Infectious diseases are usually classified by agent on the basis of biological features (categories of metozoa, protozoa, bacteria, fungi, rickettsia, viruses) (Benenson, 1990). The agent enters the human host through portals such as the respiratory tract, gastrointestinal tract, genitourinary tract, conjunctiva, or skin and tissue. Studies in intensive care units (ICUs) have shown that Gram-negative colonization of tracheal aspirate, mouth, peri-anal skin or urine significantly increases the morbidity and mortality of ICU patients (Shekelle et al, 1999).