The prevalence of an antibiotic-resistant strain of Salmonella Typhimurium definitive phage type 104 (DT104) has increased dramatically in recent years resulting in increased morbidity and mortality in both animals and humans. Colonization and shedding of Salmonella Typhimurium DT104 was studied in broiler chickens in two trials. In trial 1, 180 day-of-hatch chicks (n = 60 per group, n = 30 per replicate) were challenged with 106 CFU DT104 (wild-type isolate from poultry) or were commingled with a seeder chick challenged with 106 CFU DT104. In trial 2, 360 day-of-hatch chicks (n = 120 per treatment, n = 30 per rep) were divided into three groups. Chicks in the susceptible group were commingled with two seeder chicks that were orally challenged with 107 CFU/bird of a pan-sensitive strain of Salmonella Typhimurium DT104. Chicks in the resistant group were commingled with two seeder chicks that were orally challenged with 107 CFU/bird DT104 used in trial 1. For both trials, a control group was not exposed to DT104, composite fecal samples were evaluated twice weekly for levels of Salmonella shedding and 20 chicks per group were necropsied weekly and their cecal contents were cultured. At hatch all groups were colonized with naturally occurring Salmonella Senftenberg and Salmonella Mbandaka (trial 1) or Salmonella Senftenberg and Salmonella Ohio (trial 2) prior to exposure to DT104. Throughout the study, the level of Salmonella spp. shedding in feces (trial 1 means 3.1, 2.9, and 3.0 log10 CFU per g feces for challenged, seeder, and control groups, respectively) or ceca (trial 2 means 2.9, 2.9, and 2.5 log10 CFU per g ceca for resistant, susceptible, and control groups, respectively)did not differ among groups. In trial 1, colonization of DT104 remained constant at higher levels in the challenged group (mean 87%, P < 0.01), increased over time in the seeder group (10 to 50%, P < 0.02) and was not recovered from the control chicks. Salmonella Mbandaka colonization remained steady within each group with challenge and seeder groups maintaining higher levels of colonization than the control group. Salmonella Senftenberg colonization levels tended to decline (P = .058) over time in the challenged group (20 to 0%) and significantly decreased (P < 0.01) over time for both the seeder (80 to 0%) and control chicks (85 to 10%). In trial 2, the percentage of chicks colonized with susceptible DT104 declined (r = 0.90, P < 0.05) over the course of the trial from 45 to 0%, while recovery of the resistant DT104 persisted at a mean percentage of 27%. DT104 was not recovered from the control chicks. Salmonella Ohio colonization levels tended to decline (r = 0.79, P > 0.05) over time in the control group (75 to 20%) and significantly decreased (P < 0.05) over time in both susceptible and resistant groups (40 to 10%, r = 0.82 and 55 to 5%, r = 0.85, respectively). Salmonella Senftenberg was recovered from the control group at low frequency throughout the trial and was not recovered from the other groups. For either trial, no apparent affect on morbidity or mortality was observed. Introduction of DT104 by commingling may induce colonization resulting in persistent high levels of shedding in flocks simultaneously with other Salmonella species.
Mobile Colistin Resistance Gene mcr-1 Detected on an IncI2 Plasmid in Salmonella Typhimurium Sequence Type 19 from a Healthy Pig in South Korea
