Temperate phages in Salmonella enterica serovar Typhimurium: implications for epidemiology.

Salmonella enterica serovar Typhimurium is the most common Salmonella serovar isolated from humans in Australia. The most common definitive phage types (DT) include 9, 64 and 135. Induction of lysogenic phages from DT 64 with mitomycin C followed by cesium chloride gradient purification, resulted in separation of two populations of phage particles. DNA extracted from these particles that was digested with SmaI showed two distinct patterns of banding. Transmission electron microscopy showed that both phage particles belong to the podovirus family of the C1 morphotype. One of the phages, ST64T is capable of mediating both generalized transduction and bacteriophage type conversion. Crude phage lysate induced from S. Typhimurium DT 64 was capable of phage type conversion. S. Typhimurium DT 9 was converted to DT 64 and DT 135 was converted to DT 16. S. Typhimurium DT 41 was also converted to DT 29. Amplified-fragment length polymorphism revealed differences between the original isolates and the convertants. Phage type conversion raises the question of the stability of the bacterial phage types in natural settings and the possibility of its occurrence during an outbreak scenario.