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Microbiology: The Investigation And Diagnosis Of Infective Endocarditis

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Infective endocarditis (IE) is a microbial infection of a heart valve (native or prosthetic) or the mural endocardium, leading to tissue destruction and formation of vegetations. It is primarily a disease of the heart, but by virtue of its haematogenic spread, it is also a multisystem disorder. The aim of this article is to review the epidemiological and microbiological profile of IE, as well as pathophysiology, clinical presentation, and management of complications.

Clinical Details

A 36 year old male patient was presented to Tallaght University Hospital (TUH) with night sweats, weight loss and lethargy. No focal signs of infection. No antibiotics were given. No recent travel. The patient denies being intravenous drug user (IVDU). Liver imaging was suggested in case there was anything to be drained and checked for C&S. Eventually, patient was referred to Saint James Hospital (SJH) as mitral valve was in bad shape.

Laboratory investigations

Upon admission, 2 blood culture samples were taken from patient, aerobic and anaerobic. These samples were sent to Microbiology lab within TUH. When received in the main lab, samples are first checked for patient details, then labeled and booked in. After that, the samples are brought to blood culture section to be loaded into the BacT ALERT.

The BacT ALERT is an automated system allowing incubation, agitation and continuous monitoring of blood culture bottle incubated with clinical material. The BacT alert bottles for use with the system contain a media to allow growth of target microorganisms. If microorganisms are present in the test sample metabolism of the substrates in the bottle media will produce CO2. As CO2 is produced the colour of the indicator/ sensor at the bottom of each bottle will change from dark to light. A light emitting diode projects light onto the indicator. Reflected light is measured by a photo-detector. As more CO2 is produced more light is reflected. The intensity of the reflected light is compared to values obtained when the sample is loaded onto the machine. A high initial value or a high/ sustained rate of CO2 production will cause the sample to be flagged as positive. If the level of CO2 does not change significantly over the defined period of incubation the sample will be flagged as negative. Negative bottles are removed from the system at the end of their incubation period and culture result is reported as no growth within that period. Bottles are disposed of in accordance with laboratory protocol.


Infective endocarditis (IE) is a systemic life-threatening disease mainly affecting patients with heart valve disease, prosthetic valve, intracardiac devices, and i.v. drug abusers. Clinical findings, echocardiography, and blood cultures are the cornerstone of IE diagnostics, and serological tests and polymerase chain reaction may be useful in culture-negative patients.

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The incidence of IE is 1.7–7.2 cases per 100,000 person-years. The female to male ratio has remained stable over the years at 1:2.2 However, the median age of endocarditis patients has increased from 30–40 to 47–69 yr and rheumatic heart disease is no longer the main risk factor for IE in Western countries. Increasing longevity, degenerative valve disease, and medical treatment, including prosthetic heart valves and indwelling devices such as pacemakers and implanted defibrillators, are the main factors responsible for these substantial changes in the epidemiological profile over the last few decades.

The majority of cases of IE are caused by gram-positive bacteria. Staphylococcus aureus is now more common than oral Streptococci (formerly Streptococcus viridans) and it has become the most frequent microorganism causing IE (31–54%). Gram-negative microorganisms can also cause IE. The slow-growing HACEK (Haemophilus parainfluenzae, Aggregatibacter aphrophilus, Cardiobacterium hominis, Eikenella corrodens, and Kingella kingae) group is a well recognized but unusual cause of IE, responsible for 1.8– 3% of cases.

IE originates at sites where the endothelium is damaged by high blood velocity or mechanical damage and on foreign bodies in the circulation. Clinical suspicion and prompt investigation of IE is imperative. A multidisciplinary team involving microbiologists, cardiologists, neurologists, anaesthetists, surgeons, and intensivists should be involved in caring for these patients

A positive blood culture is still the best method for the identification of the microorganisms causing IE and it is considered to be a major diagnostic criteria. Blood cultures are positive in about 80% of cases, but may be negative in cases of intracellular or fastidious pathogens or after previous antibiotic treatment. Therefore, whenever IE is suspected (i.e. temperature . 388C, new regurgitant murmur, and history of valvular disease), it is mandatory to perform blood cultures before starting antibiotic treatment. When the antimicrobial agents have been administered before blood cultures are obtained, the recovery rate of bacteria is reduced by 35–40%. Three sets (including at least one aerobic and one anaerobic), obtained from a sterile site, are normally sufficient to identify the usual microorganisms, but some patients may need repetitive sampling.

Microbiology advice should be sought in all cases. Early antimicrobial therapy is paramount; empirical treatment (flucloxacillin and gentamicin) is started in most cases and antibiotics are later adjusted according to the sensitivity of the microorganism. The addition of an aminoglycoside is associated with side-effects including nephrotoxicity and levels should therefore be measured. Once-daily aminoglycoside regimens are now widely used for other infections, but data regarding their efficacy in endocarditis are limited. For patients with intracardiac prosthetic material or suspected MRSA vancomycin is recommended (adjusted to renal function), and its levels should be also monitored.


A 36 year old male patient was presented to Tallaght University Hospital (TUH) with night sweats, weight loss and lethargy. The microbiology laboratory contributed to the diagnosis of the patient’s condition, Infective endocarditis. The blood culture tests had a great impact on this diagnosis particularly due to the three subsequent positive sets of blood culture. Patient was sent to SJH, as mitral valve was in bad shape. The patient is currently undergoing treatment with ceftriaxone and penicillin.

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Cite this Page

Microbiology: The Investigation And Diagnosis Of Infective Endocarditis. (2022, February 21). Edubirdie. Retrieved November 29, 2022, from
“Microbiology: The Investigation And Diagnosis Of Infective Endocarditis.” Edubirdie, 21 Feb. 2022,
Microbiology: The Investigation And Diagnosis Of Infective Endocarditis. [online]. Available at: <> [Accessed 29 Nov. 2022].
Microbiology: The Investigation And Diagnosis Of Infective Endocarditis [Internet]. Edubirdie. 2022 Feb 21 [cited 2022 Nov 29]. Available from:
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