88 BACTERIOLOGY Jan. 22, 1963 of water which shows the presence of a Gram negative motile rod fermenting lactose, how would you further study it to prove the absence of Escherichia coli? In the country with flush toilets, how would you provide for sewage piping it to a nearby stream were undesirable? stulates and the experimental difficulties encountered in fulfilling Principal mechanisms which prevent the entrance of bacteria into the What cells clear the blood of invading bacteria? Where do these clear after ingestion of the bacteria? Invasiveness, minimal lethal dose, average lethal dose. What methods are used to increase virulence of a culture of pathogenic antibody, complement, simple hapten, complex hapten. Contrast endotoxins. an adequately equipped laboratory and more advanced