A significant association between erythromycin resistance and ability of bacteria to enter human respiratory cells has been documented in group A streptococci (GAS) isolated in Italy from children with pharyngitis. The occurrence of strains combining cell invasiveness with erythromycin resistance raised serious concern because they could escape penicillin by virtue of intracellular location and macrolides by virtue of resistance, resulting in difficulty in eradication.
Thirty-one pharyngeal cell-invasive, erythromycin-resistant (ER) GAS, representing that many clones recently identified among Italian GAS carrying the internalization-related gene prtF1, were investigated for intracellular persistence inside cultured respiratory cells (A549) by means of a survival assay and by staining and polymerase chain reaction assays on infected cells.
All tested ER GAS could be recovered in culture from infected cells 24 hours from infection with logarithms exceeding 4.0 (4 strains). The highest survival rate (>5.0) was exhibited by strain SP1900 [erm(A)/iMLS-B; high-level erythromycin resistance [minimum inhibitory concentration, ≥128 μg/mL)], the most widespread clone, which was cultivable from infected cells up to day 5. As long as SP1900-infected cells could be maintained in culture, the presence of multiple cocci inside cells was consistently revealed by microscopy. During the same time, DNA sequences internal to both genes prtF1 and erm(A) continued to be amplified by polymerase chain reaction.
These results suggest that the ER GAS are capable of persisting in human respiratory cells. This might contribute to clinical problems such as relapsing infection and persistent carriage despite antibiotic treatment and might have facilitated the widespread diffusion of ER GAS in Italy.