93%) cases by blood cultures, serological tests, and examination of the materials obtained from cardiac valves, respectively, in 348 (81.5%), 34 (8%), and 15 (3.5%) definite cases of IE. Concomitant infection with streptococci and C. burnetii was seen in two cases. The results of serological and rheumatoid factor evaluation reclassified 38 (8.9%) possible cases of IE as definite cases. Systematic serological testing improved the performance of the modified Duke criteria and was instrumental in establishing the etiologic diagnosis in 8% (34/427) cases of IE.

Diagnosis of infectious endocarditis in patients undergoing valve surgery

PURPOSE

Histologic examination of valve samples is considered as the gold standard for the diagnosis of infectious endocarditis. Molecular tools are also very promising for patients with negative-culture endocarditis. Thus, we studied the contribution of valvular histology, culture, and 16S rRNA PCR amplification plus sequencing to the diagnosis of infectious endocarditis in patients undergoing valve surgery.

SUBJECTS AND METHODS

We performed culture, histological examination, and broad-range PCR amplification plus sequencing on valve samples taken from 127 patients with infectious endocarditis and from 118 patients without endocarditis. The sensitivity and specificity of these tests for the diagnosis of endocarditis in patients undergoing valve surgery were studied.

RESULTS

The sensitivity of PCR was of 61% (64/105) whereas that of histological examination was of 63% (62/98) and that of valve culture was of only 13% (14/105). All 68 positive PCR results considered reliable according to an interpretation scheme were from patients with infectious endocarditis, resulting in a 100% (118/118) specificity of the interpreted molecular approach. The specificity of histology was also of 100% (118/118) when using stringent criteria (ie, presence of vegetation, microorganisms, and/or valvular inflammation with mainly polymorphonuclear cells). PCR identified an etiological agent in 38% (5/13) of definite blood culture-negative infectious endocarditis.

CONCLUSION

We show that valvular histology with stringent criteria is the gold standard for the diagnosis of infectious endocarditis. Broad-range amplification of 16S rRNA gene is indicated for infectious endocarditis of unknown etiology, whereas valve culture is of limited sensitivity.

PCR detection of bacteria on cardiac valves of patients with treated bacterial endocarditis

We used broad-range PCR amplification and sequencing to detect and identify bacterial DNA in 156 valves of patients treated for infective endocarditis (IE). Bacterial DNA was found more frequently in patients who underwent valve replacement while on antibiotic treatment for IE (60%) than in patients who had completed antibiotic treatment for IE (37%; P = 0.02). We found specific bacterial DNA in valves removed from 11 of 30 patients who had completed antibiotic treatment for IE. Six had no histological evidence of IE. The presence of DNA was significantly correlated with the presence of histologic lesions (P = 0.001) and with the presence of bacteria detected by Gram staining (P < 0.001). Bartonella and streptococci were detected for much longer after antibiotic treatment by PCR than other species (P = 0.047 and 0.04, respectively), and coagulase-negative staphylococci were detected for much shorter periods (P = 0.02). The finding that bacterial DNA was more likely to be detected in valves of patients with active IE than in patients who had completed antibiotic treatment for IE shows that bacterial DNA is cleared slowly. There was no significant correlation between the duration of antibiotic therapy and the presence of bacterial DNA in valves. Since the persistence of bacterial DNA in valves does not necessarily indicate the persistence of viable bacteria, the detection of bacterial DNA in valves from IE patients should be interpreted with caution, in particular in those patients with a past history of treated IE.

Quantitative Histological Examination of Mechanical Heart Valves

BACKGROUND

Histological demonstration of microorganisms, vegetations, or active endocarditis in cardiac valve tissue is included in the Duke criteria and is considered to be a criterion of confirmed infective endocarditis. However, the histological features that characterize infective endocarditis are not accurately defined at the qualitative and quantitative levels.

METHODS

Pathologic analysis of tissue adjoining mechanical cardiac valves was undertaken retrospectively for 21 patients who underwent surgical removal of a mechanical valve because of suspected infective endocarditis and 69 patients who underwent surgical removal of a mechanical valve because of noninfectious dysfunction. To better define the histological criteria for infective endocarditis, we used quantitative image analysis to compare these 2 groups of patients with respect to valvular fibrosis, calcifications, vegetations, patterns of inflammation, and vascularization.

