Abstract Can normal-sized bacteria pass through small holes, and if so how? We have found that common, potentially pathogenic, bacteria (which are nominally larger than 0.2μm) can cross a 0.2μm nylon membrane. All of the bacteria crossed from the upper membrane surface to the solid medium below the membrane; this ability was highly repeatable and did not depend on the make of membrane used. Bacteria growing below the membrane exhibited normal size and morphology. We suggest that the ability of common bacteria to pass through small holes has important implications in relation to their role as pathogens.