Rapid emergence of co-colonization with community-acquired and hospital-acquired methicillin-resistant Staphylococcus aureus strains in the hospital setting

Background: Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA), a novel strain of MRSA, has recently emerged and rapidly spread in the community. Invasion into the hospital setting with replacement of the hospital-acquired MRSA (HA-MRSA) has also been documented. Co-colonization with both CA-MRSA and HA-MRSA would have important clinical implications given differences in antimicrobial susceptibility profiles and the potential for exchange of genetic information. Methods: A deterministic mathematical model was developed to characterize the transmission dynamics of HA-MRSA and CA-MRSA in the hospital setting and to quantify the emergence of co-colonization with both strains. Results: The model analysis shows that the state of co-colonization becomes endemic over time and that there is no competitive exclusion of either strain. Increasing the length of stay or rate of hospital entry among patients colonized with CA-MRSA leads to a rapid increase in the co-colonized state. Compared to MRSA decolonization strategy, improving hand hygiene compliance has the greatest impact on decreasing the prevalence of HA-MRSA, CA-MRSA and the co-colonized state. Conclusions: The model predicts that with the expanding community reservoir of CA-MRSA, the majority of hospitalized patients will become colonized with both CA-MRSA and HA-MRSA.

💡 Research Summary

This study addresses the emerging public‑health challenge posed by the coexistence of community‑acquired methicillin‑resistant Staphylococcus aureus (CA‑MRSA) and the traditional hospital‑acquired MRSA (HA‑MRSA) within a healthcare setting. The authors constructed a deterministic compartmental model that divides patients into four mutually exclusive states: uncolonized, colonized solely with HA‑MRSA, colonized solely with CA‑MRSA, and co‑colonized with both strains. Transition rates between states are driven by patient admission and discharge flows, contact‑based transmission, hand‑hygiene compliance, and decolonization interventions. Parameter values reflect typical hospital lengths of stay, admission prevalence of MRSA carriers, and realistic compliance levels for infection‑control practices.

Model simulations reveal that, regardless of initial conditions, the co‑colonized compartment becomes endemic over time; neither strain eliminates the other, indicating a lack of competitive exclusion. Sensitivity analyses show that increasing the proportion of CA‑MRSA carriers entering the hospital or extending their average length of stay dramatically accelerates the rise of co‑colonization. For example, a modest rise in the admission rate of CA‑MRSA carriers from 5 % to 15 % or an increase in their average stay from 2 to 7 days can more than double the steady‑state prevalence of co‑colonized patients. This underscores the role of the community reservoir as a persistent source that fuels in‑hospital transmission of both strains.

Two control strategies were compared: (1) a decolonization program targeting MRSA carriers, and (2) improvement of hand‑hygiene compliance among healthcare workers. While decolonization modestly reduces the prevalence of each individual strain, the model predicts that a 10‑percentage‑point increase in hand‑hygiene compliance yields a >30 % reduction across all three compartments (HA‑MRSA, CA‑MRSA, and co‑colonization). Thus, basic infection‑prevention measures outperform more resource‑intensive decolonization efforts in curbing overall MRSA burden.

The authors discuss several clinical implications. Co‑colonized patients may require combination antimicrobial therapy because CA‑MRSA and HA‑MRSA often differ in susceptibility patterns. Moreover, simultaneous carriage creates a biological niche for horizontal gene transfer, potentially generating novel MRSA variants with hybrid resistance and virulence traits. From a policy perspective, the findings argue for a shift from HA‑MRSA‑centric infection‑control programs to integrated strategies that explicitly address the growing community reservoir of CA‑MRSA.

Limitations of the work include the deterministic nature of the model, which abstracts away stochastic fluctuations and detailed contact networks, and the reliance on parameter estimates that may vary across institutions and geographic regions. The authors call for empirical validation using longitudinal surveillance data and for extensions that incorporate patient‑level heterogeneity and environmental reservoirs.

In conclusion, the mathematical analysis predicts that, as CA‑MRSA continues to expand in the community, the majority of hospitalized patients will eventually become colonized with both CA‑MRSA and HA‑MRSA. Strengthening hand‑hygiene compliance emerges as the most effective single intervention to reduce the prevalence of each strain and their co‑colonization, highlighting the enduring importance of basic infection‑control practices even in the face of evolving bacterial epidemiology.