The drug-resistant bac­te­ria known as MRSA, once con­fined to hos­pi­tals but now wide­spread in com­mu­ni­ties, will likely con­tinue to exist in both set­tings as sep­a­rate strains, accord­ing to a new study.

The pre­dic­tion that both strains will coex­ist is reas­sur­ing because pre­vi­ous pro­jec­tions indi­cated that the more inva­sive and fast-growing com­mu­nity strains would over­take and elim­i­nate hos­pi­tal strains, pos­si­bly pos­ing a threat to pub­lic health.

Researchers at Prince­ton Uni­ver­sity used math­e­mat­i­cal mod­els to explore what will hap­pen to com­mu­nity and hos­pi­tal MRSA strains, which dif­fer genet­i­cally.  Orig­i­nally MRSA, which is short for methicillin-resistant Staphy­lo­coc­cus aureus, was con­fined to hos­pi­tals. How­ever, community-associated strains emerged in the past decade and can spread widely from per­son to per­son in schools, ath­letic facil­i­ties and homes.

Both com­mu­nity and hos­pi­tal strains cause dis­eases rang­ing from skin and soft-tissue infec­tions to pneu­mo­nia and sep­ticemia. Hos­pi­tal MRSA is resis­tant to numer­ous antibi­otics and is very dif­fi­cult to treat, while com­mu­nity MRSA is resis­tant to fewer antibiotics.

The new study found that these dif­fer­ences in antibi­otic resis­tance, com­bined with more aggres­sive antibi­otic usage pat­terns in hos­pi­tals ver­sus the com­mu­nity set­ting, over time will per­mit hos­pi­tal strains to sur­vive despite the com­pe­ti­tion from com­mu­nity strains. Hospital-based antibi­otic usage is likely to suc­cess­fully treat patients infected with com­mu­nity strains, pre­vent­ing the new­comer strains from spread­ing to new patients and gain­ing the foothold they need to out-compete the hos­pi­tal strains.

The researchers made their pre­dic­tions by using math­e­mat­i­cal mod­els of MRSA trans­mis­sion that take into account data on drug-usage, resis­tance pro­files, person-to-person con­tact, and patient age.

Pub­lished Feb­ru­ary 28 in the jour­nal PLOS Pathogens, the study was con­ducted by post­doc­toral researcher Roger Kouyos, now a scholar at the Uni­ver­sity of Zurich, and Eili Klein, a grad­u­ate stu­dent who is now an assis­tant pro­fes­sor in the Johns Hop­kins School of Med­i­cine. They con­ducted the work under the advise­ment of Bryan Gren­fell, Princeton’s Kathryn Briger and Sarah Fen­ton Pro­fes­sor of Ecol­ogy and Evo­lu­tion­ary Biol­ogy and Pub­lic Affairs at Princeton’s Woodrow Wil­son School of Inter­na­tional and Pub­lic Affairs.

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Kouyos R., Klein E. & Gren­fell B. (2013). Hospital-Community Inter­ac­tions Fos­ter Coex­is­tence between Methicillin-Resistant Strains of Staphy­lo­coc­cus aureus. PLoS Pathogens, 9 (2) e1003134. PMID: 23468619

RK was sup­ported by the Swiss National Sci­ence Foun­da­tion (Grants PA00P3_131498 and PZ00P3_142411). EK was sup­ported by Prince­ton Uni­ver­sity (Harold W. Dodds Fel­low­ship), as well as the Mod­els of Infec­tious Dis­ease Agent Study (MIDAS), under Award Num­ber U01GM070708 from the National Insti­tute of Gen­eral Med­ical Sci­ences. BG was sup­ported by the Bill and Melinda Gates Foun­da­tion; the Research and Pol­icy for Infec­tious Dis­ease Dynam­ics (RAPIDD) pro­gram of the Sci­ence and Tech­nol­ogy Direc­torate, Depart­ment of Home­land Secu­rity; and the Fog­a­rty Inter­na­tional Cen­ter, National Insti­tutes of Health.

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By Cather­ine Zan­donella, Office of the Dean for Research
Princeton University