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Staphylococcus pseudintermedius

Staphylococcus pseudintermedius is a Gram-positive, coagulase-positive coccus belonging to the Staphylococcus intermedius group (SIG), formally recognized as a distinct species in 2005 after being previously misclassified as S. intermedius.^[1] It primarily colonizes the skin, nasal cavities, and mucous membranes of dogs and cats, with carriage rates in healthy dogs ranging from 37% to 92%, often acquired vertically from mothers at birth.^[1] As an opportunistic pathogen, it is a leading cause of infections in companion animals, including pyoderma, otitis externa, surgical site infections, urinary tract infections, and more severe conditions like pneumonia and osteomyelitis in dogs.^[2] In cats and other species such as horses, foxes, and wildlife, it is less common but has been isolated from similar sites, indicating a broadening host range beyond primary canine reservoirs.^[1] The bacterium's pathogenicity is driven by virulence factors analogous to those in Staphylococcus aureus, enabling adhesion, immune evasion, and tissue damage.^[2] In humans, S. pseudintermedius exhibits zoonotic potential, with at least 69 documented cases of infection and colonization reported globally from 2006 to 2022, and additional cases identified since, predominantly skin and soft tissue infections (SSTIs) but also rare invasive cases like bacteremia and endocarditis, almost exclusively linked to close contact with infected dogs or cats.^[3]^[4] Transmission occurs through direct contact or fomites, with household pet ownership identified as a key risk factor.^[5] Antibiotic resistance, particularly methicillin resistance mediated by the mecA gene (resulting in MRSP), poses significant therapeutic challenges, with prevalence exceeding 60% in some regions like Japan, Taiwan, and Singapore, and multidrug resistance affecting up to 63% of clinical isolates in studies from Brazil.^[1] Resistance to critical agents like vancomycin, daptomycin, and linezolid is rare but has been reported in some isolates, necessitating ongoing surveillance.^[6] The emergence of novel staphylococcal cassette chromosome mec (SCCmec) elements in S. pseudintermedius highlights its role as a reservoir for resistance genes with implications for both veterinary and human medicine under the One Health framework.^[7] Improved diagnostics, such as MALDI-TOF mass spectrometry, have enhanced identification and surveillance, underscoring the need for integrated approaches to mitigate its spread across species.^[1]

Taxonomy and Morphology

Classification History

Staphylococcus pseudintermedius was first described as a distinct species in 2005, when Devriese et al. proposed its classification as a novel coagulase-positive staphylococcus within the Staphylococcus intermedius group (SIG), comprising S. intermedius, S. pseudintermedius, and S. delphini.^[8] This separation was based on phenotypic differences, including negative clumping factor activity, variable DNase reactions, and lack of trehalose acidification compared to S. intermedius and S. delphini, as well as genetic analyses showing 16S rRNA gene sequence similarities exceeding 99% but DNA-DNA hybridization values below 70%, confirming distinct species status.^[8] The type strain, LMG 22219^T, was isolated from feline lung tissue, with additional strains from canine sources like otitis externa, highlighting its primary association with companion animals.^[8] Subsequent studies refined identification through species-specific genetic markers, particularly differences in the thermonuclease (nuc) gene sequences. Sasaki et al. (2007) developed multiplex PCR assays targeting unique nuc regions, enabling differentiation of SIG species where phenotypic methods often failed, as many veterinary isolates previously misidentified as S. intermedius were reclassified as S. pseudintermedius. Combined with 16S rRNA analysis, these markers established S. pseudintermedius as the predominant canine-adapted member of the SIG, distinct from the pig-associated S. delphini and the less host-specific S. intermedius. Phylogenetically, S. pseudintermedius occupies a position among coagulase-positive staphylococci, forming a tight cluster with the SIG separate from the S. aureus clade, yet sharing biochemical traits like coagulase production that reflect convergent evolution for host adaptation.^[8] Its close relation to S. aureus is evident in genomic features such as similar accessory gene regulators, but core genome analyses underscore canine host specificity, with most isolates from dogs exhibiting adaptations like enhanced adhesion to canine corneocytes. Recent whole-genome sequencing efforts up to 2025 have further elucidated its taxonomy, revealing a clonal population structure dominated by complexes such as CC71, an epidemic lineage often linked to methicillin resistance and global spread in veterinary settings. Pan-genomic studies on UK and North American populations have identified over 13 clonal complexes, including emerging ones like ST496, refining the understanding of intraspecies diversity and evolutionary dynamics without altering its species delineation.^[9]^[10]

