Microbiology

Staphylococcus aureus; Streptococcus viridans; groups A, C, and G streptococci; and enterococci are the most common microorganisms that cause endocarditis (5-7).

S. viridans accounts for 50% to 60% of subacute diseases; Streptococcus anginosus group causes either acute or subacute infection, accounting for 15% of streptococcal cases. Approximately 5% of subacute cases are due to nutritionally variant streptococci. Most cases of Enterococcus spp., which is the third most common cause of endocarditis, are subacute and are generally of gastrointestinal or genitourinary tract source and often reflecting underlying abnormalities of the large bowel (e.g., ulcerative colitis, polyps, cancer). Group B streptococci are generally seen in pregnant women and older patients with underlying diseases (e.g., cancer, diabetes, and alcoholism). Groups A, C, and G streptococcal endocarditis resemble that of S. aureus (30-70% mortality rate) with suppurative complications. S. aureus is the most common cause of all forms of endocarditis, has a high mortality rate of 40% to 50%, is associated with intravascular lines and many of the isolates are currently methicillin resistant. Coagulase-negative Staphylococci causes a subacute disease, and Pseudomonas aeruginosa generally induces acute infection where surgery is commonly required for cure. The HACEK organisms (i.e., Haemophilus aphrophilus, Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, Kingella kingae) usually cause subacute disease and account for approximately 5% of cases. Fungi (Candida and Aspergillus spp.) mostly cause subacute disease; and Bartonella spp. is common in individuals with extremely substandard hygiene.

Anaerobic bacteria are an uncommon but important cause of endocarditis accounting for 2% to 16% of all cases. Most cases of anaerobic endocarditis are caused by anaerobic cocci, Propionibacterium acnes, and Bacteroides fragilis (8). Predisposing factors and signs and symptoms of endocarditis caused by anaerobic bacteria are similar to those seen in endocarditis with facultative anaerobic bacteria with the following exceptions: there is a lower incidence of preexisting valvular heart disease, a higher incidence of thromboemboli events and a higher mortality rate with anaerobic endocarditis.

The probable increase in the number of reported cases of anaerobic endocarditis noted in recent years may be explained by: the increased frequency of polymicrobial bacteremias (9), the decreased frequency of "culture-negative" cases (10,11), the increased use of prosthetic intravascular devices, and improvements in microbiological methods. Polymicrobial endocarditis is more common in addicts (2-9% of cases) (2).

In a review of 1046 cases of endocarditis from 1963 to 1969, a total of 14 (1.3%) cases were caused by anaerobes (1): 12 were due to anaerobic streptococci, one was caused by Bacteroides species, and one by a diphtheroid. An additional 33 new cases were also presented. Polymicrobial infection was observed in eight (24%) patients—mostly due to P. melaninogenica or peptostreptococci together with facultative streptococci. Nastro and Finegold (12) reviewed 37 cases of anaerobic endocarditis; where polymicrobial infections were found in five (13.5%). In another review of 66 cases, seven (10.6%) were caused by anaerobes and three of seven were polymicrobial (2). Cohen et al. (13) described 11 cases of endocarditis due to Bacteroides spp., while Kolander et al. (14) reported one case of Clostridium bifermentans endocarditis, and reviewed 16 other cases of clostridial endocarditis. None of the patients had conditions predisposing to infection.

The role of Propionibacterium spp. in endocarditis in 36 patients was recently summarized (15). In most cases, infection was protracted, with minimal signs in the early stages. Fourteen cases (42.4%) involved native valves, 16 (48.5%) involved prosthetic valves and three (9.1%) were associated with other intracardiac prosthetic material. Intracardiac abscesses were commonly encountered, with Propionibacterium endocarditis occurring in 28.6% of native valve infections and 52.9% of prosthetic valve infections. Most of the cases (70.6%) required surgical intervention. Several factors delayed institution of appropriate therapy and may have contributed to abscess formation, including an indolent clinical course, negative or delayed culture results, and the tendency to consider this organism as a blood-culture contaminant. The data supports careful clinical evaluation before disregarding a blood-culture isolate of Propionibacterium spp. as a skin contaminant, and consideration of this bacterium as a potential cause of apparently culture-negative endocarditis.

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