The present invention discloses the use of a protease inhibitor in the preparation of a medicament for the treatment of microbial infections. The invention further discloses the use of a compound binding to a cross-.beta. structure or an antibody specific for a cross-.beta. structure in the preparation of a medicament for the treatment of microbial infections. The invention further discloses methods for attenuating microbial pathogens by deleting at least part of a gene encoding a cross-.beta. structure forming protein. The invention also discloses an antimicrobial composition, and a kit for detecting microbial contamination in a solution or a substance.