Hey Beck,
I did preliminary research into enrotril and found some information that might be of value to you. It appears enrotril is used to treat a spectrum of bacteria including:
E.coli, Proteus spp., Klebsiella spp, Pseudomonas aeruginosa, Staph spp and Group D Streptococcus, Streptococcus Spp, Pasteurella spp, Klebsiella spp, Pseudomonas spp, Bordetella bronchiseptica, Staph. Intermedius, wounds, abscesses, discharging sinuses, deep pyodermas, osteomyelitis, and Gram negative septicaemias.
Enrotril Oral Solution is useful for treating serious antibiotic resistant infections of the respiratory tract or genito-urinary system, particularly chronic urinary tract infections in cats. It also reads that the medication should be administered at 1mL per 5kg of animal body weight.
I guess propylene glycol is to assist in the dissolution of enrotril in the axolotl's system as its body compostion is predominately water and will not react with a hydrophobic medication. Somebody will definitely chime in if I'm wrong. As for the toxicity...well, I don't have a clue!
It appears to me that because the vet has not ascertained the strain of bacteria (if this is the case) causing the infection, s/he is attempting to cover all bases by perscribing enrotril. Given the vet has told you the tests are expensive (which I disagree with), s/he could at least try some "cheaper" alternatives to narrow down the search. Such tests include Gram staining to determine if the bacterium is resistant to antibiotics and also a Ziehl-Neelsen stain if the bacteria are acid-fast bacilli and resistant to Gram staining.
On a separate, but equally related issue, I don't mean to be negative but I am hesistant to say that Copper is suffering from just a bacterial infection because she doesn't seem to be exhibiting the symptoms characteristic of a bacterial infection. I would be more inclined to suggest that Copper has a metabolic disorder, and that if there is a bacterial infection, it is probably secondary to the metabolic disorder.
I do commend your efforts because it does seem like you are trying all that you possibly can.
- Jay