Transfer of penicillin resistance between Neisseriae in microcosm

Horizontal gene transfer between commensal and pathogenic Neisseriae is the mechanism proposed to explain how pathogenic species acquire altered porti ons of the penA gene, which encodes penicillin binding protein 2, These cha nges resulted in a moderately penicillin-resistant phenotype in the meningo cocci, whose frequency of isolation in Spain increased at the end of the 19 80s, Little has been published about the possibility of this gene transfer in nature or about its simulation in the laboratory, We designed a simple m icrocosm, formed by solid and liquid media, that partially mimics the upper human respiratory tract, In this microcosm, penicillin-resistant commensal strains and the fully susceptible meningococcus were co-cultivated, The ef ficiency of gene transfer between the strains depended on the phase of bact erial growth and the conditions of culture. Resistance of penicillin was ac quired in different steps irrespective of the source of the DNA, The presen ce of DNase in the medium had no effect on gene transfer, but it was near z ero when nicked DNA was used. Cell-to-cell contact or membrane blebs could explain these results. The analysis of sequences of the transpeptidase doma in of PBP2 from transformants, and from donor and recipient strains demonst rated that the emergence of moderately resistant transformants was due to g enetic exchange between the co-cultivated strains. Finally, mechanisms othe r than penA modification could be invoked to explain decreased susceptibili ty.