Resident bacteria of the mucosal surfaces (such as nasal or oral) and skin of dogs could easily be transmitted to in-contact persons or owners by direct contact or from the household environment (Trinh et al., 2018). Moreover, transmission from humans to pets could also be possible (Orsini et al., 2022). These events (zoonosis and anthroponosis) represent a growing public health problem (Rahman et al., 2020; Orsini et al., 2022).

Two main groups of staphylococci are defined based on rabbit’s plasma coagulase activity, viz, coagulase-negative staphylococci (CoNS) and coagulase-positive staphylococci (CoPS) (Carroll et al., 2021). Of the two groups, CoPS are more pathogenic (Carroll et al., 2021). Staphylococcus aureus is the archetype of the CoPS and relevant animal and human pathogen (Tong et al., 2015); however, other CoPS related to animals have been described. One of the emerging CoPS of clinical importance is S. pseudintermedius, a member of the Staphylococcus intermedius group (SIG) that comprises four other species, viz.; Staphylococcus intermedius, Staphylococcus delphini, the recently described Staphylococcus cornubiensis, and S. ursi (Murray et al., 2018; Perreten et al., 2020; Carroll et al., 2021). In addition to these CoPS, Staphylococcus coagulans is another species that was originally described in 1990 as Staphylococcus schleiferi subsp. coagulans before being classified, based on genomic studies, into a separate species in 2020 (Madhaiyan et al., 2020).

Staphylococcus aureus is an opportunist pathogen frequently found in humans, which colonizes the anterior nose and nasal vestibule of 25–50% of healthy people (Liu et al., 2015; Sakr et al., 2018). S. aureus nasal colonization in humans and animals could place individuals at risk of opportunistic infections, and the carriage of specific virulence genes by the isolates can increase their pathogenic potential. In this respect, the following virulence genes are especially relevent in S. aureus: tst (encodes the toxic shock syndrome toxin, associated with sepsis), eta and etb (encode the exfoliatin A and B, associated with the staphylococcal scalded skin syndrome), and the luk-S/F-PV (encodes Panton Valentine Leucocidin, associated with abscesses and community-acquired pneumonia) (Tam and Torres, 2019; Hu et al., 2021).

The population genetics of S. aureus have shown the presence of several clonal complexes (CCs). Remarkably, some of these CCs have somewhat animal host specificity, while others are less host-specific (Chaguza et al., 2022). Methicillin-Resistant S. aureus (MRSA) is a globally epidemic bacterium in nosocomial settings and is referred to as healthcare-associated MRSA (HA-MRSA) (Cuny et al., 2010; Cuny et al., 2022). MRSA has also emerged in the community without any relation to the healthcare facilities [community associated (CA)-MRSA] and in the last decade, some CCs (mainly CC398) have been associated with livestock animals (LA-MRSA) (Matuszewska et al., 2022).

Staphylococcus aureus frequently colonizes the skin and nasal cavity of healthy humans, while S. pseudintermedius appears to predominantly colonize pets (especially dogs) but could occasionally be found in humans (Cuny et al., 2022). Although infrequent, S. pseudintermedius has been also isolated from human skin lesions (Lozano et al., 2017), due en ocassions to dog bites (Börjesson et al., 2015), as well as causing septicemia (Somayaji et al., 2016). Previous reports on nasal colonization in humans by S. pseudintermedius in dog-owning households have reported a prevalence of 25–65.9% in dogs and 3–4.5% in humans (Gómez-Sanz et al., 2013c; Han et al., 2016; Rodrigues et al., 2018). Certain isolates of S. pseudintermedius have become a major veterinary pathogen of concern due to their frequent multidrug-resistance phenotypes (Priyantha, 2022). Also, severe human infections with methicillin-resistant S. pseudintermedius (MRSP) have recently been reported (Priyantha, 2022).