RESULTS

Histologically, infective endocarditis in patients with mechanical valves was characterized by the demonstration of microorganisms, vegetations, and significant neutrophil-rich inflammatory infiltrates with extensive neovascularization. In contrast, valve tissue specimens from patients with mechanical valves that were removed because of noninfectious complications showed significant rates of extensive fibrosis and, when present, inflammatory infiltrates that were mainly composed of macrophages and lymphocytes. A neutrophil surface area with a cutoff value of > or =2% of the total valve tissue surface is highly predictive of (90%) and specific for (98%) infective endocarditis.

CONCLUSIONS

When no microorganisms are detected and vegetations are lacking in tissue adjacent to a mechanical valve, neutrophil-rich inflammation and extensive neovascularization might better histologically define the term "active endocarditis" in the Duke criteria. This definition would allow differentiation between infective endocarditis and inflammatory noninfectious valve processes in patients with mechanical cardiac valves.

Causative organisms of infective endocarditis according to host status

A prospective study of infective endocarditis (IE) was conducted between 1994 and 2000 in Marseilles, France, and included 170 definite cases diagnosed with the use of modified Duke criteria. Classification of IE based on the aetiological agent was related to epidemiological characteristics, including age, gender and the nature of the injured valve. Enterococci and Streptococcus bovis were identified more frequently in older subjects (p 0.02), and S. bovis was also associated with mitral valve infection (p 0.03). Streptococcus spp. were found to be associated with native valves (p < 10(-3)), whereas coagulase-negative staphylococci and Coxiella burnetii were associated with intracardiac prosthetic material (p < 0.05). S. bovis and Staphylococcus aureus were the predominant species associated with presumably healthy valves (p < 0.05), whereas oral streptococci caused IE exclusively in patients with previous valve damage. The basic host status of IE patients has been linked to specific microorganisms, and this may be of value when empirical treatment is needed in patients who have received previous antibiotic therapy and whose blood cultures are negative.

Diagnostic methods. Current best practices and guidelines for identification of difficult-to-culture pathogens in infective endocarditis

IE is a serious, life-threatening disease. Because treatment must often be adapted to the pathogen involved, rapid identification of the etiologic agent is critical to successful management of each patient. When difficult-to-culture pathogens are involved, routine microbiologic tests, including blood culture, may remain negative. Because such cases may account for up to 31% of all IE cases, alternative diagnostic approaches are necessary. Among the etiologic agents of culture-negative endocarditis, C burnetii and Bartonella spp play a major role; each is responsible for up to 3% of episodes of IE. The authors therefore recommend the systematic use of specific serologies in all cases of clinically suspected endocarditis. The cross-reactivity between C burnetii, Bartonella spp, and Chlamydia spp is of diagnostic importance because all are potential etiologic agents of endocarditis. However, given that the levels of specific antibodies observed in Bartonella endocarditis are extremely high, low-level cross-reactions with other antigens should not lead to misdiagnosis, provided serology for all suspected agents is performed. When serologic test results are negative for both Bartonella spp and C burnetii, special staining by the Gram, Giemsa, Gimenez, PAS, Warthin-Starry, and Grocott methods may guide the use of new diagnostic tools such as PCR and tissue culture for isolation and identification of the causative agent. Such novel approaches may lead to more comprehensive patient evaluations and the discovery of new etiologic agents of IE.

Molecular diagnosis of culture negative infective endocarditis: clinical validation in a group of surgically treated patients

OBJECTIVE

To assess the clinical validity of polymerase chain reaction (PCR) based molecular methods in the microbiological diagnosis of culture negative infective endocarditis in a group of surgically treated patients.

DESIGN

Retrospective case-control study.

SETTING

Reference cardiovascular surgical centre. PATIENTS AND SAMPLES: 15 culture negative patients with infective endocarditis classified according to Duke criteria, with 17 heart valve samples; 13 age and sex matched control patients without infective endocarditis, with 13 valve samples.

INTERVENTIONS

Medical records were reviewed and clinical, demographic, and microbiological data collected, including results of molecular detection of bacteria and fungi from valve samples. The clinical validity of molecular diagnosis was assessed, along with the sensitivity and speed of the systems.

RESULTS

In the study group, 14 patients were PCR positive (93%). Organisms detected were streptococci (3), staphylococci (2), enterobacter (1), Tropheryma whippelii (1), Borrelia burgdorferi (1), Candida albicans (1), and Aspergillus species (2). Three cases were positive on universal bacterial detection but the pathogen could not be identified because of contaminating background. One case was negative. All but two positive cases showed clinical correlations. These two cases had no symptoms of infective endocarditis but there was agreement with the surgical findings. All control cases were PCR negative. Results were available within eight hours, and if sequencing was necessary, within 48 hours.