Cellular and Colonial Characteristics

Staphylococcus pseudintermedius is a Gram-positive bacterium characterized by spherical cocci measuring approximately 0.5–1.5 μm in diameter, typically arranged in grape-like clusters.^[11]^[12] These cells are non-motile and non-spore-forming, consistent with other members of the Staphylococcus genus.^[12] As a facultative anaerobe, it thrives in both aerobic and anaerobic conditions.^[11] On blood agar, S. pseudintermedius forms small, round, creamy-white to greyish-white, opaque colonies, 1–2 mm in diameter, with smooth margins and complete β-hemolysis, often appearing as a double zone after incubation at 37°C for 24–48 hours.^[11]^[3] The bacterium is catalase-positive, producing bubbles upon exposure to hydrogen peroxide, but oxidase-negative, failing to produce a color change with oxidase reagent.^[11] It demonstrates growth on selective media such as mannitol salt agar, tolerating the high salt concentration, though mannitol fermentation may be weak or delayed.^[13]^[14]

Habitat and Epidemiology

Primary Reservoirs and Carriage

Staphylococcus pseudintermedius primarily resides as a commensal bacterium in dogs, where it colonizes the skin and mucous membranes of healthy individuals. Carriage rates in pet dogs range from 37% to 92%, with up to 90% reported in some studies for nasal, oral, and skin sites.^[1]^[15] This high prevalence underscores dogs as the main reservoir, where the bacterium persists asymptomatically without causing disease in most cases. In addition to dogs, S. pseudintermedius has been isolated from other animals, though less frequently. Cats show lower carriage rates, around 2-12%, while horses and wildlife such as foxes also serve as hosts. The bacterium appears opportunistically in livestock like pigs and in birds, including pigeons, but these are not primary reservoirs.^[1]^[16] Colonization in dogs often occurs persistently in specific sites, including the perianal and inguinal regions, as well as perinasal and perioral areas, with perineum and mouth being the most common at 65-66% carriage. Transmission happens primarily through direct contact between animals or via fomites such as shared bedding. Asymptomatic carriage prevalence in pet dogs is estimated at 37-92%, with higher rates observed in multi-pet households due to increased contact opportunities.^[17]^[18]^[15]

Prevalence in Companion Animals

Staphylococcus pseudintermedius is recognized as the predominant bacterial pathogen in various infections affecting companion animals, particularly dogs, where it is the most frequently isolated species in cases of pyoderma, otitis externa, and surgical site infections.^[19] In canine pyoderma, it accounts for approximately 42% of bacterial isolates, while in otitis externa, isolation rates range from 20% to 94% among staphylococcal cases.^[20] For surgical site infections, it represents up to 90% of staphylococcal isolates recovered from affected dogs.^[21] Several risk factors contribute to the development of S. pseudintermedius infections in dogs, including underlying conditions such as allergies (e.g., atopic dermatitis), immunosuppression, and breed predispositions. Breeds like bulldogs, pugs, boxers, and shar peis are particularly susceptible due to their skin fold conformations and predisposition to hypersensitivities, which compromise the skin barrier and facilitate bacterial overgrowth.^[22] Endocrinopathies, such as hypothyroidism, and ectoparasite infestations further exacerbate vulnerability by altering skin integrity. Prevalence data indicate that S. pseudintermedius is implicated in up to 90% of superficial bacterial folliculitis cases in dogs, highlighting its role as a primary opportunistic pathogen in recurrent skin conditions.^[21] In cats, infections are less common but emerging, particularly in wound sites, with isolation rates around 6% in wound samples and MRSP accounting for up to 24.7% of such isolates.^[23] Asymptomatic carriage on the skin and mucosae often serves as a precursor to these opportunistic infections in both species.^[1] Additionally, multi-drug resistant strains, including methicillin-resistant S. pseudintermedius (MRSP), have shown a marked increase since the 2010s, complicating treatment in veterinary practice across regions.^[20]

