Isolation of a Capnophilic and Extended-Spectrum β-Lactamase-Producing Proteus mirabilis Strain from the Urine of an Octogenarian Male Patient with Acute Pyelonephritis.

A capnophilic Gram-negative rod-shaped bacterium was recovered from the urine of an octogenarian male patient with acute pyelonephritis. The isolate was found to produce CTX-M-2-type extended-spectrum β-lactamase. Interestingly, the isolate failed to grow on modified Drigalski (BTB) and MacConkey agar media, even under CO₂-enriched atmosphere. Our analysis revealed that the pH-indicator dyes, bromothymol blue, and/or crystal violet that were incorporated into the agar media inhibited the growth of the isolate. Although routine identification methods using Vitek® 2 Compact systems were unsuccessful, the isolate was identified as Proteus mirabilis by 16S rRNA sequencing and MALDI-TOF MS analysis. The carbonic anhydrase (CA) region spanning approximately 2,000 bp upstream to 2,000 bp downstream, which is responsible for the CO₂ requirement, was not amplified, which could be attributed to the large-scale deletion or mutation of the DNA sequences containing the CA gene region. In fact, revertants with the ability to grow without CO₂ were not detected. However, a revertant that was capable of growing in both BTB and MacConkey agar was detected at frequencies less than 10^-9. Therefore, the genes responsible for the highly sensitive reactions of the isolate to pH indicator dyes is not likely to be linked to the CA genes.