Data about the co-colonization of dogs and in-contact persons with CoPS at the community level are still scarce. Hanselman et al. (2009) reported the presence of S. pseudintermedius, S. aureus, and S. schleiferi subsp. coagulans (including methicillin-resistant isolates) in dogs at the household level. In addition, concurrent animal and human colonization by indistinguishable S. pseudintermedius and S. aureus isolates have been observed (Hanselman et al., 2009). In a previous study performed a decade ago, our research group investigated the household nasal carriage of CoPS in dogs and humans, determining the prevalence of genetic lineages and their recovery rate over a year in animals and owners (Gómez-Sanz et al., 2013c). Nowadays, the abundance of dog ownership in Spain has significantly changed and the number of households with pets has increased. According to available data, there were 20 million registered pets in Spain in 2019 and about 26% of all households own dogs (Asociación Madrileña de Veterinarios de Animales de Compañía, 2019). By implication, sharing and transmission of nasal microorganisms (such as CoPS) between dogs and dog-owning households could be more frequent (Van Balen et al., 2017).

Nasal carriage of CoPS in healthy dogs could be an indicator of increased risk of colonization/infection for people in-contact with these animals, especially if S. aureus carries key virulence determinants of special relevance such as lukS/F-PV, tst-1, eta and etb (González-Martín et al., 2020). However, the influence of pet ownership on the diversity of CoPS isolates circulating among dog owners still needs to be fully elucidated (Van Balen et al., 2017). Consequently, this study sought to understand the current epidemiological situation of CoPS nasal carriage in dog-owning households by characterization of the genetic lineages, relatedness between isolates from the same households, antimicrobial resistance (AMR) determinants, virulence factors, and immune evasion cluster (IEC) types of S. aureus isolates.

Several studies have reported the transmission of CoPS between pets and their owners (Gómez-Sanz et al., 2013c; Han et al., 2016; Sahin-Tóth et al., 2021; Cuny et al., 2022). However, the present study is among the few that studied intra-species and within-host genetic diversities of CoPS in these hosts. Such information can better illustrate the complexity of challenges in the control of AMR in healthy dog-owning households.

The potential influence of dog-ownership on the nasal staphylococcal community (especially S. aureus and S. pseudintermedius colonization) of dogs and humans needs continuous surveillance. The present S. aureus household carriage rate (at least one dog and/or one human) in our study (44.4%) is lower than the 51.2% detected in a previous study performed a decade ago in Spain (Gómez-Sanz et al., 2013c), and the S. aureus carriage rate in humans in our study was also lower (34.1% vs 41.8%) (Gómez-Sanz et al., 2013c). Concerning studies from other European countries, our S. aureus prevalence in humans was higher than those reported in Germany (22%) and Hungary (23.8%) (Holtfreter et al., 2016; Sahin-Tóth et al., 2021).

Data on S. aureus nasal carriage rate among healthy dogs in community settings are sparsely available, varying between 2–8% according to different sources (Fazakerley et al., 2010; Rubin and Chirino-Trejo, 2010; Walther et al., 2012; Sahin-Tóth et al., 2021). Our results further support these numbers: we found a 5.9% carriage rate in dogs. However, in some African and Asian countries, high S. aureus nasal carriage is reported in healthy dogs, such as in Nigeria (36.9%), Indonesia (48.0%), Bangladesh (25.0%) and India (35.0%) (Mustapha et al., 2016; Sekhar et al., 2017; Rahman et al., 2018; Decline et al., 2020). The wide variation in nasal S. aureus carriage in dogs across the continents could be influenced by the local epidemiology of the S. aureus, differences in methodologies, dogs’ hygiene, environmental sanitation, antibiotic use in animals, and/or the health status of owners (Collignon and Voss, 2015; Fletcher, 2015; Valiakos et al., 2020).

As it was observed in this study, human S. aureus carriers were much more prevalent than dog carriers and indicating that transmission between humans and dogs occurs less frequently when compared to possible intrahousehold human-to-human transmission. Taking into consideration the molecular typing results, the genotypic profiles of S. aureus between our studied dog and human isolates were not similar, but those originating from humans in the same households were closely related in several cases, strongly suggesting human-to-human transmission. Moreover, there was a significant association between the household densities and nasal carriage of S. pseudintermedius in households with > than 1 dog and >than 1 human, pointing out for the possibility that a relatively higher household population is related to a higher detection rate of S. pseudintermedius.