Global Distribution Patterns

Staphylococcus pseudintermedius is widely distributed in developed regions with high rates of companion animal ownership, particularly in Europe and North America, where isolation rates from veterinary clinical samples typically range from 10% to 30%. In Europe (Germany), a large-scale analysis of 175,171 veterinary samples reported a 25.6% isolation rate for S. pseudintermedius, with 35% in canine specimens. Similarly, in Europe, proportions of methicillin-resistant S. pseudintermedius (MRSP) in clinical isolates often exceed 10% outside low-prevalence areas like Scandinavia, reflecting its role as a common opportunistic pathogen in pets.^[24]^[20] Reports of S. pseudintermedius have increased in Asia and Australia since 2015, attributed to the expansion of the international pet trade facilitating bacterial dissemination. In Asia, MRSP lineages such as clonal complex (CC) 45 predominate, with isolation frequencies reaching up to 67% in certain veterinary settings in countries like Japan. Australia has documented diverse MRSP sequence types, including ST71 and novel variants, in canine samples, signaling emerging clonal diversity linked to global pet movements. In contrast, occurrences in Africa remain rare, likely due to underreporting stemming from limited diagnostic capabilities and surveillance in veterinary practices.^[25]^[25]^[26]^[25] Veterinary laboratory surveillance data highlight the clonal spread of MRSP lineages across continents, with ST71 (part of CC71) originating in Europe and disseminating to North America and beyond through infected animals. Multilocus sequence typing (MLST) analyses from international multicentre studies confirm this intercontinental transmission, with CC71 accounting for over 50% of European MRSP isolates. Recent genomic surveillance efforts as of 2025, including phylogenomic analyses of thousands of global isolates, have identified multidrug-resistant lineages like CC551 with potential for dissemination via international pet movements, underscoring the need for enhanced border controls on animal movements. As of 2025, studies continue to report increasing MRSP prevalence in Europe and Asia.^[27]^[25]^[28]^[29]^[19]

Pathogenesis

Adhesion and Invasion Mechanisms

Staphylococcus pseudintermedius initiates infection through specialized adhesins that mediate attachment to host extracellular matrix components and plasma proteins. The clumping factor A (ClfA), a sortase-anchored surface protein, binds specifically to the α-chain of fibrinogen, facilitating bacterial clumping and adhesion to fibrinogen-coated surfaces in wounds or mucosal sites.^[30] Similarly, the fibronectin-binding proteins SpsD and SpsL, also anchored via the LPXTG motif by sortase enzymes, interact with fibronectin in the extracellular matrix, promoting stable colonization of epithelial and damaged tissues. These adhesins exhibit growth phase-dependent expression, with SpsD predominant in early exponential phases and SpsL active across multiple stages, enhancing versatility in host attachment.^[31] Biofilm formation further supports persistent adhesion and evasion of clearance mechanisms. The polysaccharide intercellular adhesin (PIA), produced via the icaADBC operon, forms a protective matrix that interconnects bacterial cells, enabling the development of robust, multilayered biofilms on skin and implant surfaces. This PIA-dependent process is prevalent in clinical isolates, correlating with enhanced resistance to shear forces and antimicrobial penetration during chronic canine pyoderma.^[32] Invasion into host cells is driven by sortase-anchored surface proteins, particularly SpsD and SpsL, which not only bind fibronectin but also induce uptake into canine epithelial cells. By engaging α5β1 integrins on host cells, these proteins activate signaling cascades that internalize bacteria, allowing dissemination from initial adhesion sites into deeper tissues and contributing to the severity of wound infections. Mutants lacking these proteins show significantly reduced invasion efficiency, underscoring their essential role.^[33] Adaptation to the canine host is exemplified by the iron-regulated surface determinant (Isd) system, which facilitates heme acquisition tailored to canine physiology. The IsdB receptor demonstrates species-specific binding to canine hemoglobin, enabling efficient heme extraction under iron-limiting conditions prevalent in infected tissues. This evolutionary divergence from human-specific IsdB in Staphylococcus aureus enhances S. pseudintermedius survival and virulence in dogs, promoting tropism to this primary reservoir.^[34]

Immune Evasion Strategies

Staphylococcus pseudintermedius employs Protein A (SpA), a key surface-anchored protein, to evade host immune responses by binding the Fc region of immunoglobulin G (IgG) antibodies. This interaction prevents opsonization, thereby inhibiting phagocytosis by neutrophils and macrophages, and also interferes with complement activation, reducing bacterial clearance.^[35] Studies have demonstrated that SpA expression in S. pseudintermedius mirrors its function in Staphylococcus aureus, where it cross-links IgG molecules to form immune complexes that shield the bacterium from antibody-dependent cellular cytotoxicity.^[36] The production of capsular polysaccharides, often manifesting as an antiphagocytic slime layer composed of polysaccharide intercellular adhesin (PIA), further contributes to immune evasion by S. pseudintermedius. This extracellular matrix reduces bacterial uptake by neutrophils, embedding cells within biofilms that hinder access to phagocytes and promote persistence during infections. Slime-producing isolates are more prevalent in diseased companion animals, underscoring its role in facilitating chronic colonization and resisting innate immunity.^[37] Superantigen-like proteins (SSLs) secreted by S. pseudintermedius modulate host cytokine responses, diverting immune signaling to favor bacterial survival. These proteins interfere with innate immune pathways, such as those involving Toll-like receptors, leading to dysregulated cytokine release that promotes chronic inflammation, particularly in canine skin infections like pyoderma. By altering the inflammatory milieu, SSLs enable prolonged bacterial presence without triggering effective adaptive immunity.^[38] Recent genomic analyses have highlighted leukocidin genes, such as lukF-I and lukS-I encoding the canine-specific leukotoxin Luk-I, which specifically lyse neutrophils in dogs, impairing the primary line of innate defense. This two-component pore-forming toxin targets polymorphonuclear leukocytes, causing rapid cell death and release of neutrophil contents that may exacerbate tissue damage while clearing key immune effectors. The presence of these genes in clinical isolates from 2024 underscores their contribution to virulence in opportunistic infections.^[39]^[40]

