Experimental nasal colonization of piglets with methicillin-susceptible and methicillin-resistant Staphylococcus aureus

Virulence attributes of successful methicillin-resistant Staphylococcus aureus lineages

Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of severe and often fatal infections. MRSA epidemics have occurred in waves, whereby a previously successful lineage has been replaced by a more fit and better adapted lineage. Selection pressures in both hospital and community settings are not uniform across the globe, which has resulted in geographically distinct epidemiology. This review focuses on the mechanisms that trigger the establishment and maintenance of current, dominant MRSA lineages across the globe. While the important role of antibiotic resistance will be mentioned throughout, factors which influence the capacity of S. aureus to colonize and cause disease within a host will be the primary focus of this review. We show that while MRSA possesses a diverse arsenal of toxins including alpha-toxin, the success of a lineage involves more than just producing toxins that damage the host. Success is often attributed to the acquisition or loss of genetic elements involved in colonization and niche adaptation such as the arginine catabolic mobile element, as well as the activity of regulatory systems, and shift metabolism accordingly (e.g., the accessory genome regulator, agr). Understanding exactly how specific MRSA clones cause prolonged epidemics may reveal targets for therapies, whereby both core (e.g., the alpha toxin) and acquired virulence factors (e.g., the Panton-Valentine leukocidin) may be nullified using anti-virulence strategies.

Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) spa type t127, Sequence Type (ST)1, quickly spreads and persists among young pigs

Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) clones other than Clonal Complex (CC)398, as CC1, have been isolated in pigs in some countries, and appeared to be prevalent in Italy. The aim of this study was to evaluate the capability of Sequence Type (ST)1, CC1, LA-MRSA clone to colonize and to be transmitted among piglets. Eighteen caesarean-derived/colostrum-deprived piglets of 35 days of age were assigned randomly to three groups: four seeder piglets were contaminated with a spa type t127, ST1, SCCmec V, MRSA (Group A), 10 MRSA-negative piglets were exposed to Group A after 2 days post-contamination, dpc (Group B) and 4 piglets were used as control group (Group C). Piglets were evaluated until 44 dpc (Group A) or at 42 days post-exposure, dpe (Group B) and then euthanized and necropsied. All nasal and skin cultures of Group A resulted MRSA-positive throughout the experiment starting from two dpc, while Group C tested always MRSA-negative. At first sampling, all Group B piglets became positive and remained positive throughout the experiment. This is the first colonization/transmission study with a CC1 LA-MRSA in pigs. The results add further knowledge on the ability of CC1 LA-MRSA to colonize pigs, and on colonization/transmission patterns, both suggesting good host adaptation.

Persistent and Transient Airborne MRSA Colonization of Piglets in a Newly Established Animal Model

Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) was first found in 2005 and is up to date widespread in animal husbandry reservoir – focusing on pig farming. The regular detectability of MRSA in the air of pigsties as well as in exhaust air of pig farms (mean count: 10² cfu/m³) poses the question whether an airborne spread and, therefore, a MRSA colonization of animals via the airborne route exists. To answer this question, we exposed three groups of nine MRSA-negative tested piglets each to a defined airborne MRSA concentration (10², 10⁴, and 10⁶ cfu/m³) in our aerosol chamber for 24 h. In the following observation period of 21 days, the MRSA status of the piglets was monitored by taking different swab samples (nasal, pharyngeal, skin, conjunctival, and rectal swab). At the end of the experiment, we euthanized the piglets and investigated different tissues and organs for the spread of MRSA. The data of our study imply the presence of an airborne MRSA colonization route: the animals exposed to 10⁶ cfu/m³ MRSA in the air were persistent colonized. The piglets exposed to an airborne MRSA concentration of 10⁴ cfu/m³ were transient, and the piglets exposed to an airborne MRSA concentration of 10² cfu/m³ were not colonized. Consequently, a colonization via the airborne route was proven.

An experimental Staphylococcus aureus carriage and decolonization model in rhesus macaques (Macaca mulatta)

Our human model of nasal colonization and eradication of S. aureus is limited by safety issues. As rhesus macaques are closely related to humans and natural hosts for S. aureus, we developed an experimental decolonization and inoculation protocol in these animals. Animals were screened for nasal carriage of S. aureus and 20 carriers were selected. Decolonization was attempted using nasal mupirocin (10 animals) or mupirocin plus trimethoprim/sulfadiazine intramuscularly (10 animals) both once daily for 5 days, and checked by follow-up cultures for 10 weeks. Intranasal inoculation was performed with S. aureus strain 8325–4 in culture-negative animals. 11/20 animals, of which 5 received mupirocin and 6 the combination treatment, became culture-negative for S. aureus for 10 weeks and these 11 animals were subsequently inoculated. Swabs were taken once a week for 5 weeks to test for the presence of the inoculated strain. In 3 animals, strain 8325–4 was cultured from the nose 1 week after inoculation, indicating short-term survival of this strain only, a finding similar to that previously found in our human model. These data demonstrate that rhesus macaques may constitute a relevant animal model to perform S. aureus eradication and inoculation studies with relatively limited invasive handling of the animals.

