Plasmid characterization and pulsed-field electrophoretic analysis demonstrate that ampicillin-resistant strains of Salmonella enteritidis phage type 6a are derived from Salm. enteritidis phage type 4.

Plasmid incompatibility studies have demonstrated that strains of Salmonella enteritidis phage type (PT) 6a resistant to ampicillin possess a 36 megadalton incompatibility group (Inc) X plasmid coding for resistance to ampicillin which is capable of converting strains of Salm. enteritidis belonging to PTs 1 and 4 to PT 6a, and PT 8 to PT 13. However, pulsed-field gel electrophoresis (PFGE) has demonstrated that all clinical isolates of PT 6a have a characteristic XbaI pulsed-field profile which is distinct from that of PT 1 and which can only be differentiated from that of PT 4 by the presence of plasmid-associated fragments of less than 45 kb. It is concluded that ampicillin-resistant strains of Salm. enteritidis PT 6a are derived from strains of Salm. enteritidis PT 4 by acquisition of an Inc X ampicillin resistance plasmid.