Toxin Production and Tissue Damage

Staphylococcus pseudintermedius produces several exotoxins that contribute to direct tissue damage during infections, primarily in canine hosts. Among these, leukocidins such as Luk-I (comprising LukS-I and LukF-I subunits) are bicomponent pore-forming toxins homologous to the Panton-Valentine leukocidin (PVL) of S. aureus. These toxins target and lyse neutrophils and other leukocytes by binding to the CXCR2 receptor, leading to pore formation in cell membranes and subsequent cell death, which promotes abscess formation and necrotizing lesions in skin and soft tissues.^[41]^[42] Luk-I is encoded on a degenerate prophage and is present in nearly all clinical isolates examined.^[39] Phenol-soluble modulins (PSMs), including δ-toxin and the canine-specific PSMε, are amphipathic α-helical peptides that disrupt host cell membranes, lyse neutrophils, and promote inflammation and biofilm formation. These toxins enhance bacterial dissemination and contribute to the severity of skin and soft tissue infections in dogs by damaging epithelial barriers and evading innate immunity.^[41] Enterotoxins and exfoliative toxins are less prevalent but play roles in severe dermatological conditions. Enterotoxin genes like sea (17% prevalence) and sec (57% in clinical strains) encode superantigen-like proteins, such as SEA homologs, which can induce inflammation and tissue disruption, though their direct link to pyoderma remains limited.^[43] Exfoliative toxins, including SIET (detected in nearly all isolates studied), EXI (23.3% in pyoderma cases), and ExpA/ExpB, cleave desmoglein-1 in the epidermis, causing intraepidermal splitting, erythema, crusting, and severe dermatitis akin to scalded skin syndrome.^[42]^[43] These toxins exacerbate skin barrier breakdown, facilitating deeper tissue invasion and chronic infections.^[44] In addition to exotoxins, S. pseudintermedius secretes enzymes that degrade host tissues to enhance pathogenicity. Lipases hydrolyze lipids in skin and subcutaneous layers, enabling bacterial nutrient acquisition and penetration into deeper tissues, which contributes to the spread of infections like pyoderma and abscesses.^[45] Hyaluronidases break down hyaluronic acid in extracellular matrices, promoting bacterial dissemination and tissue liquefaction in infected sites.^[45] These enzymes are consistently produced across strains but do not vary significantly between healthy carriers and pathogenic isolates.^[43] Toxin and enzyme profiles exhibit strain variability, particularly among clonal complexes (CCs), with higher expression in pathogenic lineages. For instance, CC71 (common in methicillin-resistant strains) often harbors multiple enterotoxin and exfoliative toxin genes, correlating with increased virulence in severe infections, while CC84 shows lower toxin diversity.^[43] This variability influences disease severity, as isolates from deep pyoderma express elevated levels of leukocidins and exfoliative toxins compared to superficial colonizers.^[42] These cytotoxins also indirectly modulate immunity by depleting leukocyte populations at infection sites.^[41]