Development of persistent gastrointestinal S. aureus carriage in mice

One fifth to one quarter of the human population is asymptomatically, naturally and persistently colonised by Staphylococcus aureus. Observational human studies indicate that although the whole population is intermittently exposed, some individuals lose S. aureus rapidly. Others become persistent carriers, as assessed by nasal cultures, with many individuals colonised for decades. Current animal models of S. aureus colonisation are expensive and normally require antibiotics. Importantly, these animal models have not yet contributed to our poor understanding of the dichotomy in human colonisation status. Here, we identify a single strain of S. aureus found to be persistently colonising the gastrointestinal tract of BALB/c mice. Phylogenetic analyses suggest it diverged from a human ST15 lineage in the recent past. We show that murine carriage of this organism occurs in the bowel and nares, is acquired early in life, and can persist for months. Importantly, we observe the development of persistent and non-persistent gastrointestinal carriage states in genetically identical mice. We developed a needle- and antibiotic-free model in which we readily induced S. aureus colonisation of the gastrointestinal tract experimentally by environmental exposure. Using our experimental model, impact of adaptive immunity on S. aureus colonisation could be assessed. Vaccine efficacy to eliminate colonisation could also be investigated using this model.

Comparison of livestock-associated and health care-associated MRSA-genes, virulence, and resistance

Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) may colonize and infect humans with close contact to pigs. We compared phenotypic and genotypic differences in resistance and virulence of LA-MRSA isolates from farms and farmers with hospital-acquired methicillin-resistant S. aureus (HA-MRSA) and assessed carriage rates. Samples from pigs (n=330), occupationally exposed personnel (n=63), the farm environment (n=134), and hospital patients (n=220) were obtained. Approximately 50% (166/330) of pigs were MRSA positive. All LA-MRSA were resistant to tetracycline, compared to only 8% of HA-MRSA (P<0.001). In contrast, HA-MRSA isolates showed significantly higher resistance rates to quinolones (81% versus 7%; P<0.001). All strains isolated from occupationally exposed personnel (61.9%; 39/63) belonged to CC398. HA-MRSA isolates were diversely distributed, with predominance of CC5 (62.7%). Human strains carried significantly more virulence genes than porcine strains, especially exotoxins (P<0.001) and immune-evasion cluster genes (P<0.001). There were significant differences in resistance patterns and recognized genotypic virulence loci between LA-MRSA and HA-MRSA.

The Effectiveness of Bacteriophages against Methicillin-Resistant Staphylococcus aureus ST398 Nasal Colonization in Pigs

Methicillin-resistant Staphylococcus aureus (MRSA) is an important colonizer in animals and an opportunistic pathogen in humans. In humans, MRSA can cause infections that might be difficult to treat because of antimicrobial resistance. The use of bacteriophages has been suggested as a potential approach for the control of MRSA colonization to minimize the—often occupational—exposure of humans. The aim of this study was to assess the efficacy of bacteriophage treatment on porcine nasal colonization with MRSA in vitro, in vivo, and ex vivo. The effectiveness of a bacteriophage combination of phage K*710 and P68 was assessed in vitro by incubating them with MRSA V0608892/1 (ST398) measuring the OD₆₀₀ hourly. To study the in vivo effect, bacteriophages were administered in a gel developed for human application, which contain 10⁹ plaque-forming units (pfu)/mL (K and P68 in a 19.25:1 ratio) for 5 days to piglets (N = 8) that were experimentally colonized with the MRSA strain. Eight piglets experimentally colonized were used as a negative control. The MRSA strain was also used to colonize porcine nasal mucosa explants and bacteriophages were applied to assess the ex vivo efficacy of treatment. Bacteriophages were effective in vitro. In vivo, sixteen piglets were colonized with MRSA but the number of CFU recovered after the application of the bacteriophages in 8 piglets was not reduced compared to the control animals (approx. 10⁵ CFU/swab). In the ex vivo model, 10⁸ CFU were used to establish colonization with MRSA; a reduction of colonization was not observed after application of bacteriophages. However, application of mupirocin both in vivo and ex vivo resulted in a near eradication of MRSA. In conclusion: i) The MRSA strain was killed in the presence of the bacteriophages phage K*710 and P68 in vitro. ii) Bacteriophages did not reduce porcine nasal colonization in vivo or ex vivo. Physiological in vivo and ex vivo conditions may explain these observations. Efficacy in the ex vivo model matched that of the in vivo system.