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Background: Chronic sinusitis is one of the most challenging health problems of contemporary society. Although several treatment methods have been defined, a comprehensive understanding of the underlying causes (e.g., antibiotic resistance) is still elusive. The aim of this study was to characterize two of the main extended-spectrum beta-lactamase genes—i.e., blaTEM and blaCTX-M genes—and investigate antimicrobial resistance in bacteria isolated from chronic sinusitis. Samples from 70 chronic sinusitis patients and 20 healthy individuals (controls) were analyzed for the presence of blaTEM and blaCTX-M resistance genes using the polymerase chain reaction (PCR) test, followed by gene sequence analysis. Results: Phenotypic and genotypic beta-lactam resistance was observed in 58.7% and 61.54% of the gram-negative isolates, respectively, with 38.46% carrying the blaTEM gene and 34.62% harboring the blaCTX-M gene. Sequencing data indicated high heterogeneity in blaCTX-M genes (69–100% similarity to reported sequences) and lower heterogeneity in blaTEM genes (93–99%). Conclusion: Broad-spectrum beta-lactam resistance is a major pathogenesis factor in chronic rhinosinusitis, and careful consideration is required for antimicrobial therapy. High blaCTX-M heterogeneity could mean high horizontal transfer rate of this gene and warrant a surveillance program. [ABSTRACT FROM AUTHOR]