Phenotypic Resistance to Beta-Lactam Antimicrobials in Bacterial Isolates from Different Animal Populations in Texas...

Background/Objectives: Beta-lactam antimicrobials are among the most prescribed drugs in veterinary medicine, yet comprehensive epidemiological assessments of resistance across animal species, bacterial taxa, and clinical specimen types remain limited. This study aimed to characterize the epidemiology of phenotypic beta-lactam resistance among bacterial isolates submitted to veterinary diagnostic laboratories in Texas and Oklahoma from 2018 to 2024 and to identify factors and organism-specimen-antimicrobial combinations associated with the overall resistance burden. Methods: A retrospective analysis was conducted using antimicrobial susceptibility testing data from 166,686 bacterial isolates collected between 2018 and 2024. Following data cleaning and quality assessment, 789,871 susceptibility observations across seven beta-lactam antimicrobials were analyzed. Descriptive analyses, univariable and multivariable logistic regression, bacterial diversity analysis, and resistance burden ranking were performed to characterize resistance patterns and identify factors associated with resistance. Results: Resistance varied markedly among the seven beta-lactam antimicrobials. Amoxicillin showed the highest resistance prevalence (56.3%), whereas ceftazidime (11.2%) and imipenem (17.2%) had the lowest. Resistance differed substantially across animal categories, bacterial taxa, and clinical specimen groups. Multivariable analyses identified state, animal category, bacterial group, and clinical specimen group as factors independently associated with resistance, while no consistent temporal differences were detected across 2018-2024 after multivariable adjustment. After sample-size standardization, bacterial richness was highest among zoo/wildlife/aquatic animals, while diversity and evenness varied across animal categories and antimicrobials. Staphylococcus pseudintermedius isolated from skin specimens and tested against amoxicillin contributed the greatest overall resistance burden. Conclusions: Beta-lactam resistance exhibited substantial epidemiological heterogeneity across host species, bacterial taxa, clinical specimens, and states. These findings provide an evidence base for antimicrobial stewardship, resistance surveillance, and empirical antimicrobial therapy in veterinary medicine.