Proteus mirabilis is a member of the Enterobacteriaceae family. It can be found in soil, water, and the intestinal tract of mammals including humans. P. mirabilis is an important urinary tract infection (UTI) agent among patients with complicated urinary tract, urolithiasis, or long-term urinary catheterization. One feature of P. mirabilis is the production of urease to generate ammonia. Thus, patients with UTI caused by P. mirabilis usually had alkaline pH urine due to the presence of ammonia resulting in calcium and mangensium crystallization which could in turn lead to obstruction of the lumen of indwelling catheters. P. mirabilis can also cause wound infections, septicemia, and pneumonia, mostly in hospitalized patients.

Other than intrinsic resistance to tetracycline, nitrofurantoin, and polymyxin type antibiotics, P. mirabilis used to be mostly susceptible to other antimicrobial agents. However, data from the Taiwan Surveillance of Antimicrobial Resistance (TSAR) 2002-2012 revealed that significant increase in resistance to extended spectrum cephalosporins and fluroquinolones occurred in P. mirabilis from Taiwan in the past decade, from 7.4% to 18.3% for cefotaxime and 19.9% to 46.2%, for ciprofloxain respectively.

Another noteworthy finding was that, the revised Clinical and Laboratory Standards Institute (CLSI) carbapenem (ertapenem, imipenem, and meropenem) breakpoints had significant impact on susceptibility to imipenem (99.8% vs. 55.1%) (P < 0.001). Since P. mirabilis has naturally higher imipenem MICs, our results support the recommendation that susceptibility of P. mirabilis to carbapenems should be determined by other carbapenem agents instead of imipenem (Wang JT et al., 2014).