DISCUSSION: Important organisms that cause nosocomial infections include vancomycin-resistant Enterococci, methicillin-resistant Staphylococci, and Gram-negative isolates with variable resistance patterns. Key Gram-negative resistance patterns arise from extended-spectrum beta-lactamases in enteric bacilli, high-level third-generation cephalosporin beta-lactamase resistance in Enterobacteriaceae and Citrobacter genus, and multidrug resistance in pseudomonads and Acinetobacter genus. Certain Citrobacter isolates, such as C. koseri, produce a small amount of beta-lactamase constitutively, while other Citrobacter species (such as C. amaloniticus) produce beta-lactamase which can mutate to high-level beta-lactamase resistance. Citrobacter causes endocarditis rarely, and only one report of C. koseri mitral valve endocarditis exists in the English language. Fourth-generation cephalosporins, piperacillin/tazobactam, and carbapenems are efficacious therapies for nosocomial infections caused by Gram-negative organisms with high beta-lactam resistance.