A Pseudomonas aeruginosa isolate is susceptible to piperacillin-tazobactam alone but resistant to imipenem. OXA-48 is detected by PCR; however metallo-beta-lactamase (MBL) screen with EDTA disk synergy is negative. The resistance mechanism is:

• A AmpC overexpression hydrolyzing imipenem
• B VIM metallo-beta-lactamase production
• C KPC carbapenemase production
• D Loss of OprD porin causing selective carbapenem resistance (mainly imipenem) ✓

Explanation

In Pseudomonas aeruginosa, selective imipenem resistance (with susceptibility to other carbapenems and piperacillin-tazobactam) is characteristically caused by loss/downregulation of OprD outer membrane porin, through which imipenem and meropenem (but especially imipenem) enter the cell; this mechanism does not produce MBL and is EDTA-negative. MBL (VIM, IMP, NDM) would affect all carbapenems and show EDTA synergy. AmpC overexpression causes broad cephalosporin resistance. KPC affects all beta-lactams.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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