Lxis apex brand of cephalexin. cephalexin is the first generation antibiotic of cephalosporins group. It is active against a wide range of gram-positive and gram-negative bacteria. Cephalexin is also active against streptococcus pyogenes, diplococcus pneumoniae and corynebacterium diphtheriae. The gram-positive cocci commonly responsible for acute chest infections are highly sensitive to cephalexin. In addition, the antibiotic is active against gram-negative organisms which may be a problem in mixed infections. It appears to be a safe antibiotic with a low incidence of side effects and is usually well tolerated even by patients allergic to penicillins.