Drug-Resistant, Biofilm-Forming Staphylococci Lurk in the Noses of Surgery-Bound Dogs

Deep inside the nasal passages of otherwise healthy dogs waiting for orthopaedic surgery, Italian researchers have uncovered an unsettling microbial landscape. A new study from the University of Naples Federico II, published in Veterinary Medicine and Science, reveals that two-thirds of staphylococcal bacteria recovered from the noses of surgical canine patients are resistant to multiple classes of antibiotics, and every single isolate tested was capable of forming biofilms, the slimy, protective communities that make bacteria notoriously difficult to eradicate. The findings paint the canine nasal cavity as a significant and underappreciated reservoir of antimicrobial resistance, one that could have direct consequences for surgical outcomes in veterinary hospitals and potentially for the humans who work closely with these animals.

The research team, led by Francesca Paola Nocera and Luisa De Martino, enrolled 100 dogs admitted to the Veterinary Teaching Hospital in Naples for surgical procedures, most of them orthopaedic operations. Because the animals were already scheduled for surgery under general anaesthesia, the researchers could collect nasal swabs without causing additional stress or discomfort. A single swab was inserted two to three centimetres into each nostril, rotated, and placed into transport medium before being rushed to the microbiology laboratory within two hours. The study was approved by the university’s Institutional Animal Welfare Committee, and written informed consent was obtained from every owner, underscoring the ethical care taken in a study that involved nothing more invasive than routine clinical sampling.

Back in the laboratory, the samples were streaked onto selective culture media, mannitol salt agar and Columbia CNA blood agar, and incubated overnight. Suspect colonies were then identified using matrix-assisted laser desorption ionization time-of-flight mass spectrometry, or MALDI-TOF MS, a proteomic technique that fingerprints bacterial proteins to deliver species-level identification. Every staphylococcal strain recovered in the study achieved a reliable species-level score, giving the team a confident catalogue of exactly which organisms were living in these canine noses. Of the 100 dogs sampled, 36 per cent showed no bacterial growth at all, while the remaining 64 per cent yielded bacterial colonies, from which 65 staphylococcal strains were ultimately isolated and characterised.

The species picture was dominated by one familiar name. Staphylococcus pseudintermedius, the staphylococcal species most closely associated with dogs and a notorious opportunist behind skin infections, ear inflammation and postoperative wound infections, accounted for 40 per cent of all isolates, a proportion the researchers confirmed as statistically predominant using a multinomial chi-square test. Staphylococcus aureus, the species of greatest concern in human medicine, came second at 15.4 per cent, followed by Staphylococcus epidermidis, Staphylococcus simulans and Staphylococcus warneri. In one patient, the team even recovered a co-isolation of Staphylococcus haemolyticus and Staphylococcus equorum, a reminder of the ecological diversity packed into this small anatomical niche.

What the antimicrobial susceptibility testing revealed was more troubling still. Using disk diffusion on Mueller-Hinton agar and interpreting results against European Committee on Antimicrobial Susceptibility Testing breakpoints, the researchers tested 14 antimicrobials from 12 different classes, all chosen for their relevance to small-animal clinical practice. Resistance to penicillin topped the list at 83.1 per cent of strains, followed by oxacillin at 63.1 per cent and cefoxitin at 55.4 per cent, the latter two serving as phenotypic markers for methicillin resistance. Erythromycin resistance was detected in 56.9 per cent of isolates, clindamycin in 46.2 per cent and tetracycline in 44.6 per cent. Only fusidic acid, rifampicin and linezolid retained activity against the large majority of strains, with resistance rates below 20 per cent.

Classified according to the internationally standardised definitions of Magiorakos and colleagues, 40 of the 65 strains, or 61.5 per cent, qualified as multidrug-resistant, meaning they were resistant to at least one agent in three or more antimicrobial categories. Three strains, 4.6 per cent, met the stricter criteria for extensive drug resistance, resistant to all but two or fewer categories. Together, the combined multidrug-resistant and extensively drug-resistant prevalence reached 66.2 per cent. Among Staphylococcus pseudintermedius specifically, the picture was even starker: 69.2 per cent of its isolates were multidrug-resistant, and one was extensively drug-resistant, although the difference between this species and the other staphylococci combined did not reach statistical significance. The authors caution that the wide confidence intervals around some of these estimates, particularly for the rarer resistance categories, reflect the limited sample size and warrant careful interpretation.

