A compound which is a cyclic oligosaccharide of formula or a pharmaceutically acceptable salt thereof, for use in a method of preventing or treating bacterial infection by removing or reducing the thickness of the capsular polysaccharide layer of a bacterium, which method comprises administering to a subject a single agent active in preventing or treating bacterial infection, said agent being said compound which is a cyclic oligosaccharide of formula or pharmaceutically acceptable salt thereof, or a mixture of two or more said compounds; wherein n is 3; and each S is the same or different and represents a saccharide unit, wherein one or more S units is modified at the primary carbon position by replacement of the hydroxyl group with a functional group X which is positively charged, or which bears a positive charge when in aqueous solution at pH 7; wherein each X is the same or different and represents a group of formula: -(Alk 1) p -(Ar) q -(Alk 2) r -A wherein: p, q and r independently represent 0 or 1; Alk 1 represents a straight or branched C 1-6 alkylene group wherein one or more of the CH 2 groups is optionally replaced with a heterogroup selected from -O-, -S-, -NR-, -NRCO-,-CONR, -CO-, -OCO- and -COO-, and wherein one or more of the CH 2 groups is optionally substituted with a substituent selected from OR, SR, NRR', CH 2 OR, CH 2 SR and CH 2 NRR', wherein R and R' independently represent H or a C 1-2 alkyl group; Ar represents phenyl; Alk 2 represents methylene or ethylene; and A represents -NR 2 or wherein each R is the same or different and represents H or a C 1-2 alkyl group. wherein n is 3; and each S is the same or different and represents a saccharide unit, wherein one or more S units is modified at the primary carbon position by replacement of the hydroxyl group with a functional group X which is positively charged, or which bears a positive charge when in aqueous solution at pH 7; wherein each X is the same or different and represents a group of formula: -(Alk 1) p -(Ar) q -(Alk 2) r -A wherein: p, q and r independently represent 0 or 1; Alk 1 represents a straight or branched C 1-6 alkylene group wherein one or more of the CH 2 groups is optionally replaced with a heterogroup selected from -O-, -S-, -NR-, -NRCO-,-CONR, -CO-, -OCO- and -COO-, and wherein one or more of the CH 2 groups is optionally substituted with a substituent selected from OR, SR, NRR', CH 2 OR, CH 2 SR and CH 2 NRR', wherein R and R' independently represent H or a C 1-2 alkyl group; Ar represents phenyl; Alk 2 represents methylene or ethylene; and A represents -NR 2 or wherein each R is the same or different and represents H or a C 1-2 alkyl group. p, q and r independently represent 0 or 1; Alk 1 represents a straight or branched C 1-6 alkylene group wherein one or more of the CH 2 groups is optionally replaced with a heterogroup selected from -O-, -S-, -NR-, -NRCO-,-CONR, -CO-, -OCO- and -COO-, and wherein one or more of the CH 2 groups is optionally substituted with a substituent selected from OR, SR, NRR', CH 2 OR, CH 2 SR and CH 2 NRR', wherein R and R' independently represent H or a C 1-2 alkyl group; Ar represents phenyl; Alk 2 represents methylene or ethylene; and A represents -NR 2 or wherein each R is the same or different and represents H or a C 1-2 alkyl group.