Heterogeneity of Tn 5253 -Like Composite Elements in Clinical Streptococcus pneumoniae Isolates

Abstract

ABSTRACT Several drug resistances in Streptococcus pneumoniae are associated with mobile genetic elements, which are loosely subdivided into a group of smaller (18- to 27-kb) and a group of larger (>50-kb) elements. While the elements of the former group, which typically carry the tetracycline resistance determinant tet (M) and whose prototype is Tn 916 (18 kb), have been studied extensively, the larger elements, whose prototype is Tn 5253 (∼65.5 kb), are not as well explored. Tn 5253 is a composite structure consisting of two independent conjugative transposons, Tn 5251 (which is virtually identical to Tn 916 ) and Tn 5252 (∼47.5 kb), with the former inserted into the latter. Tn 5252 , which so far has only partially been sequenced, carries an integrase gene, driving its site-specific insertion into the host cell genome, and the chloramphenicol resistance cat pC194 determinant. This study investigated 20 clinical isolates of S. pneumoniae , which were selected on the basis of cat pC194 -mediated chloramphenicol resistance. All 20 isolates harbored a Tn 5253 -like element. The composite elements (some of which have been completely sequenced) demonstrated considerable heterogeneity that stemmed from a dual variability: in the Tn 5252 -like element, due primarily to differences in the integrase gene but also to differences in cargo genes and in the overall genetic organization, and in the Tn 916 -like element, with the possible involvement, besides Tn 916 , of a number of Tn 916 family pneumococcal elements carrying different erythromycin resistance genes. In mating experiments, only one composite element, containing a less typical Tn 916 family element, appeared to be nonmobile. Being part of a Tn 5253 -like composite element may confer on some Tn 916 -like transposons, which are apparently nontransferable as independent genetic elements, the ability to be mobilized.