Introduction

Although aerobic Gram-negative rod pneumonia was first recognised over a century ago, aetiological agents other than Klebsiella pneumoniae did not feature until the last three decades. Their entrance into the pulmonary arena was heralded by the evolution of health care, particularly the vast explosion in high technology medical practice such as intensive care, the paraphernalia of invasive procedural devices such as endotracheal intubation and intravascular devices, iatrogenic immunosuppressive therapy for malignancy, use of antimicrobials and the ever-increasing population of the elderly with their relatively immunocompromised status. This scenario has permitted expression of the opportunistic potential of many commensal organisms. It is impossible to present precise incidence figures, as these vary from centre to centre and are influenced by diagnostic sensitivity, patient populations, etc. However, the figures quoted are generally taken from larger or more representative sources. In contrast to the fall in the aetiological contribution of the pneumococcus to community acquired pneumonia which now stands at approximately 30–40%, compared to over 80% 30 years ago, the incidence of Gram-negative bacillary pneumonia has increased from between 0.65 to 8.5% before the mid-1960s up to at least 20% since that time. In hospital-acquired pneumonia, the contribution is even more striking, with enteric Gram-negative bacilli causing between 50 and 60% of all nosocomial pneumonias and contributing substantially to the 15.5% of hospital deaths caused by nosocomial pneumonia.