The second pillar of the study concerned biofilms, the structured microbial communities that bacteria build by embedding themselves in a self-produced extracellular matrix. Within a biofilm, bacterial cells shift their metabolism, shield themselves from antibiotics and evade host immune defences, which is why biofilm-associated infections are so persistent and so difficult to treat. The team quantified biofilm formation using the classic crystal violet assay, growing each strain in 96-well polystyrene plates, staining the adherent biomass and measuring absorbance at 570 nanometres, with all assays performed in triplicate and repeated three independent times. The results were unambiguous: not a single strain failed to produce biofilm. Nearly a quarter were classified as weak producers, but 56.9 per cent were moderate producers and 18.5 per cent were strong producers.

Perhaps the most scientifically interesting finding emerged when the researchers asked whether drug resistance and biofilm capacity travel together. Within the Staphylococcus pseudintermedius subset, they compared the biofilm phenotypes of multidrug-resistant strains against those of fully susceptible ones using Pearson’s chi-square test. The answer was no: biofilm-forming ability occurred independently of resistance status, with a p-value of 0.236. In fact, one strong biofilm-producing strain was susceptible to every antimicrobial tested. This statistical independence matters, because it suggests these two threatening traits are being acquired and maintained through separate evolutionary routes, meaning a strain can be dangerous in one dimension while benign in the other, and that controlling resistance alone would not necessarily eliminate the persistence advantages conferred by biofilm production.

The clinical implications reach beyond the individual dog. The nasal cavity is a well-established ecological niche where staphylococci persist and spread, and animals carrying resistant, biofilm-competent strains into a hospital setting may seed the environment, contaminate surgical sites or transfer organisms to veterinary staff and owners. Previous work has already linked methicillin-resistant Staphylococcus pseudintermedius carriage to surgical site infections following orthopaedic procedures in dogs, and biofilms are known to promote environmental persistence on hospital surfaces. The authors suggest that preoperative nasal screening for multidrug-resistant and biofilm-forming staphylococci might be worth considering in high-risk surgical patients, potentially guiding infection control measures and refining prophylactic antibiotic protocols before the first incision is made.

The study is not without limitations, and the researchers are candid about them. The numbers of certain staphylococcal species were small, the cross-sectional design cannot establish whether nasal colonisation precedes infection, and no information on prior antimicrobial treatment was available to identify risk factors. Crucially, methicillin resistance was assessed phenotypically only, without molecular confirmation through detection of the mecA or mecC genes, leaving open the possibility that some borderline isolates were misclassified, since mechanisms such as beta-lactamase hyperproduction can mimic methicillin resistance in disk diffusion tests. Even so, the overall message stands firm: the noses of dogs entering veterinary operating theatres harbour a complex staphylococcal population in which drug resistance is the norm rather than the exception, and biofilm formation is universal. As antimicrobial resistance continues its global rise, this study adds companion animals to the growing list of reservoirs that surveillance programmes cannot afford to ignore, and it makes a data-driven case for looking up dogs’ noses before, not after, the scalpel comes out.

Subject of Research: Nasal colonization by antimicrobial-resistant and biofilm-producing Staphylococcus species in dogs undergoing surgery in Italy

Article Title: Nasal Colonization of Antimicrobial‐Resistant and Biofilm‐Producing Staphylococcus spp. in Surgical Canine Patients From Italy

Article References: Nocera, F. P., Schena, R., Romano, A., Cavalli, S., Cortese, L., Fatone, G., & Martino, L. D. (2026). Nasal Colonization of Antimicrobial‐Resistant and Biofilm‐Producing Staphylococcus spp. in Surgical Canine Patients From Italy. Veterinary Medicine and Science, 12(5), Article e71235. https://doi.org/10.1002/vms3.71235

Image Credits: AI Generated

DOI: 10.1002/vms3.71235

Keywords: Staphylococcus pseudintermedius, antimicrobial resistance, biofilm, dogs, nasal colonization, methicillin resistance, veterinary medicine, multidrug resistance, MALDI-TOF, surgical patients, One Health, Italy