An Intrinsic Pathogenicity Index for Microorganisms Causing Infection in a Neonatal Surgical Unit

The intrinsic pathogenicity index (IPI) for a particular microorganism is defined as the ratio of the number of patients infected by the microorganism to the number of patients colonised in the digestive tract by identical microorganisms. A microorganism with an IPI of 1 will practically always cause infection following colonisation ('high' pathogen), whilst a microorganism with an IPI of 0 is unlikely to be involved in infection ('low' pathogen). IPIs were calculated for microorganisms causing infection in 40 neonates admitted to a neonatal surgical unit. The indices of the predominant organisms involved in infectious problems in this group, Pseudomonas and Candida species, were 0·38 and 0·31 respectively, whilst those for coagulase-negative staphylococci and enterococci were respectively 0·03 and 0. Possible applications of the IPI are in further assessment of the value of surveillance cultures and as the basis of antibiotic policies in populations with a high risk of infection.