Background: The role of blaSHV expression in beta lactam resistance among clinical isolates of Pseudomonas aeruginosa is not fully understood. Objectives: This study aimed to investigate the expression level of the blaSHV gene in beta lactam resistant Pseudomonas aeruginosa clinical isolates obtained from hospitals in Mashhad. Methods: In this cross sectional study, 103 clinical isolates of P. aeruginosa were recovered from different hospital wards and clinical specimens. Antimicrobial susceptibility testing was performed against several β lactam antibiotics, including third and fourth generation cephalosporins. Based on beta lactam resistance profiles, particularly resistance to cephalosporins, 20 representative isolates were selected for analysis of blaSHV gene expression using real time quantitative PCR (RT qPCR). Total RNA was extracted, converted to cDNA, and gene expression levels were normalized to the 16S rRNA reference gene. Relative expression was calculated using the 2⁻ΔΔCt method with P. aeruginosa PAO1 as the calibrator strain. Statistical analysis was performed to compare expression levels with the reference strain.

Results: High resistance rates to several cephalosporins were observed among the isolates, including ceftriaxone (100%), ceftiofur (93.2%), ceftazidime (69%), and cefepime (68.2%). All analyzed isolates showed a significant upregulation of blaSHV expression compared with the PAO1 reference strain (p < 0.05). Increased blaSHV expression was also detected in some imipenem susceptible isolates, suggesting that transcriptional upregulation of this gene is not directly associated with carbapenem resistance. Conclusion: Upregulation of blaSHV is associated with resistance to extended spectrum cephalosporins in beta lactam resistant P. aeruginosa clinical isolates from Mashhad. Further studies are required to clarify the clinical implications of blaSHV overexpression and its contribution to beta lactam resistance mechanisms.