Streptococcal necrotizing fasciitis: development of an animal model to study its pathogenesis

Necrotizing fasciitis is a serious and increasingly common human disease which can be caused by an infection with beta-haemolytic streptococci (BHS) of Lancefield groups A, C or G, spreading rapidly in the loose connective tissue over the muscle fascia. To facilitate study of its pathogenesis, we have developed an animal model for the production of a spreading infection with BHS in the loose connective tissue over the muscle layer in the skin of New Zealand White rabbits. Intradermal injection of group A BHS alone into the flank was unsatisfactory in that a spreading lesion occurred on only 12% of occasions. When the group A BHS were co-injected with cultures of Staphylococcus aureus, the results depended on the strain of S. aureus used: an abscess-producing strain isolated from pigs gave rise to a spreading lesion on 50% of occasions. When BHS were injected with the alpha-lysin of S. aureus at a titre which produced inflammation without necrosis, spreading lesions occurred on 75% of occasions. However, both inoculated and uninoculated broth acted synergistically with the alpha-lysin in potentiating the spread of the streptococci. This demonstration of synergy between BHS and alpha-lysin of S. aureus may reflect the clinical situation in the human, as both organisms have been found to occur together at sites where spreading streptococcal infections have originated.