Background:Prior research indicates altered basalbowel permeability in IBS patients and aberrations inthe expression of tight junction proteins (TJP).Aim:We therefore studied the electrophysiologicalcharacteristics of colon biopsies using square wavecurrent analysisin vitro.Methods:Sigmoid colonic biopsies (unpreparedcolon) from 35 IBS patients [female=27, median age29 years (27–37)], (Rome III; IBS-C=4, IBS-D=6,IBS-M=25) and 20 healthy controls [female=14,median age 36 years (27–47)] were mounted in minia-turized Ussing chambers and a solution with or with-outE. coli(19108CFU/mL) was added to theluminal side. The electrical properties of the mucosawere assessed by square wave current analysis thatenables quantification of epithelial/tight junction resis-tance, net current generated by the membrane, andmembrane capacitance reflecting the process of exocy-tosis. To induce epithelial chloride secretion and/orrelease of mucin from goblet cells, carbachol (Cch,1 mM) was added on the serosal side. In addition,TJP, claudin and occludin expression in 114 IBSpatients [female=72, median age 32 years (27–39)](Rome III; IBS-C=14, IBS-D=26, IBS-M=74) and37 healthy controls [female=25, median age 27 years(23–35)] was analysed using qRT-PCR. Immunohisto-chemistry was used to determine the tight junctionprotein ZO-1 (TJP1) in 56 IBS patients [female=38,median age 30 years (24–36)] and 19 healthy controls[female=12, age median age 33 years (26–48)]. Datais shown as median, range 25–75 percentile.Results:Carbachol evoked similar changes in electro-physiological parameters in IBS patients and healthycontrols, with no differences seen between IBS sub-groups. However, IBS biopsies exposed toE. colihadan increased basal membrane resistance (10.4 Ohm(9.2–12.5) versus 8.3 Ohm (7.5–10);P=0.01) andnet membrane current (203.7lA/cm2(311.9 to115.9) versus121lA/cm2(210.3 to79.7);P=0.005) compared to unexposed IBS biopsies.E. coliexposed IBS biopsies also had an increasedresponse to CCh compared to unexposed IBS biopsies(321.5lA/cm2(411.3 to216.5) versus236.8lA/cm2(335.8 to102.3);P=0.01) and a reducedcapacitance response (4.3lF/cm2(2.5–8.2) versus 7.8lF/cm2(6.2–11.7);P=0.04). qRT-PCR analyses dem-onstrated a tendency towards lower expression ofclaudin (P=0.07) and TJP1 (P=0.06) in IBS patientscompared to healthy controls. Occludin expressionwas similar in IBS patients and healthy controls, butwas higher in IBS-C compared to IBS-M (P=0.004).No significant difference was seen in levels of TJP1ZO-1 protein expression between IBS patients andhealthy controls (P=0.4).Conclusions:Colonic mucosa from IBS patients reactsdifferently to mucosal exposure toE. coli, withincreased basal membrane resistance and net mem-brane current as well as an increased reactivity to themuscarinic agonist CCh. The epithelial mechanismsbehind this pattern remain unknown.