SPI1 and infection: We determined the degree of systemic and enteric colonization by wild type ATCC14028 in single infection, and we quantify the distribution of this isolate and an otherwise isogenic SPI-1 mutant (ΔinvA) during competitive infection in the ceca of 1-week-old White Leghorn chicks and 8-week-old Salmonella-resistant CBA/J mice.