Gram Negative Organisms

Gram-negative IE is much less common (approximately 2-5%) but is typically much more difficult to treat than gram-positive infections. Fastidious organisms, such as the HACEK group, tend to be seen most commonly, causing 3% of all IE.34 This group consists of Haemophilus spp. (primarily H. paraphrophilus, H. parainfluenzae, and H. aphrophilus), Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, and Kingella kingae. The clinical presentation of IE by these organisms is subacute, with approximately 50% of patients developing complications. These complications are primarily due to the presence of large, friable vegetations and numerous emboli along with the development of acute congestive heart failure often requiring valve replacement.35,36 It is important to allow cultures sufficient incubation time (often 2-3 weeks) in order to isolate these organisms. Often these organisms may not be isolated on culture and thus present as culture-negative IE.

Other gram-negative organisms, such as Pseudomonas spp., cause IE, especially in IVDUs and patients with prosthetic valves. Additionally, IE may be caused by Salmonella spp., Escherichia coli, Citrobacter spp., Klebsiella spp., Enterobacter spp.,

1 37

Serratia marcescens, Proteus spp. and Providencia spp. '

Gram-negative IE typically has a poor prognosis with high mortality rates (as high as 83%).1 Treatment usually consists of high-dose combination therapy, with valve replacement often a necessity in many patients.

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