The diverse pattern of the CCs of the MSSA isolates from dogs and in-contact humans corroborated the findings from a previous study in Germany and Spain (Gómez-Sanz et al., 2013c; Holtfreter et al., 2016). Here, a few human S. aureus isolates were scn-negative which indicates animal-adapted (perhaps dog-adapted) CCs of S. aureus. Nevertheless, dogs were carriers of MSSA-scn-positive isolates (CC8-IEC-type G and CC15-IEC-type C) suggesting an anthroponotic route of transmission. This is consistent with findings from a previous longitudinal study on S. aureus colonization dynamics in humans and dogs in Spain (Gómez-Sanz et al., 2013b). When compared to the current study, this suggests that humans are the major reservoir of S. aureus contributing to transmission to dogs. However, it is important to mention that zoonotic transmission from dogs to humans is also possible (Abdullahi et al., 2022).

Concerning S. pseudintermedius, 2.4 and 32.4% nasal carriage rates were detected in our studied humans and dogs, respectively. The S. pseudintermedius carriage rate among dogs in this study was higher than previously reported for dogs from Spain (22.7%) (Gómez-Sanz et al., 2013c), but lower than those reported in Canada (46%), Germany (37.5%), Australia (85.4%), Korea (65.9%), or Hungary (34.3%) (Hanselman et al., 2009; Bean and Wigmore, 2016; Han et al., 2016; Sahin-Tóth et al., 2021; Cuny et al., 2022). This suggests that the transfer between nasal S. pseudintermedius among healthy dogs could depend on the number of dogs in the household. This corroborated the previous reports of diverse MSSP lineages from Germany and France (Haenni et al., 2020; Cuny et al., 2022). Moreover, we did find nasal carriage of S. pseudintermedius among a person living with dogs in one of the investigated households. However, this is lower than previously reported in Korea, Spain, and Canada (Hanselman et al., 2009; Gómez-Sanz et al., 2013c; Han et al., 2016). It appears that humans are not natural hosts for S. pseudintermedius, but adaptation to humans could occur. A recent large study has shown diversity between the S. pseudintermedius isolates of human and dog infections with similar pathogenicity islands and virulence gene-containing prophages (Phumthanakorn et al., 2021). The household in our study with both human and dog S. pseudintermedius carriers strongly suggested intrahousehold transmission, as the isolates had no SNP (zero) difference and were confirmed as clones by our genomic analyses. To our knowledge, this lineage (ST1115) has not been reported so far for MSSP from dogs.

In our study, no MRSP was detected among healthy dogs and similar results were obtained in an study in Sweden (Börjesson et al., 2015). However, other studies refer low rates of MRSP nasal carriage in dogs, as 0.9% in Germany, 4.5% in Canada and 2.6% in Norway (Hanselman et al., 2009; Kjellman et al., 2015; Cuny et al., 2022). Moreover, a pooled 4.6% MRSP was reported among healthy dogs in Spain (Gómez-Sanz et al., 2011); of the nine MRSP nasal carriers, one was from a household dog, while the remaining eight were from stray dogs (Gómez-Sanz et al., 2011). The absence of MRSP in our study (healthy animals and household members) and the previously low MRSP in healthy dog studies are remarkably different to the high prevalence in dogs receiving treatment in veterinary clinics in France (16.9%) (Haenni et al., 2014). MRSP seems to be associated with animal-hospital-lineages (Ruiz-Ripa et al., 2021b), whereas isolates that are susceptible or have low AMR levels may represent natural colonizers of dogs. However, among our 19 isolates, three were found to harbour dfrK, a gene that is rarely detected in S. pseudintermedius, which could be linked to the mobile genetic element Tn559 (Ruzauskas et al., 2016; Ruiz-Ripa et al., 2021b). Further characterization of our isolate employing long-read sequencing could help to elucidate this possibility.

