Cross-infection by non-encapsulated Haemophilus influenzae

From October, 1988, to January, 1989, 18 patients admitted to an acute medical chest ward were infected with an ampicillin-resistant β-lactamase-producing strain of Haemophilus influenzae. All 18 isolates were non-encapsulated strains of biotype III and showed identical cell envelope protein profiles, as judged by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The organism was not isolated from repeated environmental samples but there was strong circumstantial evidence that a spirometer was a common iatrogenic source of the cross-infection.