CONCLUSIONS

PCR based molecular detection of pathogens in valve samples from surgically treated culture negative infective endocarditis patients is fast, sensitive, and reliable. The technology, combined with thorough validation and clinical interpretation, may be a promising tool for routine testing of infective endocarditis.

Diagnostic methods current best practices and guidelines for identification of difficult-to-culture pathogens in infective endocarditis

Culture-negative endocarditis currently represents a diagnostic challenge for physicians. Traditional methods such as histology, serology, and culture have been improved and new molecular techniques have been developed to improve the detection of difficult-to-culture agents. Serologic tests for the two most frequent etiologic agents, Coxiella burnetii and Bartonella spp, should be performed first because they can usually be identified easily in this way. The sensitivity of culture for intracellular bacteria has been improved by inoculation of samples in shell vials and by the use of novel tissue cell lines. Recently, universal and species-specific primers have been designated to amplify bacterial DNA directly from resected valves, allowing positive identification.

Diagnostic methods current best practices and guidelines for histologic evaluation in infective endocarditis

Infective endocarditis (IE) often presents diagnostic and therapeutic challenges and continues to cause high morbidity and mortality. Confirmation of the diagnosis of IE is important for the purposes of epidemiologic and clinical studies and is crucial for patient management. Despite recent advances in diagnostic techniques, about 10% of IE cases remain culture-negative. Because pathological examination of cardiac valves to demonstrate vegetations and valvular inflammation remains the gold standard for the diagnosis of IE, the role of the pathologist is often decisive, especially when bacteriologists fail to isolate a microorganism or when a microorganism that has been isolated may be a contaminant. Furthermore, the pathologist may play an important role in identification of previously unknown infectious agents.

Molecular diagnosis of Granulicatella elegans on the cardiac valve of a patient with culture-negative endocarditis

We herein report one case of culture-negative infectious endocarditis (IE) where the organism, Granulicatella elegans, was identified by molecular analysis using broad-range PCR primers complementary to the 16S rRNA gene on the removed valve. The results and utility of this method are discussed.

Clinically and histologically silent Q fever endocarditis accidentally diagnosed by PCR

A case of Q fever endocarditis was diagnosed in a patient with no sign of active endocarditis by performing PCR targeting eubacterial 16S rDNA on the resected mitral valve. The diagnosis was confirmed by detection of high levels of anti-Coxiella burnetti antibodies, positive immunohistologic analysis of the valve tissue with specific antibodies and culture of C. burnetti from the valve tissue. As this patient had an unexplained aggravation of valve dysfunction, we recommended routine serologic testing for C. burnetti to allow the diagnosis of Q fever endocarditis at a very early stage.

Endocarditis due to rare and fastidious bacteria

The etiologic diagnosis of infective endocarditis is easily made in the presence of continuous bacteremia with gram-positive cocci. However, the blood culture may contain a bacterium rarely associated with endocarditis, such as Lactobacillus spp., Klebsiella spp., or nontoxigenic Corynebacterium, Salmonella, Gemella, Campylobacter, Aeromonas, Yersinia, Nocardia, Pasteurella, Listeria, or Erysipelothrix spp., that requires further investigation to establish the relationship with endocarditis, or the blood culture may be uninformative despite a supportive clinical evaluation. In the latter case, the etiologic agents are either fastidious extracellular or intracellular bacteria. Fastidious extracellular bacteria such as Abiotrophia, HACEK group bacteria, Clostridium, Brucella, Legionella, Mycobacterium, and Bartonella spp. need supplemented media, prolonged incubation time, and special culture conditions. Intracellular bacteria such as Coxiella burnetii cannot be isolated routinely. The two most prevalent etiologic agents of culture-negative endocarditis are C. burnetti and Bartonella spp. Their diagnosis is usually carried out serologically. A systemic pathologic examination of excised heart valves including periodic acid-Schiff (PAS) staining and molecular methods has allowed the identification of Whipple's bacillus endocarditis. Pathologic examination of the valve using special staining, such as Warthin-Starry, Gimenez, and PAS, and broad-spectrum PCR should be performed systematically when no etiologic diagnosis is evident through routine laboratory evaluation.