Diagnosis

Specimen Collection and Initial Examination

Specimen collection for Staphylococcus pseudintermedius primarily involves obtaining swabs from infected sites in companion animals, particularly dogs, where it commonly causes pyoderma and other skin infections. Preferred specimens include swabs from intact pustules, epidermal collarettes, crust edges, ear canals, or open wounds, as these sites yield high bacterial loads while minimizing contamination from commensal flora.^[46]^[47] For deep pyodermas involving nodules or furuncles, fine-needle aspiration or small punch biopsies (3-4 mm) may be used to access subcutaneous material.^[46] Collection must be performed aseptically to prevent extraneous contamination: clip surrounding hair with sterile scissors (avoiding clippers to reduce skin trauma), avoid surface disinfection of lesions, and use sterile needles or forceps to lance pustules or lift crusts before swabbing exudate with a sterile cotton-tipped swab.^[46]^[48] Swabs should be immediately placed in a suitable transport medium, such as Amies or Carry-Blair, to inhibit overgrowth by non-target bacteria and maintain viability.^[48] Specimens are best stored at 4°C and processed within 24 hours to preserve bacterial integrity, though some media like ESwab support up to 48 hours at ambient temperature.^[48] Initial examination typically begins with cytological evaluation via direct microscopy of impression smears or tape preparations from lesion surfaces. Gram staining reveals characteristic Gram-positive cocci arranged in clusters, often phagocytosed within degenerate neutrophils, indicating active infection.^[47]^[49] Modified Wright's or Diff-Quik staining highlights inflammatory cells, such as neutrophils and macrophages, alongside extracellular and intracellular cocci, providing rapid assessment of bacterial morphology and host response.^[47]^[49] Direct microscopic screening estimates bacterial load but has limited sensitivity in low-density infections, where bacteria may not be visible despite clinical signs, necessitating confirmatory culture in such cases.^[47] Cytology alone cannot speciate S. pseudintermedius from other coagulase-positive staphylococci, underscoring the need for follow-up microbiological testing when infection is suspected.^[49]

Culture-Based Identification

Staphylococcus pseudintermedius is typically isolated from clinical specimens using non-selective or semi-selective media such as 5% sheep blood agar or Columbia colistin-nalidixic acid (CNA) agar, which inhibit the growth of many Gram-negative bacteria while supporting staphylococcal proliferation.^[50] Primary plating involves streaking the specimen onto these media and incubating aerobically at 37°C for 24–48 hours, allowing for the development of characteristic colonies.^[51] On blood agar, colonies appear as small (1–3 mm), round, smooth, opaque, and creamy gray to white, often exhibiting complete (beta) hemolysis, which contributes to presumptive identification as a potentially pathogenic coagulase-positive staphylococcus.^[15] Presumptive identification relies on standard microbiological tests to confirm staphylococcal characteristics and coagulase activity. The organism is Gram-positive, catalase-positive, and forms cocci in clusters, but key tests include the coagulase assays: a positive tube coagulase test using rabbit plasma (clotting within 4–24 hours) is reliable, while the slide coagulase test (bound coagulase detection via clumping) may yield weak or negative results due to lower clumping factor expression.^[15] The beta-hemolysis pattern observed on blood agar further supports this presumptive classification, distinguishing it from non-hemolytic staphylococci.^[50] To differentiate S. pseudintermedius from the closely related Staphylococcus aureus, additional biochemical tests are employed. The DNase test, performed on DNase agar with toluidine blue at 37°C, yields positive results for S. pseudintermedius, producing a clear zone around colonies after flooding with HCl, similar to S. aureus but confirming extracellular nuclease production.^[51] Novobiocin susceptibility testing, using disks on Mueller-Hinton agar, shows sensitivity (inhibition zones ≥16 mm at 5 μg concentration), which aligns with the profile of coagulase-positive staphylococci but helps rule out novobiocin-resistant species like certain coagulase-negative staphylococci.^[52] These tests, while useful, require careful interpretation as phenotypic overlap with S. aureus can occur. Culture-based methods have limitations, including the potential overgrowth of contaminating staphylococci or other flora on less selective media, which may obscure S. pseudintermedius colonies and necessitate subculturing for isolation of pure colonies.^[53] Additionally, variable expression of certain traits, such as coagulase reactivity, can lead to initial misidentification without confirmatory steps.^[50]

