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.