Cephem compounds and ESBL-detecting reagents containing the same

A cephem compound or pharmaceutically acceptable salt thereof represented by the formula I: ##STR1##

wherein R₁ and R₂ may be the same or different and each represent hydrogen atom, nitro or cyano; R₃ represents C₁-C₆ alkyl which may be substituted with carboxyl; R₄ represents hydrogen atom or amino; X represents S or SO, there being no case where both of R₁ and R₂ are simultaneously hydrogen atom.