How do bacteria clog medical devices? Very quickly

The find­ing could help shape strate­gies for pre­vent­ing clog­ging of devices such as stents -- which are implanted in the body to keep open blood ves­sels and pas­sages -- as well as water fil­ters and other items that are sus­cep­ti­ble to con­t­a­m­i­na­tion. The research was pub­lished in Pro­ceed­ings of the National Acad­emy of Sciences.

Click on the image to view movie. Over a period of about 40 hours, bac­te­r­ial cells (green) flowed through a chan­nel, form­ing a green biofilm on the walls. Over the next ten hours, researchers sent red bac­te­r­ial cells through the chan­nel. The red cells became stuck in the sticky biofilm and began to form thin red stream­ers. Once stuck, these stream­ers in turn trapped addi­tional cells, lead­ing to rapid clog­ging. (Image source: Knut Drescher)

Using time-lapse imag­ing, researchers at Prince­ton Uni­ver­sity mon­i­tored fluid flow in nar­row tubes or pores sim­i­lar to those used in water fil­ters and med­ical devices. Unlike pre­vi­ous stud­ies, the Prince­ton exper­i­ment more closely mim­ic­ked the nat­ural fea­tures of the devices, using rough rather than smooth sur­faces and pressure-driven fluid instead of non-moving fluid.

The team of biol­o­gists and engi­neers intro­duced a small num­ber of bac­te­ria known to be com­mon con­t­a­m­i­nants of med­ical devices. Over a period of about 40 hours, the researchers observed that some of the microbes -- dyed green for vis­i­bil­ity -- attached to the inner wall of the tube and began to mul­ti­ply, even­tu­ally form­ing a slimy coat­ing called a biofilm. These films con­sist of thou­sands of indi­vid­ual cells held together by a sort of bio­log­i­cal glue.

Over the next sev­eral hours, the researchers sent addi­tional microbes, dyed red, into the tube. These red cells became stuck to the biofilm-coated walls, where the force of the flow­ing liq­uid shaped the trapped cells into stream­ers that rip­pled in the liq­uid like flags rip­pling in a breeze. Dur­ing this time, the fluid flow slowed only slightly.

At about 55 hours into the exper­i­ment, the biofilm stream­ers tan­gled with each other, form­ing a net-like bar­rier that trapped addi­tional bac­te­r­ial cells, cre­at­ing a larger bar­rier which in turn ensnared more cells. Within an hour, the entire tube became blocked and the fluid flow stopped.

The study was con­ducted by lead author Knut Drescher with assis­tance from tech­ni­cian Yi Shen. Drescher is a post­doc­toral research asso­ciate work­ing with Bon­nie Bassler, Princeton's Squibb Pro­fes­sor in Mol­e­c­u­lar Biol­ogy and a Howard Hughes Med­ical Insti­tute Inves­ti­ga­tor, and Howard Stone, Princeton's Don­ald R. Dixon '69 and Eliz­a­beth W. Dixon Pro­fes­sor of Mechan­i­cal and Aero­space Engineering.

"For me the sur­prise was how quickly the biofilm stream­ers caused com­plete clog­ging," said Stone. "There was no warn­ing that some­thing bad was about to happen."

By con­struct­ing their own con­trolled envi­ron­ment, the researchers demon­strated that rough sur­faces and pres­sure dri­ven flow are char­ac­ter­is­tics of nature and need to be taken into account exper­i­men­tally. The researchers used stents, soil-based fil­ters and water fil­ters to prove that the biofilm streams indeed form in real sce­nar­ios and likely explain why devices fail.

The work also allowed the researchers to explore which bac­te­r­ial genes con­tribute to biofilm streamer for­ma­tion. Pre­vi­ous stud­ies, con­ducted under non-realistic con­di­tions, iden­ti­fied sev­eral genes involved in for­ma­tion of the biofilm stream­ers. The Prince­ton researchers found that some of those pre­vi­ously iden­ti­fied genes were not needed for biofilm streamer for­ma­tion in the more real­is­tic habitat.

This work was sup­ported by the Howard Hughes Med­ical Insti­tute, National Insti­tutes of Health grant 5R01GM065859, National Sci­ence Foun­da­tion (NSF) grant MCB-0343821, NSF grant MCB-1119232, and the Human Fron­tier Sci­ence Program.