Tetracycline is a front line antibiotic for the treatment of chlamydial infections in both humans and animals, and the emergence of tetracycline resistant (tet[superscript R]) Chlamydia is of significant clinical importance. Recently, several tet[superscript R] chlamydial strains have been isolated from swine (Sus scrofa) raised in production facilities in Nebraska. Here, the intracellular development of two C. suis tet[superscript R] strains, R19 and R27, is characterized through the use of tissue culture and immunofluorescence. The strains grow in tetracycline up to 4 ��g/ml, while a tetracycline sensitive (tet[superscript S]) C. suis swine strain (S45) and a C. trachomatis strain of the human serovar L2 (LGV-434) grow in tetracycline up to 0.1 ��g/ml. Although inclusions form in the presence of tetracycline, many contain large aberrant RBs that do not differentiate into infectious EBs. The percentage of inclusions containing typical developmental forms decreases with increasing tetracycline concentrations, and at 3 ��g/ml of tetracycline, 100% of inclusions contain aberrant RBs. However, upon removal of the tetracycline, the aberrant RBs revert to typical RBs and a productive developmental cycle resumes. In addition, inclusions were found that contained both C. suis R19 and C. trachomatis L2 after sequential infection, demonstrating that two biologically distinct chlamydial strains could both develop within a single inclusion. / Graduation date: 2002