The invention provides methods useful for identifying polynucleotides expressed
in vivo by a microbe during infection of a host. Antibodies against antigens that
are expressed by the microbe in vivo and in vitro are adsorbed with cells or cellular
extracts of the microbe that have been grown in vitro. Unadsorbed antibodies are
isolated and are probed against a phage display library of the microbe's DNA or
RNA. A polynucleotide of the microbe that is expressed in vivo is isolated and identified.