Antibiotics excreted into the intestinal tract may promote colonization by antibiotic-resistant pathogens due to disruption of indigenous microflora or have a protective effect by inhibiting pathogen growth. Previous studies showed ceftriaxone, piperacillin-tazobactam, and ertapenem did not promote ESBL-producing Klebsiella pneumoniae colonization during treatment in mice, while piperacillin-tazobactam inhibited vancomycin-resistant enterococci establishment but promoted their persistent overgrowth after high-density colonization. This study tested the hypothesis that piperacillin-tazobactam and ceftriaxone would promote the persistence of established high-density colonization by ESBL-producing K. pneumoniae strains with intermediate or high-level resistance to these agents, and that ertapenem would inhibit carbapenem-susceptible strains. Two ESBL-producing K. pneumoniae strains (P62 and P10045) were used in a mouse model where high-density colonization was established via subcutaneous clindamycin and orogastric gavage of test strains. Mice received subcutaneous treatment with saline (control), imipenem-cilastatin, ertapenem, piperacillin-tazobactam, or ceftriaxone daily for 9 days, and stool densities of ESBL-producing K. pneumoniae were measured. Compared to saline controls, ceftriaxone treatment promoted persistent overgrowth of both strains (P < 0.001), ertapenem suppressed levels of both strains (P < 0.001), imipenem-cilastatin treatment showed no difference in pathogen densities, and piperacillin-tazobactam promoted persistent overgrowth of the more resistant P10045 strain (P < 0.001) but not the more susceptible P62 strain. These results suggest that the ability of antibiotics to inhibit colonization by gram-negative pathogens is reduced after high-density colonization is established, particularly with more resistant strains. Ertapenem suppressed colonization by both strains, while imipenem-cilastatin did not promote or suppress colonization, likely due to limited intestinal excretion and lack of microflora disruption. Ceftriaxone and piperacillin-tazobactam promoted persistent overgrowth of resistant ESBL-producing K. pneumoniae strains in mice with established high-density colonization.