The MSSA-CC398 was a predominant lineage in our study, although detected only in humans. Evolutionarily, there are two clades of the CC398 based on the acquisition of SCCmec mobile element (carrying mecA), Tn916 (carrying tet(M)), or prophage φsa3 (carrying IEC), viz: (a) MRSA-CC398, often considered the predominant LA-MRSA clade (mostly IEC-negative) in Europe, (b) livestock-independent (human-adapted) clade of the MSSA-CC398 (often IEC-positive) (Price et al., 2012; Matuszewska et al., 2022). However, on very rare occasions, MRSA-CC398 carrying the φsa3 (scn-positive) and MSSA-CC398-scn-negative have been reported among certain spa types within the CC398 lineage (Price et al., 2012). Hence, these phenomena make it difficult to categorically classify the CC398 lineages into animal-adapted or human-adapted clades. However, it has been established that the CC398 lineage originated in MSSA from humans, acquired the tetracycline (tet(M)) and methicillin (mecA) genes and spread to livestock (Price et al., 2012). Thus, the absence of tet(M) in all the S. aureus and the detection of MSSA-CC398 clade could be inferred as humans-adapted strains. In many cases, MSSA-CC398 clade is associated with the predominant spa type t571 and the macrolide resistance gene ermT (Mama et al., 2021b). Worryingly, this MSSA-CC398 human clade has been recently considered an emergent lineage in invasive human infections in Spain and other countries (Laumay et al., 2021; Mama et al., 2021b). Concerning MSSA-CC398 in dog-owning households, a previous study by Gómez-Sanz et al. (2013c) also reported MSSA-CC398 of the spa type t571. An important similarity between the previous study and the current is that here, MSSA-CC398 isolates were only detected in humans and all were IEC type C of the spa type t571 and t1451. Recently, increased detection of penicillin susceptibility phenotype has been detected among invasive MSSA human isolates that have been causing clinical infections (Mama et al., 2021a). This phenotype has been frequently found among CC398 isolates (Mama et al., 2021a), however, most of our isolates are phenotypically resistant to penicillin. In another study by Gómez-Sanz et al. (2013a), about 7.1% of 98 kennel dogs also carried MSSA-CC398-scn-negative isolates but of different spa types (t034, t5883 and t108), and all were pan-susceptible. Our MSSA CC398 isolates were resistant at least to one antibiotic and worryingly 50% of the isolates were MDR. A major difference between the MSSA-CC398 isolates reported by Gómez-Sanz et al. (2013a) and ours was that here, human isolates were scn-positive (IEC type C). Recently in France, the MSSA-CC398 lineage (t571, t1451 and t18379) was also reported in 14.6 and 27.3% of dogs and cats, respectively (Tegegne et al., 2022). Worthy mentioning is the detection of MSSA-CC398-t571-scn-negative in a cat (Tegegne et al., 2022). The reason for this variation is not fully understood, however, it could be attributed to the spa type associated with the MSSA CC398 isolates or due to the carriage status of the Sa3 prophage (Gómez et al., 2020). The loss of Sa3int prophages in the scn-negative isolates is a major determinant for the human-to-animal transmission of MSSA CC398 (Price et al., 2012; Matuszewska et al., 2022). The findings of Gómez-Sanz et al. (2013a,c) and our study suggest the persistence of MSSA CC398 in humans and dogs.

Another finding of special epidemiological relevance is the dual MRSA/MSSA carriage detected in our study as both MSSA-CC30 and MRSA-CC5 were identified in a human household member. In a previous Spanish study, simultaneous carriage of both MRSA and MSSA of the CC398 lineage was reported in a farm worker with occupational exposure (Gómez et al., 2020). In another study among healthcare students in Portugal, concurrent detection of MRSA and MSSA in a single person was also reported (Coelho et al., 2021).

