Epidemiology and attack index of gram-negative bacteria causing invasive infection in three special-care neonatal units and risk factors for infection.

Of 13 consecutive episodes of gram-negative septicemia (Escherichia coli eight, Klebsiella oxytoca four, Klebsiella pneumoniae one) among 113 infants in three special-care neonatal units studied, five episodes were epidemiologically related according to a novel fingerprinting method for enterobacteria. In ten episodes the invasive phenotype was found in the fecal flora of up to 54% of the fellow infants in the same ward and for periods of up to 70 days. Two units exchanged patients, which further promoted the transmission of invasive strains. The attack index was highest for certain E. coli strains, generally low for K. oxytoca strains, but lowest for other E. coli strains. The infants contracting septicemia had lower birth weight (p = 0.04) or were more often classified as high-risk infants than matched non-infected fecal carriers of the invasive strains (p = 0.04). In summary, gram-negative neonatal septicemia was either due to an apparently high-virulent strain capable of attacking the single full-term infant carrier or a high-colonizing phenotype of lower apparent virulence, which occasionally attacked a high-risk infant among a large number of infants colonized.