Factors Influencing Fluoroquinolone Resistance

no. ISTN3X; 96% identical), comprising one of Tn3-like inverted repeats and putative coding regions for transposase, resolvase (also called repressor), and ampicillin resistance. The resistance gene encodes a TEM-1 type β-lactamase. (The sequence has been registered to DDBJ/GenBank/ EMBL with accession no. AB103092.) Conjugative transferability of p981123 between S. Enteritidis strains was examined by using the parental S. Enteritidis RDNC-a R-AS strain as a donor, and three independent S. Enteritidis strains (PT1; PT4; and PT21) resistant to nalidixic acid (RN) as recipients. p981123 was transferable between S. Enteritidis strains at frequencies of 10-5 to 10-4, and the resulting R-AN transconjugant showed the same lytic pattern of the typing phages as RDNC-a. Thus, transfer of p981123 could convert the phage types at least from PT1, PT4, and PT21 to RDNC-a. Pulsed-field gel electrophoresis (PFGE) was done by using XbaI or BlnI as well, and RDNC-a strains showed a variety of PFGE profiles. These results suggest emergence and prevalence of the 50kb R-plasmid converting phage types to RDNC-a in S. Enteritidis in Japan. Previous studies reported correlation between R-plasmids and phage types of S. Enteritidis, where, for example, a 34-MDa R-plasmid of incompatibility group N (IncN) (8) and a 36-MDa R-plasmid of IncX (pDEP57) (6) were described. Both kinds of plasmids encoded ampicillin resistance as well as that in this study, but both were identified in PT6a isolates. Preliminary sequence data of the region of p981123 essential for replication indicated a gene coding for a protein similar to protein p1 of R6K (IncX) plasmid (9), which suggests that p981123 may be related to pDEP57. However, the reactions to the typing phages in RDNC-a strains were different from those in PT6a. Therefore, the R-plasmid in this study seems to have different features from previous ones. In addition, S. Enteritidis PT6d resistant to ampicillin was recently reported (10). Relationship between RDNC-a in this study and PT6d is unknown, and further investigations will be needed. Transfer of an R-plasmid is a common way for bacteria to acquire drug resistance, and it often affects other aspects such as sensitivity of bacteriophages, as described in this study. Molecular based surveillance for drug resistance in S. Enteritidis needs to continue.