Molecular and Biochemical Confirmation

Following initial isolation through culture-based methods, definitive identification of Staphylococcus pseudintermedius relies on molecular and biochemical techniques that provide species-level precision, distinguishing it from closely related coagulase-positive staphylococci such as S. aureus and other members of the S. intermedius group (SIG).^[54] Polymerase chain reaction (PCR) assays targeting species-specific genes are widely employed for rapid confirmation. A key target is the nuc gene, which encodes a thermostable nuclease unique to S. pseudintermedius; primers amplify a 99-bp fragment in real-time PCR formats using SYBR Green detection, achieving high specificity when validated against reference strains.^[54] Alternative molecular approaches may target the phosphotransacetylase (pta) gene, which exhibits sequence divergence facilitating species discrimination via PCR-restriction fragment length polymorphism (RFLP) analysis, though nuc-based methods remain more routinely adopted due to their simplicity and established protocols.^[55] Biochemical confirmation utilizes commercial identification systems that assess enzymatic profiles and metabolic reactions. The API Staph system (bioMérieux) identifies S. pseudintermedius with 83-98% accuracy based on a 20-test strip, typically yielding a profile positive for alkaline phosphatase, acid phosphatase, esterase (C4), and esterase-lipase (C8), but negative for urease, β-glucuronidase, and Voges-Proskauer reaction; these patterns differentiate it from S. aureus (urease-negative but often β-glucuronidase-positive) and other SIG members.^[54] Similarly, the VITEK 2 system (bioMérieux) employs automated card-based assays for Gram-positive cocci, confirming S. pseudintermedius through comparable enzyme activities and susceptibility patterns, with overall agreement exceeding 90% when cross-validated against PCR. These panels are particularly valuable in veterinary diagnostic labs for processing isolates from canine pyoderma or otitis, where phenotypic ambiguity is common. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) offers a high-throughput alternative by generating species-specific spectral profiles from ribosomal proteins. Systems like VITEK MS or Bruker MALDI Biotyper can identify S. pseudintermedius, often as part of the S. intermedius group (SIG), from S. aureus, with log(score) values >2.0 indicating confident identification at the group level; database limitations may group it with S. intermedius, requiring molecular confirmation for species-level separation.^[56] This method's speed (minutes per sample) and low cost make it ideal for routine confirmation, outperforming traditional biochemical tests in accuracy for veterinary samples.^[57] Recent advances as of 2025 emphasize whole-genome sequencing (WGS) integrated with multi-locus sequence typing (MLST) for not only species confirmation but also clonal tracking. WGS assembles full genomes to verify core SIG markers while applying MLST schemes targeting seven housekeeping genes (e.g., abcZ, adk), identifying prevalent clones like ST71 (dominant in Europe) and ST68 (dominant in North America)—methicillin-resistant lineages associated with recurrent infections in companion animals.^[58] This approach, enabled by platforms like Illumina or Oxford Nanopore, supports epidemiological surveillance and outperforms single-gene PCR in resolving outbreak strains, though its adoption remains limited to reference labs due to cost.

Antimicrobial Resistance

Mechanisms of Resistance

Staphylococcus pseudintermedius exhibits antimicrobial resistance through multiple genetic and biochemical mechanisms that enable survival in the presence of antibiotics. One primary pathway involves beta-lactam resistance, mediated by the mecA gene, which encodes the penicillin-binding protein 2a (PBP2a). This altered protein has a low affinity for beta-lactam antibiotics, allowing continued cell wall synthesis despite exposure to drugs like methicillin and oxacillin. The mecA gene is typically integrated into the staphylococcal cassette chromosome mec (SCCmec), a mobile genetic element that facilitates its dissemination among bacterial populations.^[59]^[1]^[60] Multidrug efflux pumps represent another key resistance strategy in S. pseudintermedius, actively expelling antibiotics from the bacterial cell to reduce intracellular concentrations. NorA-like transporters, belonging to the major facilitator superfamily, are particularly implicated in resistance to fluoroquinolones such as ciprofloxacin and norfloxacin by pumping these hydrophilic compounds out of the cell. Similarly, efflux systems like Tet(K) and Tet(L) contribute to tetracycline resistance by exporting drugs like doxycycline, thereby diminishing their bacteriostatic effects. These pumps are often chromosomally encoded but can be overexpressed under selective pressure from subinhibitory antibiotic concentrations.^[61]^[62]^[63] Biofilm formation further enhances resistance in S. pseudintermedius, particularly in chronic infections where bacterial communities adhere to host tissues or medical devices. Within biofilms, extracellular polymeric substances create a physical barrier that limits antibiotic penetration, while slow-growing persister cells exhibit intrinsic tolerance to antimicrobial agents. This matrix-associated resistance can increase minimum inhibitory concentrations by factors of 10 to 1000 compared to planktonic cells, complicating eradication in conditions like canine pyoderma or otitis. Diagnostic tests for resistance, such as broth microdilution, may briefly reference biofilm effects but primarily assess planktonic growth.^[64]^[65]^[66] Horizontal gene transfer amplifies resistance dissemination in S. pseudintermedius through mobile genetic elements like plasmids and transposons. Plasmids often carry genes for efflux pumps or aminoglycoside-modifying enzymes, enabling conjugative transfer between staphylococcal species during co-infections. Transposons, such as Tn916-like elements, mobilize tetracycline resistance determinants via transposition events, integrating into the chromosome or other plasmids. These mechanisms promote interspecies exchange, including with Staphylococcus aureus, heightening the risk of multidrug-resistant strains emerging in veterinary and human settings.^[67]^[68]^[69]

