Cotrimoxazole is bactericidal in vitro to a wide range of Gram-positive and Gram-negative organisms, including Streptococcus, Staphylococcus, Pneumococcus, Neisseria, B. catarrhalis, Escherichia coli, Klebsiella, Proteus spp., Haemophilus, Salmonella, Shigella, Vibrio cholerae, Brucella, Pneumocystis carinii, Nocardia and Bordetella. A particularly high degree of activity is exhibited against Haemophilus influenzae, E. coli and Proteus spp., making Cotrimoxazole particularly suitable for the treatment of chronic bronchitis and urinary tract infections. Cotrimoxazole exerts its bactericidal action by the sequential blockade of two bacterial enzyme systems in the biosynthesis of Folinic acid in the micro-organisms. The synergy thus produced accounts for the high degree of bactericidal activity.