Infection characteristics of Salmonella enterica serovar Brandenburg in vitro

Abstract

Salmonella enterica serovar Brandenburg (S. Brandenburg) is primarily recognized as a pathogenic agent causing acute diarrhoea in humans and livestock. In New Zealand, it also causes abortion and septicaemia, particularly in pregnant ewes. The disease outcome and severity may depend on serovar- and host-specific factors. In this study, the uptake and persistence characteristics (adhesion, invasion and replication) of selected S. Brandenburg isolates were investigated, using primary cell cultures derived from ovine and bovine oviduct and intestine, in an attempt to help explain why the agent is so pathogenic in sheep. In the first study, different profile 14 (epidemic) S. Brandenburg isolates 3684, 3062 and 4468 and non-profile 14 S. Brandenburg isolate 4527 were added into ex vivo and in vitro cell cultures derived from sheep oviduct. In three independent assays adhesion, invasion and replication were measured at 1, 2, and 24 h respectively. In the second study, ex vivo or in vitro cell cultures from sheep oviduct were infected with either S. Brandenburg isolate 3684 or S. Typhimurium isolate 1979 and adhesion, invasion and replication characteristics were compared. In the third study, S. Brandenburg isolate 3684 was added to ex vivo and in vitro the ovine oviduct epithelial cell (OOEC) or bovine oviduct epithelial cell (BOEC) cultures and same time course measurements were made. In the fourth study, S. Brandenburg isolate 3684 was added to in vitro cell cultures derived from either ovine intestine cells (OIECs) or ovine oviduct cells (OOECs) and the same time course measurement were compared. Bacterial counts were estimated after 10 fold serial dilution onto LB agar plates and overnight incubation at 37°C. The adhesive, invasive and replicative characteristics of the different Salmonella isolates were compared using a Student two-sample t-test. S. Brandenburg profile 14 isolates 3062, 3684, 4468 shared similar adhesive, invasive and replicative capabilities in both ex vivo and in vitro OOECs. Invasion of non-profile 14 isolate 4527 was less than profile 14 isolates 3684, 3062 and 4468 in both ex vivo and in vitro OOECs (P<0.01). Secondly, S. Brandenburg isolate 3684 more readily adhered to, and replicated within, OOECs than S. Typhimurium isolate 1979, in ex vivo OOECs (P<0.01). Thirdly, S. Brandenburg isolate 3684 more readily replicated within OOECs (P<0.01) than BOECs in both ex vivo and in vitro assays. Fourthly, S. Brandenburg isolate 3684 more readily invaded and replicated within in vitro OOECs than OIECs (P<0.01). Together, these results suggest that S. Brandenburg profile 14 isolates share some phenotypic characteristics regarding infection and suggest that virulence of S. Brandenburg in vivo is associated with its infectivity characteristics in vitro and ex vivo. In addition, the characteristics of S. Brandenburg field infections in sheep may be due to its preference for infecting in vitro OOECs, not BOECs or OIECs.