Persistence of Two Salmonella enterica ser. Montevideo Strains Throughout Horn Fly (Diptera: Muscidae) Larval and Pupal Development

Abstract Strains of Salmonella enterica can be subdivided into clades that differ in their genetic composition, influencing microbial ecology and bacterial transmission. Salmonella serovar Montevideo strains 1110 and 304, representatives of two different clades, were used to evaluate interactions with the various stages of horn fly development. Sterilized cattle dung was inoculated with Salmonella monocultures, and horn fly larvae were exposed to 103, 105, and 107 colony-forming units (CFU)/g per strain. Salmonella supported horn fly development, and concentration-dependent differences in pupal survival suggested that Salmonella Montevideo 304 impacts adult emergence when larvae are reared in a high concentration. Viable bacteria of each strain were quantified from larvae, pupae, and newly emerged adults. Both strains were cultured from larvae at a mean ~105, regardless of concentration, and both strains survived pupation. Quantities of Salmonella 1110 were stationary through the midpupal stage, after which quantities declined in pupae reared in 105 and increased twofold in pupae reared in 107 CFU/g. Quantities of Salmonella 304 remained stationary throughout pupal development when reared in 105, yet increased 29-fold when reared in 107 CFU/g. At high densities, properties of Salmonella 304 may influence its interaction with horn fly larvae, enabling the bacteria to evade degradation during larval gut histolysis and to subsequently proliferate during the late stages of pupal development. This may account for the observed effect on adult emergence. The Salmonella strains were rarely cultured from newly emerged adults, indicating that transstadial carriage to the adult stage is inefficient.