Methicillin-Resistant Strains (MRSP)

Methicillin-resistant Staphylococcus pseudintermedius (MRSP) emerged as a significant veterinary pathogen around 2006, with the first reports documented in Europe from clinical cases in dogs.^[60] Since then, MRSP has become globally disseminated, particularly in companion animal populations, where it accounts for 10–30% of S. pseudintermedius isolates in many veterinary clinical settings.^[25] This resistance is primarily mediated by the acquisition of the mecA gene, which encodes a penicillin-binding protein conferring resistance to methicillin and other β-lactam antibiotics.^[60] The epidemiology of MRSP is dominated by specific clonal lineages that have facilitated its rapid spread. In Europe, the sequence type ST71 clone, often associated with SCCmec types II–III, was predominant until around 2015, while in North America, the ST68 clone carrying SCCmec type V remains most common; both lineages exhibit multidrug resistance profiles beyond β-lactams.^[58] As of 2025, emerging clones such as ST551 have become dominant in parts of Europe, replacing ST71, with additional lineages like ST258 and ST726 reported in Europe and Asia, contributing to ongoing outbreaks and intercontinental spread through pet travel and trade.^[70]^[71] These clones have contributed to outbreaks in veterinary hospitals and have been detected across continents through pet travel and trade.^[28] Key risk factors for MRSP colonization and infection in pets include prior exposure to antibiotics, particularly β-lactams and fluoroquinolones, which select for resistant strains in canine populations.^[72] Hospital-acquired transmission is also prominent, with MRSP spreading within veterinary clinics via contaminated environments, equipment, and direct animal contact, leading to nosocomial infections in affected facilities.^[73]

Treatment Implications and Alternatives

Treatment of methicillin-susceptible Staphylococcus pseudintermedius (MSSP) infections primarily relies on beta-lactam antibiotics, with cephalexin (22–30 mg/kg PO q12h) or amoxicillin-clavulanate (12.5–25 mg/kg PO q12h) as first-line options for canine pyoderma, administered for 3–6 weeks until clinical resolution plus one week.^[74]^[75] These agents are selected due to their efficacy against the majority of MSSP isolates and favorable safety profiles in dogs.^[74] For methicillin-resistant S. pseudintermedius (MRSP), beta-lactams are ineffective owing to mecA-mediated resistance, necessitating susceptibility-guided therapy with alternatives such as doxycycline (5–10 mg/kg PO q12h) or potentiated sulfonamides like trimethoprim-sulfadiazine (15–30 mg/kg PO q12h), provided in vitro susceptibility is confirmed.^[76]^[74] Topical therapies, including 2–4% chlorhexidine washes (applied every 1–3 days) and mupirocin ointment for localized lesions, form the cornerstone of MRSP management to minimize systemic drug exposure and reduce bacterial carriage.^[77]^[75] Adjunctive measures enhance outcomes across both MSSP and MRSP cases; topical antiseptics like chlorhexidine are recommended universally to decrease skin colonization and shorten systemic treatment duration.^[74] For deep pyoderma or abscesses, surgical debridement is advised to remove necrotic tissue and facilitate healing.^[77] The International Society for Companion Animal Infectious Diseases (ISCAID) guidelines, updated in 2025, emphasize antimicrobial stewardship by prioritizing topical therapies, mandating culture and susceptibility testing for recurrent or severe infections, and limiting systemic use to confirmed cases to mitigate resistance development.^[74]^[75]

Zoonotic Potential

Transmission Routes to Humans

_Staphylococcus pseudintermedius, primarily a pathogen of companion animals such as dogs, transmits to humans mainly through zoonotic pathways involving close animal contact. The bacterium colonizes the skin, mucosae, and gastrointestinal tract of dogs at rates up to 90%, establishing them as the principal reservoir for human exposure. Direct transmission occurs predominantly via physical contact with infected or colonized pets, particularly through handling of wounds, exposure to saliva during bites or licks, and skin-to-skin interactions. This route poses the highest risk to veterinarians, dog owners, and individuals with frequent pet handling, as evidenced by colonization rates of 4.5–5.6% among dog guardians and veterinary staff.^[78]^[79] Indirect transmission is less common but facilitated by contaminated environments and fomites, such as grooming tools, kennel surfaces, or household items shared with pets. Studies have detected S. pseudintermedius on veterinary clinic equipment and in animal housing areas, enabling transfer to humans via touch.^[80] Airborne or foodborne routes are rarely documented and considered negligible, with no substantial evidence supporting their role in human acquisition. Molecular typing methods, including pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST), have confirmed transmission by identifying identical strains in dog-human pairs, such as ST71 clones matching between pets and their owners.^[81]^[82] Human infections with S. pseudintermedius were first documented in 2006, with subsequent reports indicating low overall incidence but a potential increase linked to rising pet ownership and closer human-animal interactions. A case series of 24 infections highlighted pet exposure in 95.4% of cases, underscoring the zoonotic link without widespread community spread.^[83]^[79] Transmission remains uncommon, occurring primarily in households or veterinary settings where direct or fomite-mediated contact predominates.

