Our canine best friends could spread our bacterial worst nightmare, according to a recent study.

The problem with drug-resistant bacteria is well known. Overused, poorly used, and naturally adaptive bacteria clearly have us outnumbered. As science drives hard to find alternative drugs, therapies, and options to treat increasingly resistant infections, humans are treading water, hoping our drugs of last resort work until we figure out better strategies.

A study published in the journal Antimicrobial Agents and Chemotherapy reports that Spanish researchers have identified the first known cases of drug-resistant pathogens infecting dogs.

While two dogs with germs may not sound significant, the infections were in two unrelated dogs and caused by human pathogens that are resistant to a human drug of last resort, tigecycline.

Approved by the US Food and Drug Administration in 2005, tigecycline is an antibiotic that is usually only administered intravenously in hospitals as a last-chance way to tamp down multi-drug resistant infections. Marketed as Tygacil, the drug is not without its own risks, and is mostly used only for intractable, life-threatening bacterial infections.

The multi-drug resistant infections identified in this study were Klebsiella pneumoniae, a dangerous pathogen usually spread in healthcare settings to immune-compromised patients, usually those with in-dwelling catheters or on ventilator machines.

Colorized scanning electron microscope image of two rod-shaped, drug-resistant Klebsiella pneumoniae. Image via NIAID

In this study, scientists from Complutense University in Madrid, Spain, wanted to see if they could find antibiotic-resistant bacteria in companion animals. The team was surprised when they tested pathogens from two dogs: One receiving treatment at the University's veterinary hospital, and the other, which was an outpatient visitor to the clinic.

The study first looked for — and did not find — canine resistance to antibiotics commonly used in veterinary procedures. Researchers then looked at canine resistance to antibiotics prescribed for humans.

Scientists identified tigecycline-resistant K. pneumoniae bacteria in urine samples from two dogs. The dogs were not from the same household, nor had they shared physical contact. Analysis of the samples confirmed the two bacteria strains were not genetically similar, so the two animals weren't likely infected from the same source, but the bacteria was related to K. pneumoniae strains widely found in hospitals. Though never treated with tigecycline, these dogs carry tigecycline-resistant bacteria.

To the best of our knowledge, this is the first identification of tigecycline-resistant bacteria isolated from companion animals. — Study authors (via Antimicrobial Agents and Chemotherapy)

This straightforward assessment raises real concern about the transfer of resistant bacteria from humans to dogs, and then potentially back to humans.