Though all the MRSA isolates had an MDR phenotype, more than 20% of the MSSA and MSSP isolates were also MDR. Generally, the AMR rate was moderate, but the most common AMR in S. aureus isolates were to penicillin, aminoglycosides, and erythromycin-clindamycin. Conversely, similarly to previous studies in S. pseudintermedius, the predominant AMR phenotype was to sulfamethoxazole-trimethoprim, erythromycin, and tetracycline (Rynhoud et al., 2021). Novel mutations (G2261A & T1584A) in the domain V region of the 23SrDNA of one MRSA isolate was observed, and although the predicted in silico resistance did not reveal a currently known AMR phenotype attributable to this mutation, the strain was phenotypically linezolid resistant. The inability to detect the linezolid phenotype from the genome database could be due to that this mutation has not been fully characterized (not previously reported and deposited in the genome database), as opposed to the most frequently detected 23S rDNA point mutation in staphylococci (G2576T) (Gostev et al., 2021; Ruiz-Ripa et al., 2021a). However, recently, novel point mutations in 23S rRNA associated with linezolid resistance in staphylococci have been reported in S. epidermidis in Austria (Huber et al., 2021) and in S. capitis in China (Han et al., 2022). It could be those novel mutations in domain V of 23S rRNA are silently emerging and mandates the need for close surveillance.

Regarding S. aureus virulence factors, all were negative for TSST-1, PVL, ETA and ETB encoding genes. All except one of the nasal S. pseudintermedius isolates of dogs and human origins carried the lukS/F-I, siet, and expA genes. These leucocidins and exfoliatins are responsible for host-specific clinical infections in dogs (Gharsa et al., 2013; Gómez-Sanz et al., 2013c).

Contrary to the findings of Penna et al. (2013), which reported S. schleiferi subsp. schleiferi carriage among ~32% of healthy dogs in Brazil, only 2 dogs from our study had S. schleiferi nasal carriage (5.4%). This finding is similar to the S. coagulans-positive dogs (4.9%) from the study of Lee et al. (2019) in Korea, but relatively higher than the 1.0% previously reported in Spain (Gómez-Sanz et al., 2013c). Since its first identification in humans in 1988 (Freney et al., 1988), several S. coagulans infections have been reported in humans and pets (May et al., 2005; Abraham et al., 2007; Yarbrough et al., 2017). Furthermore, the recent emergence of S. coagulans among healthy and pyodermic dogs has been a relevant global health issue in veterinary medicine due to its high AMR and diverse virulence factors (May et al., 2012; Abraham et al., 2007; Lee et al., 2019). Contrary to this assertion, all S. coagulans identified in our study were susceptible to antibiotics tested, likely because the isolates were cultured from healthy dogs.

It is worth mentioning a limitation of this study. As this was a one-point prospective study on the potential transmission of CoPS between dogs and dog-owner, randomly selected households were used. The sample size was relatively small and this could affect the detection of rare AMR phenotypes and virulence genes of CoPS in healthy individuals such as MRSP, eta and tst.

The nasal carriage of S. aureus and S. pseudintermedius in healthy dogs’ households were moderate. Low rates of MRSA and S. coagulans, and no MRSP carriage were detected. Human-to-human MSSA and dog-to-human MSSP transmissions were identified in this study. S. pseudintermedius isolates had a homogeneous profile of virulence determinants. The detection of MSSA CC398, an emergent clade that has been implicated in invasive human infections is a relevant health concern and suggests the need for its continuous surveillance of humans, different species of animals and their shared environment from the One Health perspective.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary material.

The studies involving human and animal participants were reviewed and approved by Ethic Committe of the University of La Rioja, Spain. The patients/participants provided their written informed consent to participate in this study.

IA and CT: conceptualization, methodology, and writing — original draft preparation. IA: laboratory experiments and software analysis. CT, IA, MZ, and CL: validation. IA, CT, MS, and AS: formal analysis and data curation. CT, IA, MS, AS, MZ, and CL: writing — review and editing. CT, CL, and MS: supervision. CT: project administration. CT, MZ, and IA: funding acquisition. All authors contributed to the article and approved the submitted version.

This work was supported by the project PID2019-106158RB-I00 of the MCIN/AEI/10.13039/501100011033 of Spain. Also, it received funding from the European Union’s H2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement N° 801586.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.