Clinical Manifestations in Humans

Staphylococcus pseudintermedius primarily causes skin and soft tissue infections (SSTIs) in humans, such as abscesses and cellulitis, particularly among individuals with close contact to dogs or those who are immunocompromised.^[79] These infections often arise from wounds, including dog bites, and are typically mild to moderate in severity, allowing management on an outpatient basis with oral antibiotics.^[15] In a series of 24 human cases, 75% involved SSTIs, with the majority resolving without hospitalization.^[79] Systemic infections by S. pseudintermedius are rare but can occur in vulnerable populations, such as the elderly, diabetics, or those with underlying malignancies, manifesting as bacteremia, surgical site infections, or otitis.^[84] For instance, bacteremia has been documented in immunocompromised patients with indwelling devices, sometimes leading to endocarditis or pneumonia requiring inpatient care.^[15] Otitis cases, including externa, have been reported in elderly individuals with pet exposure.^[84] Unlike the pyoderma commonly seen in canine hosts, human infections lack an equivalent primary skin condition and show a stronger association with traumatic wounds.^[85] As of 2023, at least 87 human cases have been documented in the literature, often initially misidentified as Staphylococcus aureus due to biochemical similarities, which can delay appropriate diagnosis.^[1] A 2023 review (as of December 2022) identified 97 publications on human infections since the first report in 2006, with most cases mild but highlighting the pathogen's potential for severity in at-risk groups.^[15] Cases continue to be reported into 2025, including rare instances of severe infections like aortitis.^[86] Transmission from companion animals, especially dogs, is a common predisposing factor.^[79]

Risk Factors and Prevention Strategies

Individuals at higher risk for zoonotic transmission of Staphylococcus pseudintermedius include those with frequent close contact to infected or colonized companion animals, particularly dogs. Veterinarians and veterinary staff face elevated occupational exposure risks, with studies showing carriage rates of methicillin-resistant S. pseudintermedius (MRSP) up to 4% among small animal dermatologists due to direct handling of infected animals.^[87]^[88] Immunocompromised individuals, such as those with weakened immune systems from underlying conditions or treatments, are particularly vulnerable to opportunistic infections from the bacterium.^[85] Elderly persons with pet contact also exhibit increased susceptibility, as evidenced by case reports of severe infections like bacteremia in older adults cohabitating with carrier dogs.^[85] Children, especially young ones engaging in close play with pets, may face heightened risks through direct contact, necessitating targeted hygiene education for households.^[89] Conditions compromising skin integrity, such as atopic dermatitis, further amplify susceptibility by facilitating bacterial colonization and entry in humans with frequent animal exposure.^[90] Prevention strategies emphasize basic infection control measures to interrupt transmission from pets to humans. Rigorous hand hygiene, including thorough washing with soap and water or use of alcohol-based sanitizers after pet handling, is the cornerstone of personal protection and has been shown to significantly reduce carriage risks in high-exposure settings.^[91]^[92] In households with infected pets, prompt wound care—such as cleaning and covering any breaks in the skin—is essential to prevent bacterial ingress, particularly for at-risk family members.^[91] For pets identified as carriers, decolonization protocols involving topical antiseptics like chlorhexidine baths can effectively reduce bacterial loads on the skin and mucous membranes, thereby lowering household transmission potential.^[93] Public health efforts focus on surveillance and education to mitigate broader zoonotic threats from S. pseudintermedius, especially MRSP strains. Routine monitoring of bacterial prevalence on surfaces and in air within veterinary clinics helps identify and contain outbreaks, with environmental sampling revealing persistent contamination risks in high-traffic areas.^[94] Educational initiatives targeting MRSP risks have been promoted since 2010, when updated susceptibility testing guidelines were introduced to enhance diagnostic accuracy and awareness among veterinary professionals.^[95] As of 2025, the World Health Organization and aligned frameworks advocate One Health approaches, integrating human, animal, and environmental health surveillance in pet-dense urban areas to address antimicrobial resistance spread from companion animals.^[96]^[97]

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